The intricacies of high pressure



Even when the weather appears fairly uninteresting during a settled spell, there's usually some things to look at - this plot (below) from midday on 8th January 2018 at Herstmonceux (East Sussex) of temperature with height, measured by instruments attached to a weather balloon, highlights some interesting features.

Click on image to enlarge

A temperature inversion is present, a result of dry air sinking through the upper/middle portions of the atmosphere courtesy of an area of high pressure to the east of the British Isles. But this dry, sinking air doesn't reach all the way down to the ground - instead, there is a shallow layer of cold, moist air trapped near the surface. Notice how the temperature at 0.7 miles (3,600 ft) above ground is actually warmer than that near ground level.

The closer the temperature (solid) and dewpoint (dashed) lines are together, the moister the air is. When the air becomes saturated (both lines touching), cloud or fog forms. in this example, a layer of cloud can be determined, its base around 1,000 ft (300 m) above ground. Any hills above 300m would likely experience 'hill fog'.

Click on image to enlarge Image courtesy of NASA Worldview

This layer of stratus cloud can be seen in the satellite image above, its progress northwards blocked somewhat by the higher ground of Wales which sticks out above the inversion.

Dan Holley  8th January 2018