REVIEW OF SPRING 1984 OVER THE NORTHERN HEMISPHERE

MARCH

The north Pacific low on the monthly mean surface chart was 16mbar deeper than usual and centred near the date line at about SOON. On the other hand the Iceland low was almost absent and a strong ridge of high pressure extended from the Azores across the British lsles and northern Europe into Russia. At 500mbar the strongest flow was in the Pacific and around the Canadian trough near 75"W. There was a marked upper trough in Europe at about 20"E. The circumpolar vortex was centred slightly on the Canadian side of the pole and was a little deeper than usual.

The month was colder than usual over most of Europe and the eastern half of north America but it was warmer than normal over the USSR west of 90"E and over the western parts of north America (especially Alaska).

Rainfall over Europe was very variable but it was mainly less than usual over Germany and central USSR. Canada was drier than average except for the far north and parts of Labrador but in the USA most areas had above average rainfall. Notable weather events included 16 tornadoes on one day in north and south Carolina; the subsequent storm moving north-north-east brought up to 30cm of snow to parts of New York State and Pennsylvania and resulted in at least 75 deaths. Snow and gales affected Scotland and the high ground of northern England on the 2nd. A 319 day drought in Bahrain was broken.

Central England Temperature 4.9"C ( - 0.7 degC)

England and Wales rainfall 65 mm ( 1 1 1 per cent) (provisional)

APRIL

The monthly mean surface chart was somewhat similar to that of March with the north Pacific low centred near the Aleutians and 6 mbar deeper than usual while the Azores high again ridged strongly north-eastwards across Britain and southern Scandinavia into central Russia. At 500mbar the strongest flow was in the Pacific at about 40"N but the Canadian trough at 50"N was displaced eastwards t o be at about SOOW. The upper trough which had been over Europe in March virtually disappeared in April. The circumpolar vortex was situated over the Russian Arctic with a subsidiary centre over the northern Davis Strait. Over North America monthly mean temperatures were below average south of about 45"N but i t was warmer than usual farther north with anomalies up to + 7 deg C in the Canadian wheatlands west of Hudson's Bay. In Europe mean temperatures were above average in Scandinavia and eastern Europe but a large area of Russia centred on 65"N 85"E was very cold (anomaly - 9 degC): monthly mean temperatures there were colder than in February or March.

Most of western Europe was very dry especially central and southeast Britain and western France where some places had less than 10 per cent normal rainfall and there was concern for water supplies. London had its driest April for ten years in contrast t o last year which was one of the wettest on record. Parts of southern Britain were also very sunny with over 200 hours of sunshine and it was the warmest Easter since 1949. Saharan dust fell in southwest England and south Wales on the 22nd. By contrast it was very wet in Greece and Turkey with over 200 per cent of normal rainfall in places.

Canada was relatively dry except in Baffin Island and around the northern shores of Hudson's Bay but the USA was in general rather wet except near the Great Lakes.

Central England Temperature 8.2"C ( + 0.2 degC)

England and Wales rainfall 12mm (21 per cent) (provisional)

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MAY

The monthly mean 500mbar chart showed acut-off low south of the Aleutians and a strong jet stream near 40"N in the east Pacific extending across most of the USA. Downstream the pattern was more meridional with the Atlantic jet stream displaced northwards with a + 12 dam anomaly at 50"N in mid Atlantic. There was a very sharp upper trough extending from Iceland across Britain to Spain with a cut-off upper low over France and a - 12dam anomaly over Spain. East of this trough a strong upper ridge gave a positive anomaly of 12 dam south of the White Sea. The monthly mean surface chart reflected these features. The Iceland low was displaced to southern Greenland and the Aleutian low extended southwards. The Azores high was bigger than usual with a strong ridge to the north near 30"W. There was a closed low over eastern France of 1008 mbar and negative anomalies covered the whole of Europe. An anticyclone south of the White Sea extended south to the Caspian.

The month was colder than usual over practically all of Europe with anomalies of -4 degC over central Spain and west central France but it was abnormally warm in Scandinavia and western USSR with a + 5 degC anomaly near the White Sea. It was colder than normal over Greenland ( -5 degC), Alaska (-3 degC) and in a broad belt across north America from the eastern USA to Vancouver with anomalies of - 3 degC near the Great Lakes and in the med-west. Southwest USA was warmer than usual, anomalies reaching + 5 degC in inland parts of California. The Sahel region and most of India were also warm (anomaly + 2 degC).

It was a wet month over most of Europe except northwest Britain and southern parts of Greece and Italy. France had over 250 per cent of normal rainfall in many places. Eastern north America was also wet as was most of the west except for California and Oregon. Most of central USA west of the Mississippi was drier than usual together with areas near the Great Lakes and Hudson's Bay.

The cool unsettled weather over Europe went a long way towards relieving the water shortage in Iberia and also gave unseasonable snowfall in the Alps with a 75 cm fall on the Simplon pass on the 29th. The Spring Bank Holiday in Britain was exceptionally cool and wet. Floods in northeast Bangladesh were responsible for 300 deaths and there were 17 deaths due to floods in eastern USA in the last week of May.

Central England temperature 10.0"C ( - 1.2 degC) England and Wales rainfall 70mm (104 per cent) (provisional)

SPRING AS A WHOLE

The season was perhaps most remarkable for the generally cool, wet weather over much of eastern USA, cool weather over Europe and unusual warmth in western USSR. In Europe, the contrast between very little rain in April and a very wet May (especially in France) was also notable. West Wales escaped the wet weather of May and had a very dry spring.

Central England temperature 7.7"C (-0.6 degC) England and Wales rainfall 147mm (79 per cent) (provisional)

R. A. S. RATCLIFFE

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