Rain Records? No.

The 54mm of rain that have fallen in Morden during the last 4 days have certainly made the ground soggy. However, the first 23 days of the month only yielded 7mm of rain. So no records, at least so far. During the last 110 years, there have only been 2 Mays when rainfall exceeded 100mm in south London, namely in 1932 and 1979. Statistics will show this to be a wet month, but for much of the time it has been dry and warm. May 1992 was similar in many respects. It was a very warm month with plenty of sunshine, but there were 65mm of rain, 54mm of which fell on just 3 days. At Heathrow Airport, there was a total of 95mm, with nearly 62mm falling on the 29th. The wettest May day in the last 100 years occurred on the 31st May 1911. At Banstead, over 91mm of rain fell, over 81mm fell at Harrow, and nearly 73mm fell at Epsom, of which almost 62mm fell in just 50 minutes. This became known as the 'Derby Day' storm, on account of the 5 people that were killed by lightning at the end of the Epsom Derby. Several other people were struck by lightning, including a police cyclist who died in Garth Road, Morden.

Warming up again

Now it's warming up again, the risk of air frost has probably gone until the autumn. The minimum temperature (2.2 Celsius) that occurred the night before last, was the equal lowest during the second half of May in Morden for at least 20 years. A similar minimum occurred on the night of the 28th/29th May 1994. An air frost did occur in a few places on Monday night, with minus 0.3 Celsius recorded at Farnborough (Hants). The cold air originated north of the Arctic Circle, and when it crossed Finland, minima of minus 10 Celsius occurred, resulting in much damage to apple blossom.

The Return of the Rain

The warm weather abruptly ceased yesterday as the first measurable rain for 11 days occurred. The heaviest rain fell before 0900 UTC and was accredited to the previous day. Generally, amounts of rain in the London area were fairly small, with the more persistent rain occurring over counties to the north of the Capital. Further heavy rain is forecast, but looking through the records for south London during the last 20 years, high rainfall totals are not particularly common. There have been only 10 days during May with rainfall above 15mm. The wettest May day, with 24mm, occurred on the 27th last year.

Continuing May Warmth

Nearly one third of the month has gone and temperatures are heading towards a new record, but are they? The year 1992 had the warmest May on record in south London, with May 1989 only a fraction of a degree cooler. In May 1992, most of the very warm days occurred during the second half of the month, with several decidedly chilly days during the first fortnight. It is very unlikely that the present warm spell will persist, but a very pleasantly warm May, so far.

Late Spring Warmth

The temperature exceeded 22 Celsius for the third consecutive day today. The last time this happened was between the 5th and 7th September last year. During May last year there were 4 consecutive days late in the month with highs above 22 Celsius and there were also 3 consecutive days above 22 in April. The longest sequence during the last 20 years of 22+ days (9) in May occurred during 1998, from the 8th to the 16th. After a maximum temperature of 21.4 on the 17th, there were then another 3 consecutive days with highs above 22 Celsius. By way of contrast, in May 1994 the temperature failed to rise above 22 Celsius all month!

May Frosts?

Last night the temperature failed to fall below 13 Celsius (55F), the mildest night since 16th October. No doubt the Garden Centres will increase their sales of bedding plants, helped by the occurrence of the holiday weekend. However, before last night, there were 2 nights with ground frost. In this part of the country, there has only been 1 May during the last 20 years when ground frost was not reported, and in 2 years there were damaging air frosts with the temperature falling below minus 1 Celsius (30F). A few years ago, a well-respected local gardener said 'never put in frost-prone plants before 17th May'. Good advice, well received.

A thundery start to May

The thundery start to May is nothing unusual. This late Spring month often has a spell with thunderstorms, and it's now 12 years since the last thunder-free May. In both 2001 and 2002 there were 5 days when thunder occurred. Today, a line of showers continued to feed northeast across London. Similar lines of showers occasionally cause problems during the Wimbledon Tennis Fortnight, and like this afternoon, skies were fairly clear both north and south of the Capital. Although southwest winds blew today, May is frequently a month when northeasterly winds dominate. We await the rest of the month with interest.