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Atmosphere

What Is Humidity?

Imagine: You take a nice, long, hot shower. Stepping out, you feel it’s pretty muggy and warm in the bathroom, and the mirror is foggy. You wipe your hand across the mirror and suddenly, your hand is all wet. That’s because of the moisture in the bathroom!

This digital graphic shows a right hand wiping a foggy mirror from right to left. The area that has been wiped is a darker blue color than the surrounding foggy area, which is colored a light blue.

Water vapor in the bathroom allows us to draw on the foggy bathroom mirror after a hot shower! Image credit: NASA/JPL-Caltech

This happens because the foggy mirror is actually tiny droplets of water that have formed on the mirror. When you take a hot shower, water vapor in the bathroom increases due to evaporation from the shower. When that water touches the cool mirror, it condenses back to water, in the form of tiny droplets.

But, what does this all mean? What is humidity? Why does it get humid outside? And how does humidity affect our weather?

First, we have to talk about water vapor. Not knowing what water vapor is would be a MIST opportunity! (Get it?!)

What is water vapor?

Water is a super special substance. It is basically everywhere on Earth – in our soils, our oceans, and our air.

Water exists in three states – liquid, solid, and gas. Liquid water helps hydrate us during a hot day or after a long sports game. Solid water, or ice, is colder and gives us a surface to ice-skate on. Water in its gas form is water vapor, or moisture.

A man in a light-gray t-shirt is outside, holding an open plastic water bottle over his head, pours the remaining water inside the bottle over his forehead to cool off in the humid heat. The water runs down his face and onto his shirt.

Oppressively hot summer days often evoke the expression, "it’s not the heat, it’s the humidity." That sticky, tropical-like air combined with high temperatures is more than unpleasant. Image credit: U.S. Air Force Photo/Staff Sgt. Josie Walck

There is water vapor in our atmosphere because of evaporation. Evaporation happens when water changes from a liquid to a gas. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain.

This graphic shows how water moves through the water cycle. According to this graphic, water evaporates from bodies of water like oceans and lakes. This movement is shown using yellow arrows pointing upwards from these bodies of water. Condensation happens in the clouds, and precipitation falls from the clouds.

The water cycle. Water moves through the Earth’s soil, atmosphere, and bodies of water by processes of evaporation, condensation, precipitation, transpiration, and many more! Image credit: Dennis Cain/NWS

So, what is humidity? Well, there are actually two different types of humidity!

This digital graphic is labeled "Evaporation". This cartoon is a simple line drawing. At the bottom are three wavy lines, representing a body of water. Wavy arrows point upwards from the waves toward a cloud at the top of the cartoon.

Evaporation happens when water changes from a liquid to a gas. Liquid water evaporates into the air from all sorts of bodies of water! Credit: NASA/JPL-Caltech

What is absolute humidity?

Absolute humidity is the actual amount of water vapor in the air. The higher the amount of water vapor, the higher the absolute humidity.

What is relative humidity?

Relative humidity also measures water vapor in the air, but it is compared to the maximum amount of water vapor that can exist in the air at its current temperature. It is written as a percent.

For example, warm air can hold more water vapor, or moisture, than cold air. So, with the same amount of absolute humidity, cold air will have a higher relative humidity and warm air will have a lower relative humidity.

The relative humidity is 100% when the air is completely full of water vapor.

This digital graphic is labeled "Relative Humidity". The graphic says, "RH represents amount of water in air in percentage". The cartoon shows four cups, each with an increasing amount of water inside them. On the far left is an empty glass with no water, labeled "Relative Humidity 0%". Next is a glass that is half full, labeled "Relative Humidity 50%". Third is a glass that is ¾ full, labeled "Relative Humidity 75%". On the far right is a glass that is completely filled with water, labeled "Relative Humidity 100%".

Relative humidity is a measure of water vapor compared to the amount of water vapor that air at a certain temperature can hold. Credit: NASA/JPL-Caltech

What is dew point?

The dew point marks the temperature at which water vapor will turn into liquid water droplets. This process is called condensation. Condensation is what causes clouds to form, which can then lead to precipitation such as rain, snow, and hail. The dew point will always either be the same or lower than the actual outside temperature.

At the bottom of this illustration is a small translucent cube. Next to this cube are the words "Surface, Temperature, Dewpoint", showing that, at the surface of the Earth, the temperature is 87℉ and the dew point is 65℉. A yellow arrow pointing upwards points at a medium-sized translucent cube in the middle of the illustration, showing a growing rate of saturation in this block of air. This cube is labeled with the same words, describing that this block of air is at 2,000 feet with a temperature of 76℉ and a dew point of 65℉. A second yellow arrow points upward from the medium-sized cube toward a larger translucent cube with a white cloud inside of it. Next to this cube are the same words, describing that this cube is at 4,000 feet with a temperature of 65℉ and a dew point of 65℉. Again, this drawing shows how in an ideal atmosphere, the saturation level of a block of air containing water vapor with a surface temperature of 85℉ and a dew point of 65℉ will cool to the saturation point at about 4,000 feet in elevation. At this elevation, with the temperature and dew point being equal, a water vapor will condense and form a cloud.

This drawing shows how water vapor will condense and turn into a cloud when the dew point and temperature are equal. Image credit: NASA/JPL-Caltech

The higher the dew point rises, the greater the amount of moisture in the air. This affects how "comfortable" it will feel outside. So, just reading the relative humidity can be misleading.

Quiz time! Which combination do you think would feel more humid? A 30℉ day with a relative humidity of 100%? Or an 80℉ day with a relative humidity of 50%?

Ready to check your answer? It would feel much more humid on the 80℉ day with 50% relative humidity than on the 30 degree day with a 100% relative humidity. This is because of the higher dew point.

This digital graphic is labeled "Dew Point". On the left is a square outlined in red with small circles scattered across the square’s area. These circles represent molecules of water in warm air. On the right is a square outlined in dark blue, representing cool air. Inside this square are a few molecules of water with three big water droplets, representing molecules of water in cold air that condense into larger droplets.

Dew point marks the temperature at which water vapor will turn into liquid water droplets. As air cools, moisture condenses into droplets. Credit: NASA/JPL-Caltech

So, if you want to know just how "dry" or "humid" it will feel outside, look at the dew point instead of the relative humidity. The higher the dew point, the muggier it will feel.

Check out the chart below to see the general comfort levels using dew point that can be expected during the summer months!

Dew Point Feeling
Less than or equal to 55 Dry and comfortable
Between 55 and 65 Becoming "sticky" with muggy evenings
Greater than or equal to 65 A lot of moisture in the air, very humid and uncomfortable

This digital graphic compares low dew point and high dew point. On the left is a drawing labeled "Low Dew Point". In this cartoon, a character is standing outside. Around the character are three water molecules. The cartoon says, "Sweat evaporates easily because there is room in the air for more water vapor molecules". On the right is a drawing labeled "High Dew Point". In this cartoon, the same character is sweating a lot and there are a lot of water molecules around them. The cartoon says, "Sweat builds up and won’t evaporate because the air is already full of water vapor molecules".

Have you ever been sweaty on a hot, muggy day? Maybe you noticed that the sweat didn’t completely evaporate off your skin! This is probably because of the high dew point. When the dew point is high and it is really hot out, sweat builds up on our skin and has a hard time evaporating because the air is already pretty full of water vapor molecules! If you compare hot summertime days in a humid place (like Florida) and a dry place (like Arizona), you will be more sweaty in Florida because the relative humidity is closer to 100%, meaning it doesn’t evaporate as efficiently as in the dry, low relative humidity air of Arizona. Credit: NASA/JPL-Caltech

How does humidity affect our weather?

Basically, the more water vapor in our atmosphere, the more possibility of precipitation.

Precipitation happens when the air rises and causes condensation of water vapor into liquid water droplets. This is the beginning of the formation of precipitation! These little droplets grow through a process called "collision-coalescence". During collision-coalescence, these droplets come together and form larger drops. Once the drops are a certain size, they become too heavy for the air and the drops fall as precipitation from the clouds.

This digital graphic shows how little cloud droplets come together with other droplets to form large cloud droplets and raindrops. The graphic labels a small cloud droplet, a large cloud droplet, and a rain cloud droplet. The graphic shows larger cloud droplets fall and collide with smaller droplets, coalescing into even larger droplets.

Wondering how rain happens? Check out this drawing! Little cloud droplets come together with other small droplets to form large droplets. This forms rain! Credit: NASA/JPL-Caltech

How is NOAA studying humidity?

The National Oceanic and Atmospheric Administration, or NOAA, measures water vapor with its Geostationary Operational Environmental Satellites (GOES) satellites in several ways. For example, NOAA uses a measure called total precipitable water (TPW) to provide information to weather forecasters and hydrologists (scientists who study the waters of the Earth and atmosphere). This information helps improve forecasting for events such as heavy rain, flash flooding, and more.