8.40 a.m. THIS MORNING’S snow, settling briefly on the Blackthorn blossom by the wood when I drew this, seems out of season after the weeks of dry, settled weather we’ve been had, recently with summer temperatures. Cold air is moving in from the continent as a couple of high pressure systems over the Atlantic weaken and retreat. At least that’s as I understand it from the Met Office sequence of maps that I’ve been looking at. Warm fronts (indicated on the map by a red line with semi-circles facing in the direction of travel) followed by cold fronts (indicated by a blue line with triangles) have been moving down across the country but today the map shows an occluded front (a purple line with alternate triangles and semi-circles) across southern England.
An occluded front occurs when a second mass of air moves in so quickly that it overtakes the first. If colder air is catching up with a warm front it will plough under it, wedging itself beneath the warmer air.
If warm air is forced upwards it’s going to cool, resulting in rain, and if the rain falls through the wedge of colder air, that would, I guess, turn it to snow.
Weather maps show the situation from above but to understand a swirling meeting of air masses like an occluded front, you’d really have to see it from the side or in three dimensions.
Occlude means to block, stop up or obstruct and in today’s case I think that would refer to the cold wedge of air swirling in and cutting off the warm air from the centre of the low pressure system, a cyclone, which in the northern hemisphere moves in an anti-clockwise direction when seen from above.