Today’s weather came from the passage of a cold front: where cold air pushing in, lifts the warmer, moister air in front of it, to create rain. Although the front was not particularly remarkable, it fitted the textbook very well so is worth reporting.
Firstly, the analysis chart shows the situation. Over Europe there is still the high pressure which has dominated our weather for some time, bringing light winds from the south and south-east. This air has been in place for a while, so was warm and humid. The air held quite a lot of water moisture, but there was no rain as the high pressure meant that air was descending and there was no mechanism to lift the air to create clouds and rain.
Behind the cold front, a very different air mass is pushing in from the atlantic. The air is much colder and so holds much less moisture. The front is where the colder air is pushing at the warmer air, lifting it from the surface and so making rain.
That’s the “synoptic view” (which means “seeing it all”). What did we see here in Horley, on the ground? My weather station saw a sudden drop in temperature, a sudden drop in dew point (which reflects how much moisture is in the air) and a change in wind direction. There was also a sudden increase in wind speed at the front itself.
There was a drop in temperature from about 14C to about 10C over less than an hour. The dew point dropped from about 15C to about 10C in the same time. A clear change in air mass.
The wind speed peaked and the wind direction changed at the same time. This was when the bulk of the rain (3mm of the total of just over 4mm today) fell.
After such a long period of dry and warm weather, it is tempting to see this clear event as the arrival of autumn. We shall see.