Dew Point: Show Me Some Respect!

August 3, 2007 at 2:30 am | Posted in Uncategorized | Leave a comment

Do you know what the dew point is? Would you believe me if I told you that it might be the single most important number that I disect every night when I’m putting together my forecast for Channel 5? It’s true. In fact, if you take away all of my computers and models and maps and tell me the temperature and the dew point, I can make a pretty good estimate of what the weather is doing right now and might even be able to give you a pretty good forecast.

The air temperature tells us whether it’s hot or cold. Pretty easy, right? The higher the air temperature, the hotter it is. Well, the dew point temperature tells us whether it’s moist or dry. The higher the dew point, the more moist it is. Have you stepped outside today? Did you get hit with that wall of moisture. You couldn’t see it, but you could feel it? Now watch the weather report. The dew point is in the 70s. That’s incredibly high and means there’s a ton of moisture available. The higher the dew point, the more uncomfortable it is, especially in the summer.

I wish it were that easy, but the dew point gets even more complicated. It’s really defined as the temperature at which condensation occurs. In other words, whenever the air temperature drops to the dew point, the relative humidity is 100% and condensation occurs. That means clouds, fog, dew, frost or some type of precipitation. Another way to look at it: When the temperature drops to the dew point, we go from feeling the water in the air to actually seeing it.

That brings me to relative humidity: A very misunderstood number and at times, very over-rated. The problem with relative humidity is the “relative” part. It is the humidity “relative” to the temperature. The temperature is always changing which means the relative humidity at one temperature is completely different from the relative humidity at a different temperature even when the actual humidity or moisture in the air hasn’t changed.

To get the relative humidity, there is a mathematical equation whose main components are the temperature and, yep, you guessed it, the dew point.

We already know when the dew point and temperature are the same, we get a relative humdity of 100% and some type of condensation. The complicated part is when the numbers DON’T equal each other, which is MOST of the time. On a typical day, unless a front comes through the area or we see a big wind shift, the dew point will not change by more than one or two degrees. Whatever the dew point is in the morning will be the dew point throughout the entire day. But the temperature is ALWAYS changing. The sun comes up and it gets warm; then sun goes down and it gets cool.

Let’s say the temperature at 7am is 80 degrees and the dew point is 75 degrees (a typical mid-summer, humid morning). The relative humidity at 7am is about 80 or 90 percent. Sounds pretty high, and it is. At 3pm, the temperature soars to 100 degrees with the afternoon sun but the dew point DOES NOT CHANGE because it’s summer and there aren’t any fronts coming through. Guess what the relative humidity is now. Under 40 or 50 percent, which sounds low. Because it’s the humidity RELATIVE to a much higher temperature than at 7am. But it feels just as humid at 3pm as it did at 7am. That’s because the dew point is the same. And after sunset, the relative humidity will start going right back up.

The lesson: When you want to know what it FEELS like outside, look at the dew point, not the relative humidity. The higher the number, the more humid it is. Remember, when it’s 100 degrees, 30% humidity is incredibly high. And if the relative humidity is 90 percent in the morning, that number will keep going down during the day, even though the actual moisture in the air and the way it feels has not changed.


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