Warmer temperatures along with higher humidity are in the forecast. Get ready to sweat more as you go outside through the weekend as well as into next week.
Sweating is a good thing for our bodies. The answer to the title question — why do we sweat? — is so our bodies can cool down. It all sounds pretty simple. It gets hot, and then our body reacts by producing sweat. We cool down.
Have you ever wondered why our bodies actually cool down from sweating? The process is tied to chemistry and physics, as well as math. Let’s take it one step at a time.
It’s a hot afternoon and you are enjoying a nice air-conditioned room. Your body is not hot or cold. You are just right with your body temperature and feel very comfortable. You decide you need to go outside to take out the trash. The inside of your house is a comfortable 70°F with low humidity. The outside is 90°F with high humidity. You walk outside and within minutes you are sticky with sweat. Why did that happen?
The good news is that you body is working as it should. It has determined that the air surrounding your skin is hotter, and it needs to produce sweat so you can maintain your body temperature. So the sweat cools your body? The answer is that it helps create the process to cool your body.
This is where humidity comes into the equation.
Let’s review. You walk outside on a hot day and you start to sweat. Now, you have sweat on your skin. This sweat will do one of two things: it will either linger on your skin or it will evaporate.
The higher the humidity, the slower sweat evaporates. The lower the humidity, the faster sweat evaporates. This is why relative humidity is known as “The Drying Power of Air” in meteorology — how fast, or slow, can it evaporate water?
This still does not explain why sweat cools our body. This is where chemistry and physics come into the process. We now know that when our bodies get hot, we sweat. The sweat then tries to evaporate into the air around our bodies. This could be a fast process if the relative humidity is low or a slow process if the relative humidity is high.
The keyword is “evaporate” in the cooling process. When a liquid changes to a gas from our skin, it is known as evaporation. This “evaporation” process within the phase change from a liquid to a gas is very important in relation to the process of cooling our bodies off.
There are two types of processes that happen when molecules change their bonds to form a gas, liquid or solid. These processes are related to chemistry and physics. To keep it simple, the process either takes away heat from the surrounding air or adds heat to the surrounding air in order to let the molecules do their job to create one of the three states (gas, liquid or solid).
The two processes are known as endothermic and exothermic. An endothermic reaction will absorb heat from the surrounding air, causing a cooling impact to the region it pulls from. An exothermic reaction will give off heat to the surrounding environment, causing a warming impact to the region it is near.
Think of exothermic as a fire that is combusting. It heats the air around it. When ice is created, it is exothermic too. It releases heat to the environment around it. Condensation is exothermic, releasing heat around it.
Evaporation is endothermic. The water molecules need to take in heat from the area around them to change to the state of gas. When this heat is taken from the surrounding area, it cools the region around the water molecules as it evaporates.
It all sounds pretty confusing when you throw in all the science. To make it simple, think of it this way.
Your body sweats when it gets hot. The water on your skin evaporates into the air around it. It is the process of evaporation that creates cooler temperatures for your skin because of the endothermic process pulling the heat from the surrounding air to complete the evaporation process.
The bottom line is that it is the actual evaporation process that makes the cooler air around your skin. Your body uses that cooler air to reduce your body temperature. That is why we sweat. We need the sweat in order to make evaporation so we can cool.
You also notice this when you jump out of a swimming pool and you are wet with water. You may feel cold until you dry off. It is because all the water on your skin is evaporating and it is causing a cooling due to the endothermic process.
If the relative humidity is high, it will limit the evaporation around your skin. That is why you feel hotter on a humid day. Your body can’t cool as easily. This is reversed on a dry day with low relative humidity. Your body may sweat all day long and dehydrate you, but you may not notice sweat. This is known as a dry heat. You would find this more often in the southwest United States.
So, to answer the initial question, we sweat so our bodies can create evaporation. It is the endothermic reaction in the evaporation process that cools our bodies down.