costs and benefits of meteorology on both national and international scales but the problems are great and no attempt has yet satisfied all the critics. However, benefit- cost ratios of at least 10 are common and higher figures have been suggested. In the particular case of disasters such as storm surges in areas such as the Bay of Bengal, hurricanes crossing the coast of the USA and similar tropical revolving storms elsewhere, the costs in life and property are very considerable. The financial and social benefits of early warnings have been well demonstrated in developed countries where the infrastructure permits quick response and it was estimated in 1984 that an average 20000 people lost their lives because of tropical cyclones and that the damage caused may reach US$6000-7000M. The incalculable costs of environmental hazards such as climate variability to the world community can be minimised only if an early assessment of the probable changes and their distribution can be provided by meteorologists - indeed it is probably not too much to say that the survival of human societies depends on appropriate meteorological knowledge. Let it be noted, too, that much of the meteorological background to matters of present interest derives from work first undertaken decades ago within international programmes. It is important that pro- posals for work in both national and international fields should not be scrutinised purely in terms of the presently foreseen gains since the eventual importance may not be discernible.

REVIEW OF SPRING 1992 IN THE NORTHERN HEMISPHERE IMPORTANT CLIMATOLOGICAL EVENTS

Warren Spring Laboratory have been taking measurements of atmospheric pol- lution at over 360 sites in Britain. Results show that nitrogen oxides pollution has increased by 35 per cent in the last 5 years. It is estimated that a third of the UK population now live where air quality fails to meet EC guidelines for nitrogen oxide pollution limits. At a few sites in London, Sheffield and Manchester such pollution is near the level above which there is a risk to health.

The Oxford University Environmental Change Unit has been estimating the effect on world crop production of the likely climatic changes which are expected to occur in the future as a result of the increase of greenhouse gases in the atmosphere.

Crop increases in the relatively high latitudes of Canada and Russia are expected to be more than offset by a decline in food production in the tropics and subtropics.

Drought is a serious problem in much of Iberia and in several regions of France where there have been inadequate amounts of rain for four consecutive years.

Scientists in the European Arctic Stratospheric Ozone Experiment have announced a 10-20 per cent loss of stratospheric ozone over Europe last winter. This is believed to be due partly to debris from the eruption of Pinatubo last year and partly to the unusual anticyclonicity in Europe.

Britain has developed a robot submarine which is currently undergoing trials and is expected to be operational next spring. It will be able to study the variations in the thickness of Arctic ice and also the chemistry of the oceans.

The British Pavilion at Expo-92 in Spain will feature instruments carried by the European satellite ERS-1 launched last July. These include an instrument able to measure sea surface temperature to 0.3 deg C and a radio altimeter capable of measuring height to about 10 cm. The latter will enable accurate contour mapping of the Antarctic ice sheet to be made for the first time (the satellite is in near-polar orbit).

MARCH

On the 500mbar monthly mean chart there was a broad trough in the central North Pacific with a strong jet stream around it. There was also a strong upper trough at 70"W and the Atlantic jet stream was strong but displaced northwards with an upper ridge extending from the Azores towards southern Britain.

315

On the monthly mean surface chart the Aleutian low was deeper than usual and the Iceland low was weak and split, with part south of Greenland and part north of Norway. The Azores high was displaced north-eastwards with a ridge into France.

It was a warmer than usual March over Europe (anomaly up to +5 degC) and most of Asia north of 40"N except for an area east of the Caspian Sea. It remained unusually cold in the Middle East and in Iraq the monthly mean anomaly was -4 degC in places. North America had monthly mean anomalies up to +8 degC west of Wow but it was colder than usual in the east.

March was a dry month in south and south-west Europe but precipitation was mostly above average in Europe north of 45"N and in Asia north of 40"N. In North America rainfall amounts were unremarkable though it was rather dry between 45 and 55"N.

There were a few damaging tornadoes in the USA, notably at Zero (Mississippi) on the 13th.

Central England Temperature 7.5"C (+1.9degC) England & Wales Rainfall (prouisionaf) 78.7 mm (133 per cent)

APRIL

On the 500 mbar monthly mean chart the circumpolar vortex was split with part over Baffin Island and part over Siberia. The jet stream was very far south over the North Atlantic and the North Pacific, and 500 mbar heights were unusually high over the Bering Strait and the Arctic north of Asia. There were well defined upper troughs at 150"W and 55"W but the normal European trough was weak and displaced to about 35"E. On the monthly mean surface chart the Aleutian low was much less deep than usual and the Iceland low was displaced somewhat southwards.

It was a warm April over Europe to about 25"E and over the Arctic to the north of Norway, also over Asia between 35 and 55"N. However, Asia between 55 and 70"N had an unusually cold month with anomalies as much as -8°C. In North America mean temperatures were below average over Canada and the north-east of the USA but southern and western states (except Florida) were warmer than usual.

Rainfall was above average over most of Europe and Scandinavia and much of central Asia, also over much of Canada. Western and southern parts of the USA and Alaska had a drier April than normal. There were severe floods in northern England and the Scottish Border areas on the 2nd. A cyclone caused extensive damage and casualties in north-east Assam on the 21st.

Central England Temperature 8.7"C (+0.7degC) England & Wales Rainfall (prooisionof) 75.5 mm (130 per cent)

MAY

On the monthly mean chart 500 mbar heights were higher than usual virtually all around the hemisphere between 45 and 55"N but were unusually low in the high latitudes. As a result the upper westerlies were very strong and displaced somewhat northwards. The normal Canadian trough was displaced to about 6O"W. On the monthly mean surface chart pressure was unusually high over most of Eurasia and also in the North Pacific.

It was a warm May over western Europe and central parts of North America but north Canada and southern USA were colder than usual as was much of central Asia.

In the North Atlantic sea temperatures were below normal south of Greenland and Iceland (anomaly - 1.5 degC) but the North Sea, Baltic Sea and the Atlantic near 50"N were warmer than usual. There was a well developed El Nifio in the tropical Pacific with sea surface temperature anomalies over 2 degC over a wide area.

There were some very heavy falls of rain in central parts of England late in the month; almost 100mm fell in 36 hours near Luton. A new record maximum tem-

316

perature for Scotland in May (28.9"C) was set on the 14th in Edinburgh and 25°C was reached somewhere in Britain every day from 19 May to 1 June.

Central England Temperature (provisional) 14.0"C (+2.7degC) England & Wales Rainfall (provisional) 53.7 mm (80 per cent)

SPRING AS A WHOLE

It was a very warm spring in central England, probably the third warmest in the last 100 years after 1893 and 1945. It was also warmer than usual in each month over western Europe and much of Asia north of about 35"N and also over much of western North America.

Central England Temperature (prouisional) 10.0"C (+ 1 .7deg"C) England & Wales Rainfall (prouisional) 207.9 mm (114 per cent)

R. A. S. RATCLIFFE

Readers' Forum: Readers are inviied to contribute short questwns on any melcwdogical topic. We wi l l endeavour to

obtain answers to all submirted questions.

Over the weekend I read the latest of a series of recent articles about so-called crop formations in which a meteorologist is claimin4 that these are made by vortices.

Science was never my best subject at school, but I think I know enou to be pretty certain

rectangles, any an es at all, in fact; neither can they make symmetrically accurate straight hnes.

could let me know exactly what a vortex can and cannot do! London Nw1 RUTH REES

Without theprior knowledge of Dr Brugge or the Weather Board, Dr Brugge's reply has been seriously misquoted elsewhere: We are glad to take this opportunity of printing the correct version of his reply to Miss Reeds letter.

that while vortices can possibly create circular shapes, it is impossible fort f2h em to form trian$es,

But perhaps t 'm wrong, after all. I should be very grateful, and indeed enlightened, if you

Roger Brugge re lies: I presume &at the 'vortex' shapes referred to by Ruth Rees are similar to the one presented

in the August 1991 edition of Physics World. This shows a line of four large and two small circular patterns, connected by a straight line (where the crop has also, presumably, been flattened). Also connected to this line are shapes resembling the letters ' E and 'C', together with a set of short parallel lines straddling the main straight hne. Visible nearby are other very small clusters of circles.

Almost certainly, such patterns cannot (as Miss Rees points out) be the result of atmospheric vortices, due to the sharp angles that appear to be present in the shapes and also because of the elaborate, or anised nature of the patterns. It seems very likely that they are the result of the activities of a foaxer . Real vortices possess rather indistinct ed es where the velocity gradients

adually decay outwards from the centre - and they are E a s G y circular in cross-section. gattening of the crop along a straight line could result from a travelling vortex - but then we would expect the line width to be similar to that of the circle diameter.

ssessin vorticity. There are many types and sizes of meteorological vortices, ranging from t c large tuch as depressions) to the small (tornadoes) and even smaller. Vorticit is a three-dimensional property of the field of motion of a fluid. For many scales of motion t ie component of vorticity that meteorologists are chiefly interested in is that which occurs in the horizontal plane (i.e. rotation about the vertical axis) - the vertical component of vorticity.

A practical example of such a horizontally rotatin vortex is the 'bath plug' vortex. In a horizontal plane through the vortex, the motion may !x represented by a pattern of closed streamlines (at least close to the centre of the vortex); streamlines are lines arallel to the flow at any given instant in time. In addition to this horizontal motion there is Jrn present a vertical component to the velocity (the water is draining down the plug-hole) together with a horizontal meandering of the whole vortex (with its base remaining rooted above the plug-hole).

317

A vortex is, in its most general sense, any fluid flow