What to Do With Dew Points

Read...

While the focus of our discussion in this lesson has been on evaporation rates and condensation rates as they relate to net condensation and net evaporation, I want to refocus the discussion on moisture variables that meteorologists commonly measure or calculate (namely dew point and relative humidity). In this section, I'm going to focus on dew points. You may have heard dew points mentioned in weather reports or articles, but how do we interpret them? What can we do with dew points? You've encountered some of this information about dew points already in the course, but reviewing the basics and applying them to common weather situations will help you make practical use of dew points.

For starters, recall the definition of dew point: the approximate temperature to which the water vapor in the air must be cooled (at constant pressure) in order for it to condense into liquid water drops. In addition, you've also learned that (assuming air pressure doesn't change) the dew point temperature is an absolute measure of the amount of water vapor present. In other words, the higher the dew point, the more water vapor molecules in the air. The lower the dew point, the fewer water vapor molecules in the air.

As you've learned, when more water vapor molecules are in the air, the likelihood that any water vapor molecule will condense onto a surface increases. So, more water vapor molecules in the atmosphere (higher dew points) mean higher condensation rates. When fewer water vapor molecules are in the atmosphere, dew points are lower, and the likelihood that any water vapor molecule will condense onto a surface decreases. So, lower dew points mean lower condensation rates.

What constitutes "high" and "low" dew points? At the surface of the earth, the lowest dew points tend to be found during winter, in bitterly cold, dry continental Arctic (cA) air masses. In cA air masses, dew points can be well below 0 degrees Fahrenheit. On rare occasions, dew points in cA air masses in the northern United States can drop to -50 degrees Fahrenheit or lower! Dew points in cA air masses are so low because low evaporation rates over the bitterly cold ice and snow-covered grounds of polar latitudes mean that few water vapor molecules enter the air.

The most obvious way to increase the dew point is to evaporate water into the air, and that explains why the highest dew points tend to be found during summer in warm, moist, maritime Tropical (mT) air masses. In the summer, mT air masses sometimes cause air with dew points above 70 degrees Fahrenheit to overtake much of the eastern United States. On occasion in the United States (usually for short periods of time), dew points can even rise into the low 80s, but extremely rarely climb higher than that. But, the region of the world with the highest dew points is near the Persian Gulf in the Middle East, where dew points in the summer can exceed 90 degrees Fahrenheit on rare occasions. Such high dew points correspond to some of the highest water vapor concentrations on Earth!

To see an example of a warm, moist, maritime Tropical air mass in the eastern U.S., check out the analysis of surface dew points below from 19Z on August 21, 2017. Dew points greater than 75 degrees Fahrenheit extended from the Gulf Coast northward to central Missouri. Dew points greater than 65 degrees reached north of the Canadian border in the Great Lakes region.

The surface dew point analysis from 19Z on August 21, 2017 shows that high dew points, characteristic of a maritime Tropical (mT) air mass, had spread throughout much of the eastern U.S.

Credit: University of Illinois

Regions like the eastern U.S. (and the Persian Gulf) are prone to high dew points with mT air masses because the waters of the Gulf of Mexico (and the Atlantic Ocean near the southeast U.S. coast) are very warm in the summer, which leads to high evaporation rates. The high evaporation rates from the Gulf of Mexico lead to high concentrations of water vapor in the atmosphere and high dew points. This high dew-point air can then spread throughout the eastern U.S. by the wind. That's right, just as the wind can bring warmer or cooler air into a region (temperature advection), it can also bring moist or dry air into a region (moist advection or dry advection, respectively). On August 21, 2017, note the general wind flow off the Gulf of Mexico and off the warm Atlantic Ocean waters into the Southeast U.S., which over a period of days had helped usher moist air into much of the eastern U.S.

Of course, when dew points climb during the summer, the air can begin to feel very humid or "muggy." Obviously, how something "feels" is somewhat subjective, but dew point can help you determine whether the air will feel uncomfortable. Recall the table below, which shows how the air feels to most humans, based on dew points.

So, once dew points creep into the middle or upper 60s, most folks start to feel like the air is "muggy" or "sticky," and when dew points climb into the 70s, most folks find the air to be truly uncomfortable and stifling. By itself, dew point is a much more useful number to gauge human comfort than relative humidity (which depends on temperature, as you know). Also, because high dew points signal a high concentration of water vapor in the atmosphere, they may signal the potential for heavy rain and flooding from intense rainfall rates if showers and thunderstorms develop.

Weather forecasters always keep tabs on dew points because they're a critical part of making and communicating weather forecasts. Dew points are useful for everything from describing comfort levels to providing a piece of the puzzle in determining whether net condensation will occur. But, forecasters can't just concentrate on dew point when it comes to assessing the potential for net condensation and cloud formation; they have to be concerned with the evaporation rate (which depends on temperature), too. As you know, we have a variable that is useful for comparing the condensation rate and the evaporation rate--relative humidity. We'll wrap up our lesson next by discussing the best uses of relative humidity and some applications to everyday weather. Read on!