Just been triggered by a post from someone surprised by a hot car:

Even if the air outside is still cool, the sun's strength right now is about the same as it will be in late August.

We're < 2 months from mid summer, well-insulated cars with big windows allow that radiation in but not out.

So the radiative heat from the sun can still be pretty hot right now in the northern hemisphere! You can already get sunburned. The air and sea takes a while to catch up to the strength of the sun, is all.