The Question: “Why do you feel colder when the air is humid? I understand why higher humidity, which suppresses evaporation cooling, makes you feel warmer at normal temperatures. But people keep telling me opposite is true when it gets near freezing or below. I am not buying this. All other things being equal (same wind, no rain, dry clothes, etc.) I suspect moisture in the air will always make one feel warmer, even at 20 degrees. Help me settle this argument.”
Limiting Case Experimental Answer
In science, the ultimate arbitrator for answering questions is experiment rather than theoretical arguments. Physicists also often use the limiting case to gain insight into a particular situation. The limiting case of high humidity is 100% humidity, which means its raining. Anyone who has ever been caught in a cold rain while wearing inadequate clothing knows that 100% humidity during cool weather makes us feel much colder than if it were dry at the same temperature.
This experiment strongly suggests, but does not prove, that high humidity (dampness) during cold weather makes us feel even colder. The same mechanisms that make people feel colder during a cold rain contribute to making cold damp weather feel colder.
Why Humidity Makes a Hot Day Feel Hotter
It helps to first understand why high humidity on a hot day makes the perceived temperature higher. Sweating is a cooling mechanism. When the humidity is low, sweat evaporates easily. Evaporation requires thermal (heat) energy, so evaporation is a cooling process. When our sweat evaporates it cools our bodies. On a hot humid day, sweat does not evaporate as easily, so the body’s cooling mechanism does not work as well. The limited evaporation in humid conditions is not enough to cool the body.
When it is cool and humid, the body does not need a cooling mechanism, so the body sweats less. The high humidity does not therefore limit evaporation to keep the body warm as it does on a hot humid day. Additionally, on a cool dry day, the low humidity does not increase the body’s cooling rate as it does on a hot dry day because most people do not sweat significantly when it is cool.
Therefore the mechanism that causes a humidity to make a hot day feel hotter does not apply in cool weather.
Why Dampness Makes a Cool Day Feel Colder
On a cold rainy day the falling rain soaks our clothing to make us feel colder. On a cool damp day, it is less obvious, but our clothing can also absorb some moisture from either the damp air or our bodies. Whether it is raining or simply damp, wet clothing does not keep us as warm as dry clothing for a few reasons.
Even if it is humid, some of the moisture in our clothing can evaporate. Evaporation still serves as a cooling mechanism. This effect is usually small.
Our clothing keeps us warm on cool days by trapping air between our bodies and clothing. The clothing, and layer of trapped air, prevents our bodies from losing heat by convection currents, which transfer heat by circulating air like a cool breeze on a hot day. Air trapped by clothing cannot easily circulate to transfer heat and cool our bodies. The body must first warm this layer of trapped air to keep us feeling warm.
On a very cool damp day, however, this layer of trapped air contains water molecules. If it is damp, our clothing is also likely to contain some water molecules. It takes more heat energy to warm water than air. In physics parlance, water has a higher specific heat capacity than air. If the layer of air next to the skin is damp, it therefore takes more of the body’s heat energy to warm it. Hence the perceived temperature is cooler.
Finally, liquid water conducts heat better than air, although humid air does not conduct heat better than dry air. If the dampness causes some liquid water to form on our skin or in our clothing, the water can conduct heat away from our bodies.
Another effect contributes to the cooler feeling outdoors on a damp day versus a dry day: A damp day is more likely to be overcast than a dry day. On a dry sunny day, the body is warmed by radiant heating from the Sun. A damp day is more likely to be overcast and therefore have less radiant solar heating. It will therefore feel cooler.
For a variety of reasons dampness can make a cool day feel even cooler.© Copyright 2011 Paul A. Heckert, Ph.D., All rights Reserved. Written For: Decoded Science