Designing Beyond Hot and Cold – Exploring Full-Body Heat Experiences in Sauna

The design of thermal experiences, often only associated with hot and cold sensations, encompasses a much wider range of qualities that can evoke emotional and sensory effects on users extending beyond this basic binary nature. Through a phenomenological study of a traditional Finnish sauna, we delve into how individuals perceive and express full-body heat sensations using both verbal and non-verbal methods and infer dimensions and parameters of heat that can inform future thermal experience design. Findings from participants’ expressions lead to formulating experiential dimensions such as the dynamic nature of heat, the aesthetics of discomfort, considerations of texture and heaviness, interoception, and personal memories that expand our understanding what heat as a design material consists of and the various possibilities it may hold. Furthermore, we propose three thermal parameters of motion, timbre, and distribution which can contribute to designing more intricate heat experiences and pave the way for further research in temperature interfaces.


P8 P2
Figure 1: Two body maps (each with front and back perspective) by two study participants visualizing their diverse heat experience of Finnish sauna

ABSTRACT
The design of thermal experiences, often only associated with hot and cold sensations, encompasses a much wider range of qualities that can evoke emotional and sensory effects on users extending beyond this basic binary nature.Through a phenomenological study of a traditional Finnish sauna, we delve into how individuals perceive and express full-body heat sensations using both verbal and non-verbal methods and infer dimensions and parameters of heat that can inform future thermal experience design.Findings from participants' expressions lead to formulating experiential dimensions such as the dynamic nature of heat, the aesthetics of discomfort, considerations of texture and heaviness, interoception, and personal memories that expand our understanding what heat as a design material consists of and the various possibilities it may hold.Furthermore, we propose three thermal parameters of motion, timbre, and distribution which can contribute to designing more

INTRODUCTION
Alongside multi-sensory technologies such as smell and taste interfaces there is a growing interest in researching and designing thermal experiences, as evidenced by an expanding body of research [5] and recurring conference workshops in Human-Computer Interaction for researchers to share technologies and methods [4].Thermal experience relates to the subjective and multi-dimensional perception of temperature-related sensations provided by thermal stimuli.Temperature feedback finds application in enhancing levels of immersion and realism in virtual environments [15,42].Researchers have explored its further usage as a means of communication to convey emotional content [11,30,57], provide notifications [41,58] or guide awareness [25,34].Moreover, temperature stimuli have been incorporated into devices to render wetness sensations [48].However, the primary emphasis in temperature-related studies within the HCI domain has predominantly revolved around the technological aspects of generating thermal feedback.Only a limited number of studies have identified experiential or aesthetic qualities of temperatures that go beyond the basic intensities of hot and cold [25,30,52,57].The sensory experience of temperatures holds many more potential dimensions such as richness, emotional impact, and captivating nature [18].For instance, sitting by a cozy fireplace on a chilly evening can evoke a sense of relaxation, contentment, and nostalgia.The warmth from the fire can create a calming atmosphere that may encourage social interaction and a feeling of togetherness.Immersing in cold water, such as swimming in a cold lake or ocean, can be exhilarating and promote a sense of adventure and vitality.How can we utilize these distinct qualities of temperatures in the design of thermal feedback?Nevertheless, the experiential qualities of thermal feedback, as highlighted by Jonsson et al. [25], remain largely unexplored due to their complexity and subjectivity.One reason for this gap is the challenge posed by the unfamiliarity of expressing and articulating thermal sensations beyond simple hot-cold descriptions [25].This presents a hurdle for designers in terms of creating, conveying, and evaluating thermal content.Similar challenges and missing experiential vocabularies have been observed in the haptic design of vibrotactile [38], ultrahaptic feedback [9] and electromagnetic stimulation [27].
Approaching this from a human experience perspective, we address this gap by asking: RQ1 How do individuals perceive a full-body heat experience of Finnish sauna and what are the experiential dimensions of heat?
RQ2 How can these experiential dimensions inform the design of heat experiences?
In a user study with ten individuals, we capture verbal, visual and tactile expressions of heat sensations through interviewing, body mapping and tactile exploration to identify and translate common experiential dimensions of heat.Given its cultural and physical significance as a place for heat experiences, we work with the traditional Finnish sauna as a use case for exploring thermal experiences.We employ both verbal and non-verbal methods of expression to access participants' tacit knowledge about thermal sensations of sauna and identify design qualities and parameters of heat.
Our research makes significant contributions to the TEI community in three key areas.Firstly, it enhances our understanding of the factors that contribute to a lived heat experience.Secondly, it sheds light on different modes of expression (verbal, visual, tactile) in enabling individuals to articulate their sensations related to heat.Lastly, it puts forward recommendations and potential avenues for future research and design in the field of thermal experiences.In summary, this paper serves as an initial stride towards assisting designers and engineers in creating intricate thermal experiences.

BACKGROUND AND RELATED WORK
In this section, we provide an introduction to thermal perception, review prior studies investigating the expressiveness of thermal feedback, and outline the methods employed to capture participants' thermal experiences including their relevant previous research in HCI.

Physiological principles of thermal perception
This section delves into the physiological principles of thermal perception, underscoring their vital role in designing for thermal experiences.By gaining a comprehensive understanding of these principles, we can better grasp the intricacies of thermal sensations and their effects on the human body.This knowledge can form a solid foundation for creating more effective and meaningful thermal interactions aligning with the human sensory system.Generally, temperature is a measure of the amount of thermal energy, or heat, in an object or a substance.When the temperature of an object or substance is different from the temperature of the skin, thermoreceptors send signals about that difference of temperature to the brain.The brain then interprets these signals and we perceive the temperature as being either hot or cold [50].Similar to tactile perception the skin contains different densities of thermal receptors across the body which affect our sensitivity levels to thermal stimuli differently [32].The skin is three-to ten-fold times more sensitive towards cold than hot stimuli due to an increased amount of cold receptors and a closer location to the skin's surface [16].There are low threshold receptors firing from 15 to 45°C and high threshold receptors that sense temperatures higher than 45°C and lower than 15°C causing pain.Extreme temperatures above 50°C and below 0°C will trigger pain receptors (nociceptors) and cause harm to human skin tissue.The body also has ways of regulating its own temperature through processes such as sweating and shivering.When the body is too hot, it will produce sweat to cool down the skin, and when it is too cold, it will produce goosebumps and shiver to generate heat [14].This is a homeostatic mechanism that is responsible for regulating and maintaining the body's core temperature, which is essential for overall well-being and survival [2].This interoceptive ability is closely linked to the thermoregulatory system and is an integral part of our overall perception of thermal environments [12].To conclude, our perception of temperature is the result of the interactions between the temperature of the environment including relative humidity, mean radiant temperature, and air velocity and the temperature receptors in our skin, as well as the body's internal temperature regulation mechanisms, physical activity, clothing and time [45].
The study of the human experience cannot be accomplished by relying on physiological activities alone [55].Hence, it is crucial to complement this understanding by investigating subjective user experiences to gain a comprehensive perspective on the intricate nature of thermal perceptions and experiences.

Expressiveness of thermal feedback
While studies in thermal HCI research often focus on how to generate and control thermal feedback from a technological perspective exploring different ways to generate thermal feedback in wearable systems [15,26] or investigate users' accuracy of recognizing various intensities and locations from a psychophysical standpoint [42,58] there is limited work on how heat can be used in more expressive ways than only hot and cold descriptions.Here, previous authors address the affective qualities of temperature feedback to influence users' valence and arousal [11,46,52] while others explore more ambiguous and subjective qualities [25,30,57].
Proposing the concept of 'thermal expression' Lee and Lim [30] present one of the first studies to highlight the need to understand and utilize thermal sensation in interactive systems in order to control heat and its impact on interpersonal communication, emphasizing the design opportunities and emotional value of heat as a communication medium.They define four expressive parameters to be controlled for thermal expression, namely temperature, duration, temperature change rate, and location.In previous work, they discuss different characteristics, values and potential of thermal expression for designers to consider [29].
In terms of affective qualities of expression, Tewell et al. [52] explore emotional concepts such as valence and arousal of thermal feedback in combination with text messages and find out that temperatures invoke arousal highlighting the possibility of using thermal feedback to augment text messages.El Ali and colleagues [11] find that thermal feedback can increase arousal and respectively de-/increase valence of incoming text messages.Salminen et al. [47] report increasing unpleasantness, arousal and dominance in participants when perceiving warmer and dynamic stimuli.In the work by Jain and colleagues [24], thermal patterns displayed on the back were explored to induce an experience of aesthetic chills and examine the effect of prior experiences.The study finds an increased chance of perceiving chills coupled with higher pleasure and engagement ratings when the thermal pattern matched participant's prior bodily experiences.
Further potential thermal qualities are presented by Wilson et al. [57] who report that participants linked warm feedback received at the palm of the hand to more ambiguous concepts such as presence, activity and quality while cool stimuli symbolized absence and poor quality.
Through interviews and body drawings by individuals evaluating an interactive heat mat during a Feldenkrais activity, Jonsson et al. [25] formulate experiential qualities of heat concerning the felt experience from within, subjectivity, subtlety and material qualities relating to inherent material attachment, slowness and heat transfer between two bodies (e.g.heating element and adjacent materials).They adopt a comprehensive body perspective in their study, considering multiple regions of the body.Their device allows stimulation of the shoulders, hips, and calf regions on the back side of the body.
Considering environmental conditions, Halvey and colleagues [17] examined the role of ambient temperature and humidity on the perception of thermal feedback.They prove that particularly ambient temperature has a significant effect on the detection and perception of thermal stimuli provided by a mobile interface.
While many of these studies focus only on individual spots on the body such as the palm [17,46,57], the wrist [17,30], and arm [52], we sought to develop approaches to capture participants subjective experiences further by embracing multimodal means of expression of perceiving immersive, full-body heat.

Accessing thermal experiences
Our objective is to draw insights from authentic heat experiences (RQ1), which can subsequently guide the design and research of more intricate heat experiences (RQ2).

Methodological background.
As part of the third wave of HCI, which emphasizes designing interactive systems that are more context-aware, embodied, and integrated into our daily routines [3], the emerging research fields of embodied design [10] and soma design [21] are giving greater importance to the sensory and experiential aspects of the body when designing technologies that interact with the human body.These fields draw among others from scientific disciplines such as phenomenology and somaesthetics to inform their approach.Phenomenology seeks to understand the essence of human experience by exploring the subjective and conscious aspects of phenomena [35].Somaesthetics offers a framework to understand and enhance the aesthetic and sensory dimensions of somatic experiences, encompassing both physical sensations and their impact on our overall well-being and self-awareness [49].
McCarthy and Wright [33] define experience as comprising four threads: the sensual, the emotional, the spatio-temporal, and the compositional thread.In the context of experiencing technology, these aspects encompass the sensory aspects or how an experience may feel, look, sound, taste or smell like, the emotional responses it may elicit in users, the influence of when and where an experience takes place, and the overall composition or unfolding of events during that experience.
Acknowledging the challenge of using verbal descriptions to express bodily experiences, which can be limited by a lack of appropriate vocabulary and unfamiliarity in discussing tactile sensations [38,39], our aim is to access participants' tacit and bodily knowledge of heat experiences of a Finnish sauna through the use of both verbal and non-verbal methods.

Tacit knowledge.
Tacit knowledge is characterized as knowledge that is difficult to express or codify in a systematic manner, as it is often acquired through personal experience, observation, and practice, and stored in the form of mental models, skills, and intuition [21].For the design of novel technologies Prpa et al. [44, p.1] recognizes the need 'for an in-depth understanding and articulation of users' experiences, as lived and felt, multi-dimensional, everpresent, and ever-changing phenomena, comprised aesthetic, affective, embodied, tangible, and intangible qualities'.HCI researchers can here turn towards second person research methods such as proposed by (micro-)phenomenology [43] in order to access users' experiences.However, acknowledging the difficulty to articulate these subjective experiences we seek to explore additional, alternative modalities of expression beyond verbalization, such as visual or tactile means, to capture the complex, entangled, and dynamic experience of temperatures.
Below, we present the three selected methods and their previous application in HCI to capture somatic experiences and access tacit knowledge.

Micro-phenomenological interview.
As phenomenology defines the scientific field of studying human experience, microphenomenological interviewing is a technique to access and document participants' intricate experiences and perceptions ( [43] based on Explicitation interview by [56]).Its purpose is to carefully guide and direct the interviewee's focus towards the experience being explored, guiding them through the process of recalling the experience, and encouraging them to reflect on specific aspects of it.Relating to the experiential threads [33], Petitmengin [43] structures experiences into diachronic and synchronic elements.Diachrony describes how an experience unfolds over time through its main elements, while synchrony defines the sensory and emotional aspects or how an experience is perceived by an individual in-depth.This interviewing technique has been used by various researchers in HCI to investigate the nuances of different user experiences.Prpa and colleagues [44] shed a light on five HCI and design experts and their use of the method.In the field of tactile interfaces, researchers utilized micro-phenomenology to understand individuals' experiences and articulations about novel haptic sensations such as electric muscle stimulation [27] or mid-air haptics [9,38].In these studies, participants were typically asked 'What words would you use to describe how this feels?','Does this relate to any previous experience?','How would you describe this to someone who has never experienced this?'.Furthermore, participants descriptions get constantly repeated or reformulated by the interviewer to help them diving deeper into these singular experiential moments of investigation.

Body mapping.
Soma experiences are inherently subjective and complex, making them challenging to describe due to their bodily and emotional interconnections with the world.Body mapping is a tool that allows for the visualization of bodily sensations, emotional information, and other subjective experiences [13].Individuals are asked to identify on a body outline where they experience specific sensations or emotions, such as pain, comfort, tingling, or warmth.This tool is useful in enabling individuals to become more self-aware of their internal experiences.Creating a body map typically involves drawings or scribbles that are based on the individual's intuition and connected to their personal experiences.Commonly, participants are allowed to use words in combination with the drawings for further expression.Body mapping has found increasing use in the HCI and TEI community [1,23,25,36,51,54], particularly, for designing technologies that come into contact with the body.Cochrane et al. [1] provide various examples of design research utilizing body maps as a tool to capture soma experiences.Jonsson and colleagues [25] use body maps to document thermal experiences of participants using their heat mat.In the context of walking meetings, [23] ask participants to reflect on their own experiences, but also their relationship to others and the environment.In a TEI studio, [36] propose to advance body maps into the third dimension by using tangible materials for expression.Furthermore, Tennent et al. [51] explore the temporal aspect of soma experiences and propose trajectories of bodily experiences as a way to represent change over time.And most recently, Vidal and co-authors [54] call for enhancing body maps as a body-centric research tool by considering aspects such as temporality, sociality, representativeness, granularity, context, and focus.

Sensual evaluation instruments.
As an additional and third modality of expression that can facilitate participants in expressing temperature experiences through tactile means we use the Sensual Evaluation Instruments (SEI) that consist of eight differently shaped clay objects [22] (see Figure 2).By evoking immediate and affective responses in participants these shapes can represent different levels of arousal and valence.Initially, the shapes were inspired by emotional states of confusion, frustration, fear, happiness, surprise, satisfaction, contentment, stress, and flow.Furthermore, they serve as an additional channel of non-verbal expression and can be used to quantify sensory properties and preferences of different materials, products or experiences, as perceived by human senses.SEI have been used in HCI for evaluating taste [37] and thermal experiences [25].In the study by [25], however, the objects were removed from the protocol because participants experienced difficulties in utilizing them for expression.Our objects for the study were derived from the original shapes and we subsequently handcrafted our own versions using clay (1: pseudopod, 2: bubbly, 3: doubleball, 4: spiky, 5: barba papa, 6: stone, 7: anteater, 8: ball; names originate from [22]).[22] In this background section, we have introduced the physiological aspects of thermal sensation, emphasizing their significance in the assessment and creation of thermal experiences.Additionally, we have provided an overview of relevant research that delves into the expressive and emotional aspects of thermal feedback.Furthermore, we have introduced both our methodology and previous work that leveraged techniques to access tacit knowledge about heat experiences.Now, as we move forward, we will delve deeper into our study of exploring individuals' experiences of full-body heat in the traditional Finnish sauna.Our aim is to identify additional heat qualities that can contribute to the creation of more complex and refined thermal experiences in the space of multi-sensory technologies.

CAPTURING HEAT EXPERIENCES IN SAUNA 3.1 Context
The sauna is a significant cultural institution in Finnish society and is recognized as an important part of the country's national identity.Its popularity extends worldwide, with many people acknowledging its potential health benefits.The traditional Finnish sauna involves heating a small, enclosed space to high temperatures (80-100℃) using a wood-fired stove or electric heater and then generating steam by pouring water over heated stones.Sauna-goers typically sit on wooden benches in the hot, humid environment and may alternate between sweating and cooling off with cold water by taking a shower or a dip in a nearby lake.The Finnish sauna has also been the subject of scientific study, with research suggesting various health benefits, including improved cardiovascular function, decreased inflammation, and a reduced risk of dementia [28].Furthermore, as a case study, the Finnish sauna is particularly intriguing for investigating thermal experiences due to the fast and dynamic changes in extreme temperatures that visitors experience.
To assist participants in narrowing their focus and immersing themselves in a specific heat-related moment in the sauna, we chose löyly (Finnish) as our subject of phenomenological investigation.Löyly (also translated as 'Aufguss' in German) refers to the moment when water is poured over the heated rocks, releasing hot steam that begins to fill the room (see Figure 3).

Study procedure
Prior to the study, ethical clearance was obtained from the university ethics committee.Firstly, participants were introduced to the aim of the study and signed an informed consent form.Each study session lasted between 60 and 75 minutes and consisted of two sauna sittings of approximately ten minutes each, with a break in between sittings.During the break the participants took a shower and then cooled down to a neutral state before going back into the sauna for a second time.To obtain similar thermal conditions for each participant, the temperature in the sauna was kept between 80 and 90℃.Within the ten minutes sauna sittings four regular-sized scoops of water were poured on the hot stones (see Figure 3), two scoops in the beginning of the sitting and two more scoops after approximately five minutes.We did not use any scents or oils such as eucalyptol or menthol since these can have significant effects on tactile and thermal perception as proven by [6].The study was conducted in two saunas of similar size and layout.Audio recordings were taken during the interviews and eventually transcribed for thematic analysis.
We employed a triangulation of methods by utilizing three distinct modes of expression: verbal, through an interview that was inspired by micro-phenomenology; visual, through body mapping; and tactile, through Sensual Evaluation Instruments (similar as in [25,38]).This multimodal approach allowed individuals to express themselves in various ways, providing access to participants' implicit bodily experiences related to heat (see Figure 5).The integration of diverse methodologies and data types has been a longstanding practice in the field of social science [19].With triangulation researchers are able to enhance their understanding of the research subject, as it enables the inclusion of various methods that can capture distinct facets of the phenomenon being investigated.This approach contributes to a more comprehensive and profound perspective, facilitating a more refined and holistic analysis.
Our intention was to prevent participants from basing their drawings and object selections on the verbalization they made during the interview.Hence, in order to randomize the sequence of methods we divided the participants into two groups, each group following a different sequence of methods (see Figure 4).Participants in Group A (verbal -visual -tactile) were first interviewed in the sauna, following a body mapping exercise in the break, and, eventually, were asked to explore the SEIs.While Group B (tactile -visual -verbal) would still first go to sauna, however, without being interviewed we asked them focus on their bodily experiences silently, then in the break selected the SEI objects first, created the body map and finally were interviewed in the second sitting.
Aligning with micro-phenomenology and to help participants concentrate on a specific moment of a thermal experience, we chose to exclusively examine the moment of 'löyly', which is when water is poured onto the sauna stones.In the interview, participants were asked to describe their bodily sensations in real-time while the principal researcher was present in the sauna room.The participants were prompted with questions such as 'What words would you use to describe how this feels?','Does this relate to any previous experience?',and 'How would you describe this to someone new to sauna?' (adapted from [27,38]).We also frequently reformulated the descriptions provided by the participants to ensure a consistent focus on their experiences.For body mapping, participants were given markers and sheets of paper with illustrations of the body in a seated position, where they could visualize their experiences through drawings.They were allowed to make adjustments to their drawings after the second sitting if they had discovered new aspects of the experience.Afterwards they were asked to explain their body map.For tactile exploration, participants were asked to blindly feel the eight different clay shapes and pick one, multiple or none that may represent the felt heat experience.Again afterwards, they were asked to explain their selection.
For each participant, the study concluded with a questionnaire about participants' demographic information and their ratings of the different methods of expression.Each method was rated based on its difficulty level of expressing their experiences on a Likertscale from 1 to 5. Finally, we asked for an overall ranking of the methods on intuitiveness.For safety reasons, we monitored participants' well-being throughout the study session and reminded them that they can leave the sauna at any time of feeling unwell.All participants reported that they had experience with high temperatures in the sauna beforehand.

Participants
Given the nature of this study, which delves into intimate full-body experiences, the first author opted to invite only participants with whom they have a personal connection.This decision was made to create an environment of trust and familiarity, enabling participants to express their subjective experiences with greater openness and depth.By having a personal relationship with the participants, the main author sought to cultivate a sense of comfort and understanding, encouraging them to share their intimate experiences more freely [53].
Ten individuals (four women and six men) participated in the study, with an age range from 26 to 56 years (mean age is 31.6 years).Nine out of ten reside in Finland.As sauna is a common recreational activity in Finland, all participants are regular sauna visitors, attending at least bi-weekly.Most participants reported no particular features related to their sense of touch or temperature.P2 mentioned difficulty in distinguishing certain textures like wool or cotton, while P7 reported slight abnormalities in the feet region.Despite these reports, we decided not to exclude them from the data analysis as we believe that these sensory features did not significantly impact the overall heat experience.Participants were from Guatemala, Colombia, Russia, Finland, Estonia, Spain, the Netherlands, and Germany.P10 is Finnish and exhibited a deeper connection to the sauna experience, evident through numerous memories and emotional associations linked to this cultural tradition.None of the participants is a native English speaker, but all felt confident in using English during the interviews.P8 possesses expertise as a visual communication designer, while P3 and P6 work as journalists.Their professional backgrounds might have shaped their capacity to express their thoughts and sensations, using their visual or verbal skills.To encourage participation, each subject received a gift card valued at €20.

Analysis of interviews
To analyze the interviews, we utilized thematic analysis [7].Initially, the authors transcribed the data and familiarized themselves with its content, identifying meaningful codes.Through iterative discussions, the codes were revised, and experiential dimensions were generated as themes.Additionally, we conducted an analysis of the specific verbal, visual, and haptic language used by participants to express their thermal sensations.For body maps and SEI objects, we closely examined the drawings and object properties, deconstructing distinct visual and haptic elements and their significance concerning the heat experience.The participants' verbal reports regarding the body drawings and SEI choices were also taken into account during this analysis.).Furthermore, we present the participants' evaluations of the methods.Following that, we delve into an exploration of the overall experiential dimensions that emerged from the thematic analysis.By analyzing the thermal expressions and dimensions, we identify parameters that can guide the design of thermal experiences (see Figure 6).

Heat expressions
4.1.1Verbalizing.During the study, participants used a range of metaphors to vividly articulate their heat sensations.The most commonly used metaphor was that of a blanket, suggesting the encompassing and enveloping nature of the heat experience.This blanket was described by one participant as if someone is placing a blanket over their face while sitting on a sunny beach (P5) while another one described its heaviness as that of a heavy winter blanket (P1).Additionally, participants compared the approaching of heat to a wave, emphasizing its gradual and immersive arrival (P1, P5, P6, P10).Some participants described the heat as if it was a cloud engulfing them (P3), conveying a sense of being completely surrounded by its presence.Other textile metaphors emerged with participants expressing how the heat felt like wearing a hoodie (P2).These metaphors not only add depth and imagery to the descriptions of heat sensations but also reveal the participants' attempts to convey the multi-sensory and atmospheric qualities of their experiences.
4.1.2Visualizing.Body mapping as a tool allowed for a better understanding of how, primarily, heat intensity and temperature varied across the body through differently colored body parts.In terms of wording, some participants explained the colors as representing 'intensity levels of heat' and for others the 'levels of temperature from hot to cool'.Further sensations reported through body maps were pressure, tingling, sparks, movement of temperature, and discomfort being a more abstract sensation.Participants used colors, lines, dors, arrows and shapes as visual elements to represent bodily sensations of temperature (see Figure 7).The color red often represents the most intense or hottest body regions, mainly in the head and upper body region.Other colors such as orange, yellow, green, blue or purple were used to visualize cooler regions of the body or even internal bodily sensations.Lines were mostly used to visualize pressure, a sense of burning, for example, in the area of the ears, a feeling of alertness or general movement of water and sweat across the body.While dots represented sweat sensation and water drops, arrows helped to depict the movement or direction of heat.Participants added shapes to the body maps as a way to represent water droplets, or body parts that were missing in the drawings (e.g.lungs, eyes or nose).Overall, the body map provided way to analyze thermal sensation in relation to the body's spatial layout.
4.1.3Tactilizing.The SEI figurines helped participants in expressing more abstract concepts such as time or (dis-) comfort.Participants reported phasic changes, for instance, from initial discomfort to comfort and relaxation.This was explained by picking shape 5 (see numeration in Figure 8) representing time or selecting two different objects with round and sharp edges (such as shape 4 and 5).Comfort was linked to round, soft and smooth properties of the objects (1, 2, 3 or 5).While pain and discomfort was mostly represented in the sharp figurine 4. Furthermore, metaphors that were already used in the interviews were re-emphasized in the properties of the shapes: a 'cloud' was found in shape 1 or 'sparks' in shape 4. Shapes (such as 3 or 5) that fit ergonomically well into participants' hands were compared with the heat shaping its way around the human body.In terms of number of selected objects, every other participants picked two shapes representing two distinct qualities of thermal sensations (for instance discomfort and comfort), four out of ten individuals chose only one object and one person selected three different ones.To conclude, the tactile aspects that were used by participants to describe their perception of heat included roundedness, sharpness, multifarious, asymmetry, volume and weight (Figure 9 shows the iconic depiction of these elements and their representations in relation to heat).This indicates that drawing was evaluated as the easiest method, followed by tactile exploration and verbal expression.In addition, participants were asked to rank the three methods in terms of naturalness or intuitiveness for expressing thermal sensations.Six participants ranked drawing as the most natural method, while three participants considered talking as the most natural, and one participant favored touching as the most natural approach (see Figure 10).Overall, these findings indicate that most of the participants found visualizing as the easiest and most natural way of expression thermal sensations.Surprisingly, verbal and tactile methods were ranked with similar levels of difficulty, without one surpassing the other.However, individual opinions differed, as P2, for instance, reported that visualizing with body maps felt as 'downstreaming or reducing my feelings to simply lines and dots' while tactile exploration helped them to relive the experience internally but still posed a challenge for them to externalize sensations to a third person.Hence, they ranked verbalizing as most intuitive for articulating sensations.

Experiential dimensions of heat
Building upon the previous analysis of expressive elements of each method and a general thematic analysis, we identified four main experiential themes about heat that emerged: (1) the dynamic nature of heat, (2) the aesthetics of discomfort, (3) texture and heaviness, (4) interoceptive quality, and additional observations relating to participants' personal memories and emotional reactions.

Heat is dynamic.
The dynamic nature refers to how heat was perceived by participants in terms of time and physical location.It involves the dynamic aspects of heat, including its duration, intensity and distribution in time.
Many participants described a sensation of movement of heat across the body: 'it starts from the head and then goes slowly down the torso and legs and arms' (P7).The following verbs were used as descriptors for heat movement: 'descending' (P1), 'going down' (P4, P5, P7), 'traveling' (P8), 'creeping', 'moving' (P9), 'coming and growing' (P3), 'rising', 'spreading' (P2).Furthermore, this was emphasized by participants using movement metaphors in the interview such as experiencing a 'wave' (P7, P8) or 'pouring a bucket of heat over me' (P7).In addition, participants described this sensation as 'allencompassing', 'engulfing' (P1), and 'tight hugging' (P10).With their SEI selection, participants addressed this dynamic sensation by, for instance, picking shape 5 (see Figure 8) that represents different phases of heat (P5, P8).This sensation of movement was also affected through distinct body parts being affected by the heat differently.The heat was often accumulated in head and upper body region while legs and feet were perceived as less intense or colder.This was mentioned in the interviews but also in the drawings represented through the upper body regions that were often colored with red or purple colors symbolizing higher levels of heat or intensity.Moreover, lines represented the dynamic movement   of heat across the body (see Figure 11).The SEI figurines represented the diversity of heat experience across the body through their asynchronic or diverse shape (shapes 2 and 5) (P5, P6, P8).Based on the interviews with participants, the experience of discomfort followed by a sense of relief played a vital role in shaping the overall heat experience.The temperatures induced in almost all participants a state of relaxation in both the body and the mind that promoted a sense of well-being, calmness and tranquility: 'warmth is comfortable and peaceful' (P3), 'I feel cozy, relaxed, warm, happy' (P5).But this state was only reached after going through an initial, short period of intense or almost uncomfortable heat.Participants described how their body fell into an alarming state after water was thrown onto the stove: 'I want to kind of close my eyes and hide my face with my palms' (P5) or 'there is a sense of alertness in expectation what is about to come' (P1).However, P8 describes 'you have to be patient enough to endure the initial intense heat until it feels nice and warm' (P8).With regards to time, 'comfort settles in over the period of 30 seconds' (P1).Particularly, the SEI figurines depicted this discomfort-comfort interplay through many participants picking two very opposite objects in terms of shape, one representing pain through sharp and spiky edges (shape 4) and one round figurine for comfort, relaxation and well-being (shape 3 or 5).

4.2.3
Heat possesses texture and heaviness.Participants described the heat as having a distinct texture and heaviness that influenced their physical state.Specifically, during the initial phase when water made contact with the stones, the heat was likened to applying pressure on the face (P5).P1 describes the first moments as in 'The entire body is tingling'.When exploring the SEI objects, such as the spiky shape 4, participants reported sensations akin to 'sparks' on their bodies (P2, P4, P10).P2 even compared the sensation to electricity.These sparks were also represented in some of the drawings through small lines or dots particularly in the head regions.P10 visualized 'small needle spots' across the body with red and orange, some pressurizing the body more and some less.Individuals also mentioned the perceived weight of the heat with metaphors such as 'a heavy winter blanket' (P1).P1 reported that among the SEI objects they were missing heavier objects that would actually represent the perceived weight of the heat indicating that the heaviness of heat was a pivotal characteristic.

4.2.4
Interoceptive quality of heat.Another aspect highlighted by participants is the pervasive ability of heat to move beyond the skin surface and 'touch' the inner core of the body (similar experiences described in [25]).During the interview, participants reported the sauna experience would feel like 'heating from the inside' (P4) or 'it is not only the external heat but also inside' (P2).P8 illustrated this distinction of internal and external sensations in their drawings by picking blue color for inside relating to heavier breathing rhythms and pressure on the stomach and colors of orange, red, yellow and green for the outer sensations concerning varying heat intensities (see Figure 12).This interoceptive quality of heat, driven by the temperature contrast between the body's interior and the skin, form a fundamental component of the body's ability to maintain thermal balance and adapt to varying environmental conditions [8].For the participants, this resulted in changes of physical nature (P3: 'I want to kind of close my face with my palms.'),physiological nature (P2: 'I can feel my heart beating.'),and affection (P6: 'When the heat comes, I'm like, I am scared all the time.').Hence, individuals perceived heat not only as a bodily sensation internally and externally but also as an emotional state which ranged from discomfort or distress to comfort and relaxation.

Internal sensations
Figure 12: Example depiction of internal (blue) and external sensations (red, orange, yellow, and green) by P8 4.2.5 Further observations about personal memories and emotions about heat.'Heat brings back memories from the past' (P10).Participants expressed positive memories and emotional reactions during the sauna session.Particularly, P10 (a Finnish person who has been going to sauna for decades), remembered the warm feeling of pressure 'as being wrapped in a blanket as a baby, there is both warmth and pressure' or 'as being hugged by a friend, first slightly pressured then warmth and relaxation'.The blanket metaphor (not necessarily connected to childhood) was used by three other participants and also found representation in the body maps.Moreover, P10 described how the practice of sauna fostered a meaningful relationship with their father: 'But I actually have so many memories of sauna with my parents and it's always with my dad.It was the moment when I sort of I grew to it or connected with my dad.So otherwise we didn't do that much.You know, he didn't take me to sports or anything like that.'.This highlights the contextual and cultural significance of heat, particularly in the context of sauna, and how it intertwines with personal memories, relationships, and emotions.

Design parameters
Heat possesses specific properties that can be manipulated by a designer, including temperature, duration, location, and rate of change.By deliberately selecting and manipulating these variables, designers can create diverse effects and develop more nuanced thermal experiences beyond simply perceiving hot and cold intensities.Lee and Lim [30] call this 'thermal expression'.Based on our thematic analysis of participants' heat expressions and identified experiential dimensions and to address RQ2 ('How can these experiential dimensions inform the design of heat experiences?'),we define three new parameters (in addition to temperature, duration, location, and rate of change suggested by [30]) that can assist designers in crafting more intricate experiences: motion, timbre, and distribution (see Figure 13).These parameters can be included to address the three experiential qualities we identified above: dynamics, aesthetics of discomfort, texture and heaviness, and interoception.We define these parameters as secondary because they build upon the primary ones by [30].For instance, motion considers factors such as duration, location, and rate of change, timbre can be a combination of duration, distribution, and temperature, and distribution relies on the number of stimulated body locations.

Motion.
Motion of temperature feedback on the skin refers to the perception of temperature changes over time and location that creates a feeling of movement across the skin's surface.This can be, for instance, experienced as a gradual shift in temperature, akin to a wave-like or flowing sensation, compared to a static and constant thermal stimulus.In VR (Virtual Reality) experiences or gaming, moving thermal stimuli can simulate environmental conditions, such as wind or flowing water, adding an extra layer of realism and immersion to the virtual environment.
4.3.2Timbre.Our findings reveal that heat possesses distinct timbre or tonality perceived by participants.Just as timbre in sound refers to the unique tonal quality of different musical instruments or voices, thermal timbre can be used to describe the distinctive character of temperatures.Individuals described heat as having, for instance, sharp or 'electrifying' qualities, and they also expressed how it can exert a heavy load on the body.However, heat was also able to sooth and immerse participants into a comfortable, relaxing state of body and mind.Thermal technologies with various timbres can be integrated into, for instance, smart home devices, allowing users to personalize their thermal experiences based on their mood or activity.For example, a user may prefer a gentle and relaxing timbre during bedtime (e.g., comfortably warm and static stimulus that covers larger areas of the body), while a more invigorating tonality could be chosen for morning wake-up routines (e.g., recurring cold feedback pattern at pointed locations).

Distribution.
Distribution refers to the way in which heat is experienced, either as a localized sensation in a specific area of the body or as a more encompassing and ambient sensation that can permeate the entire body.In therapeutic settings, devices with adjustable distribution settings can be used for targeted heat therapy.For instance, a device with localized heating can be applied to specific muscle groups for pain relief, while a distributed heat mode can be used for overall relaxation and stress reduction.
These additional parameters present valuable elements that can be leveraged when designing thermal experiences.Moreover, they offer potential avenues for further exploration in future research to deepen our understanding of how these properties influence and enhance the overall temperature experience.

Motion Timbre Distribution
Figure 13: Identified heat parameters

DISCUSSION
Through our research, we have unveiled further unique qualities of heat that can be harnessed as valuable design parameters.In this section, we discuss our work from methodological standpoints, addressing limitations and potential areas for future research.

Reflections on methods
5.1.1Participant selection.For this study, we opted to include participants who have a personal connection with the first author.This decision was motivated by the hope to establish trust and openness, especially when delving into intimate bodily experiences in the context of sauna.The intention was to gain deeper insights and a more comprehensive understanding of thermal experiences within a richer contextual framework.However, it is essential to acknowledge that this sampling approach may introduce bias, as the findings could be more representative of the researcher's acquaintances rather than a diverse population.Despite this limitation, we believe that the personal connection with participants yielded more benefits than drawbacks in terms of the quality of research findings.During the study, participants felt comfortable sharing personal memories and feelings related to sauna with the researcher potentially due to their existing personal connection.This level of familiarity and trust allowed for a deeper exploration of the emotional and cultural dimensions of their thermal experiences, which might have been challenging to elicit from strangers.Nevertheless, future work should consider this aspect and explore the impact of participant familiarity on qualitative research outcomes by inviting a broader audience for a more comprehensive examination.

Cultural background.
The cultural background of participants can significantly influence their thermal experiences, as observed in our study.The Finnish participant, in particular, displayed a deeper emotional connection and nostalgia towards the heat experience in sauna due to its cultural significance in Finland.Memories of family traditions, bonding with parents, and a sense of warmth and relaxation associated with sauna contributed to a more profound and personal heat experience.This suggests that cultural context and upbringing can shape individuals' perceptions and emotional responses to thermal stimuli.Understanding these cultural variations is crucial for designing thermal experiences that resonate with different user groups, catering to their unique emotional and experiential needs.Future research could explore how cultural factors further impact thermal perceptions and how to accommodate this diversity in designing multi-sensory interfaces that include thermal feedback.7 and 9.For instance, tactile exploration allowed for more abstract explanations of sensations including transitional phases, comfort, or time.Body maps were more effective in spatially locating sensations.And verbalizing allowed for in-depth descriptions and temporal unfolding of the experience aligning with micro-phenomenology.We found that combining three methods allowed to get a better understanding thus addressing the challenge of expressing and documenting bodily sensations of heat.

Body maps.
We acknowledge the limitation of our body map, which may portray a stereotypical, slender, and male body type.We are aware of recent research taking into account the diversity of body types and will strive to make our future body maps more inclusive.Efforts have been made to ensure inclusivity and accommodate a wide range of physical characteristics, enabling more accurate representation and interpretation of bodily sensations across different individuals [54].Additionally, there has been exploration into visualizing the dimension of time and the trajectories of bodily sensations, allowing for a deeper understanding of how these sensations evolve and unfold over time [51].These advancements contribute to the refinement and effectiveness of body maps as a means of capturing and analyzing subjective bodily experiences in research.

Exploring other thermal experiences.
To help participants focus on a specific, singular thermal experience, we chose to exclusively examine the moment of 'löyly', which is when water is poured onto the sauna stones.The Finnish sauna experience involves several other phases, like stepping out for a cold shower or a plunge into the lake to cool down.We made this methodological choice inspired by the concept of micro-phenomenology, aiming to thoroughly investigate the minute details of individual experiences.Therefore, exploring other thermal encounters, both within and outside of the sauna, can offer a more holistic insight into how hot and cold sensations are perceived.Furthermore, we would like to emphasize that the identified parameters emerged from participants' experiences with the heat in sauna.Future work should examine the effectiveness of these parameters for cold sensations.Understanding the nuances of other extreme thermal experiences, including its physical and affective dimensions, can contribute to a more comprehensive understanding of human thermal perception and inform the design of immersive experiences that encompass a wider range of thermal sensations.Furthermore, adding chemical compounds such as menthol or eucalyptol to the water being thrown onto the hot stones could influence how we perceive temperatures tactilely or thermally [6].In future work, it could be examined how these scented oils might change the experience of heat in sauna.

Material carriers of heat
Heat always relies on a carrier material to be perceived such as hot air or cold water.In our study, steam served as the transfer material for heat.When water vaporizes during 'löyly' there is an abrupt rise in humidity levels, which could potentially affect the sensation of heat.Jonsson et al. [25] emphasize the importance of material qualities in shaping our perceptions and responses to thermal stimuli.This implies that heat perception of sauna cannot be completely separated from other sensory inputs.Thus, understanding the environmental factors is crucial for designing more nuanced and immersive thermal experiences.

Parameters open up future research on thermal experiences
HCI scholars and designers are only beginning to explore and investigate the ways in which thermal phenomena can be utilized and manipulated in interactive technologies to enhance user experiences and interactions.The generated parameters open up exciting new opportunities for researching thermal experiences and technologies.In this section, we provide a few directions for prospective research.
Human spatial localization of thermal stimuli is less accurate compared to tactile signals.Hence, thermal motion across the body may need longer duration or higher intensity to be perceived.Thermal referral phenomenon could be explored to achieve a sense of movement on the skin (as in [31]).Thermal referral describes the phenomenon in which a change in temperature perception in one part of the body influences the thermal perception in another part of the body [20].It occurs when an external thermal stimulus applied to one body region produces a sensation of temperature change in a different body region that is not directly exposed to the thermal stimulus.An experimental study proved that when the index, middle and ring finger are touching three thermal plates but only the two outer fingers receive actual thermal feedback, the middle finger still perceives a thermal signal even though that plate is thermally neutral [20].Hence, adding a moving tactile stimulus to the thermal signal could enable individuals to perceive thermal motion more easily.
With thermal timbre we consider that thermal stimuli can create a feeling of pleasantness or unpleasantness influencing our emotional or physical status.Thermal alliesthesia is a phenomenon where the perceived pleasantness or unpleasantness of a temperature sensation is influenced by the individual's internal body temperature [40].When an individual's core body temperature deviates from their thermo-neutral state, thermal stimuli aimed at returning to a thermally neutral body state are perceived as especially pleasant (for instance, taking a cold shower after a hot sauna creates a feeling of pleasantness).This concept highlights the role of the body's internal temperature regulation in shaping our subjective experiences of temperature.

CONCLUSION
This study continues the work in HCI of exploring the experiential dimensions of temperatures to inform thermal experience design.We present a phenomenological study of a natural full-body heat experience of sauna and infer experiential qualities and parameters.We discuss different methods to capture tacit bodily experiences, findings and propose future directions in the design and research of heat experiences.These insights enable designers to create more nuanced and meaningful heat experiences by considering factors such as motion, timbre, and distribution of heat stimuli.Understanding these experiential dimensions allows for the intentional manipulation of heat variables, resulting in the creation of richer and more immersive content beyond simplistic perceptions of hot and cold sensations.

Figure 2 :
Figure 2: Eight Sensual Evaluation Instrument (SEI) figurines out of clay based on the original ones by[22]

Figure 3 :
Figure 3: The moment of Löyly when water hits the heated rocks

FirstlyFigure 6 :
Figure 6: Structure of findings to address our research questions

Figure 7 :Figure 8 :
Figure 7: Visual elements used to describe heat with example body maps from participants

Figure 9 :
Figure9: Tactile elements (the icons do not represent the SEI shapes themselves but certain aspects that have been expressed by participants to describe their heat experiences through the tactile exploration)

Figure 10 :
Figure 10: Participants were asked to rank the three modalities of expression from most to least natural

Figure 11 :
Figure 11: Example depiction of heat dynamics in the body map with curved lines by P9 5.1.3Combining methods.We see the strength in triangulating different means of expressions and combining methods as a way to capture rich bodily experiences.As reported in the findings, different participants preferred different expression modalities in terms of intuitiveness.Nevertheless, each method revealed individual qualities for expression, as summarized in Figures