April 2011....a remarkable month

April 2011 was a sunny month, about the 15th sunniest since 1900. It was also a dry month, the 2nd driest since 1900 behind the April of 1938. It was also the driest early Spring (March/April combined) since 1938. However, the most remarkable feature about April 2011 was the temperature. It was the warmest April since reliable instrument records began in the 19th century, and in doing so beat the outstandingly warm April of 2007. To put the event into perspective statistically; the mean temperature for April 2007 was 13.0, which was a massive 1.4 Celsius higher than the previous record set in 1943. The mean temperature for April 2011 was 13.6! If the month was May instead of April it would have been ranked the 21st warmest May since 1900. The very dry ground aided overnight cooling, and although no air frosts occurred, the mean minimum temperature was only 2.5 Celsius above normal. The mean maximum temperature (19.3) was 4.9 Celsius above normal. This figure was truly incredible. Since 1900, there have only been 8 Mays with higher average maxima. None of these warmer Mays occurred between 1923 and 1988 inclusive. Not only were April 2011 days warmer than most May days, there were several summer months in recent years that have been cooler by day: namely the Junes of 1953,1954,1955,1956,1971,1972,1977,1978,1979,1980,1981,1985,1987,1988 and 1991: the Julys of 1954 and 1980; and the Augusts of 1956 and 1963. What next?

A mild February. Nature redresses the balance.

Although cold air continues to lurk uncomfortably close on the near continent, forecast charts for the remainder of the month generally suggest that mild weather will prevail. In fact the projected mean temperature in this area of 7.8 Celsius would rank February 2011 as the 4th mildest in the last 110 years, behind 2002, 1990 and 1961. In February 2002 there were no exceptionally mild days, but only 2 air frosts occurred in this snow-free month. In February 1990 there was sleet early on the afternoon of the 3rd, but only 1 air frost occurred and on the 23rd the afternoon temperature soared to a record-breaking 19.0 Celsius. In February 1961 there were several very mild, sunny days and the month was totally free of air frost and snow. Although this February is not going to be a record breaker it has certainly helped to redress the balance after the cold December.

The media snow circus has begun

The cold weather has arrived and journalists are posted in every corner of the land for the big chill. In reality, it may be the start of something big but in this area no records (yet!). A few flurries of snow around London are not unusual in November. Two years ago there was 1cm of lying snow on the morning of the 23rd. The last time we had a November day colder than today was 17 years ago, on the 22nd to be precise, when the maximum temperature was only 1.2 C. November 1993 was very different to November this year with plenty of cold weather and several very frosty nights with minima below minus 4 C. By the way, December 1993 was mild with only 4 slight frosts, and the lowest temperature only minus 1.2 C!

For snow and cold weather lovers late November 1890 was interesting. After an unremarkable early part to the month, the temperature at Kew exceeded 14 C on the 23rd. However, 5 days later the maximum was only

minus 3.6 C. The following December was the coldest on record. Although it was a relatively dry month, it was very dull with less than 1 hour of bright sunshine all month. Despite the extensive cloud cover, there were only 4 frost-free nights, and after a maximum temperature of minus 6 on the 14th, the minimum on the following night was minus 12 C, a value we haven't seen in this area since 1981!

Early sharp frost,but no record

The air temperature here fell to minus 2.7 Celsius last night.This was the lowest temperature in October since 28th October 2003 when a similar value was recorded. No monthly record was broken but the lowest temperatures have usually been logged during the last week of the month, and only on the 19th October 1926 has there been such a cold night in this area so early in the autumn during the last 110 years. At that time this area was given over to farmland!

The lowest temperatures measured in south London occurred relatively recently in October 1997. After a very dry September, some heavy rain occurred between 6th and 14th October and this took the monthly total above average. However, after mid month, rainfall totalled less than 2mm with sunshine helping to dry out the ground. The drying process accelerated during the last week as high pressure became established, the air descended and from the 28th until the end of the month the low-level inversion dissipated and all cloud, apart from some cirrus on the 30th, disappeared.

Very low humidities, light winds and dry soils combined to produce the perfect recipe for frost. Nature didn't disappoint with record-breaking low night-time temperatures on both the 28th/29th and 29th/30th. Wallington and Twickenham both recorded minus 5.2 Celsius on the 28th/29th, and here in Morden, after a minimum of minus 4.7 Celsius on the 28th/29th, the following night had a low of minus 5.0 Celsius. Perhaps more remarkable were the grass minima. Values of minus 13.2 and minus 12.8 over the 2 nights remain the lowest 2 consecutive grass minimum temperatures of ANY month since records began here in 1988. By the way, the grass minimum temperature last night was only minus 4.9 Celsius.

Today marks the 15th consecutive day without measurable rain (defined as 'no daily rainfall equal to, or exceeding, 0.2mm/0.01 ins.'). Up until the 1980s this 15-day period was known as a 'drought'. However, the rise of the mocking tabloid press, the general decline in exactitudes, and comparisons with life-threatening water shortages in the Third World quite correctly brought about the abandonment of the term 'drought' by all but a few diehards.

Nevertheless, it's interesting to examine these lengthy dry spells. The previous one occurred here from the 16th September to 2nd October 2009 (a total of 17 days), but lack of rain is generally less common during the Autumn and Winter months. A good Summer often produces a prolonged period of dry weather. In 1959 this area had 37 consecutive dry days (15th August to 20 September) and in 1976 parts of southwest England had no measurable rain for 45 days. Perhaps surprisingly there was no 15-day dry spell in 2003, but the rainfall deficit for the period February to October in that year was well over 50%.

The Spring months seem to have the most frequent dry spells. In recent years there have been several in mid Spring. In 1995 there were 19 dry days (29th March-16th April), in 1997 it was dry from the 25th March to 17th April (24 days). A total of 27 dry days occurred between 21st March and 16th April 2002, and in 2007 there was dry weather between 31st March and 22nd April (23 days).

Is there any way of connecting dry Aprils to the following Summer? Fortunately the answer is no. In 1912, parts of London had no measurable rain during April and the following Summer was one of the coldest and wettest on record! By the way, the previous 4 mid Spring dry spells were equally divided between good Summers and decidedly indifferent ones.

So how bad has the winter been?

Days are still chilly, and there was even a flurry of snow early this afternoon. However, the push from winter to spring is well under way.

For statistical purposes, winter covers the 3 calendar months of December, January and February. In this area, December was the 29th coldest since 1900, but in recent years the Decembers of 2002, 1997 and 1996 were colder. January was slightly more interesting. It rated as the 12th coldest since 1900, with colder Januaries occurring back in 1987, 1985 and 1979.

A third consecutive winter month with temperatures significantly below average is quite rare. The last occasion this occurred was during the memorable winter of 1962/3. In more recent years, the winter of 1984/5 was the 10th coldest since 1900 despite a relatively mild December. Similarly, December 1978 was not cold overall, and the winter of 1978/9 was only rated 7th coldest.

Although this current February is only ranked 50th coldest at the moment, it does look as if this figure will rise towards the top 20 over the next few days. However, the trend to milder weather later in the month is apparent from some forecast charts. If, therefore, the mean temperature at the end of the month is similar to the current value (4.9 Celsius), This February will have been the coldest since 1996 and the winter will have been the coldest since 1990/1. However, if this February is cold enough to rise to 20th, it will have been the coldest February since 1991, the 9th coldest winter since 1900, and making it the coldest winter since Jim Callaghan's 'Winter of Discontent'......1978/9.

We await with interest!

The end of the dry spell

After 17 days without measurable rain, the 0.2 millimetres that fell yesterday evening was enough to break the dry spell. Most years have a prolonged dry spell (15 days or longer) with Spring and Autumn months most likely to see one. September and the beginning of October have had 3 dry spells in recent years. 1997, 2002 and 2007 each had between 15 and 17 consecutive dry days, and back in 1969 only 3 millimetres of rain fell throughout September. The most prolonged autumn dry spell in the London area occurred from the 14th September to 9th October 1921 (26 days). However, there were several lengthy dry periods that began in August, notably 23rd August to 28th September 1929, and 15th August to 20th September 1959, both 37 days. In the extended good summer of 1959 there was a further 17 days of dry weather from the 23rd September until 9th October.