Royston (Iceni) Weather Station

Daily Weather Observations - January 2004

The year commenced with a mild and wet January but with average sunshine.

For the 6^th occasion in the last 7 years the January mean temperature was above average. Only a brief cold snap near the end of the month interrupted the generally mild conditions. It was, however, only the mildest January since 2002.

The mean daily maximum temperature in January was 7.7°C, which is 1.1°C above the 30 Year (1973/2002) average for Royston. Temperatures recovered very rapidly from 29^th which experienced the lowest day maximum temperature of the month, 3.1°C, to 31^st when the highest day maximum temperature of the month, 12.0°C, was recorded!

The mean night minimum temperature in January was 3.3°C, which is 1.2°C above average. The coldest night of the month was 28^th/29^th when a minimum temperature of –4.5°C was recorded. In contrast the mildest night of the month was 10th/11^th with a minimum temperature of 10.1°C.

The lowest grass minimum temperature of the month was –8.3°C on 29^th. Totals of 5 air frosts and 10 ground frosts were recorded in January. Air frost duration was 68.2 hours.

The mean earth temperature as observed daily at 0900 GMT in January was 5.0°C. The earth temperature reached a maximum of 6.4°C on both 23^rd and 24^th, but by 30^th had fallen to 3.2°C, its lowest value of the month.

After the rainfall deficit in 2003 a wet January to start the new year was welcome news for the water supply situation in the district. However, 2003 commenced with a very wet January which was then followed by 4 dry months! Total rainfall in January was 82.1 millimetres, which is 160% of the 30 Year (1971/2000) average. This was the result of mainly small amounts of rainfall occurring on a high number of days (number of "rain days" (0.2 millimetres or more) = 25) rather than due to any notable downpours. This is also illustrated by the high rainfall duration of 109.7 hours.

On 18 days in the month 1.0 millimetre or more rainfall was recorded. The wettest day of the month was 14^th with 10.0 millimetres rainfall.

The most notable feature of the month’s precipitation was the snowfall on 28^th. In an Arctic air stream which was establishing itself across the country on 28^th the day commenced with an overnight snowfall which had cleared by dawn. A few hours sunshine then followed but in mid-afternoon a southwards moving very active cold front reached the Royston area and gave blizzard conditions for about an hour and was accompanied by thunder and lightning. This was the first thunder heard in January in Royston since 1998; over the last 30 years in Royston thunder in January has been only a 1 in 5 year occurrence.

At 0900 GMT on 28^th a level depth of snow of 4 cm was measured. A partial thaw on 28^th plus the new snowfall as mentioned above gave a level snow depth of 5 cm on 29^th. By 30^th the snow depth had reduced to 4 cm and on 31^st the snow had all but disappeared.

The month began on a very dull note and cumulative sunshine remained below average until 29^th when a clear sky from sunrise to sunset gave a bright sunshine total of 7.7 hours, making this day the sunniest day of the month. The final sunshine total for the month of 57.1 hours was just 3% in excess of the 30 Year (1971/2000) average. Despite the average sunshine total it was nonetheless the dullest January in Royston in the last 7 years.

A remarkable feature of the January sunshine regime was the absence of any sunshine at all from Monday 19^th to Friday 23^rd. Sunny weekends either side of this period helped to compensate for this dismal spell of weather. There were 5 other days in the month with no sunshine.

The mean wind speed in January was 5.6 mph. The highest daily (00/00 GMT) mean wind speed was 15.4 mph on 31^st whilst the lowest daily mean wind speed was 0.7 mph on 26^th. The 31^st was indeed a very windy day and at 1955 GMT the highest gust speed (67 mph) in the whole month was recorded.

The analysis of daily wind directions as observed at 0900 GMT shows a strong bias towards the south and west, as follows: NORTH 1 day, NE 2, E 0, SE 1, S 7, SW 8, W 9, NW 2 and CALM 1.

The mean MSL air pressure in January was 1007.8 mB, which is 8 mB below average. The highest and lowest air pressure values in January were 1029 mB and 981 mB on 18^th and 13^th respectively.

The mean relative humidity in the month was 91.0%. The highest daily (00/00 GMT) mean relative humidity was 98.0% on 21^st whilst the lowest daily mean relative humidity was 80.6% on 13^th. The absolute lowest relative humidity reading in January was 64% at 1420 GMT on 13^th.

Fog was not a factor in January with no day having a visibility below 1000 metres at the 0900 GMT observation hour.

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

5^th February 2004

Royston Crow (Corvus Cornix)

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