Understanding Dew Point and Relative Humidity

One of my twitter followers asked me the other day if I can tweet about the difference between dew point and relative humidity. While I can certainly be succinct when I need to, I am not sure 140 characters are quite enough to tell you about this concept. If you can follow this blog entry, you’ll have a powerful understanding of this important meteorological concept.

First, if there is something unclear here let me know on Twitter @growingwisdom. I can edit and clear it up.

If you go back 20 years, no meteorologist or very few anyway were talking about dew point to the public. Everything was relative humidity. Then, at some point we all switched and began to try to convey this concept to the layperson. I know from teaching this subject in class that it’s not an easy concept to grasp, but once you do, it really quite useful.

Air
Let’s start with a box of air. Most of you know the air we breathe is made up of oxygen, carbon dioxide, nitrogen, argon etc. Within the air there is also water vapor. It’s simply water in gas form. Like anything with matter you can weigh air and you can weigh the part of air that is water vapor.

Each kilogram (1000 grams) of air has within in it, some number of grams of water vapor. So a box of air weighing one kilogram will always have about 980 grams of oxygen and nitrogen. However, the weight of the water vapor changes dramatically over the course of the year.

One of the properties of air is that at higher temperatures you can squeeze more of those water vapor molecules into each kilogram of air. This time of year, when the air is warm, we can put about 10 grams of water vapor in each kilogram of air. However, sometimes that number can go as high as 20 and on really dry days as low as 5. In the winter, the number averages around 2 or 3 grams of water vapor but can go to under a gram of water vapor.

Think about that for a moment. The amount of water vapor in the same amount of air can range from under a gram to as much as 20! Why does this change so much? It’s all about the air temperature. As the temperature goes higher, the capacity of it to hold more water vapor grows. I tell my students to think about warmer air like a big box with a lot of capacity for water vapor and cold air like a small box with much less.

The chart below shows how much many grams of water vapor you can squeeze into a kilogram box of air at various temperatures. Notice how much higher the number go as it becomes warmer. These numbers are how much water vapor is possible, but that doesn’t tell us how much is actually in it.

Dew Point
What we just looked at is the capacity of the air, but not how much is actually in there. To know how much water is actually in the air at a given time we need to look at dew point. The chart below, which is basically the same chart, shows how much water vapor is in the air at various dew points.

The dew point is in reality a temperature. So if the dew point is 68F, you have 15 grams of water vapor in a kilogram of air. That’s enough water vapor to feel it and we would say it’s a humid day. It might actually be better if meteorologists starting using the grams of water vapor as an actual variable on TV. I think people would quickly get the concept of more water versus less.

Relative humidity
With this knowledge, we can now talk about relative humidity. The word relative is the key to the term, not the word humidity. If you tell someone you won a race in your age group, they might ask how many people were in it. Coming in first is much more significant if there are 100 people in your age bracket, not two.