What Exactly Is The Dew Point?
"...the temperature at which air must be cooled at constant pressure in order for it to become saturated with respect to a plane surface of water"
-As defined in Atmospheric Science: An Introductory Survey by Wallace & Hobbs
Well, there's your definition. All right, I'll put it in simple terms for you. The dew point is a temperature at which the air becomes saturated. But as most weather definitions go, this is too simple of a definition.
Prepare yourself, what we are about to delve into actually takes some thinking. Why? Because the concept of moisture in our atmosphere, how we measure it, and how it changes, is pretty complex. Sorry about that.
However, you just may learn something
that will be on Jeopardy, so let's dig in.
So, when the air temperature and dew point are equal, does that mean it rains or snows? Not necessarily, because the process of forming rain is more complex. Also, keep in mind that rain falls from above. So, while the surface air may be saturated, that doesn't mean the air above you is saturated. In fact, before rain develops, there needs to be a cloud, and the process of cloud formation depends upon more than just saturated air. Such factors could be the type and size of aerosols which water vapor can condense upon. These aerosols are called cloud condensation nuclei (CCN). You see, it is not as simple as saying "if you have saturation, then you have a cloud or rain drop." However, it is the first step, and that's why dew point and saturation play a close role in forecasting rain.
The dew point is used a lot by TV meteorologists and seen on most current weather conditions because it's a great indicator of the moisture content of the air, or humidity. The higher the dew point temperature, the higher the humidity. Okay, this is where some thinking comes in, because the relative humidity and dew point both describe the moisture content of the air, yet they are different at the same time.
About Relative Humidity:
One thing you'll notice is that when the air temperature is close to the dew point, the relative humidity is high (often 80% or greater). But the relative humidity is "relative"or dependent. Relative humidity is dependent upon the temperature. So, if the temperature changes, the relative humidity will change. Dew point is a better "absolute" measure of the air's moisture content. Here's a good example of why:
From this table above, look at the relative humidity of 33%. Looks like a comfortable relative humidity, right?
Probably not. Sticky, sultry and oppressive are appropriate adjectives for 110 degree heat with an 80 degree dew point But why is the relative humidity so low? Because relative to that hot temperature of 110 degrees, the air is no where near saturation. Remember, we would have to cool the air down to 80 degrees (with no pressure change) in order to saturate that air mass (remember that's the definition of dew point). That's a 30 degree cooling! If our air temperature is 90 degrees, now the air is 67% saturated and we only have to cool the air 10 degrees to reach saturation. What if the air temperature was 80 degrees and the dew point was 80 degrees? The air is saturated, it 100% relative humidity and it is likely that you would be experiencing either rain, snow, mist or fog.
So, relative humidity is just that...relative, meaning it depends. It depends on the temperature of the air. Dew point temperature is a better "absolute" measure of moisture in the air.
In general, I consider dew points above 70 degrees F very uncomfortable. Most people consider dew points near 55 degrees F and higher to be necessary for severe weather. That's just a guideline though.
Check out this map above. It's a contour forecast of dew point. If you look at the yellow, that's dew points above 60 F. Since the dew point doesn't depend on temperature, I know that 60-70 degree F dew points are humid no matter what the temperature. Thus, the yellow region is the sticky and humid region. If this were a map of relative humidity, I would not be able to discern if 60% was humid or dry unless I also knew the temperature. That's another example of why the dew point is very useful in determining humidity.
The dew point is also critical when it comes to forecasting fog. Fog is likely when the surface air temperature and dew point temperature are the same. Light winds and clear skies at night are also favorable for the formation of fog.
Okay, that was a lot to think about. And now, I'm going to throw more info at you. But this is like extra credit, because at this point you probably understand the dew point rather well. At least, I'd like to think that. But if you're like me, you ask a lot of questions...
Why does cooling the air to the dew point cause saturation?
The cooler the air, the less energy it has. It takes energy for molecules to break free of the liquid bonds (or solid in the case of ice). So if you cool the air, you're reducing the energy the molecules need to escape the liquid bonds and become a gas. In turn, this encourages condensation. If you were to heat the air, you're adding energy and encouraging evaporation.
This doesn't quite explain why cooling to the dew point will cause saturation. In order to understand saturation, first understand that water in our atmosphere is constantly evaporating and condensing. H2O molecules are constantly coursing back and forth from a liquid to a gas and vice versa. What is important to the growth or decay of a water droplet is which process is dominant. For example, if you had 10,000 molecules evaporating and 1,000 molecule condensing, then evaporation is dominating and a rain drop or ice crystal is not likely to form. But when you cool the air, you reduce the energy molecules need to evaporate. At some point 5,000 molecules are condensing and 5,000 molecules are evaporating. The temperature at which this occurs, is called the dew point. It's the point at which condensation and evaporation are equal. Or you could consider it the temperature at which condensation has a chance to dominate over evaporation. And the reason that's important is because when condensation dominates over evaporation, it's finally possible to form a rain drop or ice crystal.
I mentioned several times about cooling the "air temperature", I am using the air temperature to approximate the temperature of the water vapor in the air. Although air is made of Nitrogen, Oxygen, water vapor and other stuff, we can make the assumption that the "air temperature" is also the temperature of the water droplet. It's kind of like trying to separate the temperature of the water in tomato soup from the temperature of the entire bowl of tomato soup. For all practical purposes, the temperature of the water is probably the same as the temperature of the entire soup. I mention this because it's not the temperature of the air as much as it is the temperature of the water vapor that is important.
Pictures courtesy of the NOAA photo library: http://www.photolib.noaa.gov/search.html