Wettest day of the year in London?

Yesterday was the wettest day of the year with 25.4mm (exactly 1 inch) of rain falling in the 'Rain day' that stretches from 09 to 09 GMT. It was not a record breaker, nearly 32mm fell in December 2002 and almost 30mm fell in 1995. Boxing Day 1886 appears to have been the wettest December day, but as some of the totals included melted snow a cynic may question the validity of the readings. The numbers for that day included 46mm for Kensington and Brixton and 60mm for Hampstead. In recent years, 34mm fell in parts of southwest London during December 1979. Interestingly, the whole rainfall event this December covered 2 'Rain days' and the total came to nearly 36mm. It commenced around 0100 hours on the 13th and then rained continuously until 2300 hours, a total of 22 hours. That in itself is not a record. During June 1903 parts of north London had over 58 hours of continuous rain. By the way, a few days after that rain a three week 'drought' commenced!

Sunny Weather in London

After 3 dull days the sunshine that arrived today was to be welcomed. If it wasn't for a few minutes of low cloud at either end of the day the sunshine could have been unbroken. The 1st of December is potentially the sunniest day of the month with the daylight fractionally longer than on any other December day. This potential is seldom realised of course, but in 1954, a mild and sunny December occurred, and there were 7 hours of sunshine on the opening day. Two years later it was also mild during December but there were only 9 hours of sunshine all month. The Clean Air Act of 1956 helped to reduce pollution in London and smog is now almost unheard of. Fog is also a rarity now in Greater London, and the dreadfully dismal months of the 19th and early 20th centuries are behind us. There will probably never be a repeat of December 1890 when much of London had less than 30 minutes of sunshine all month, and parts of the West End remained completely sunless.

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!

Not Another Wet July!

The first rain of the month has just fallen but there's a long, long way to go before we reach last July's total of 124mm. It was the second wettest July for at least 100 years in South London, with only July 1956 wetter. Perhaps surprisingly, none of the top 10 wettest Julys correlate with the top 10 coolest Julys. During the wet Julys, rain is often of a thundery nature, and although the weather may be cyclonic, there are often spells of warm sunshine between the downpours. However, out of the 9 Julys when 100mm was exceeded, only the Julys of 1925 and 1941 could be described as rather warm. In association with all the wet Julys, the following Augusts were cool, and sometimes very cool. Let's hope it's not another wet July this year.

Cool Junes

The last time that temperatures were below normal during June across south London was in 1991, so statistically we are well overdue for a cool June. To add to the statistical woes, 3 out of the last 5 Junes have been 'top ten' Junes. In other words, amongst the 10 warmest Junes since 1900.

During the '70s and '80s poor Junes were in the majority. In fact, only 2 Junes in each decade could be described as warm; those of 1970, 1976, 1982 and 1989. This early summer month was particularly poor in 1971 and 1977, but in 1972 the coldest June of the century occurred with the the temperature failed to rise above 20 Celsius all month. The 3 disappointing Junes in the '70s were all dull, and 1971 produced the second wettest June of the century. Rainfall was above average in June 1977, but perhaps surprisingly, June 1972 was a very dry month in South London. Looking at the synoptic charts for the month, it certainly appeared very unsettled, but Atlantic rain bands only yielded small amounts of rain, and the 'showery' air behind the fronts only produced limited amounts of convection. However, after a sunny start to these days, cloud developed during the mornings and spread across the sky to produce gloomy, but mainly dry, afternoons, the worst type of summer weather.

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.

A Wet April Day

Yesterday was the wettest April day in Morden for 15 years. Much of the rain fell today (30th) but a 'rain day' runs for 24 hours from 0900UTC.

The 17.5 mm was eclipsed by the 27.8 mm that fell on the 1st April 1993 and the 31.2 mm that was recorded on the 29th April 1991. On the 10th April 1878, a total of 99 mm of rain fell at Haverstock Hill (Highgate).

Over the last 20 years, April has proved, on average, to be the wettest Spring month, but this year it looks as if March will take the honours.