This IFOD is going to hit a few related topics: how sweating cools us,  how do fans make us feel cool, why drinking a hot drink can make us feel cooler, how do moisture-wicking clothes work, what is the heat index, how panting helps dogs and other animals cool off, and what is dew point and why it is a more relevant measure than relative humidity, Here we go . . .

How Sweat Cools Us

When the hypothalamus in our brains detects a rise in body temperature it directs more blood flow to the skin and triggers sweat glands to produce sweat. Humans typically have between 2 and 5 million sweat glands. The water itself produced by the sweat glands does not does not cool us by running down the skin or making our skin wet. Instead, water must evaporate off our skin in order for cooling to occur.

Evaporation cools our bodies as follows: our bodies heat the water in sweat which causes a transfer of energy from our bodies to the water and the water gains enough heat energy it breaks the bonds that holds it as water and it  becomes water vapor. As the water molecules leave the body as water vapor they take their energy with it, and that loss of energy provides the cooling effect. So, basically, evaporation transfers the heat that the body used to heat the sweat into the air. Its a pretty neat system and very efficient. Related IFOD on evaporation: How Do Wet Things Dry Indoors?.

Humidity and Sweating

When the air is more saturated with water, evaporation occurs more slowly and at full saturation, evaporation doesn’t occur at all. When our sweat doesn’t evaporate, we don’t experience the cooling effect of sweating. Instead, the water just pours down our skin.

Clothes and Sweating

Evaporation  is primarily determined by the moisture saturation of the air surrounding the skin. When we wear clothes, fabric can trap the moisture evaporating off the skin as we sweat and some fabrics, such as cotton, absorb the sweat and render air next to our skin fully saturated.

Moisture-wicking clothing uses fabrics that do not absorb the sweat and allow it to evaporate from the fabric. These fabrics are synthetic and are designed to have a “capillary action” and pull moisture from the skin to the surface of the fabric where it can evaporate.This reduces moisture saturation of the air near the skin and allows more evaporation than when wearing a moisture trapping fabric.

How Fans Provide a Cooling Effect

Air blown by a fan isn’t made any cooler by the fan causing the air to move. Instead what blowing air does is decreases the humidity of the air right around the skin. Explanation: when we sweat the air near the skin, if the air is still, becomes saturated with water vapor from the evaporating sweat. A fan provides a cooling effect by moving this humid air near our bodies away from our skin and replacing it with less humid air from elsewhere which allows evaporation to occur more effectively. When the air is at full humidity, a fan provides no cooling effect. That is why on a very humid day a fan has little beneficial effect.

How a Hot Drink on a Hot Day can Cool You Down

This study concludes that when the air isn’t fully saturated and evaporation can occur, drinking a hot drink actually can cool you down on a hot day. Wow. How? The study author, in an interview with Smithsonian Magazine, explained it as follows: “What we found is that when you ingest a hot drink, you actually have a disproportionate increase in the amount that you sweat. Yes, the hot drink is hotter than your body temperature, so you are adding heat to the body, but the amount that you increase your sweating by—if that can all evaporate—more than compensates for the the added heat to the body from the fluid.” The key is that the sweat needs to be able to evaporate. So, if it is a very humid day or if your clothing is hindering evaporation, drinking a hot drink is not a good idea for cooling.

The Heat Index

According to the National Weather Service, “the heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.” Basically, the heat index adjusts for how effective sweating will be based on the moisture saturation in the air.

How Panting Works

We humans cool ourselves primarily by sweating and other mammals sweat, most notably horses. Dogs and cats sweat, but only on their paw pads.

Dogs and many other mammals primarily cool themselves by panting. Panting works by forcing out warm, moisture-filled air and breathing in cool, dryer air. The cooler, dryer air increases moisture evaporation in the mucous membranes of the nasal passages, mouth, and lungs. As with sweating, this internal evaporation results in heat/energy transfer out of the animal and into the air.

Dew Point vs. Relative Humidity

Relative humidity tells the amount of atmospheric moisture present, relative to the amount that would be present if the air were fully saturated with water vapor. So, at 100% relative humidity, no more water vapor can be suspended in the air and evaporation stops.

Dew point is a related concept to Relative Humidity (“RH”) but is a better measure of how dry vs. wet the air feels. Here’s what the National Weather Service says about Dew Point:

The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold anymore water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly effects how “comfortable” it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more “humid” on the 80 degree day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

So if you want a real judge of just how “dry” or “humid” it will feel outside, look at the dew point instead of the RH. The higher the dew point, the muggier it will feel.

In general, during the summer, a dew point of 55 or less will feel great, between 55 and 65 it will feel a bit “sticky” and over 65 will feel “icky.” Right now, in St. Louis, the temperature is 73 degrees, the dew point is 72 degrees. This is a very high dew point and will feel pretty icky when I go walk my dog here in a minute. Also, the relative humidity is 96%. Feels like home.