Heat, enough already

On July 25 in an oppressively hot month, the atmosphere has shown a splendidly benign side today, and if you have a chance to spend some time outside, by all means do so.

Tomorrow, the atmosphere returns to Summer of 2012 -- and Summer of 2011 and Summer of 2010 -- mode.

Yet again, the National Weather Service has issued an excessive heat warning for Philadelphia and all the neighboring counties.

The forecast high of 96 would be well shy of a record, but heat indices in Philadelphia could reach 103, and triple figures as far north as Reading.

The difference between today and tomorrow won't be so much in the temperatures, although they indeed will be warmer, but in the water-vapor content of the air.

The dewpoint tomorrow afternoon is projected to head toward 70; today, it was in the upper 40s.

The dewpoint -- the temperature at which water vapor would condense -- is an absolute measure of moisture in the air and a far better indicator of discomfort than relative humidity.

At 3 tomorrow, the projected humidity level for Philadelphia is 45 percent. That might seem harmless, but the temperature is expected to be 96.

The higher the temperature, the more moisture the atmosphere can hold, thus the humidities at very high temperatured might not look all that scary.

But the dewpoint doesn't mislead. As it approaches 70, sweating becomes ever more ineffective. The more moisture in the air, the harder it is for sweat to evaporate.

If you were to go out into the yard right now holding a glass of tapwater -- with a water temperature of, say, 55 -- the outside of the glass would stay dry.

That's because the glass couldn't cool the surrounding environment to the dewpoint.

Do the same this time tomorrow with a glass of tapwater, and droplets of condensation will coat the outside of the glass.

Friday won't be quite as oppressive, but still quite uncomfortable, and while the temperature should back off several degrees Saturday, dewpoints will remain on the high side.

In these dry times, having the atmosphere thick with water vapor isn't all bad, because the moisture will enhance the shower chances Friday and Saturday.

The region needs more rain a whole lot more than it needs more heat.