Royston (Iceni) Weather Station

Daily Weather Observations - July 2000

JULY 2000 WEATHER REVIEW

It was a cool, wet and dull July in Royston, particularly the first half of the month. In the first 16 days of July the daily maximum temperature reached its 25 Year mean on only 2 days (2^nd and 3^rd), whilst the mean daily maximum temperature in this period was 4.0°C below normal. Rainfall reached the July average as early as 4^th, and sunshine from 1^st to 16^th averaged only 2.2 hours per day. Fortunately, the second half of July was mainly dry, and much warmer (but still with several cool days) and sunnier.

After the very dull first half of July with only 34.9 hours sunshine being recorded between 1^st and 16th, the 3^rd week saw a marked contrast with 62.6 hours sunshine in the 5 day period 17^th/21^st. This improvement was somewhat short-lived, however, as the next 4 days could only manage 1.0 hours sunshine between them!

Total sunshine in July was 143.6 hours, which is only 77% of the average. This was the 2^nd very dull July in the last 3 years in Royston. The sunniest day of the month was 18^th with 14.5 hours sunshine and the next sunniest 21^st with 13.6 hours sunshine. Surprisingly only 2 days in the month (4^th & 5^th) were completely sunless, although 23^rd, 24^th and 25^th only registered 7, 1, and 7 minutes sunshine respectively.

At the 0900 GMT observation hour there was a complete cloud cover on 18 days, including a 13 day consecutive period 4^th/16^th. This is a very high total for a Summer month, and would be unusual even in Winter!

The fact that the temperature failed to reach 20°C on as many as 16 days in July says much about the month’s temperature regime and it was, in fact, the coolest July in Royston since 1988. Several days were particularly cool, for instance 10^th when a maximum temperature of 14.5°C was recorded, and 24^th which produced a maximum of only 15.4°C.

The mean daily maximum temperature for July was 20.1°C, which is 2.1°C below the 25 Year (1973/1997) average. The mean night minimum was closer to, but still 0.7°C below, normal at 11.8°C. July thus became the first month this year to experience a below average mean temperature, with the negative anomaly of 1.4°C being the greatest since November 1998.

The highest temperature recorded in July was 25.0°C on both 30^th and 31^st. This value is the lowest maximum temperature for July since 1988. The lowest temperature recorded was 8.3°C on both 12^th and 16^th, whilst the lowest grass minimum temperature recorded was 5.7°C on 16^th.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 17.6°C. The highest and lowest earth temperature values in the month were 19.1°C on 31^st and 15.8°C on 12^th respectively.

The month commenced on a very wet note and 95% of the July rainfall was recorded in the first 10 days of the month. The remainder of the month was then essentially dry with 9 completely dry days and only small amounts or a "trace" (too small to measure) on the other days.

The total rainfall in July was 74.3 millimetres, which is 153% of the 35 Year (1961/1995) Royston average. The number of "rain days" (0.2 millimetres or more) was 13, and the wettest day of the month was 3^rd with 18.9 millimetres. It was the wettest July in Royston since 1987.

Thunder was heard on 3 days in July, these being 3^rd, 6^th and 28^th.

The wind direction as observed at 0900 GMT exhibited a northerly component on 19 days of the month. Actual daily directions at this hour were N 7, NE 7, E 0, SE 0, S 3, SW 2, W 7, NW 5 and CALM 0.

The mean wind speed in July was 2.8 mph, whilst the maximum wind gust speed recorded was 30 mph on 9^th, 10^th and 11^th. The highest daily (00/00 GMT) mean wind speed was 6.7 mph on 10^th and the lowest daily mean wind speed was 0.4 mph on 3^rd and 25^th. It can be seen from these figures that July was thus a fairly calm month with no strong winds.

The mean MSL air pressure in July was 1013.3 mb, which is approximately 5 mb below normal. The highest pressure value recorded was 1026 mb on 21^st and the lowest 989 mb on 10^th.

The mean relative humidity for the whole month was 78.3%. The highest daily (00/00 GMT) mean value was 95.5% on 4^th and the lowest 64.3% on 27^th. The absolute lowest relative humidity was 37% on both 18^th and 26^th.

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

8^th August 2000

Royston Crow (Corvus Cornix)

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