3.8 Depressions in practice

The centers of most depressions pass to the north of the UK. As a depression goes overhead, we hence generally pass under the warm front, the warm sector and the cold front. Remember that warm air is rising up along both of the fronts. Warm air rising leads to clouds and potentially rain. This should lead to a somewhat predictable sequence of weather, as shown in the diagram below.

Cross section through the warm sector, showing typical cloud formations

The first sign of the approaching depression is moist air rising high at the front edge of the warm front, causing ice crystals to be blown eastwards by the jet stream. These can be seen as clouds called cirrus, or ‘mares tails’:

Cirrus clouds over the Isles of Scilly

As the warm front becomes lower overhead, the high cloud develops into a thin layer known as cirrostratus:

Cirrus transitioning to cirrostratus over the Isles of Scilly

This cloud produces a halo around the sun or moon.

As the front comes lower still, layers of cloud form at mid levels, known as altostratus:

Altostratus over the Isles of Scilly

These clouds thicken and lower as the warm front draws near, eventually producing persistent rain near the warm front from featureless grey nimbostratus clouds:

Nimbiostratus. Image from the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2016 University Corporation for Atmospheric Research. All Rights Reserved.

Within the warm sector, humid damp air will bring poor visibility and intermittent rain or drizzle. Fog and mist is possible. However, as the depression tracks east, this weather may improve to give clear spells.

Warm sector weather over St. Marys, Isles of Scilly.

The arrival of the cold front is heralded by thicker clouds and heavier rain. The wind will tend to be gusty and there may be thunderclouds.

Arrival of a cold front over Jersey

Behind the front, the sky should clear rapidly. It may be followed by cumulonimbus clouds that bring sharp showers. In practice, depressions rarely have the perfect form and weather sequence described. However, the sequence of events described above is a useful guide.