What is Humidity? - Definition, Measurements & Effects

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• 0:06 What Is Humidity?
• 0:44 Relative Humidity
• 3:31 Dew Point
• 5:24 Humidity Affects Water…
• 6:49 Lesson Summary

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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Humidity is an important aspect of the atmosphere because it affects both weather and climate. In this video lesson you will learn about the different types of humidity and understand how temperature affects water vapor in the air.

What Is Humidity?

Ever heard the phrase 'It's not the heat, it's the humidity'? People say this because humidity, which is the amount of water vapor in the air, can make hot temperatures even more unbearable than they already are. Humidity is actually a broad term, and we can describe different types of humidity in different ways.

Humidity is an important thing to understand because it affects both weather and climate as well as global climate change. Humidity also affects indoor environments, so understanding it can help you determine the best place to store your books, clothing and other important items in your house.

Relative Humidity

You've probably heard about relative humidity in weather reports. This is the amount of water vapor in the air relative to what the air can hold. Think about it this way: if you have a cup that is half-full of water, the cup contains 50% of what it can hold. Air works the same way.

Let's say that a certain parcel of air can hold 30 grams of water vapor per cubic meter of air, but it only has 15 grams of water vapor per cubic meter of air. We simply divide the amount of water present by the amount of water possible, so 15 divided by 30, and then multiply it by 100 to get a percent. So in this case, 15 / 30 = 0.5, multiplied by 100 gives us 50%. So the relative humidity is 50%, which means the air contains about half of the total water vapor it could possibly hold.

It's important to know that warm air can hold more water than cold air. You can think of different temperature air like different size cups: smaller cups hold less water than larger cups, right? In this case, cold air is a smaller cup and warm air is a larger cup. So when the air is colder, the same amount of water vapor will produce a higher relative humidity than the same amount of water vapor in warmer air. This is because we have to consider the amount of water vapor relative to what the air can hold.

Let's look at an example. You have two cups, one that can hold a maximum of 12 ounces and one that can hold a maximum of 24 ounces. Now put six ounces of water in each cup, and you'll notice that the smaller cup is fuller than the larger cup, even though they have the same amount of water in them.

There are 6 ounces of fluid in the 12-ounce cup, so 6 / 12 * 100 = 50%. There are 6 ounces of water in the larger cup as well, but this cup can hold a total of 24 ounces. So, 6 / 24 * 100 = 25%. If the 12-ounce cup represents cold air and the 24-ounce cup represents warm air, the relative humidity for the cold air is 50%, but for the same amount of water in warm air, the relative humidity is only 25%.

How full the cup is relates to how much the cup can hold overall. When we apply this to air, we can see that for the same amount of water vapor, the relative humidity will be higher for cold air than for warm air because the cold air is 'fuller' from the same amount of water.

Dew Point

When air becomes saturated, this means that it contains the maximum amount of water vapor possible. Just like relative humidity differs with different air temperatures, saturation is also temperature-dependent.

Remember our cups from before? Saturation occurs when the cups are completely full of water. Just like the smaller cup needs less water to become full, cold air becomes saturated from less water than it takes to saturate warm air. And just like if you kept filling the cups even though they were already full, the water would have to go somewhere, so it would spill out over the cup and fill the surrounding area.

Dew is when water condenses at ground level because the air is saturated. Just like the water spills over the side of the full cup, when air is saturated, the excess water 'spills over' and builds up on leaves, cars, buildings or anything else that is surrounded by the saturated air.

Therefore, the temperature at which saturation occurs is called the dew point. You most commonly see dew in the morning because air temperature goes down overnight. And remember, cold air can't hold as much water, so it's like you're taking the larger cup (warm air) and pouring it into a smaller one (cold air). There's simply not enough room, so it 'spills over' as dew.

Interestingly, this same condensation process occurs in the sky, but when this happens, clouds are created. Clouds are water condensation above ground level due to air saturation. So when you see the condensation on your patio furniture in the morning, this is dew. However, when you see it floating by as a white, puffy shape in the sky, this is a cloud!

Humidity Affects Weather and Climate

Humidity plays an important role in both weather and climate on Earth, and we should be thankful that it does because it makes our planet hospitable for us! Water vapor is a key greenhouse gas, which helps block harmful UV rays from the sun as well as traps heat on Earth that makes life possible.

Water vapor in the air comes from water on land and in the oceans. It evaporates into the air, which is an important part of the water cycle on Earth. Without this step, we would have no rain or snow, which help cycle water back to the ground and feed our rivers, lakes and streams that we depend on for drinking water.

However, because it is such a powerful greenhouse gas, water vapor can be quite the double-edged sword. Like a nice blanket on a cold day, we depend on water vapor in the atmosphere to give us just the right amount of protection and warmth. But if someone comes along and adds more blankets on you when you don't need them, this will likely make it too warm for you to be comfortable.

The same thing happens to Earth when too much water vapor is in the air but Earth can't remove the extra blankets and simply continues to warm all over. This feeds a vicious cycle on Earth because as the planet warms, the humidity increases. But with more humidity comes more warming, which then leads to more humidity and so on.

Lesson Summary

If you've ever visited the Southeast U.S., you know how humid it gets there. Sometimes the air literally feels like you're walking through a swamp. Humidity describes the amount of water vapor in the air, and the more water vapor that is present, the more humid it is.

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