Today is to be the seventh consecutive day that high temperatures at Philadelphia International Airport will reach at least 90.
That's an impressive stretch, and we of the local media have been calling this a "heat wave."
But we can't remember a more-benign one, akin to a gentle overnight snowfall on a late March weekend that disappears when the sun comes up. So is this truly a heat wave?
"It's not," says Gary Szatkowski, meteorologist in charge of the local National Weather Service office in Mount Holly.
Traditionally, a heat wave is defined as three consecutive days of temperatures 90 or better, and that is based on a definition from one A.T. Burrows appearing in a 1900 U.S. Department of Agriculture publication.
However, the official National Weather Service definition heat wave does allow for interpretation: "A period of abnormally and uncomfortably hot and unusually humid weather. Typically a heat wave lasts two or more days."
Let's review.
Heat indices approached 100 on both Wednesday and Thursday last week, and "excessive heat warnings" were in effect. But by daybreak Friday, the comfort levels improved dramatically with the lowering of the levels of water vapor in the atmosphere.
That water vapor typically is expressed in terms of "relative humidity." But as Glenn Schwartz has lectured, the concept of "relative humidity" is just that – relative. It's the amount of moisture the atmosphere could hold at a certain temperature.
On Thursday, for example, the humidity went down as the temperature went up. It was 80 degrees at daybreak, and the relative humidity was 79 percent; at 1 p.m., 90 degrees, and 68 percent.
A better measure of oppressiveness is the "dewpoint" temperature, since that's an absolute measure of moisture in the atmosphere. It's the temperature at which invisible water vapor will condense.
Take a pitcher of iced tea outside, and it will be covered with water beads as the air around it is cooled well below the dewpoint.
When the dewpoint is 70 or better, the air is soupy enough to cause discomfort, and it got as high as 74.5 on Thursday afternoon at the airport.
By 1 a.m. Friday, the dewpoint had dropped to 67.5, and it hasn't looked back. It got into the quite-comfortable 50s Saturday, Sunday, and Monday, even though the official reading hit 90 all three days.
"I am not in the heat wave camp," Tony Gigi, a lead forecaster in the weather service Mount Holly office. "I wouldn't call three consecutive lows in late January in the low 20s a cold wave."