Royston (Iceni) Weather Station

Daily Weather Observations - January 2009

The year commenced with a cold and sunny January, with average rainfall.

The milder conditions experienced in recent years were highlighted on 6^th when a minimum temperature of –7.0°C (the lowest of the month) proved to be the lowest temperature in any month since 3^rd January 1997. The corresponding grass minimum temperature of –14.0°C on that date was the lowest such value for a much longer period, since 13^th January 1987 in fact.

The lowest daily maximum temperature in January was –1.3°C on 9^th and this was the first day in any month with a maximum temperature below 0°C since 2^nd February 2006. In contrast the highest night minimum temperature of the month was 7.0°C on 17^th.

The highest daily maximum temperature in January was 9.6°C on 12^th. This was first occasion the absolute maximum temperature in January has failed to exceed 10°C since 1979, and was only the 3^rd such occasion since at least 1972.

The mean daily maximum temperature in the month was 5.4°C, which is 1.2°C below the 30 Year (1976/2005) average. The mean night minimum temperature was similarly 1.2°C below average at 0.9°C. It was the coldest January in Royston since 1997. Totals of 11 air frosts and 18 ground frosts were recorded in January.

The first snowfall of 2009 occurred on the morning of 5^th with a level accumulation of 2 cm measured at 0900 GMT. This depth of snow was maintained for the next two mornings before thawing, but appreciably milder air did not reach Royston until 11^th. Further (slight) snowfall occurred on 7^th and a few flakes of snow occurred in rainfall on 23^rd.

Total rainfall in January was 49.3 millimetres, which is 96% of the 30 Year (1971/2000) average. The number of "rain days" (0.2 millimetres or more) was 14, whilst on 13 of these days rainfall exceeded 1.0 millimetre. The wettest day was 24^th with a modest 6.1 millimetres rainfall, confirming the relatively light nature of daily rainfall amounts. It was the driest January since 2006.

Despite January containing 6 days which were completely sunless the month’s sunshine total of 85.7 hours reached as high as 154% of the 1971/2000 average. This was partly achieved due a sunny end to the month with 7.0 hours sunshine on 27^th and 18.7 hours sunshine over the last 3 days.

The sunniest day of the month was 31^st with 7.1 hours sunshine. It was the sunniest January in Royston since 2005.

Although January was not a stormy month wind gust speeds did exceed 40 mph on 4 days and on 17^th a maximum wind gust of 53 mph was recorded which proved to be the highest of the month. The mean wind speed over the whole month (calculated from 24 hour 5 minute mean wind speeds) was 5.3 mph.

The highest daily (00/00 GMT) mean wind speed was 13.0 mph on 11^th (12.9 mph on 12^th), whilst the lowest daily mean wind speed (to nearest 1 decimal place) was 0.0 mph on 1^st.

The daily (at 0900 GMT) wind direction analysis for the month was NORTH 1 day, NE 1, E 0, SE 3, S 10, SW 9, W 2, NW 2 and CALM 3.

The mean MSL air pressure in January was 1008.2 mB , which is 8 mB below the 1976/2005 average. The highest and lowest air pressure values (reduced to mean sea-level) recorded in January were 1031 mB on 2^nd and 3^rd, and 963 mB on 23^rd respectively.

A good proportion of days in January were rather damp and a mean relative humidity for the month of 93.4% was recorded. The highest daily (00/00 GMT) mean relative humidity in January was 99.5% on 9^th, whilst the lowest such daily mean value was 84.5% on 3^rd. The absolute lowest relative humidity reading in January was 46% at 1455 GMT on 11^th.

Fog (visibility < 1000 metres @ 0900 GMT) was noted on 2 days (2^nd and 9^th) in January.

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

5^th February 2009

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