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Weather in South London during January 2017
A rather cold, sunny and frosty month, but also a few wet spells.
Temperatures

The average maximum temperature during January 2017 was 7.3 Celsius (45F), and that was 0.9 Celsius (1.5F) below normal for the month. The average minimum temperature was 0.5 Celsius (33F), and that was 2.9 Celsius (5F) below  the monthly normal. Only the January of 2010 has had lower average minimum temperatures in records spanning 28 years.     

The highest temperature recorded in January was 11.2 Celsius (52F) on the 11th, and there were only 3 other days, the 7th, 24th and 28th, when temperatures rose above 10 Celsius (50F). On most days (19) maximum temperatures were between 6 and 10 Celsius (43-50F), but there were a few colder days, these mainly occurring around the middle of the month and at the beginning of the last week. Of the 4 days with maxima below 5 Celsius (41F), only 2 recorded highs below 4 Celsius (39F). On the 25th, the temperature only reached 3.3 Celsius (38F), but on the following day the high was just 0.2 Celsius (32F). 

There were no very mild nights during January, but on 8 nights the temperature stayed above 5 Celsius (41F). Of these milder nights,  there were 5 with minima above 6 Celsius (43F), including 2 with lows above 7 Celsius (45F). The mildest nights were both the 7th/8th and 8th/9th when minima of 7.6 Celsius (46F) were recorded. The coldest nights were mostly during the second half of January, and although the nights of the 4th/5th and 5th/6th each had temperatures below minus 3 Celsius (27F), the 7 other nights with lows below minus 3 occurred after mid month. Only 2 of these colder nights fell below minus 5 Celsius (23F). On the 20th/21st a low of minus 5.4 Celsius (23F) was measured, but on the following night the temperature dipped to minus 6.7 Celsius (20F), the coldest January night for 14 years.   

Frost

There were 16 air frosts during January, and that was 9 above average. This was the highest number of January air frosts since records began in 1989, and the highest number in the area since the harsh January of 1987. Although 3 air frosts occurred during the first week of the month, these were the only ones during the first 13 days of the new year. However, between the 14th and 27th, there was only 1 night, the 15th/16th, that was free of air frost. The last 4 nights of the month were free of air frost. There were 20 grass frosts during January, amnd this was the highest number for 20 years. However, the average grass minimum temperature of minus 3.3 Celsius (26F) was the lowest since records began at this site in 1989. There were 5 grass frosts between the 2nd and 6th, with the remainder occurring between the 13th and 29th. There were some very cold nights on the grass, with 9 nights having grass minima below minus 8 Celsius (18F). The coldest nights were the 20th/21st and 21st/22nd when grass lows of minus 10.3 Celsius (13F) and minus 10.5 Celsius (12F) were measured.    

Rain

A total of 72.7 millimetres (2.9 ins.) of rain fell during January,  and this was 16% above the long-term average. There were  14 days with significant*  rain and this was 3 below average.  A total of 11 wet** days occurred during January and that was normal for the month. More rain fell on the first day of the new year than during the whole of the preceding December. The total for the day was over 9 millimetres (0.35 in.), but over the following 4 days there was no significant rain. Although pressure was initially high, the weather became, and on the 6th there were 5 millimetres (0.2 ins.) of rain. with a further 3.6 millimetres (0.15 ins.) on the 9th. The 10th and 11th were both dry, but the 12th was particularly wet with 20.4 millimetres (0.8 ins.) measured, a value only exceeded twice in January during the last 28 years. Nearly 4 millimetres (0.15 ins.) was recorded on the 14th, with over 8 millimetres (0.3 ins.) falling on the following day. No measurable rain fell between the 17th and 27th but the end of the month was changeable. On the 29th, almost 12 millimetres of rain fell.

*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 lightning soon followed by a clap of thunder close by to the southeast late on the afternoon of the 28th..        

Hail 

Light soft hail occurred in a shower just before the thunder clap on the afternoon of the 28th. 

Snow

Rain turned to sleet, then wet snow, from late afternoon to mid evening on the 12th. A slushy deposit of about 1 centimetre was measured on the following morning having been topped up by a light snow shower overnight.  The snow  disappeared by late morning. Sleet preceded rain for a little while on the afternoon of the 14th with a few snow grains on the afternoon of the 26th making up the total of 4 days with sleet or snow.

Wind

January was one of those seemingly rare anticyclonic months. The average pressure readings were the highest for 11 years and as a consequence winds were often light and variable in direction. Southwesterly winds predominated during January. That said, they have not been as infrequent since 2001. Apart from at the beginning of the month, and again later in the second week, strong winds were almost entirely absent, and gusts only exceed 20 knots (23 mph) on 4 days. The 1st of January experienced a gust of 21 knots (24 mph), also, there were gusts over 20 knots on the 11th, 12th and 13th. The highest gust was 27 knots (31 mph) from the northwest on the 13th.

Fog

High pressure and associated light winds allowed fog to form across England more frequently than normal, but in Morden fog was only observed on 2 mornings. This fog was freezing and occurred on both the 23rd and 25th. 


Monthly figures for January in, and around, London
 
Place Maximum Minimum Rainfall (mm)
Central London 11.4 -2.8 65
Heathrow A/P 11.2 -4.2 58
Gatwick A/P 11.6 -6.6 76
Croydon 10.1 -3.9 73
Northolt 11.1 -7.2 64
Farnborough 10.9 -8.1 66
Stansted A/P 9.9 -5.1 47

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