High pressure is here all week. It will often be dry with light winds but cloud amounts will vary. Where there are breaks and sunshine, it will feel pleasant and direct sunlight will bring warmth even if the air just has a slight edge. Under the thicker cloud cover, temperatures will often be around average for the time of year but there could be a little dampness and drizzle at times. At the edge of the high pressure, through the English Channel or in Shetland, there is more of a breeze. This will accentuate any coolness under cloud cover or if there is a dampness in the air. Even areas of north-east Britain which saw brilliant sunshine on Sunday were nervously watching the lurking haar or seafret as it returned from the North Sea.
It’s a very autumnal setup, but with great contrasts. By night, the clear skies allow temperatures to fall into single figures with frost forming on the grass. There will be mist and fog in places. For other areas, the blanket of stratocumulus cloud holds on and can bring a lingering greyness which is known as anticyclonic gloom.
What this high pressure does do is to block any incoming low pressures from the Atlantic. Another autumnal theme can be wet and windy weather with leaves whirling about, as happened with Storm Amy on Friday, 3rd October. Although this did bring a spell of heavy rain, we are now back into days of dry weather when reservoirs are not being replenished.
The Netweather Radar and satellite image from 10am on Monday shows a layer of cloud with just the Scottish mountains popping out above the inversion layer (darker areas). You can also spot the Mourne Mountains in Co.Down, the Lake District and Eryri National Park (Snowdonia).
An inversion is where the air temperature increases with height, rather than the usual decrease that you would expect when climbing a mountain. This can occur in a narrow layer in the atmosphere under the steady cool anticyclonic conditions of autumn. The valleys can be full of fog or cloudy skies but hill walkers can break through into brilliant sunshine, blue skies and autumn warmth from the sun’s rays. It is a common quest at this time of year as the views can be stunning.
The cool air and murk are trapped at the surface underneath the slowly descending air in a high pressure situation and the small ‘warm’ layer makes a lid. As the ground cools further overnight, the inversion is more pronounced early in the morning but can disappear by lunchtime. Alpine skiers will also have seen this phenomenon. It’s lovely up top but you do have to return back into the cooler cloud on the way down.
Aviemore in the Cairngorm mountains was the warmest place in the UK on Sunday, reaching 19.4C in the weekend sunshine. Temperatures this week across the UK will be around 14 to 17C, lowering to 13C for more areas during the week where the cloud cover sticks. Where the cloud breaks, you will feel warm in any sunshine.
Inversion photos from Lake District - Rob Davies
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