The return of high pressure this week after a lengthy absence will have been greeted with joy by many people, after the long very wet (call it winter) few months. Everyone is walking about in awe of the blue skies and sunshine, crocuses and blossom are appearing, Sunday was super for the southern half of Britain up to 20.5C (69F). This week is the second part of the Big Pedal fortnight, where as many school children as possible are encouraged to cycle (or scoot) to school. March can be a rather iffy month so it is great that that the weather is not an issue, in fact it is super cycling in that sunshine.
However, having a high pressure (anticyclone) over the UK does not always mean warm and sunny, as parts of England and Wales will vouch for this morning. Troublesome cloud is the bane of forecasting in a high.
Also rather big in the public perception stakes, if you were expecting a warm sunny day and your area happens to be at the edge of the cloud, then you could end up grey and shivering.
The air in an anticyclone is descending, subsiding gently. Clouds usually form from uplift and instability, like cumulus, so there are often clear skies in a high. Stratiform (flat layer) clouds are more likely, lifting up into a layer of Stratocumulus or feeding in at the edge of a high.
Fog often appears with high pressure, the lighter winds allowing it to form, as radiation fog (cooling at night under clear skies) hill fog (lifting Status) and sea fog as troublesome cloud drifts in from the Channel or North Sea. Keep an eye on the VIS (visible spectrum) satellite pictures and watch for low cloud in the North Sea heading towards E.Britain (likely Thurs 13
th Mar)
Snow lovers will be looking for a Cold, slow moving high, preferably closer to Scandinavia. None this year.
There are warm slow moving highs (Azores), too, both these are semi-permanent features which can be detected right up to 300hPa (jetstream level) see the Netweather charts 300hPaWinds.
These are often described as ‘blocks’ or a blocking pattern. If low pressures are like tennis balls being thrown across the Atlantic, then the high acts as large static deflection field, blocking out the incoming lows. That allows the longer spells of drier, settled weather
You do get cold mobile highs, which usually develop behind cold fronts. You won’t see a signal for these beyond 700hPa in the atmosphere; they are rather shallow affairs but can become a warm slow moving high.
As the centre of a high pressure increases say 1020mb becomes 1027mb, the high is building.
As the centre of a high pressure decreases, the high is declining.
There is a forecasting saying, never to drop an old front too quickly (from a synoptic chart) and that is very true over high pressure. Sunday saw cloud and frontal rain over northern parts of the UK as the south got Spring warmth. That front was decaying as it moved southwards for Monday but still brought grey skies to central parts of England on Monday morning, a really difference after the sunny Sunday.
Cold front frontolysis – the disappearance or marked weakening of a front shown by crosses in-between the triangles. So the Met Office synop. chart for Monday (see above) carried a frontolysing cold front, to highlight the cloud area. Opposite of Cold front = frontogenesis – the development or marked intensification of a front, shown by dots in-between triangles.
Generally, temperatures get more severe over time within a high. The coldest winters will be when the UK is sitting under a High for weeks on end. Record breaking summers are also when there have been days on end of warmth, getting stored up in the earth, allowing temps. to creep upwards each day. The primary input to surface heating is solar radiation, sunshine. Lengthy clear spells within a high allow temperatures to rise; lighter winds don’t disturb the air too much so it can continue to heat up. The soil can also store warmth, although the air will lose its heat quick enough at night. So at this time of year, we begin to expect temperatures creeping upwards as the nights shorten.
Personally as a forecaster, I can't stand weeks of high pressure (unless it is harvest time), especially doing television forecasts. It's much more difficult to hold the subtle details in your head. Give me an exciting storm to watch any day. Saying that, I am enjoying the sunshine today (Lothian), the birds tweeting and a feeling that Spring is underway. However, later in March could be bringing some colder conditions, so let's not get carried away. I'm off to get some Vitamin D whilst I can.
