By Philip Eden

Local sunshine records for this February 2008 are already tumbling, and with several days still to go it is almost certain that we will register the sunniest February on record, averaged over England and Wales. Having an extra day helps, of course, but only one of the earlier record-holders - 1949, 1971, 1988 and 1998 - fell in a leap year.

The exceptionally sunny spell may have been interesting to the climatologist for its statistical rarity, but the spectacular sunrises and sunsets which accompanied it ensured that it had a much wider audience. Stunning colours were observed from much of the country at both ends of the day, and the sequence of sunsets between last Sunday and Tuesday were the most striking that I can ever remember. We were treated, not just to the usual golds and pinks, but to a whole succession of colours which lasted for an hour after the sun had actually dropped below the horizon.

On Monday the 18th, for instance, the setting sun was first of all accompanied by a bright, pale-yellow - almost a greeny-yellow - glow across two-thirds of the sky, then, as the sun disappeared we cycled through yellowy-orange, crimson, and finally a dull orangey-purple ("nectarine" describes it nicely). As the sky darkened, the colour gradually retreated to the western horizon, but it did not finally fade until twenty past six, a full hour after the sun had gone. And there lies a clue to the explanation for the stunning display.

An hour after sunset, the sun is approaching ten degrees below the horizon, and from there it can only illuminate something which is 20-25 kilometres above the ground. That is well into the stratosphere, and indicates either stratospheric dust, or a very thin layer of ice-crystal cloud.

When a large high pressure system settles over the British Isles, as occurred last week, it is often accompanied by a sharp drop in temperature in the lower stratosphere. The anticyclone acts as a dome-shaped obstruction, not just in the horizontal, but also in the vertical. One important effect is that the tropopause - the boundary between the troposphere (where all our weather happens) and the stratosphere - is pushed upwards. As the temperature decreases with height in the troposphere, this results in a much colder than normal tropopause. In the lowest part of the stratosphere the temperature changes little with height, so it also meant a colder-than-normal lower stratosphere.

Last Monday, for instance, it was -90°C there instead of a more usual -55°C, and at such a low temperature the residual moisture in the air frequently appears in the form of an extremely tenuous veil of ice-crystal cloud. These clouds - usually known as "stratospheric cirrostratus" - are often present over Antarctica, but they are rare in our latitudes. Such clouds are so thin they are invisible most of the time and can only be seen when lit from beneath by the setting sun.

© Philip Eden