text size: small medium large Water is everywhere on Earth and it's everywhere in the air as a gas or vapor. Depending upon the air temperature, water can occupy up to 4% of the volume of air. That does not sound like much but a slight change can produce clouds, it can drive storms, or it can make us uncomfortable. There are two common ways to describe how much water vapor is floating in the air. One way is relative humidity and the other is dew point temperature, commonly called dew point and written as one word- dewpoint.
Relative humidity describes how far the air is from saturation and it's useful for figuring the amount of evaporation. It's good to know if you are involved in agriculture where you apply water or pesticides to large areas, or if you are in forestry and you are considering burning. Relative humidity is an important indicator of the rate of moisture and heat loss by plants and animals. When relative humidity is high it means the air is more saturated and not much water can evaporate into it. When the relative humidity is low, water that is on the ground, in plants, and even in people will evaporate quickly into the air.
The trick with relative humidity is it is relative to two different things: how much water is in the air; and what the air temperature is. During a typical day the relative humidity is high in the early morning when the temperature is cool. As the temperature rises until mid afternoon, the relative humidity steadily falls. This is because warm air is able to contain more water vapor than cold air. With low relative humidity it is easier for clothes to dry outside.
Look at it this way: if you take a handful of marbles and put them in a cup, the cup is relatively full. If you take that same handful of marbles and put them in a large jug, the jug will not be very full. We have the same amount of marbles but the varying size of the containers make the fullness relative. In the cup, if you want to make it 100% full of marbles you have to add a few more marbles, or you could cut the top of the cup off so that the cup is smaller. In the big jug you'd have to add a whole bunch of marbles to make it 100% full.
Think of the cup as cold air and the jug as warm air.
Back to the atmosphere, you can make the RH reach 100% by adding moisture to the air, or by cooling the air down so that it holds less water. When the air is cooled to the point that it is 100% saturated you've reached the dew point temperature. How do you know? Dew forms.
Dew point is the temperature to which air must be cooled to become saturated (without changing the pressure). There is no easy formula to relate dew point to relative humidity. Think of the dew point as a shoe size. The bigger the number, the more moisture is floating in the air. The dew point is useful in figuring out how comfortable we are and for forecasting thunderstorms, nighttime low temperatures, and the formation of dew, frost, and fog.
When the dew point equals the air temperature, the air is saturated and the relative humidity is 100%. The dew point temperature tells us nothing about how many water molecules are in the atmosphere or how close the air is to a relative humidity of 100%. To know how close the air is to saturation, we need to know the dew point and the air temperature. The closer the dew point is to the air temperature, the closer the air is to saturation. The temperature, the dew point temperature, and the relative humidity are related to one another.
Here's what confuses people- You can have a dew point of 40 degrees with relative humidity of 100%. Most people would be cool but not too uncomfortable. With a dew point of 70 degrees and relative humidity of 100%, most people would say the air is sticky. This tells us that relative humidity alone does not tell you how comfortable you are. That's what dew point can do.
Compare cities in the image below from a summer afternoon. The dew point in Boston is about the same as it is in Detroit but Boston has a RH of 100%. That tells us that it is either raining or foggy in Boston since the temperature would equal the dew point. Topeka has a lower dew point than Dallas yet it has a higher relative humidity. In the mountains both dew points and relative humidity values are low.
Along the Gulf Coast, dew points in the 20s are common in our cool seasons. You may wake to see frost whenever the dew point is under 32 on a clear, calm morning. As dew points rise into the 30s, above 32, we might wake to find dew on clear, calm mornings. When dew points are in the 40s the air is dry. When they move into the 50s you'll notice slightly higher humidity but you'll still be comfortable. Dew points in the 60s are noticeable by everyone. That's when the air is sticky. When dew points get into the 70s you become uncomfortable. In the rare cases when they rise above 80 along the Gulf Coast, the air is oppressive. There are tropical areas near the equator where dew points commonly exceed 80!
Dew points are also a good thunderstorm
index. The higher they get, the more fuel is available for storms and possibly
severe thunderstorms.
40s- thunderstorms not likely
50s- few thunderstorms possible
60s- thunderstorms
70s- thunderstorms very likely
Our dew points can get high because of the moisture flowing in from the Gulf of Mexico. In the deserts of the southwest US, the dew points don't get very high. When dew points are below freezing, they can be called frost points.
