Making Do with Dew Points

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Everyone will surely recognize that water is an important player in weather, so meteorologists must have weather variables that help them assess moisture. One such variable is dew point temperature. By definition, the dew point is the approximate temperature to which the water vapor (the gaseous form of water) in the air must be cooled (at constant pressure) in order for it to condense into liquid water drops. I emphasize here that dew point is a temperature, so it's typically expressed in degrees Fahrenheit or Celsius.

As it turns out, the dew point temperature is also an absolute measure of the amount of water vapor in the air. The higher the concentration of water vapor, the higher the dew point (and the lower the concentration, the lower the dew point). 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 air masses from the Arctic, where dew points can be well below 0 degrees Fahrenheit. On rare occasions, dew points in such air masses in the northern United States can drop to -50 degrees Fahrenheit or lower! On the flip side, the highest dew points tend to be found during summer in warm, moist, "tropical" air masses. In the summer, these air masses frequently have dew points above 70 degrees Fahrenheit. 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. If you want to learn more about extreme dew points, check out the Explore Further section toward the bottom of this page.

The fact that dew point serves as an absolute measure of the amount of water vapor in the air sets dew point temperature apart from many of the other variables that describe moisture in the atmosphere. These other variables have their uses, but they also depend on other factors beyond just the amount of water vapor present. We'll talk more about some of these other variables later in the course. Moisture is a fairly complicated topic, so we're just going to scratch the surface for now!

To better understand dew point and its applications, we should start with the characteristics and behavior or water vapor. As mentioned above, water vapor is the gaseous form of water. You probably learned at some point that matter exists in three states -- solid, liquid, and gas. Well, water is one of the rare substances that can exist in all three states naturally in our atmosphere. Water's solid (ice) and liquid forms are evident all around us, but the gaseous form (water vapor) might not be so obvious. Just like other gases (oxygen, nitrogen, carbon dioxide, etc.) water vapor is invisible.

A consequence of this is that standard photographs really don't show water vapor, even if they claim to. For example, check out this image of a steaming tea kettle. Within the effluent escaping from the spout, where is the water only in vapor form? Hint: It’s not in the part you can see. Although some water molecules are likely in the vapor state mixed within the visible “cloud,” the water that you can see is actually in the form of tiny liquid drops. If you look closely, there appears to be a gap between the tea kettle’s spout and the visible cloud (here's an annotated image of the tea kettle). This is where the water exists in a pure vapor state. In fact, this is only a portion of the effluent that is “steam,” or super-heated water vapor.

It is often helpful to think of air molecules as marbles. No one molecule can "hold on to" another.

Credit: Public Domain

Ultimately, water vapor behaves just like any other gas. On a molecular level, water vapor behaves just like oxygen, nitrogen, carbon dioxide, etc. Consider a situation where you had a box of “air” (containing all of the molecules normally found in the atmosphere). This is very much like having a box of various colored marbles. These marbles (because they have a lot of energy) are zooming around, bouncing off the sides of the box and each other. However, each marble is acting independently of the others. This means that in our box of air, the oxygen molecules are acting independently of (and oblivious to) other molecules – including water vapor molecules. The implication of all of this is: Air does not “hold” water vapor, and has no "holding capacity" for water vapor (which are common, but incorrect, phrases that are used to explain water processes). Air isn't like a sponge that can't absorb any more water once the pores of the sponge become filled with water. Indeed, all the air molecules in our box, combined, would only occupy a really tiny fraction of the space in the box, no matter what. So, there's always enough room for more water vapor molecules. We're going to expand on these ideas later in the course when we talk about topics like cloud formation, but I wanted to lay the groundwork for thinking correctly about water vapor now (it will help later on).

Now we need to discuss the processes by which water changes phase (namely to and from water vapor). When transitioning from a gas to a liquid, water undergoes a process called condensation. Likewise, when transitioning from a liquid to a gas, the process is called evaporation. We'll explore these (and other) “phase transitions” in more detail later in the course; however, at this point, I want to emphasize that evaporation and condensation events are taking place all the time, everywhere around you, even if you can't see them. Surprised? Allow me to illustrate.

Is the glass of cold water half full or half empty? You can tell by the "dew" on the outside of the glass.

Credit: David Babb

Take a look at the metal glass roughly half-filled with cold water on the right. The bottom of the glass is obviously coated with a layer of small liquid water drops (often called “dew”), while the top is not. Why is that? Molecules of water in the gas phase (water vapor) are zinging around in the air, but when a water vapor molecule strikes an object (like the side of the glass), it may “stick” (that is, condense on the surface). I say “may” because there is only some chance that the molecule is captured by the surface. If it does stick, then there is another chance that within some time frame, the molecule will become “unstuck” (that is, evaporate from the surface) and return to the gas phase. Thus, on all surfaces, there is a chance of condensation and a chance of evaporation for each gas molecule that encounters a surface, which means that we always have a rate of condensation and a rate of evaporation for every surface.

So, molecules are impacting (condensing) and leaving (evaporating) on both the top and bottom surface of our glass half-covered in dew. But, then why is the bottom covered in tiny liquid drops while the top remains dry? The answer lies in the fact that the rates of condensation and evaporation are not equal everywhere. On the bottom of the glass, the rate of evaporation is less than the rate of condensation; therefore, there is a net increase in liquid water (we say “net condensation”). On the top of the glass, the rate of evaporation is greater than the rate of condensation, meaning that there's a net decrease in liquid water (we say “net evaporation”). Since the glass is about half full of cold water, you might have guessed that temperature is playing a role here (and you're correct). The colder part of the glass has a lower evaporation rate, which allows tiny water drops to grow via net condensation (condensation occurs faster than evaporation does on this part of the glass).

Now with some background about water vapor's behavior, let’s revisit our definition of dew point temperature. We said that the dew point is the approximate temperature to which the water vapor in the air must be cooled in order for it to condense into liquid water drops, and that the dew point temperature is an absolute measure of the amount of water vapor in the air -- the higher the concentration of water vapor, the higher the dew point. Can you now see how these two ideas connect? If the air contains a high concentration of water vapor (dew points are high), then net condensation will occur at a higher temperature (that is, at a high dew point temperature). If water vapor concentrations are very low (dew points are low), then net condensation will not occur until the air is very cold (that is, at a low dew point temperature). If the dew point temperature is less than 32 degrees, the term frost point is, technically, more appropriate than "dew point" because frost will form (by a process called deposition, not condensation) instead of dew.

One final practical point about dew point. The higher the concentration of water vapor, the higher the dew point, and by itself, the dew point serves as an indicator of the way the air “feels” – whether it be dry or muggy. Since our skin temperature is regulated to some degree by evaporation of sweat, it would be logical that we would be affected to some degree by the dew point temperature. Certainly, describing how something “feels” can be a bit dicey in a science course because it’s a somewhat subjective topic, but examine the table below for a rough guide on how the air might “feel” based on dew point temperature.

Now that you know some basics about dew point and the characteristics and behavior of water vapor, let's shift gears to looking at dew points on station models, which is covered in the Key Skill section below.