Today has seen a procession of heavy convective showers moving slowly up the Bristol Channel.
The outstanding feature of July 2013 was the temperatures – last month not only produced a record (for this weather station) high temperature of 31.2C on the 19th, it also saw the highest daytime average (25.7C on the 18th) and the highest average temperature for the month of 19.1C, about 2C higher than normal.
The reason for all this warm weather was a high pressure system that established itself over the UK on the 5th and stuck with us until the 22nd, when it drifted off to Scandinavia. This anti-cyclone blocked the path of Atlantic lows, hence ensuring a prolonged dry spell with lots of clear blue skies and sunshine.
Being near the Bristol Channel, hot weather in this area is often tempered by a daytime sea breeze – the land warms up in the sunshine and heats the air above it, which then rises and draws in cooler air from the sea. On several occasions this meant our temperatures were a few degrees below those recorded further inland, and perhaps that made life a little more comfortable for us. However, for much of the hot spell there was a gentle north-easterly airflow across the country. When this was strong enough to block the sea breeze then our temperatures rose in line with inland areas, hence the record-breaking highs around the 18th and 19th.
The air in these north-easterlies was warmed as it descended from the high pressure system and passed over warm land on its way to us: but it contained little moisture and had no opportunity to absorb water from the sea. Hence there were some unusually low humidity readings, the lowest (another local record) being 23% on the 9th. This made the heat a little more bearable.
Eventually the high pressure moved away, and as the cooler, moist air of the Atlantic met the warm air over the UK there were thunderstorms. Despite some spectacular lightning and dramatic skies, rain totals were unexceptional – the wettest day was 29th July with 8.4mm, and the total for the month was 26.2mm. That total is below average if not exceptional, but the first three weeks of July were very dry indeed with just 0.3mm.
Our rainfall total so far in 2013 is 243.8mm, putting 2013 on a par with the very dry 2010 when we had recorded 210mm at this point. By this time last year, on the other hand, we had recorded 469mm of rain and had just endured a cold, wet July – how things have changed.