Back To Home
The Story Behind the Weather - By Forecaster John Ensworth M.S. - 

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

 Last updated: 09/14/2003 02:19 PM

Good day!  Remember the old saying... it's not the heat, it's the humidity.  And here comes the atmospheric moisture that will make this heat wave (well, more normal summer heat) feel like a more normal summer.   The upper air ridge axis (marked in purple below) is sitting on our heads and that means sinking and warming air. 

The surface observations (a key to the station circles can be found at the very bottom of this page) at 10am this morning show light, mainly westerly (meteorologists name the wind by where it is coming from...that gives the listener an idea what kind of air is coming into the area). Already this morning, the temperatures have climbed to the upper 80's in Philadelphia (the upper left red numbers). That is some significant heat.

But along with the heat, we have the much delayed moisture returning.  The Dew Point (the green/lower left numbers) values that were in the 40's and 50's across the state for the last few days, have now climbed to the mid and upper 60's.  The Dew Point is the temperature you need to cool the air down to, to get dew (or fog) to form.  If there is a lot of actual water in the air, then you don't need to cool the air down much to get water to condense out of it.  Dew Points in the 60's and 70's (not far to the south of PA) are quite high.   The closer the dew point is the the temperature, the higher the Relative Humidity.  Even though it is warm, the dew points and temperatures are close (since the dew points are so high).  When the Relative Humidity is high, humans can't cool themselves effectively with perspiration.  So your skin remains moist and you get uncomfortable (not to mention the spots in your clothing!). That's why we look at Dew Point as an expression of the actual amount of water in the air, and the Relative Humidity as an expression of human comfort.   

A low dew point is one less than 40 degrees F.  A high dew point is one greater than 60 degrees F. 
A low relative humidity is on less than 40%. A high relative  humidity is one greater than 60%. 

The next two maps show the dew points over the US yesterday and today at the same time. You can see the entire eastern seaboard is one category more moist than yesterday.

Cooler temperatures and a chance of rain returns when the system out in the plains and Great Lakes (see the surface map below) gets here (along with the 500mb trough you can see in the first map a top). 

The rain (and then cooler temperatures and lower dew points) will hit (according the the ETA computer model forecast below) Friday evening...

But will be quickly past us by Saturday morning.  We might might have a nice, moderate, comfortable weekend. Woohoo!

But, in the mean time, we haven't felt heat like this since last summer, so take it easy outside.  Stay in the shade and don't over exert.  Given a week of this, or so, the human body does adjust to the heat stress. Give it time to figure out what season it is. 

Meteorologist  John Ensworth

Surface Station sky cover color key: