Why do I feel hotter when its humid?

“It's not the heat, it's the humidity!" Have you ever heard someone say that on a warm, muggy day? As you'll see soon, there's a lot of truth in that statement.

To understand humidity, you first have to realize that there's water in the air all around you. “But I'm not wet!" you might say. That's true. Most of the air around you has water in the form of a gas called water vapor.

Humidity is the amount of water vapor in the air. Too much or too little humidity can be dangerous. For example, high humidity combined with hot temperatures is a combination that can be a health risk, especially for the very young and the very old.

Humidity plays an important role in our daily weather. Without water vapor in the air, our weather might be like the weather on Mars. Could you imagine life without clouds, rain, snow, thunder, or lightning?

When you hear weather forecasters talk about humidity, you may hear them talk about two different terms: absolute humidity and relative humidity. Absolute humidity is the amount of water vapor divided by the amount of dry air in a certain volume of air at a particular temperature. The hotter the air is, the more water vapor it can hold.

Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity, which will depend upon the current air temperature. Relative humidity is the term weather forecasters use most often.

A relative humidity of 100% means that the air can't hold any more water vapor. It's totally saturated. When this occurs, it can rain. In fact, the relative humidity must be 100% where clouds are forming for it to rain. However, at ground level where the rain lands, the relative humidity can be less than 100%.

So how does humidity affect us on a hot day? Humans are sensitive to changes in humidity, because our skin uses the air around us to get rid of moisture in the form of sweat. If the relative humidity is very high, the air is already saturated with water vapor and our sweat won't evaporate. When this happens, we feel hotter than the actual temperature.

Likewise, very low humidity can make us feel cooler than the actual temperature. This happens because the dry air helps sweat evaporate more quickly than usual.

If the temperature outside is 75° F (23.8° C), humidity can make it feel warmer or cooler. A relative humidity of 0% would make it feel like it's only 69° F (20.5° C). On the other hand, a relative humidity of 100% would make it feel like it's 80° F (26.6° C).

So what's a comfortable humidity level? Studies have shown that we tend to feel most comfortable at a relative humidity of around 45%. Some people use special machines called humidifiers (add humidity) or dehumidifiers (remove humidity) to keep indoor humidity at a comfortable level.

OMAHA, Neb. (WOWT) - During the summer months, I’m sure you’ve heard – or have said yourself - “It’s not the heat; it’s the humidity!”. Or when describing the extreme desert climate, “…but it’s a dry heat!”. The same air temperature can feel much different depending on a number of factors, including humidity.

Sweat is our body’s natural air conditioner. When our body temperature rises, sweat is released from the skin through evaporation. This draws heat away from the body in a process called “evaporative cooling”. As humidity increases, sweat doesn’t evaporate as quickly off our our bodies, leaving us feeling hot and sticky.

The heat index is the “apparent temperature”, or what the air actually feels like once both the air temperature and relative humidity are considered. The most notable researcher on the topic was Robert G. Steadman, who published a journal article in 1979 titled “The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science”.

Steadman’s research led to an extensive equation that can be used to calculate the heat index, based on air temperature and relative humidity. Thankfully, the complicated equation has been transferred onto easy-to-read charts and simple heat index calculators like this one: https://www.wpc.ncep.noaa.gov/html/heatindex.shtml

As you can see from the chart above, with a steady temperature of 90°, the heat index rises to 95° at a relative humidity of 50%. When the relative humidity is at 70%, the heat index jumps to 105°! The higher the heat index, the greater likelihood of heat illnesses.

It should be noted that the heat index is calculated for shady areas with a light breeze. The heat index can rise up to 15° in direct sunlight!

With dangerous heat and heat indices expected throughout the next week or two, stay hydrated and limit time outdoors - especially during the afternoon hours. You can keep track of the scorching 10-day forecast with the WOWT First Alert Weather App: https://www.wowt.com/page/get-the-first-alert-weather-app/

Copyright 2021 WOWT. All rights reserved.

Originally published on November 10, 2018.

This week's question comes to us from Lucy Narbaitz from Ottawa, who asks:

Why does humidity in the air make us feel hotter in the summer but colder in the winter?

Dr. Deepak Chandon, a post-doctoral fellow in the Department of Physics at the University of Toronto has the answer.

In the summer, the human body tries to regulate heat and maintain an optimal temperature through the process of perspiration. The sweat then evaporates into the atmosphere, taking with it the excess heat it has eliminated from the body. But under humid conditions, the atmosphere cannot absorb any more water. This means sweat does not evaporate, and its function is thwarted, making us feel hotter.

In the winter, humidity does make us feel colder, but the reasons are less well understood. One reason could be that damp air in winter, the equivalent of humidity, causes the body to lose heat more quickly than dry air. It may also be that the condensation of water vapour on the fibres of our clothes in winter interferes with the ability of clothes to trap pockets of warm air. This reduces the insulation capability of clothing and makes us feel colder.