Royston (Iceni) Weather Station
Daily Weather Observations - July 2005
July 2005 Weather Review
July 2005 was yet another drier than average month in Royston, but temperatures and sunshine were near normal.
After exceeding 30°C in both May and June it is perhaps surprising that the temperature did not quite live up to expectation and do the same in July. The highest temperature of the month was not far off this value at 29.6°C on 14th, however.
A hot spell occurred between 10th and 18th when the mean daily maximum temperature averaged 26.7°C over this period. Outside this period temperatures were rather on the cool side resulting in the mean daily maximum temperature for the month as a whole actually being 0.1°C below average at 22.1°C. The mean night minimum temperature of 13.2°C was 0.7°C above average, thus nudging the overall mean temperature for the month just into positive territory. This means that over the last 12 months only February has experienced a below average mean temperature, and then only by 0.2°C!
The lowest night minimum temperature recorded in July was 10.4°C on 5th, whilst the lowest grass minimum temperature of the month was 8.1°C on 16th. The highest night minimum temperature in July was 17.4°C on 14th/15th, in contrast to the lowest day maximum temperature of 15.8°C on 25th.
The mean earth temperature at 30 cms depth as observed at 0900 GMT daily was 18.8°C. The highest and lowest earth temperature values were 20.6°C and 17.4°C on 15th (and 16th) and 28th respectively.
For the 8th consecutive month rainfall in July was below average. For the period December 2004/July 2005 total rainfall was 271.9 millimetres, which is only 74% of the 30 Year (1971/2000) average for this period.
Rainfall in July was 33.8 millimetres, which is 78% of the 30 Year average for Royston. The number of "rain days" (0.2 millimetres or more) was 13, whilst on 7 of these days rainfall exceeded 1.0 millimetre. The wettest day of the month was 24th with a rather modest 7.6 millimetres rainfall. Rainfall duration over the whole month was 43.9 hours, indicating the light nature of much of the month’s rainfall. It was the driest July since 1997.
No thunder was heard in July in Royston, and this is the first occasion since 1984 that July has been a thunder free month.
The first 9 days (average daily sunshine 3.3 hours) and last 10 days (average daily sunshine 1.7 hours) of July were exceptionally dull for a summer month, but in between the period 10th to 21st recorded a total of 133.1 hours sunshine (daily average 11.1 hours).
This very sunny spell was insufficient to give an above average total for the month, however, since the July total of 179.7 hours sunshine was 6% below the 30 Year (1971/2000) average.
The sunniest day of July was 17th with 15.2 hours sunshine. Two days in the month (25th and 27th) were completely sunless. It was the dullest July in Royston since 2000.
July was a fairly calm month with a mean wind speed of 2.1 mph, and a maximum wind gust speed of 37 mph (on 6th). The highest daily (00/00 GMT) mean wind speed was 6.1 mph on 20th, whilst the lowest daily mean wind speed was 0.1 mph on both 23rd and 26th.
Daily wind directions (as observed at 0900 GMT) contained a westerly component on 20 days, as shown by the following analysis: NORTH 4 days, NE 3, E 1, SE 1, S 1, SW 7, W 8, NW 5, and CALM 1.
The mean MSL air pressure in July was 1013.5 mB, which is almost 4 mB below the 30 Year (1973/2002) average. The highest and lowest air pressure values in July were 1032 and 999 mB on 12th and 25th respectively.
The mean relative humidity in July was 77.1%. The highest daily (00/00 GMT) mean relative humidity was 95.8% on 27th, and the lowest daily mean relative humidity was 58.2% on 17th. The absolute lowest relative humidity reading in July was 30% which lasted between 1600 and 1745 GMT on 17th.
Fog (visibility < 1000metres @ 0900 GMT) was observed on just 1 day (25th) in July.
Richard Barker
Richard Barker
Met. Observer, Royston (Iceni) Weather Station
5th August 2005
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