By Philip Eden
The winter of 2012-13 has provided us with several wintry episodes - one in early-December, a lengthy one from January 10-25, a short cold snap from February 10-13, and the present one which began on February 20 which has so far delivered little or no snow. Most of these cold spells was introduced by a phenomenon known to meteorologists as "trough disruption".
The behaviour of the depressions and anticyclones, ridges and troughs, and the various kinds of fronts that you see on the Atlantic weather charts in your newspaper or on the television, is controlled by the flow of air in the upper atmosphere. Particularly important is the jet-stream which stretches around the middle latitudes of each hemisphere roughly six or seven miles above the Earth's surface.
Sometimes the jet-stream flows fast and straight between low pressure towards the pole and high pressure towards the tropics, and during these periods vigorous depressions and troughs sweep rapidly and frequently from west to east across western Europe. At other times the jet-stream becomes sluggish, and like a slow-flowing river it meanders widely, forming strong ridges in sub-polar latitudes, and deep troughs in the sub-tropics.
On occasion, the flow becomes too distorted to be sustained, the jet stream buckles completely, and the tops of the ridges and the bottoms of the troughs become disconnected from the flow, forming 'blocking highs' and 'cut-off lows'. Trough disruptions occur when the bottom end of a deep trough becomes cut off.
Our dry but very cold weather since February 20 has come courtesy of a blocking high which has settled over Scandinavia, feeding frigid easterly winds across the UK, while the heavy downpours which have hit many parts of the Mediterranean Basin and north Africa have been delivered by a cut-off low.
One of the most difficult forecasting problems with blocking highs concerns shallow layers of stratocumulus cloud. These layers may be only 200-500 feet thick, but they mean the difference between a sunny day and a grey day. Fortunately, in the last 10-20 years, computerised forecasts have become much better at predicting where these cloud sheets are likely to develop, so we are now rarely promised unbroken sunshine only to find dismal skies. Sunshine totals over the last four days (Feb 20-23) have contrasted between 38 hours in northern and western Scotland, and none in eastern counties of England.
By Philip Eden