During the winter months across the UK you may well hear of the term a "Dirty High" being mentioned.
In areas of high pressure the air is descending from upper levels to lower levels. As it does this, the air naturally warms. This creates clear, cloudless skies for many levels of the high pressure area.
Dirty highs are formed when air is advected North around the area of high pressure and this brings milder air at the surface into contact with cooler air either in the North Atlantic or saturated ground of the British Isles.
This cooling of the air leads to condensing at lower levels and a thin layer of cloud forms in the first 500m of the atmosphere.
This chart shows where this process is most likely to take place :-
As the air continues to descend within the high pressure area, this creates an inversion at lower levels which then "traps" the cooler, moister air near the surface which accelerates the process.
With light winds and a weak sun in the Winter months, the low level moisture is rarely burnt off which can lead to days or even weeks of this weather type as the high pressure crosses the country.
Key points in forecasting a dirty high pressure area :-
- Winter months when the sun is at its least powerful
- Origin of the high pressure area (i.e Azores)
- Temperature of ocean/land which the air is being drawn across
- Saturation of the soil which the air is being drawn across