The quality of today's air mass is so different from the weekend's that one might think the region has undergone a sudden change of seasons, or perhaps a radical change of venue.
It feels so different because it is. The temperature peaked at 93 Sunday, making it the fifth consecutive day that the official Philadelphia airport thermometer had reached at least 90.
That constituted an official heat wave, and based on a National Weather Service analysis, a heat wave this early in the season increases the odds of this being a warmer-than-normal summer.
The early heat, however, may have a positive impact. Health experts have observed that early heat may help inoculate the elderly and vulnerable to surges of heat later in the summer.