Why all of a sudden I am grinning like a fool

For the last four or five days temperature in London has been about 33 degrees in the daytime and rarely less than 20 at night, with no wind whatever and dreadful levels of humidity and pollen. Now the sky has covered, the wind is rising fast, and thunderstorms in the afternoon are promised, after which the temperature should fall by eight degrees or so. I have been gasping (literally) for this moment for days.