The sense of temperature tells how hot or cold objects and surroundings are. It works through distinct receptors for warmth and for cold, located in the skin. There are more receptors for cold than for warmth, and they are not evenly spread: the face and the hands are more sensitive than the back, for instance.
As with the sense of life and the sense of touch, the whole skin takes part. There is a difference, however. When something touches the body, one feels exactly which spot is touched. Temperature is perceived in relation to one's own warmth and to the size of the skin area exposed to cold or heat.
The larger the surface, the finer the difference
A finger put into one bucket of water and then into water three degrees warmer hardly feels the difference. A whole hand feels some difference, and a submerged forearm feels it more strongly still. The larger the area perceiving the change, the more accurately the difference is estimated.
Lying naked in a bath, a person can perceive deviations of only a fraction of a degree. A bath that has cooled a little is felt as a large change, even when a thermometer shows only a small drop.
Warmth and cold enter the body through the skin. Exposing a large area of skin to warmth lets more warmth in, and one feels warmer than when only a small part is exposed. This is why a cold wind on bare arms is felt so much more than the same wind on a face alone.
Warmth goes inside
The sense of touch tells that something lies outside the body: the surface of the object and the surface of the skin meet and stay separate. With temperature the situation is different. The cold or warmth penetrates into the body. Nor is the temperature of an object felt as belonging only to its outside; it seems to come from the whole object, radiating from within.
This is also why materials of the same temperature feel so different. A metal railing and a wooden bench standing side by side in the same cold air have the same temperature, yet the metal feels far colder. Metal carries warmth away from the hand quickly, wood slowly. What the sense perceives is the flow of warmth into or out of the body, not a number on a scale.
Warm spots and cold spots
The receptors are not a continuous layer. If the skin is tested point by point with a fine warm or cold tip, small spots appear that respond to cold and others, fewer in number, that respond to warmth, with areas in between that register neither. Cold receptors react when the skin cools, warm receptors when it warms. Beyond a certain point both give way to pain: very hot water and ice can both feel burning, and in the first moment it is sometimes hard to tell which of the two one has touched.
Certain substances fool the sense. Menthol, from mint, acts on the same receptors that respond to cold, which is why a peppermint feels cool in the mouth although its temperature is no lower than that of the tongue. The substance that makes chilli peppers hot acts on receptors that respond to heat. The mouth then reports a temperature that is not there, which shows once more that the sense perceives the body's own response rather than a measured quantity.
Relative, not absolute
The sense of temperature is closely tied to the body's own warmth. It does not measure absolute temperatures, only temperatures relative to itself. The classic experiment shows this clearly. One hand is held in water of about 10 degrees for three minutes, the other in water of about 40 degrees; then both are put into water of about 27 degrees. For a few minutes this water seems cold to one hand and warm to the other. The effect slowly fades until both hands feel the same temperature.
The receptors also adapt. Water that seemed hot on stepping into a bath soon feels simply pleasant, and a cold swimming pool becomes bearable after a few minutes. The sense reports changes most strongly, and a steady condition gradually fades from awareness.
The body's own warmth
Behind this sense stands the fact that human beings, like other mammals and birds, keep their inner temperature almost constant, at about 37 degrees Celsius in the core of the body. The skin is cooler than the core and varies much more: hands and feet can be cold while the trunk stays warm. Temperature signals from skin and from inside the body help to regulate this warmth, through shivering, sweating and the widening or narrowing of the blood vessels in the skin.
Cold-blooded animals such as reptiles depend much more on the warmth of their surroundings. A lizard basks in the sun in the morning before it becomes active. Some snakes, the pit vipers and many pythons, have developed special heat-sensitive pits in the head with which they can detect the warmth radiated by a mouse or bird, even in the dark. Here the sense of temperature has become almost a way of seeing.
Animals with thick fur or fat show the other side: they shield their warmth from the outer world. The pages on mouse, lion and cow and on pigs and hippopotamuses describe how differently mammals deal with heat, from mudbaths to hours in the shade.
Warmth in an animal's body
The warmth of an animal is not the same everywhere either. Anyone who lays the hands on different parts of a cow, for example, finds the trunk and the belly warm, and the legs, ears and horns cooler. The nose is moist and cool. The heat is concentrated in the great digestive organs in the middle of the body, and it falls off towards the extremities. In winter this is visible in the steam rising from the cow's back and mouth in a cold stable, while the horns and the tips of the ears may feel almost as cold as the air around them.
In a horse or a dog the pattern is similar, but the balance is different: after running, a horse is hot all over and sweats, while a dog, which hardly sweats, cools itself by panting. Such observations with the hands add something that a thermometer cannot give. They show where an animal is warm, where it lets its warmth go and how it deals with heat and cold.
Warmth, cold and mood
Temperature affects mood more strongly than most other senses. This is partly because the sense covers the whole body, and partly because warmth and cold make the whole body feel comfortable or uncomfortable. Cold chills, and severe cold can numb or even paralyse. People pull in their shoulders and become quiet. Warmth can make people open and enthusiastic, but too much heat causes apathy. Moderate temperatures hardly affect the mood at all, and that is exactly why they are not noticed.
Warmth between people
Warmth and cold also matter in social life. Someone who wants to get to know another person does well to radiate warmth, and can then expect warmth in return. Someone who meets coldness feels rejected. People need warmth from others; without it a community cannot hold together. Language is full of this experience: a warm welcome, a cool reception, a heated debate, being left out in the cold.
Clothing, houses and seasons
Much of human culture is a way of dealing with this sense. Clothes keep a layer of warm air around the body, houses keep the cold out, and a fire or a stove creates a centre where people gather. Wool, which holds air between its fibres, feels warm even to the touch, while smooth linen or silk feels cool. The choice of material is often made by the sense of temperature before any label is read.
The seasons are experienced largely through this sense as well. The first mild day in spring is felt on the skin before any leaf appears, and the first cold morning in autumn makes people look for a coat. What a thermometer shows as a gradual curve is experienced as a series of moments: stepping out of the door, coming in from the rain, holding a warm cup in cold hands. Each of them is a small experiment in how the body's warmth meets the warmth of the world.
In the anthroposophical tradition
Rudolf Steiner counted the sense of warmth among his twelve senses. In his arrangement it belongs to the middle group of senses, those directed at the outer world, together with sight, taste and smell. Warmth plays a central part in the anthroposophical view of the human being, where it is seen as the element in which the individual core of a person lives; the page Warmth: finding the core describes that view. Such statements belong to the tradition and are presented here as its way of looking, not as physiology.
What can be observed directly is simpler. The sense of temperature never gives a fixed value but always a relation between one's own warmth and that of the world. In that respect it resembles the sense of sight, which also sees colour only in contrast. A hand coming in from the frost feels lukewarm tap water as burning.
Questions
Where is the sense of temperature located?
It works through separate receptors for warmth and for cold in the skin. There are more cold receptors than warm ones, and they are spread over the whole body surface, though not evenly.
Why does the same water feel warm to one hand and cold to the other?
Temperature is felt relative to the skin's own temperature. A hand that has been in cold water feels lukewarm water as warm, while a hand that has been in hot water feels it as cold, until both have adapted.
Why does a larger area of skin feel temperature differences better?
More receptors take part and more warmth flows in or out. A small difference that a finger hardly notices becomes clear to a whole hand or forearm, and very clear to the whole body lying in a bath.
Why does metal feel colder than wood at the same temperature?
Metal conducts heat away from the skin much faster than wood. The sense of temperature registers the flow of warmth out of the body rather than the temperature of the object itself.
How is the sense of temperature different from the sense of touch?
Touch tells where the body ends and an object begins. With temperature the warmth or cold enters the body, and the warmth of an object seems to come from its whole inside rather than from its surface.
Why does temperature affect mood so strongly?
The sense covers the entire body, and warmth and cold make the whole body feel comfortable or uncomfortable. Cold makes people withdraw and stiffen, pleasant warmth opens them up, and too much heat makes them listless.
Do animals have a sense of temperature too?
Yes. Some snakes, such as pit vipers, even have special pits between eye and nostril that detect the warmth radiated by prey. Warm-blooded animals also use temperature signals to keep their own body warmth constant.
What is the sense of warmth in Steiner's list of twelve senses?
In the anthroposophical tradition, the sense of warmth is one of the twelve senses described by Rudolf Steiner. It is grouped with the senses directed at the outer world, together with sight, taste and smell.
Why do people speak of a warm or cold atmosphere?
The experience of warmth and cold carries over to how people meet each other. Expressions such as a warm welcome or being left out in the cold describe social experiences in the language of this sense.