Royston (Iceni) Weather Station
Daily Weather Observations - January 2001
January 2001 Weather Review
It was an exceptionally sunny January in Royston, with mean temperatures and rainfall being a little below and a little above average respectively.
Relatively mild weather characterised the first week of January, but then a progressive decline in temperatures set in with frequent night frosts and cold but mostly sunny days. This anticyclonic spell gave way to milder conditions again by 22nd, in the process thawing a light snow cover which had occurred on 20th.
Overall it was the coldest January since 1997, and the first January since 1997 to record a below average mean temperature. The mean daily maximum temperature was 5.6°C, which is 0.8°C below the 25 Year (1973/1997) average for Royston. The mean daily minimum temperature was 1.0°C below average at 0.9°C.
The highest temperature recorded in January was 10.5°C on 23rd. This is not a particularly high value for January, having been exceeded in 23 years out of the previous 28. No "ice days" (maximum temperature below 0°C) occurred this January, the lowest 24 hour maximum temperature being 1.0°C on 19th.
On 16th the month's lowest minimum and grass minimum temperatures of –5.6°C and –9.0°C respectively were recorded. The equivalent temperatures on 17th were almost as low at –5.4°C and –8.8°C respectively. In contrast, on the night of 1st/2nd the temperature did not fall below 8.5°C. In total, air frost was recorded on 12 days in January, and ground frost on 17 days.
The cold days and frosty nights in mid-month resulted in the earth temperature at 30 cms depth (as observed at 0900 GMT) falling to its lowest value of the month, 1.8°C, on the 4 days 20th/23rd. The earth temperature peaked at 5.9°C on 5th.
Rainfall was heading for a below average total for the month until moderate rainfall swept in from the south-west on 26th, turning to snow in the evening and quickly building up a 5 cm ground cover.
This boosted the month’s rainfall total to 59.6 millimetres, which is 116% of the 30 Year (1971/2000) average. The wettest day of the month was 26th with a total of rainfall and rainfall equivalent of snow of 17.7 millimetres. The number of "rain days" in January was 13, with the rainfall exceeding 1.0 millimetres on 11 of these days.
Snow fell on 3 days (19th, 21st and 26th) and the ground was more than 50% covered with snow on 6 days (20th, 21st, and 27th/30th).
As mentioned at the beginning of this report it was a very sunny January, despite 9 days experiencing no sunshine at all. By 28th a sunshine total for the month exceeding 100 hours appeared in prospect, but then the last 3 days of the month were completely sunless! The accumulated sunshine total of 95.8 hours on 28th thus became the final total for the month, and this was 177% of normal.
The sunniest day of the month was 24th with 7.5 hours sunshine, whilst 8 other days recorded a sunshine total of over 6 hours. The 9 sunless days were 4th, 10th, 12th, 20th/22nd, and 29th/31st. The last 4 Januarys have all seen above average sunshine with year on year increases from 1998 to 2001 of 13, 12, and 9 hours i.e. from 62.1 hours in January 1998 to the 95.8 hours in January 2001. It seems fairly safe to assume that this trend will not continue!
The mean wind speed in January was 4.0 mph. The maximum wind gust recorded was 48 mph on 23rd. On 16th and 17th very calm conditions were experienced for the whole 2 day period with a mean wind speed of 0.0 mph (to nearest one place of decimal) on both days, and maximum gusts of only 7 mph on 16th and 5 mph on 17th. The following 2 days, 18th and 19th, were almost as calm with mean wind speeds of only 0.2 mph and 0.7 mph respectively. At the other end of the scale the windiest day of the month was 24th with a mean wind speed of 10.1 mph and a maximum gust of 44 mph.
As regards wind direction (as observed at 0900 GMT) the daily totals were NORTH 1, NE 5, E 1, SE 2, S 4, SW 7, W 5, NW 0, and CALM 6. This is the highest total for ‘CALM’ in any month in the last 4 years.
The mean MSL Air Pressure was 1008 mB, which is 8 mB below the normal for January, and the lowest value in January since 1988. The highest and lowest pressure values recorded were 1037 mB on 14th and 977 mB on 24th respectively.
The mean relative humidity for the whole month was 92.0%. The highest daily (00/00 GMT) mean relative humidity value was 99.0% on 21st and the lowest daily mean relative humidity was 80.5% on 11th. The absolute lowest relative humidity reading in January was 60% at 1410 GMT on 24th.
Fog (visibility < 1000 metres) was observed at the 0900 GMT observation hour on 3 days (5th, 20th and 29th) in January.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
4th February 2001
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