Wettest day since......?

Yesterday (10th November) nearly 24mm of rain fell. This made it the wettest day of the year so far and the wettest day since November 19th last year. November ranks, on average, as the wettest month of the year in many parts of the country, but in the last 20 years, in this area, October has been a wetter month. A daily rainfall total of 24 mm in November is actually not uncommon here. It has occurred 4 times during the last 20 years, and 3 of those occasions have been during the last 8 years. On each of the last 3 very wet November days, more than 30 mm of rain have fallen. The wettest day was the 5th November 2000 with 33 mm of rain. The wettest day in the area from historic records appears to have been the 25th November 1938 when nearly 42mm of rain was measured.

Frosty October Nights

The grass temperature fell to minus 5 last night, the 8th 'grass frost' of the Autumn so far. The minimum temperature forecasts published in newspapers and featured on radio and television broadcasts are, quirkily, based on the expected temperatures in a louvred white box standing in an open position between 1.25 and 2 metres above the ground. Anyone small enough to be sitting in one of those boxes last night would have experienced a temperature of minus 0.3 Celsius, making it the first air frost of the season. During the last 20 years there have been 12 years when air frost was absent during October, but in 1997 air frost occurred on 5 nights during October. The earliest frost was on the night of the 15th/16th October 1993, but in the suburbs of London frost has occurred much earlier in past years. Aside from the notable frost hollows at Chipstead (Surrey) and Rickmansworth (Hertfordshire), one of the earliest suburban air frosts occurred on the night of the 18th/19th September 1952. There was also a frost in southwest London on the 29th/30th September 1919. In October 1888 there were 9 or 10 air frosts across much of London. The lowest October temperature recorded in Morden during the past 20 years occurred on the night of the 29th/30th 1997 when a minimum of minus 5.0 Celsius was measured. A value that could be hard to beat.....maybe!

Last of the Summer Days?

Time to get the potatoes in before Autumn truly arrives. Although temperatures above 20 Celsius in October are not unusual, after the disappointingly cool September in London, this short warm spell is much welcomed. The maximum temperature of 22.2 Celsius today is over 1 degree warmer than any maxima recorded during September, and is the highest this late in October since 2001. There were only 4 days during August that were warmer, 13 days in July and 7 in June. In October 1921, the first 11 days of the month had maximum temperatures above 21 Celsius, and on the 5th and 6th of October in that year the high was close on 29 Celsius at several places in London, including Kensington Palace, Greenwich, Camden Square and St James's Park.

Although rain threatens for the next couple of days, it's now 11 days since any measurable rain fell in Morden. With little, if any, rain likely during the last week, it looks as if September is going to be another dry month. That will make 3 out of the last 4 months with below average rainfall; and 6 out of the last 7 months have had temperatures below the 20 year average. The scientific community regularly assure us that the very wet weather (but not everywhere!) is part of global warming, and the El Nino is responsible for the lower temperatures. Of course, the threat of Global Warming cannot be ignored, but perhaps the trend towards sixties fashions has been followed by a return to sixties weather. Let's hope not. Apart from the severe winter of '62/63 most of the weather was instantly forgettable. Some of the fashions and much of the music appear to have lasted in the memory somewhat longer.

The Screen has a make over.

Every now and again the screen housing the thermometers needs renovating. With no spare available, a little improvisation is called for to maintain reliable temperature readings. The 'Official' box, temporarily absent for repair, spent many years housing thermometers on State House, a high-rise office block in High Holborn, London. The block succumbed to concrete rot and has since been replaced, and the 'London Weather Centre' that used to take readings from it's base opposite in Penderel House has long since moved on. The Stevenson Screen, the official name for the louvred 'bee-hive' box was invented by Thomas Stevenson in 1864. He was a civil engineer responsible for the design of many lighthouses but the screen is maybe his enduring legacy. The World Meteorological Organisation recognize the Stevenson Screen as the standard housing for meteorological instruments. It has to be in an open area between 1.25 and 2 metres above the ground. Originally this screen was around 80 metres above street level. It has led a Jeckyll and Hyde existence, perhaps appropriate considering the inventor was the father of Robert Louis Stevenson, the author of The Strange Case of Dr Jekyll and Mr Hyde

To put it simply, there are 3 main types of thunderstorm in this country. The most common of these is the all-year-round storm which starts with a build up of cloud during the morning, a couple of rumbles from a cumulonimbus cloud in the afternoon, followed by a fine evening. In the winter months this type of storm is usually found on windward coasts. The second type is formed by the destabilisation of the air well above the surface, usually 6,000 feet AMSL or higher. This commonly occurs when cool Atlantic air is about to bring an end to a hot spell. In recent years this form of storm has been fairly uncommon, although one occurred here on the 28th July. The first sign of these type of storms is usually several afternoon reports of thunder from Brittany, followed by a quick migration northeastwards, reaching Southampton in the early evening and London bewtween 9 and 10 pm. For the good folk of Kent last night's storms were closest to the second category. Although very little surface wind blew in association with the storms, it was certainly not true that they were slow moving. In fact the storm clouds were moving at around 35 knots. Rather than one storm lasting a long time, it was a series of storms generating within a band that ran from the East Sussex coast towards Essex. Now, the third type of storm is the 'swing-seat' one. This occurs on a warm summer's evening when several storms develop or move into the area, produce a vivid display of lightning, numerous rumbles of thunder, but are never overhead. Thus, it remains dry and warm, and a free display of nature's power can be observed accompanied by a glass of claret and a packet of peanuts. That was the thunderstorm that occurred over this part of South London yesterday evening.

Where is the Summer Heat?

Yesterday (Thursday) was the warmest day of the 'Summer' so far. Here in Morden, the temperature reached 27.6 Celsius, central London and and Northolt reached 27.5 Celsius, and Heathrow Airport managed 28.2 Celsius. Today is less warm, but humid, and over the next few days it looks like being warm but probably not hot. In the London area there is usually at least one day in the Summer when 28 Celsius is comfortably exceeded, and, of course, there is still plenty of time for high temperatures. In recent years, some record-breaking maxima have occurred during August. However, if the 27.6 Celsius should prove to be the highest maximum, it would then be the lowest yearly maximum since 1978!