Royston (Iceni) Weather Station

Daily Weather Observations - July 2002

The mean temperature and sunshine were very close to average in July in Royston, but it was a very wet month.

Rainfall had exceeded the July normal by 8^th of month, then conditions turned drier and in an 18 day spell from 12^th/29^th inclusive only 0.7 millimetres rainfall was recorded. This very dry period came abruptly to an end, however, on 30^th when the most significant July rainfall event of at least the last 30 years occurred.

Between about 0900 and 1700 GMT on 30^th thunder was heard at regular intervals in Royston but rainfall was minimal. This was followed in the early hours of 31^st by a torrential thunderstorm (but with less thunder than during the day) which increased the rainfall total for the day (24 hours ended 0900 GMT on 31^st) to 48.8 millimetres. The 30^th thus became the wettest July day in Royston since at least 1972.

This very high daily total added to the rainfall recorded in the early part of the month resulted in a rainfall total for the month of 104.7 millimetres, which is 242% of the 1971/2000 average. It was the wettest July in Royston since 1968.

The number of "rain days" (0.2 millimetres or more) in July was 13, with the rainfall exceeding 1.0 millimetre on 11 of these days. Thunder was heard on 6 days (3^rd, 5^th, 20^th, 29^th, 30^th and 31^st) in July, this total being the highest in July since 1983. Rainfall duration in July was 58.4 hours.

Rainfall for 2002 to the end of July was 348.2 millimetres, which is 111% of average.

July started off on a cool note and the temperature failed to reach 20°C during the first week. This is the first time this has happened since 1978. Further, the 25 Year (1973/1997) daily mean maximum temperature was not reached on any day until 13^th.

However, higher temperatures in the second half of the month brought the mean temperature precisely up to the 25 Year average for Royston. The mean daily maximum temperature in July was 22.0°C, which is 0.2°C below average. The mean night minimum temperature in the month was 0.2°C above average at 12.7°C. It was a cooler July than in 2001, but warmer than in 2000.

The highest temperature recorded in July was 30.8°C on 29^th, this being the first day of the Summer to exceed 30°C. The last such occasion was on 25^th August 2001. At the other end of the scale the lowest day maximum temperature of the month was 16.5°C on 3^rd.

The lowest night minimum temperature of the month was 9.0°C on 14^th, whilst the lowest night grass minimum temperature was 6.4°C, also on 14^th. The warmest night of July was 29^th/30^th with a minimum temperature of 17.2°C.

The mean earth temperature at 30 cms depth as measured at 0900 GMT daily was 18.4°C. The earth temperature ranged from a maximum of 21.1°C on 30^th to a minimum of 16.4°C on 4^th.

Sunshine averaged only 2.4 hours per day over the first 9 days of the month, an extremely poor daily average for July. As with temperatures, matters improved subsequently and by the end of the month the cumulative sunshine total was only just short of the monthly average.

Total sunshine in July was 180.1 hours, which is 97% of the 1961/1990 average. At least some sunshine was recorded on every day of the month, the lowest daily total being only 0.3 hours on 8^th, however. The sunniest day of the month was 15.0 hours on 14^th. The next highest daily total was 13.9 hours on 13^th.

The monthly analysis of wind directions as observed at 0900 GMT daily was NORTH 6 days, NE 1, E 0, SE 1, S 3, SW 8, W 6, NW 5, and CALM 1 day.

July was a relatively calm month, the mean wind speed of 3.2 mph being the lowest for any month so far this year. The highest daily (00/00 GMT) mean wind speed was 8.7 mph on 1^st, whilst the lowest daily mean wind speed was 0.4 mph on 30^th. The highest wind gust speed recorded in July was 39 mph on 23^rd.

The mean air pressure (reduced to mean sea-level) in July was 1014.5 mB, which is approximately 3 mB below average. The highest air pressure value recorded in the month was 1029 mB on 14^th, whilst the lowest was 991 mB on 3^rd.

The mean relative humidity in July was 76.7%. The highest and lowest daily (00/00 GMT) relative humidity means were 94.3% and 63.8% on 5^th and 14^th respectively. The absolute minimum relative humidity reading in July was 30% at 1420 GMT on 26^th.

At the time of writing, August has commenced in a similar vein to July, dull and wet! Sunshine has averaged only 2.8 hours per day over the first week, and rainfall had reached 36.4 millimetres by 7^th (average for whole month 49.6 millimetres).

Richard Barker

Richard Barker

Met. Observer, Royston (Iceni) Weather Station

8^th August 2002

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