Haytor climate data

I have now been recording the weather at Haytor for 14 years and so it is useful to summarise the actual data. Such a summary of period averages comprises the local climate of Haytor. I hope you find it both interesting and useful.

2003-2016

Average monthly rainfall (mm)

Average maximum temperature (C)

Average minimum temperature (C)

Average number of days snow lying (more than half cover)

Average number of days sleet or snow falling

Average number of days with an air frost

Total number of days with thunder over the years

January

208

7.2

2.1

3.3

6.7

7.5

9

February

137

7.0

1.6

2.4

6.8

8.8

1

March

115

9.4

2.4

1.0

4.1

6.3

4

April

102

12.5

4.2

0.1

1.4

2.1

3

May

102

15.0

6.8

0.0

0.1

0.1

5

June

94

18.1

9.4

0.0

0.0

0.0

10

July

101

19.6

11.2

0.0

0.0

0.0

6

August

123

19.2

11.2

0.0

0.0

0.0

12

September

82

17.4

9.7

0.0

0.0

0.0

4

October

187

13.6

7.7

0.0

0.1

0.6

11

November

179

10.0

4.6

0.4

1.1

2.4

8

December

189

7.8

2.6

1.7

3.5

6.7

2

YEAR

1617

13.1

6.1

8.9

23.8

33.3

75

               
Extremes 2003-2016            
               
Highest temperature 29.8C (13/07/13)        
Lowest temperature -8.4C (22/12/10)        
Warmest month July 2013          
Coldest month December 2010        
Warmest year 2014          
Coldest year 2010          
               
Wettest day 06/02/16 (72.1 mm)        
Wettest month November 2009 (418 mm)        
Wettest year 2012 (2192 mm)        
Driest year 2003 (1165 mm)        
               
Most snow in a day 22 cm (13/01/10)        
Snowiest month December 2010 (15 days snow lying)      
Snowiest year 2010 (36 days snow lying, 37 days snow falling)      
Least snowiest year 2012 (0 days snow lying, 17 days snow falling)      

I have now derived some data in graphical form for each month or winter period from 2003 to 2016. Below are three graphs showing:

1. Winter snowfall

2. Monthly rainfall

3. Average monthly temperatures


Winter snowfall

Not surprisingly snow has never fallen here in Haytor during June, July, August and September since I moved here in 2003. It has been very rare too in May and October, but I have chosen the period October to May as the "winter" period. The graph below shows the number of days in each winter when sleet or snow fell between 0000 and 2359 hours. Also shown are the number of days when snow was lying covering more than half the ground area at 0900 GMT each morning. Days with patchy snow cover were not recorded. If snow was lying then the maximum depth (in cms) for the winter is shown.

This graph clearly shows that in the last decade snowfalls increased and got heavier, that trend has now reversed somewhat. However, the winter of 2011/12 is interesting as in that winter there were no days of snow lying (just temporary patchy coverings) but there were still 20 days with sleet or snow falling. What the graph also shows is that snow is guaranteed to fall in Haytor every winter and on most winters it lies at least for a few days. This is not so for low-lying coastal parts of Devon where snow is very rare.

Monthly rainfall

The graph below shows the amount of rain recorded (in mms) each month.

The wettest month was November 2009 with 418mm (equivalent to more than half an inch a day every day). The driest month was September 2014 with just 3.5mm, which was exceptional. Clearly rainfall is copious at Haytor with most falling during winter and autumn.

Monthly temperature

The graph below shows for each month the average maximum temperature, average minimum temperature and mean temperature.

The warm summers of 2003, 2006, 2013 and 2014 clearly stand out. The very cold December in 2010 also stands out with a mean temperature just above freezing.


Estimated climatological averages for 1971-2000

These averages have been estimated using long term records for Teignmouth and Princetown and then checked against nearby Yarner Wood for consistency. Haytor is almost precisely midway between Teignmouth and Princetown, so the averages have been derived by interpolating linearly in height above sea level.

1971-2000

Rainfall (mm)

Maximum temperature (oC)

Minimum temperature (oC)

January

183

6.8

1.8

February

142

6.7

1.6

March

133

8.3

2.7

April

92

10.5

3.8

May

86

13.6

6.7

June

95

16.4

9.3

July

88

18.6

11.6

August

110

18.4

11.7

September

129

15.9

9.7

October

175

12.6

7.2

November

188

9.4

4.5

December

209

7.8

2.9

YEAR

1629

12.1

6.1

With these it is instructive to compare my actual data for 2003-2016 to the estimated data for 1971-2000 to see if there are major differences.

Plotted below are graphs of monthly mean temperature differences and average monthly rainfall difference.

A warming trend is evident (+0.5C mean), but Decembers have got colder and Springs much warmer. Rainfall shows a mixed signal, overall marginally drier.

December 2015 was exceptionally mild and probably unprecedented in long term records for Dartmoor. This is nicely illustrated in the following diagram which shows average maximum and minimum temperatures at Haytor from 2003 to 2016.


Finally, and in addition to the numerical averages, I am gradually building up experience of other various weather elements:

WIND - the site is relatively sheltered for winds from all directions except from the easterly sector. To the east there is a long sea fetch along the English Channel from Belgium. However, depressions can still bring periods of westerly or southwesterly gales at times and the most damaging wind direction locally is a gusty westerly.

RAIN - rain falls often and in copious quantities, although not as much as on the western side of Dartmoor. Often it consists of many small droplets and drizzle is common. September has become notably drier over the years.

SHOWERS - these occur at all times of the year and tend to be most frequent from mid-morning to early evening. Haytor tends to miss the heaviest of the showers in north or northwest airflows as they preferably run down the Teign valley to the east. In westerly winds the high moor gives shelter but frequent showers can occur from other directions. Rainbows are common and are often vivid and double.

SNOW - Major, disruptive snowfalls with drifting are rare, although in 2009/10 Haytor was completely cut-off for a short time with moorland roads remaining difficult for a week or so. Small falls of 1-2 cm in northerly airstreams are not uncommon but these tend to melt during the day. Sleet or wet snow 'falling but not settling' occurs quite often in winter. The level for lying snow seems to be around 230m asl with places below that seeing very little in most winters.

FROST - Very low temperatures are rare but days with penetrating frosts with temperatures just below freezing accompanied by wind are not uncommon. On the exposed moor wind chill needs to be considered if out walking in winter. Autumn and late spring frosts seem to be infrequent. Generally there is usually too much wind and cloud for persistent frosts.

THUNDERSTORMS - These can occur at any time of year and can be spectacular in winter. Major storms are rare, however, in summer local storms occasionally develop just to the east giving torrential downpours. Very occasionally high level thunderstorms will migrate north from France, usually overnight and early in the morning.

SUNSHINE - During summer most sunshine occurs early in the morning and in the evenings. During the day cloud often builds over the moors while the coast stays clear. Anticyclones and ridges often bring the longest sunny spells.

FOG - Hill fog is common at all times of the year but especially in autumn and winter in southwesterly airflows when it can be dense and long lasting. Quite often radiation fog can occur lower down but remain wonderfully clear at Haytor above the temperature inversion.

CLOUDS - Clouds of all types are commonly observed. Sunrises can be spectacular and lighting effects can be dramatic at any time of the year. Cumulonimbus anvils are frequently observed, especially in winter and spring.

 

Will Hand

Chief Meteorologist for Haytor Weather Station

May 2017