When you've finished this section, you should be able to define and interpret relative humidity as it relates to net condensation, describe the effects of increasing and decreasing temperature on relative humidity, and be able to define and discuss the lifting condensation level (LCL).
There's no doubt that dew points can tell a meteorologist (or any weather-savvy person) quite a bit about moisture. But, it's probably not the most commonly cited moisture variable in weather reports. My guess is that you've heard weather forecasters mention relative humidity many times in weather broadcasts or weather-related articles.
While relative humidity is not an absolute measure of how much water vapor is present (it doesn't tell us about the concentration of water vapor in the air), it's still an extremely useful variable. Let's review a few important points you've already learned:
- The equation for relative humidity, , shows that its essentially a comparison between the condensation rate and the evaporation rate.
- Since relative humidity depends on both the condensation rate and the evaporation rate, it depends on both dew point and temperature. The larger the difference between dew point and temperature, the lower the relative humidity. The smaller the difference, the higher the relative humidity.
- When the condensation rate equals the evaporation rate at equilibrium (the dew point equals the temperature), relative humidity is 100 percent.
I'll discuss some practical applications for relative humidity shortly, but first I want to mention a little quirk about relative humidity observations. Relative humidity values calculated from standard weather instruments range from as low as near 1 percent when the evaporation rate greatly exceeds the condensation rate (a huge difference between temperature and dew point), to 100 percent when the evaporation rate equals the condensation rate (temperature and dew point are equal). But, you already know that for net condensation to occur, the condensation rate has to be slightly greater than the evaporation rate. In other words, the temperature has to fall slightly below the dew point. But, the standard instruments that we use to make measurements are not precise enough to accurately measure the small difference between dew point and temperature when net condensation is occurring. Still, in reality, when net condensation is occurring, the dew point is ever so slightly higher than the temperature (even if we can't measure it). This leads to relative humidity values slightly greater than 100 percent within clouds, for example.
So, for all practical purposes, the temperature does not measurably fall below the dew point and we don't see relative humidity values greater than 100 percent reported. Since the dew point serves as a lower bound for temperature, on clear, calm nights when dew points won't change much, weather forecasters sometimes use the dew point as a guide for what the nighttime low temperature might be. Remember that, by definition, the dew point is 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. Once the temperature falls to the dew point, relative humidity increases to 100 percent, and the measurable cooling ceases as long as dew points don't decrease further. Net condensation occurs onto condensation nuclei when there's a little extra cooling that we can't measure with a standard thermometer.
Interpreting Relative Humidity, Temperature, and Dew Point
I hope that by now, you understand that relative humidity, dew point, and temperature are all closely intertwined. After all, relative humidity depends on both dew point (which is connected to condensation rates) and temperature (which is connected to evaporation rates). As the temperature nears the dew point, the evaporation rate and condensation rate become increasingly similar, and relative humidity increases. On the other hand, if the difference between temperature and dew point grows, relative humidity decreases.
To see this relationship in action, watch the short video (2:44) below, in which I discuss temperature, dew point, and relative humidity trends in State College, Pennsylvania from 00Z October 6, 2016 through 00Z October 7. From the video, you should clearly see how relative humidity changes based on trends in temperature and dew point, as well as how the changes in relative humidity impact the observed weather.
You should have noticed in the video that when the relative humidity was 100 percent for an extended period of time, fog was reported. Did you notice, however, that rain was never reported? Sometimes, students assume that if they surface relative humidity is 100 percent it must be raining, but that's not necessarily true. When relative humidity is 100 percent at the surface, the temperature equals the dew point, and it's very likely that net condensation may be occurring around hygroscopic condensation nuclei suspended in the air. If net condensation occurs for a long enough period of time, the end result is essentially a cloud at (or very near) the ground. In other words, fog!
When it's raining, the relative humidity must be near 100 percent somewhere, and it is -- up in the clouds! That's where net condensation is occurring as tiny cloud drops grow. Larger rain drops that fall from the clouds actually develop from a variety of processes, but once the rain drops fall beneath the cloud, they're usually falling into an environment with relative humidity that's less than 100 percent. If the relative humidity near and above the surface is too low, most or all of the rain drops can evaporate before reaching the ground (remember that low relative humidity values indicate major net evaporation), paving the way for significant evaporational cooling, as we've already discussed. But, even when rain does reach the ground, usually some drops have evaporated partially or entirely along the way. In other words, for rain to reach the ground, relative humidity need not be 100 percent in the lowest part of the atmosphere; it just can't be too low, or else all of the rain drops will evaporate before reaching the ground.
Relative Humidity, Lifting, and Clouds
Another practical application of relative humidity is that it gives us a basic idea of whether a little or a lot of cooling is needed for net condensation to occur. If relative humidity is high (near 100 percent) very little cooling is needed in order to achieve net condensation (there's a small difference between temperature and dew point). If relative humidity is low (say, less than 50 percent), then quite a bit of cooling is needed to achieve net condensation because a large difference between temperature and dew point exists.
To see what I mean, take a look at the two simplified station models below. The station model on the left has a temperature of 85 degrees Fahrenheit and a dew point of 50 degrees Fahrenheit. The station model on the right has a temperature of 40 degrees Fahrenheit and a dew point of 35 degrees Fahrenheit.
Now, let's apply our knowledge of temperature, dew point, and relative humidity to recap some main ideas from this lesson and see what we can determine from these station models:
Question: Which station has a higher concentration of water vapor in the air?
Answer: Station A has a higher concentration of water vapor in the air as evidenced by the fact that the dew point is higher at Station A.
Question: Which station has a higher relative humidity?
Answer: Station B has a higher relative humidity because the difference between temperature and dew point is much smaller (only a 5 degree Fahrenheit difference, compared to a 35 degree Fahrenheit difference at Station A). The actual calculation for relative humidity based on temperature and dew point is complex, but you can easily calculate the exact relative humidity at each station with this handy relative humidity calculator to confirm that the relative humidity at Station B is higher. You should find that Station A has a relative humidity of about 30 percent, while the relative humidity at Station B is about 82 percent.
Question: Would more cooling be required for net condensation at Station A or Station B?
Answer: We know that more cooling would be required for net condensation at Station A because of its lower relative humidity and larger difference between temperature and dew point. Meanwhile, less cooling would be required at Station B, because the relative humidity is higher.
The answer to this last question has practical implications for some types of cloud formation. Remember that the most common way for clouds to form is by cooling the air by lifting it until the temperature decreases to the dew point, which increases relative humidity to 100 percent, paving the way for net condensation to begin. Meteorologists call the altitude at which net condensation begins in these situations the lifting condensation level (or "LCL" for short), which marks the cloud base. So, imagine for a moment that air is being lifted from the ground to form clouds at Stations A and B. At which station will clouds form first, at a lower altitude? Clouds will form at a lower altitude at Station B because less cooling is needed for net condensation, so less lifting is required.
Ultimately, the LCL (or altitude of the cloud base) depends on surface relative humidity when air parcels are rising from the surface to form clouds. When relative humidity values are low, there's a large difference between temperatures and dew points, which means that a lot of cooling must occur before the temperature drops to the dew point (which requires lifting the air to higher altitudes). So, the LCL will be high (cloud bases will be high) when surface relative humidity values are low. When surface relative humidity is high, the LCL will be lower (cloud bases will be at a lower altitude) because the difference between temperature and dew point is small, so not much cooling is required for the temperature to equal the dew point (not as much lifting is required).
The bottom line that I want you to take away from these applications is that relative humidity is useful for assessing the difference between temperature and dew point, and for assessing how much cooling is needed for net condensation to occur (useful for predicting cloud and fog formation). Relative humidity won't tell you how much water vapor is in the air, or give you an idea of how humid the air might feel by itself, but when you see clouds in the sky or fog near the ground, you're seeing the results of 100 percent relative humidity and net condensation!