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Weather in South London during May 2015
No summer warmth in a rather cool and damp month.
Temperatures

The average maximum temperature during May 2015 was 17.2 Celsius (63F), and that was 0.9 Celsius (1.5F) below normal. The average minimum temperature was 8.3 Celsius (47F), and that was 0.3 Celsius (0.5F) below the monthly normal.  

The highest temperature recorded  was 21.2 Celsius (70F) on the 11th, and this was the lowest monthly maximum in May for 21 years. There were only 4 other days when the temperature exceeded 20 Celsius (68F). On most days, maxima were between 15 and 19 Celsius (59-66F), but on 6 days the temperature failed to rise above 15 Celsius (59F). There were 3 of these cooler days during the first week, but on each occasion maxima were above 13 Celsius (55F). In fact 5 of the 6 cool days had highs above 13 Celsius (55F); the only exception was the 14th when 10.3 Celsius (50F) was recorded, the coolest day of the month.

There were no especially mild nights during May, indeed only 8 nights had minimum temperatures above 10 Celsius (50F). There were only 2 nights with minima above 12 Celsius (54F), the 4th/5th, with a low of 12.2 Celsius (54F), and the 22nd/23rd when 13.4 Celsius (56F) was recorded. The vast majority of nights had lows between 5 and 10 Celsius (41-50F) but there were 3 chillier nights. On the 12th/13th a low of 4.2 Celsius (40F) occurred, but the lowest temperatures occurred on consecutive nights, the 19th/20th and 20th/21st with 3.1 Celsius (38F) and 3.7 Celsius (39F) respectively.   

Frost

There were no air frosts during May but ground frosts occurred on consecutive nights at the end of the third week. Around dawn on the 20th a grass reading of minus 2.4 Celsius (28F) occurred with minus 0.4 Celsius (31F) measured on the following morning.    

Rain

A total of 51.2 millimetres (2 in.) of rain fell during May,  and this was over 10% above the long-term average. There were 14 days with significant*  rain and this was 3 above average, and there were 9 wet** days during May and that was 1 day above average for the month. After a dry day on the 1st, the remainder of the first week of May was changeable. Much of the rain was light, but steady rain fell late on the 4th and continued overnight producing almost 7 millimetres (0.3 ins) in total. Unsettled weather extended through until mid month, and although daily rainfall amounts were generally small, the 14th was exception. Heavy rain, particularly during the afternoon of this cool and miserable day, boosted the monthly total by almost 16 millimetres (0.6 ins). Between the 15th and 28th less than 10 millimetres (0.4 ins) of rain fell, with over 6 millimetres (0.25 ins) of the total falling in showers on the 19th. No measurable rain fell between the 21st and 28th, but the last 3 days of the month were very changeable with over 13 millimetres (0.5 ins) of rain recorded, including almost 9 millimetres (0.35 ins) on the 29th.

*Significant rain is defined as one that has 0.2mm or more. ** A wet day is defined as one that has 1mm, or more, of rain 

Thunderstorms

There were 2 days when thunderstorms were observed during May. On the 19th many thunderstorms occurred around Greater London from late morning until evening. In Morden, there was thunder heard  early in the afternoon and again early in the evening. On the 29th, just after midday, one flash of lightning was quickly followed by a loud and very prolonged rumble of thunder.      

Hail 

On the 19th there were 3 showers when small hail was observed. The first one occurred in the early afternoon, followed by another one mid afternoon, with the last one accompanying the thunderstorm in the early evening. Late in the afternoon of the 29th there was a short-lived hail shower. The hailstones were up to 12mm in diameter, and although there was no noticeable,damage, they were the largest 'hard' hail stones to have fallen at this site in 27 years of records. 

Snow

No sleet or snow was observed in Morden during May.

Wind

In a month when northeasterly winds have been prominent in recent years, the prevailing southwesterly winds were dominant in this May and accounted for half the daily observations. It was generally quite a breezy month especially during the first week and again around mid month. Overall, there were 10 days when gusts exceeded 20 knots (23 mph), and on 2 of these windier days there were gusts in excess of 30 knots (34 mph). On the 6th,  there was a gust of 33 knots (38 mph), but the previous day, the 5th, was the breeziest,  when south to southwesterly winds gusted to 36 knots (41 mph).

Fog

No fog was observed in the Morden area during May. 


Monthly figures for May in, and around, London
 
Place Maximum Minimum Rainfall (mm)
Central London 22.7 5.4 43
Heathrow A/P 21.6 4.0 42
Gatwick A/P 20.6 0.8 68
Croydon 19.5 3.7 60
Northolt 21.2 3.1 43
Farnborough 19.7 1.0 52
Stansted A/P 20.7 2.5 37

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