Monday, June 12, 2017

Why we need to be concerned with the dew point temperature

It's a question I get asked every spring, summer and fall - 'Why do you always show the dew point temperature?  I want to know what the relative humidity is.'

It's important to pay attention to the dew point temperature because that number is a direct correlation to how much moisture is in the atmosphere.  The higher the dew point temperature, the more moisture is present in the atmosphere.  It's also an indicator of how 'uncomfortable' a person may feel on a hot and humid day. 

Relative humidity, on the other hand, represents how close the air is to saturation and can change drastically from morning to afternoon.  Dew point is more constant throughout the day.  The relative humidity will be highest in the morning when the air temperature falls near the dew point temperature.  But as the air heats up and the temperature rises, the relative humidity will decrease because there is a larger difference between the dew point and air temperature. 

Let's take Monday evening for example.  The air temperature at 7pm was 91 degrees and the dew point was 64 degrees.  This produced a relative humidity of 41% - not overly high.  But when you step outside, it 'feels' muggy and humid.  That is a result of the dew point temperature nearing the mid-60's.  Dew point temperatures in the upper 60's and low 70's are nearing tropical air mass levels and just makes it feel oppressive.

The dew point temperature tonight won't drop much past the mid 60's - which will actually keep our overnight lows in the low 70's.  Moisture 'pooling' near a stationary boundary Tuesday could cause the dew point temperature in a few locations to reach 70 degrees for a time during the afternoon.

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