Right now in Philadelphia, the air is about 67°F, but the model estimates the asphalt under your dog's paws is closer to 67°F — the surface and air have nearly converged because the pavement is shedding the day's stored heat after dark. That reading puts Philadelphia in the clear for an ordinary walk. Air temperature alone would have told you none of this — the pavement, not the forecast, is what your dog actually stands on.
Across the 18 hours left today, the pavement in Philadelphia is projected to top out near 121°F around 3 PM, when the air will be about 81°F — a 39°F gap between the forecast your weather app shows and the surface your dog walks on. Today breaks into 2 separate safe stretches rather than one continuous window, so missing the first does not cost you the day.
In an average August, Philadelphia sees highs around 85°F and overnight lows near 68°F. Even at those moderate highs, a dark surface in direct sun can run 40–60°F hotter than the air, which is enough to tip midday walks into the caution range. As a seasonal rule of thumb, the safest windows are before about 11:00 AM and after about 4:00 PM.
Philadelphia loses its all-day safe window in 3 months of the year — June through August — and August is one of them. The year peaks in July at an average high of 88°F against 43°F in January, a 45°F swing that decides how much of the day belongs to your dog.
The nearest place we track is Wilmington, DE, about 25 miles away. That is close enough to move together in a heat wave but far enough that a passing cloud bank or a coastal breeze can put real distance between the two readings, so check Philadelphia's own page rather than assuming.
Whatever the number says, confirm it with your hand: press the back of it flat on the pavement for seven seconds. If you can't hold it there, neither can your dog. Conditions in Philadelphia are on the gentler side right now, but keep the same habits: water before and after, grass over asphalt, and a close eye on flat-faced breeds, seniors, and puppies, who overheat and burn sooner than a healthy adult dog.