The sequence of weather as a depression passes

Overview

How pressure, temperature, wind, cloud, precipitation and visibility each change at a station as a whole depression passes overhead.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe how atmospheric pressure changes at a station as a depression passes overhead.

   Model answer

   Pressure begins to fall slowly as the first high cloud appears, then falls more and more steeply as the warm front approaches, giving the fastest fall of the whole sequence. As the warm front passes, the fall levels off and pressure stays low and almost steady throughout the warm sector, reaching its minimum near the centre of the low. When the cold front passes, pressure begins to rise sharply, and it continues to rise more slowly as the depression moves away eastwards.

2. 2

   Describe how temperature changes at a station as a depression passes overhead.

   Model answer

   Ahead of the warm front the station is in cold polar maritime air, so temperatures are low and change little, perhaps falling slightly as prolonged rain and cloud cut off any sunshine. As the warm front passes there is a distinct and often rapid rise, sometimes of five to eight degrees Celsius within an hour or two, as tropical maritime air arrives. Temperatures then stay mild and steady through the warm sector. As the cold front passes there is a sharp fall back to cool conditions as polar maritime air returns, and behind the front temperatures stay low, though they may recover a little in sunny intervals by day.

3. 3

   What do the terms veering and backing mean?

   Model answer

   Veering means the wind direction changes in a clockwise sense, for example from south-east to south-west to north-west. Backing means it changes in an anticlockwise sense, for example from west to south-west to south. In the Northern Hemisphere the wind veers as a warm front passes and veers again as a cold front passes, and it usually backs ahead of an approaching warm front.

4. 4

   Describe how wind speed and direction change at a station as a depression passes overhead.

   Model answer

   As the system approaches from the west, winds freshen and back towards the south or south-east, strengthening as the isobars tighten. When the warm front passes, the wind veers, typically from south or south-east to south-west, and often eases a little. Through the warm sector it remains a steady, moderate to fresh south-westerly. When the cold front passes, the wind veers again, typically from south-west to west or north-west, and there is usually a squally increase in gust strength at the front itself. Behind the cold front it slowly moderates as the isobars open out.

5. 5

   Describe the sequence of cloud a station sees as a warm front approaches.

   Model answer

   First high wispy cirrus appears, often as mares' tails, 500 kilometres or more ahead of the surface front. This thickens into a milky sheet of cirrostratus, thin enough for the sun to shine through and often producing a halo around the sun or moon. Next comes altostratus, a grey layer through which the sun appears only as a watery disc. Finally the cloud base lowers into thick dark nimbostratus, from which steady rain falls, and ragged low stratus may form beneath it in the rain.

6. 6

   Describe how precipitation changes at a station as a whole depression passes overhead.

   Model answer

   There is no rain under the first cirrus and cirrostratus. Rain begins lightly as the altostratus thickens and becomes steady and moderate, lasting perhaps six to twelve hours, as the nimbostratus of the warm front arrives. As the warm front passes the rain eases to drizzle or stops. In the warm sector there is light rain, drizzle or hill fog. At the cold front there is a short, very heavy burst of rain, perhaps an hour long, sometimes with hail and thunder. Behind the cold front the rain becomes showery, with heavy bursts of cumulonimbus rain separated by bright dry intervals.

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