To give people a little hope in these trying times... am I the only one here doubting it will hit 40 today? Was just outside and yeah it's warm, but I don't feel it in my bones...
Salt. Specifically (in this context) the salt of sweat.
Seriously, if the cloud cover persists then the temp will probably stay low. As soon as the sun comes out the temp will soar. Looking at windy.com the cloud cover is pretty patchy so I don't hold out much hope for a reduced temperature.