Prioritize…
After this section, you should be able to
- define and interpret dew point temperature in terms of condensation rates and measuring the amount of water vapor present,
- explain what the Clausius-Clapeyron relationship is and how water vapor in the air is tied to temperature
- sketch a distribution of precipitable water across Earth's latitudes with the knowledge that polar regions are cool and equatorial regions are warm
Read…
So far, we have primarily focused on the interplay between evaporation rates, condensation rates, and the concept of net condensation and net evaporation. However, let's shift our attention to essential moisture variables - specifically, dew point and relative humidity.
Dew Point
To begin, let’s define the “dew point temperature” – you have probably encountered this term when watching a local weather forecast on TV. It represents the approximate temperature at which the water vapor in the air must cool (at constant pressure) to condense into liquid water droplets. Remember, the phase transition between liquid water and water vapor (and vice versa) is associated with a certain amount of energy and energy is tied to temperature. So, cooling a parcel of air that contains water vapor will eventually induce a phase transition of the water to liquid. This relationship is governed by something known as the Clausius-Clapeyron relationship, which tells us that as the temperature increases, the maximum amount of water vapor the air can contain at equilibrium at a given temperature also increases. Therefore, the dew point temperature serves as an absolute measure of the amount of water vapor present. In simple terms, a higher dew point indicates a greater concentration of water vapor molecules in the air, whereas a lower dew point signifies fewer water vapor molecules.
The Clausius-Clapeyron Relationship
The Clausius-Clapeyron relationship underlines the crucial role of temperature in controlling moisture in the atmosphere. It is one of the single most important principles in climate science! As temperatures rise, the air can hold more gaseous water vapor (see my sidebar below). Consequently, when dew points are higher, the air can support a more significant quantity of water vapor. This elevates the likelihood of water vapor molecules condensing onto surfaces, resulting in higher condensation rates. Conversely, lower dew points indicate lower saturation-specific humidity levels, which translate to lower condensation rates. See the graphs below. The x-axis contains temperature, and the y-axis is a value called “equilibrium vapor pressure” – for our purposes, we can consider this the maximum amount of vapor a parcel of air can hold at that temperature before it must condense (or deposit in the case of ice). As we move from left to right on the bottom axis, we increase the temperature. We also see that as air gets warmer, the amount of water vapor it can hold prior to saturation increases.
To Put It Another Way…
The phrase “warmer air can hold more moisture” is a commonly used shorthand, but it’s not entirely accurate in the strict scientific sense. Here's the nuanced explanation:
When I said that “warmer air can hold more moisture,” I was referring to the fact that at higher temperatures, the evaporation rate increases, and the air can contain more water vapor before net condensation occurs. However, this doesn’t mean the air has a fixed capacity to “hold” water vapor like a sponge.
Instead, it’s all about the balance between evaporation and condensation rates:
At higher temperatures, water molecules move faster, which means more molecules stay in the vapor phase, increasing the amount of water vapor in the air without condensation occurring.
As the temperature increases, the rate of evaporation increases, so more water vapor can exist without forming liquid droplets. Still, the air isn't “holding” it—it's just that the higher energy of the water molecules prevents condensation from happening as quickly.
So, while warmer air does lead to more water vapor in the atmosphere, it's not because the air has some magical “holding capacity” for moisture, like a box that needs to be filled — it's because of the physics of evaporation and condensation rates that we just learned about.
In summary: Higher temperatures = higher evaporation rates, which means more water vapor can exist without condensation, but air doesn’t literally "hold” water vapor like a sponge. The phrase “warm air holds more moisture” is a convenient simplification -- and if that's the way you want to think about it, I think you'll be just fine, but now you know some more of the gory details!
Relative Humidity
While the dew point tells us something about the mass of water in the air, it doesn’t necessarily tell us how close we are to reaching the maximum amount of water vapor that air can hold. That is where relative humidity comes in. Although not an absolute measure of water vapor concentration, relative humidity is an exceptionally valuable variable when studying the atmosphere. As you have previously learned, it serves as a comparison between condensation rates and evaporation rates, expressed as a percentage. Relative humidity depends on both dew point and temperature, with the gap between these two variables influencing the outcome. A larger difference results in lower relative humidity, while a smaller difference yields higher relative humidity. When the condensation rate equals the evaporation rate at equilibrium (when the dew point equals the temperature), relative humidity reaches 100 percent.
So, the blue curve in our above graph shows “how much water vapor is held in a parcel of air when the relative humidity is 100 percent.” It also shows how much water vapor is in the atmosphere when the dew point temperature and temperature match – the further right you get on the curve, the further up the y-axis (water vapor) you go. You may have heard someone say at some point “warmer air can carry more moisture,” this is what they were referring to. Note, that’s a bit simplistic, the correct framing is “warmer air can hold more water vapor given the same relative humidity” but now you can tell them it’s thanks to this thing you learned about at Penn State called the Clausius-Clapeyron relationship!
As you see, dew points, temperature, and relative humidity are intimately connected atmospheric variables. Relative humidity depends on both the dew point and temperature. As the temperature nears the dew point, the evaporation, and condensation rates become increasingly similar, and relative humidity increases. On the other hand, if the difference between temperature and dew point grows, relative humidity decreases.
We’ll see that this relationship is very important in the climate system for determining where and how hard it rains and how our weather- particularly extreme precipitation- may change as our climate warms. We’ll talk more about this in the next chapter, but I want to leave you with one last piece of information that ties this together. All other things being equal, a warmer atmosphere can hold more water vapor. We also know that it is warmer near the equator than it is at the poles. So, if this is true, we should be able to look at a graph of water vapor as a function of latitude and see that it is highest where it is warmest. Do we see that?
Above is a zonally averaged graph (we talked about these earlier!) showing a variable called total precipitable water. Precipitable water is the amount of water vapor (remember, vapor = the gaseous form) above you if you were standing on a 1 square meter patch of Earth. We call it “precipitable water” because the idea is it’s the amount of vapor that could be converted to precipitation if we cool the atmosphere sufficiently to “wring” all the water out of it. It is indeed highest at the equator – warm areas have more water vapor in the atmosphere than cool areas. We’ll find that this will be important in the next chapter.