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Weather in South London during September 2015
A sunny end to a cool and wet month.
Temperatures

The average maximum temperature during September 2015 was 18.3 Celsius (65F), and that was 1.6 Celsius (3F) below normal. The average minimum temperature was 8.5 Celsius (48F), and that was 2.4 Celsius (4F) below  the monthly normal. September nights 2015,  on average, were the coolest in the area for 29 years. Overall, it was the coolest September in Morden for 22 years.  

The highest temperature recorded in September was 22.2 Celsius (72F) on the 11th. There were only 3 other days when the temperature exceeded 20 Celsius (68F), the 10th, 12th and 20th. On most days maxima were between 17 and 20 Celsius (63-68F), and although there were no exceptionally cool days, on 7 occasions the highest temperature was below 17 Celsius (63F). Unusually, the majority of these cooler days occurred before mid month, including the consecutive days of the 3rd, 4th and 5th. However, the 21st and 22nd were the coolest days of the month with highs of 14.9 Celsius (59F) and 14.2 Celsius (58F) respectively.

There were no exceptionally warm nights during September, although the minimum temperature on the 11th/12th was 15.6 Celsius (60F). This was the only night with a minimum above 13 Celsius (55F), and remarkably, there were only 6 other nights when the temperature remained above 10 Celsius (50F). On most of the other nights (15) lowest temperatures were between 7 and 10 Celsius (45-50F), but on 4 nights minima were below 5 Celsius (41F). The 5th/6th was 1 of these colder nights, but the other 3 occurred during the last week of September. On 2 consecutive nights, the 25th/26th and 26th/27th,  the  temperature dipped below 4 Celsius (39F). The coldest night was the 25th/26th with a low of 3.1 Celsius (37F).   

Frost

There were no air frosts during September but grass frost occurred on the night of the 26th/27th with a minimum temperature of minus 0.6 Celsius (31F). On both the preceding, and the following, night the grass temperature fell below 1 Celsius (34F).    

Rain

A total of 87.1 millimetres (3.4 in.) of rain fell during September,  and this was over 60% above the long-term average. It was the wettest September in Morden for 15 years. There were 14 days with significant*  rain and this was 2 above average, and there were 9 wet** days during September and that was 1 day above average for the month. There were heavy showers on the first day of the month with nearly 8 millimetres (0.3 in.) of rain recorded. The following 3 days were showery, but although the week beginning on the 5th only yielded 0.3 millimetres (0.01 in.) of rain, the weather was still unsettled and often cloudy. Further showers on the 13th gave nearly 7 millimetres (0.3 in.) of rain, but on the 16th showers were replaced by more persistent rain. The afternoon was particularly wet and the total for the day was 27.3 millimetres (1.1 in.). Heavy showers occurred on the 18th, but torrential rain fell early on the 22nd, and combined with spells of rain on the previous afternoon, the total for the rainfall 'day' 0900-0900 UTC on the 21st was 28.4 millimetres (1.1 in.). There was local flooding on the 22nd, but then pressure rose and the last 6 days of the month were dry with plenty of sunshine.

*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 was only 1 thunderstorm in Morden during September. It was very brief and occurred mid way through the evening on the 2nd.      

Hail 

No hail was observed in Morden during September. 

Snow

No sleet or snow was observed in Morden during September.

Wind

Although southwesterly winds occurred at times from the end of the second week through to the beginning of the fourth week, much of the month was dominated by winds from a northeasterly direction. Generally, breezes were lighter than normal and gusts only exceeded 20 knots (23 mph) on 4 days. The windiest weather was mostly around the middle of the month when pressure across southern England became exceptionally low for September. The evening reading of 983 millibars on the 16th was the lowest recorded in September since records began in 1988. However, the highest gust on that day was only 21 knots (24 mph), with the windiest days being the 12th and 14th when 27 knots (31 mph) were measured.

Fog

No fog was observed in the Morden area during September. 


Monthly figures for September in, and around, London
 
Place Maximum Minimum Rainfall (mm)
Central London 22.0 6.6 70
Heathrow A/P 22.3 5.7 66
Gatwick A/P 22.3 2.5 76
Croydon 20.1 4.7 83
Northolt 22.5 3.9 73
Farnborough 21.6 2.2 66
Stansted A/P 21.2 6.0 74

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