For the first time this season the Philadelphia Corp. for the Aging dusted off its Heatline Thursday, as the season's first potentially dangerous heat wave of the season ripened over the region.

That oppressive air also is swollen with water vapor, which inhibits the evaporative-cooling effects of sweat and also is a reason the Storm Prediction Center is saying strong thunderstorms are possible.

A severe thunderstorm watch — that would be for wind gusts approaching 60 mph and/or hail — is in effect until 10 p.m. for Philadelphia and for Bucks, Montgomery, Burlington, and Camden Counties.

With a high of 95 just before 4 p.m. at Philadelphia International Airport, this has become officially the fifth heat wave of the season, defined as three consecutive days of 90-plus temperatures.

But that's a technicality. The first four heat waves lasted only three days; this one means business and is expected to lap into the weekend.

Thursday marked only the second  "excessive heat warning" of 2017 for the region by the National Weather Service — it remains in effect until 8 p.m. Friday — and it's possible that the short dosages of heat so far this season might provide a measure of protection for the vulnerable.

"There is some truth to that," Sarah Johnson, a lead forecaster at the National Weather Service, told us last week.

But this hot spell bears watching. Significant stretches of steamy days and warm nights constitute a serious health threat, and have resulted in heat-related deaths in the past.