‘Humid’ enough for you? Philly sets soupy standard in a September to remember for rain

With a spritz of rain on Monday, the first 17 days of September constituted the second-wettest such period in Philadelphia records dating to 1874, passing 1960, the year historic Hurricane Donna directly impacted the region.

The only other rainier Sept. 1-17 period occurred in 1999 — the year of Hurricane Floyd.

By mid-morning Tuesday, the month also had set an official standard for atmospheric soupyness unmatched since 1970. That has something to do with why it's been raining so much.

"It certainly has been humid," said Alex Staarmann, meteorologist with the National Weather Service in Mount Holly, invoking the term most accepted by the public to describe moisture in the air.

But more to the point: It has been the dew points, a superior measure of water vapor that meteorologists prefer, that have been so high.

The dew point, the temperature at which water vapor comes out of hiding and condenses — think of beads of water on a cold glass on a hot day — is an absolute measure; humidity is the percentage of moisture in the air relative to what the atmosphere could hold at a given temperature. The hotter it is, the more vapor the air can hold.

Dew points steam up the windows, curl the hair, add jewels to spider webs, and drive heat indexes. When sweat evaporates, it gives off a cooling effect to the skin, but high dew points retard evaporation.

When the Eagles played at Tampa Bay on Sunday, the temperature was in the low 90s and the humidity was around 60 percent.

While those conditions might not seem extreme in and of themselves, at 92 degrees, the air can hold buckets of water vapor, so 60 percent humidity would mean a steam bath.

Dew points during the game were in the stratosphere — the mid- to upper 70s — and so were the heat indexes, 106 to 108.

Things haven't been quite that oppressive around here, but the air has been consistently swollen with water vapor this month.

The 70-plus dew point hourly readings passed the 190 mark on Tuesday, the most since September 1970, which featured 244 of them.

The accelerating fall of brittle brown leaves is providing quite a contrast to the grass and other foliage made luxuriant by the rain. Water evaporating from the foliage in turn is helping to moisten the atmosphere.

But the reign of rain is about to end, and nature is about to turn off the soup pot.