Why Does Lukewarm Water Feel Hot to Cold Hands?
– Your Skin Measures the Difference From Itself, Not the Temperature
You come home on a cold day, wash your hands, and plain lukewarm water feels scalding. The water isn't hot. Your skin has no thermometer. It senses how different the water is from your own temperature, and how fast heat is pouring in.
It's an evening with a biting wind. You forgot your gloves and cycled home. Your fingertips are numb and red.
At the sink you run the hot tap, put your hands in, and snatch them back: "Ouch!" But a family member tries it and says, "What? It's lukewarm."
Same water, but one says hot and the other says lukewarm. Is one of your hands lying?
There are only two reasons
Heat flows from the warmer thing to the cooler one. The rate is proportional to the temperature difference. A cold hand and warm water differ a lot, so heat rushes in.
Skin's temperature sensors get used to a steady temperature and go quiet. Their baseline drifts toward your own temperature, so the same water feels different to different hands.
So your skin isn't reporting "what temperature the water is". It reports "how much hotter than me it is, and how fast I'm warming up". Let's take these one at a time.
The bigger the gap, the harder the heat rushes in
A normal palm is said to be around 33℃. When 38℃ water touches it, the gap is 5℃. Heat creeps in slowly, and it feels pleasantly warm.
But fingers after a walk in midwinter can cool to about 15℃. The gap to the same 38℃ water is a full 23℃. The gap is about 4.6 times bigger, so the rush of heat is about 4.6 times stronger too.
Look at Figure 1. As you lower the hand's temperature with the slider, the arrow showing the gap to the water grows longer. The bar on the right shows "what water temperature a warm hand would need to feel the same gap". With a 15℃ hand, that is 56℃. No wonder it feels hot.
The sensors quickly "get used to it"
Skin has nerves that respond to cold and nerves that respond to warmth. Both fire hardest at the moment the temperature changes. If the temperature stays the same, the signal fades sharply within tens of seconds to a few minutes.
A cold hand is already used to its cold. For its nerves, 15℃ has become "normal". Then the temperature suddenly jumps by 23℃, and they send a big signal: "Alarm, it's hot!"
The speed of the rise matters too. When the gap is large, skin temperature shoots up, and that speed is itself a strong stimulus. The stinging in cold fingers is also thought to come from blood flow, which had slowed, returning.
Conversely, if you keep your hand in the water for about a minute, the heat fades away. The water's temperature hasn't changed. Your hand has warmed up, the gap has shrunk, and the sensors have adapted to the new temperature.
In a book of 1690, the English philosopher John Locke described a thought experiment. Put one hand in hot water and the other in cold water, then place both in the same lukewarm water. The same water feels cold to one hand and warm to the other. He used it in an argument that sensation is not a property of the object itself.
If you probe skin point by point with a thin rod, you find spots that sense only cold and spots that sense only warmth. Cold spots are said to outnumber warm spots several times over. The body seems to be built to be more sensitive to getting cold.
Useful things to know
If you check a baby's or an elderly person's bath with cold hands, it tends to feel hotter than it really is. After a long soak, your hands will feel it as cooler. For bath temperature, a thermometer is more reliable than your hand.
Also, badly chilled, numb fingers may not sense heat correctly. To warm frozen hands, it is said to be better to use lukewarm water and warm them slowly, not hot water.
Summary
Skin is not a thermometer. It takes your current temperature as the baseline and reports the gap from it and the rush of incoming heat. For a cold hand, the gap to 38℃ water is simply too large.
"Hot" is decided not by the water's temperature
but by the gap between your hand and the water.
For why metal feels colder than wood at the same temperature, see "Why does metal feel colder than wood?". For why numb fingers can't do up a button, see "Why can't you do up a button when your hands are numb with cold?". For the story of how the nose is sensitive to "change", head to the article on the scent of osmanthus.
- Prepare three basins. Put cold tap water in the left one, rather hot water (only as hot as feels comfortable to touch) in the right one, and lukewarm water in between in the middle one.
- Hold your left hand in the cold water and your right hand in the hot water for about a minute.
- Move both hands into the middle lukewarm water at the same time. Your left hand should feel "warm" and your right hand "cold". If you wait about 30 seconds, you can also see the two hands' feelings come into line.
Keep the hot water on the right to a temperature that feels pleasant when you put your hand in. Never use water that is too hot. When children try it, an adult should check the temperature for them.
For those who want more – terms, formulas and links to textbooksEach part is labeled by level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics or biology
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (heat transfer engineering, sensory physiology)
- ResearchNot yet taught even at university as settled fact; researchers are still investigating it
Middle schoolTerms: this phenomenon has names
- Warm spots and cold spots: points on the skin that sense only warmth or only cold. Cold spots are said to be more numerous.
- Adaptation: when the same stimulus continues, the signal from the sensory nerve weakens. It also happens with smells and brightness.
- How heat travels (conduction): heat moves from the hotter object to the cooler one between things in contact.
Middle schoolHigh schoolCheck with a formula: how many times more heat flows into a cold hand?
Let's compare the rate of heat entering the skin from the water for a warm hand and a cold hand. In the symbols, q is the heat flowing in per second per square meter (unit: watts per square meter), h is a coefficient for how easily heat transfers, Tw is the water temperature, and Ts is the skin temperature (both in ℃).
| In symbols | q ＝ h × ( Tw − Ts ) |
| In words | Rate of heat flowing in ＝ ease of heat transfer × (water temperature − skin temperature) |
| Where it comes from | Newton's law of cooling: heat passing through a contact surface is proportional to the temperature difference. For the same water and the same hand, h is the same, so to compare you only need to look at the temperature difference |
| Water temperature Tw | 38℃ |
| Normal palm temperature | said to be about 33℃ |
| Temperature of fingers chilled outdoors in midwinter | said to drop to about 15℃ |
| Gap between warm hand and water | 38 − 33 ＝ 5 |
| Gap between cold hand and water | 38 − 15 ＝ 23 |
| Ratio of heat flow rates | 23 ÷ 5 ＝ 4.6 |
| Water temperature giving a warm hand the same gap | 33 ＋ 23 ＝ 56 |
Heat flows into a cold hand about 4.6 times as fast as into a warm hand. This is the same temperature gap as a warm hand touching 56℃ water. That is far above 43℃ or so, the rough point where pain begins, so it is natural to feel "hot" and "stinging". Real sensation also depends on how far the nerves have adapted and on blood flow, so treat this as a rough guide.
High schoolHigh school+Senses emphasize "change"
High schoolIn biology you learn that when a receptor gets a stimulus, a nerve sends electrical signals, and the stronger the stimulus, the more often it fires. Temperature receptors are the same: the firing rate jumps at the moment skin temperature changes, then settles down to a lower rate.
High school+This "strong at first, then weaker" response is split into a dynamic response and a static response. When skin temperature is in the range of roughly 30–36℃, you stop feeling both warmth and cold after a while, and this range is called the "neutral zone" (thermoneutral zone).
UniversityEase of heat transfer, and the molecules that sense temperature
The h in the formula is called the heat transfer coefficient in heat transfer engineering, and it changes a lot depending on whether the water is moving or still. Stirred water feels hotter because h gets larger. In sensory physiology, TRPM8 (an ion channel that also responds to menthol) is known to be involved in sensing cold, and TRPV1 (which also responds to capsaicin) in the pain of heat. Cold signals are mainly carried by thinner myelinated nerves (Aδ fibers), and warm signals by unmyelinated nerves (C fibers).
📖 Derivation of the formula, and further reading: Newton's law of cooling (Wikipedia) ／ Heat transfer coefficient (Wikipedia)
ResearchWhat is still not clear
- The molecule that receives "warmth" has no clear answer yet. Compared with TRPM8 for cold, for the molecules behind pleasant warmth there are said to be only a few candidates so far.
- What the stinging pain really is. When cold fingers are warmed, how much the painful itch comes from the nerves recovering and how much from blood flow recovering has not been clearly separated.
- The brain's "temperature map". How the brain combines temperature signals from the skin into an overall feeling such as "this room is hot" is still being studied.
In other words, this article too is "an explanation of what is known so far". Our understanding of temperature sensing at the molecular level only began to grow quickly in the 21st century.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, year 2: "Stimulus and response"; how heat travels | Warm and cold spots; heat flows from higher to lower |
| High school | Physics "Heat"; biology "Receptors and nerves" | Heat flow proportional to temperature difference; firing rate of signals |
| High school+ | Advanced biology (adaptation) | Dynamic and static responses; neutral zone |
| University | Heat transfer engineering, sensory physiology | Heat transfer coefficient; TRPM8 and TRPV1 |
| Research | Molecular biology of temperature sensing | Molecule for warmth; integration in the brain |
| ― | Link to daily life | Check bath temperature with a thermometer; warm cold hands with lukewarm water |
- John Locke, An Essay Concerning Human Understanding (1690), Book II, Chapter 8 – the thought experiment of the three waters
- TRPM8 (English Wikipedia)
- Newton's law of cooling (Wikipedia)
- McKemy, D. D., Neuhausser, W. M., Julius, D. (2002) Identification of a cold receptor reveals a general role for TRP channels in thermosensation. Nature 416
- Ken'ichi Honma et al., Standard Physiology (標準生理学), Igaku-Shoin – chapter on somatic sensation (temperature sense)
*This article is a general-audience science explanation. The figures given are rough guides to help you understand the mechanism. Body and skin temperatures vary from person to person. If strong chilling, numbness or pain in your hands or fingers persists, consult a medical professional.