Dew Point: The Most Important Number In Weather

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By Steve MacLaughlin on May 21, 2013, 3:15pm Last modified: May 22, 2013, 9:04am

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I have posted this article a few times before on days when the "dew point" is so vital to our forecast. Today is one of those days. For the first time this year, yesterday and today actually "felt" like summer with very high humidity. If you look at the hourly observations around Connecticut, you will see dew points between about 65 and 70 degrees. That's really high. In the summer, dew points range from about 50 on a really dry, comfortable day up to 80 degrees on the most oppressively humid days. Winter dew points rarely get out of the 40s and can be as low as below-zero when we are getting our coldest and driest weather. So when we are up at 70 degrees, it is very sticky. So, with that in mind, here's why the dew point is so important:

We all live and die by the temperature or wind or humidity. But the forgotten, but in my opinion, most-important number in weather is the dew point. It can tell you almost anything about what the weather is doing and what it 'will' do.

The nice thing about the dew point is that it is absolute. The higher the dew point, the more moisture in the air. While relative humidity is constantly changing based on the temperature, it doesn't mean the actual moisture is changing. A hot summer day might have a morning relative humidity near 100% with fog or dew but by the afternoon it is oppressively humid with a relative humidity of 50%. But...what didn't change the entire day was the dew point. If it was 70 at wake up it was probably 70 during the afternoon and I don't care what the temperature is, if it's sunny or cloudy or raining, a dew point of 70 means nasty humidity.

Because the dew point determines the amount of moisture in the air, it also determines how quickly the temperature rises and falls. More moisture means a slower rise and fall. A humid night will be warmer while a dry night can see a quick drop in temperature. And, since the temperature can never be lower than the dew point, the dew point is a great "bottom" for where temperatures might go on a clear, calm winter night.

Of course, the dew point also tells us when we can have condensation, perhaps its most known function. The temperature drops to the dew point and the relative humidity hits 100% which means fog or dew or frost or precip. But it also tells us what will happen to rain or snow as it falls to the ground. It might be raining on radar or above our heads in the sky, but if dew points are very low, that precip will evaporate.

Dew points also help us find fronts when the temperature change isn't big enough. Look for the drop in dew point and you don't even need to look at temperatures or wind...picking out the front is a breeze.

Dew point also controls what happens to your weather where you live. Live along the shoreline? Higher dew points will keep your temp higher at night in the winter than inland where it is drier. And the same lower dew point inland in the summer can help temperatures sore while the shoreline remains cooler.

Wanna know what the weather is doing? Look at the dew point each hour for the last day. Is it high or low? Is it going up or down? Although we have a thousand things that help us figure out the weather, if I had to look at just one number and how it's changing, I would pick dew point...it's amazing how many things you can learn by looking at this often-dismissed number.

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Steve MacLaughlin

Town: New Haven, CT  

Reporting for WXedge since January 2012.

Articles: 122

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