Monday, September 22, 2014
Inquirer Daily News

Summer coming, in a hurry.

This weekend will feel like the Fourth of July, and it's not just the temperature.

Summer coming, in a hurry.

The temperature forecast for the Memorial Day weekend has a decidedly July look, but what impresses us is the outlook for discomfortl.

Dewpoints are expected to reach the upper 60s, and with temperatures in the upper 80s, heat indices will creep toward the middle-90s.

It's a shame that the dewpoint, a measure of the absolute moisture content of the atmosphere, is so poorly understood. It is a handy way to get an instant read on discomfort, as opposed to the "relative humidity" and the heat index.

The heat index gives a general idea of discomfort, but it can't speak to all human beings in all situations. The dewpoint has no such pretensions.

A heat index of 100 means something very different to the lemonade-sipping slender woman in a bathing suit sitting under a tree by the pool, from what it means to the overweight, fully-clothed male walking in the sun after consuming a double cheeseburger and French fries.

As for relative humidity, it actually can be a confusing concept. It represents the amount of moisture the atmosphere can hold at a given temperature.

The higher the temperature, the more moisture the air can hold and the lower the relative humidity tends to be. For example, at 3 p.m. Saturday in Philadelphia the forecast high is 88, and the forecast humidity is a relatively harmless sounding 52 percent.

The dewpoint, however, is expected to be 68 at 3 p.m., which indicates the air will be generously stuffed with invisibile, clingy water vapor. The higher the dewpoint, the harder it is for sweat to evaporate.

If the dewpoint were 68, and the temperature were the same, the relative humidity would be 100 percent, since the air could hold no more moisture at that temperature. The moisture would be forced out of hiding, perhaps as dew or fog.

The weekend dewpoints are likely to have a salubrious effect on cash registers at the Shore and other escapists' destinations. Triple-A Midatlantic is projecting that 400,000 people in the region will be on the road this weekend, high gas prices be damned.

The atmosphere, said Tony Gigi, meteorologist at the National Weather Service in Mount Holly, "should get everyone in the mood for summer."

 

 

 

 

Tony Wood Inquirer Weather Columnist
About this blog

Everyone talks about the weather, and here we write about it.

When we’re around and conditions warrant, we’ll keep you updated about what’s coming, but we will do our best always to discuss weather and climate developments in context and remind you that nothing in the atmosphere happens in a vacuum.

Tony Wood has been writing about the atmosphere for The Inquirer for 26 years.

Reach Tony at twood@phillynews.com.

Tony Wood Inquirer Weather Columnist
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