Hottest Day of the Year

Today was, in traditional terms, a scorcher. The temperature topped out at 30.5C, the hottest this year and the only time we have managed to register a "tropical" day (with Tmax > 30C) this year. That's well down on last year's six days, the big difference being a slightly cooler July here, though other parts of the UK (not so affected by sea breezes) had some exceptionally hot days then.

Uk Met Office, Surface Pressure Chart noon today.

This chart explains why it was so warm today. The low pressure areas over the near continent and south of Ireland show where very hot air has advected (moved) off of the land mass towards the end of the summer. The isobars (pressure lines) are far apart, associated with light winds at the surface and aloft. The hot air has moved and is now sitting over us as it has for some days, with the sun still powerful enough to heat it further. We are in a kind of temporary oven.

Thunderstorms did break out over the Midlands and NE England today, but they are too far away to make a difference here. Cloud moved in around sunset here (associated with the torugh marked on the chart as a line) MEt Office radar showed light rain falling from this cloud, but so far there has been none at the ground: most likely the rain is evaporating before it ever falls far enough for us to see and hear it.

That adds to the unpleasant humidity. Hot air can hold more water vapour than cooler air and the air is more than warm enough to pick up lots of moisture from the sea around us, as well as from any rain that evaporates.  There is so much mositure n the air that we had thick mist yesterday morning (complete with foghorns locally) when the temperature was over 20C. The air could not hold any more water, so it condensed into tiny droplets to form mist. The temperature at which this happens is called the dew point and is rather high in this air mass.

Change is coming. Cooler, and therefore less humid, air currently West of Ireland will move in over the next day or two, also bringing cloud. Temperatures will fall to more reasonable levels and it will be much easier for people to sleep.

This article was updated on Tuesday, 27 August 2019