Depressions And Anticyclones
These notes offer additional information about the British weather, and may be found helpful in the following of official forecasts, and in estimating flying conditions beforehand. Depressions are storms bringing wet weather. In form they are roughly oval, and sometimes measure nearly 1,000 miles from end to end and perhaps 200-300 miles across. They begin in the neighbourhood of Newfoundland, and move from west to east across the Atlantic, travelling along regular paths, according to the seasons. In summer the path lies farther north than it does in winter. The speed of travel varies. It may be sometimes about 700 miles per day. At other times one may be at a standstill for a few days. As to duration, generally speaking a depression originates and fades out within the space of one week. On an average 50 depressions move across the British Isles in a year. They occur more often in winter than in summer.
A depression is also defined as being a system with low pressure at its centre, from which pressure increases in all directions. The air at the centre is warmer than the air surrounding it, because warm air is not so dense or heavy as cold air, and consequently it exerts less pressure. The wind blows spirally towards the centre in an anti-clockwise direction in the northern hemisphere. Certain signs herald the approach of a depression: a falling barometer, a southerly wind, a cloudy sky, and at night perhaps a halo round the moon. As the centre of the depression moves nearer, the wind may drop, the clouds become lower and threatening, and rain falls - light and scattered at first, but developing to heavy and continuous. When the storm centre has passed, the barometer gradually rises, the wind may change to northwesterly, low stratiform cloud and drizzle give way to clearing skies and sunshine. The depression gradually fades out over north-west Europe.
This description applies to a depression only in a general way, because actually no two are identical. There are variations of the general pattern, and therefore the unexpected can happen. Again, a given area may not be in the direct path of the storm centre, but to the north or south of it. In these cases there could be variation of rainfall and of wind direction. In contrast to a depression, an anticyclone is associated with fine weather, warm to hot in summer and clear and frosty in winter. These periods of fine weather might last for days or even weeks. However, as will be seen in a moment, this is not all which may be said about anticyclonic conditions. An anticyclone is a system in which the centre of high pressure is encircled by low pressure. The winds blow outwards from the centre in a clockwise direction. They are usually gentle winds. Anticyclonic weather varies according to the position of the centre of pressure. If it is over the British Isles in summer, then there will be summery weather: clear skies, light winds and warm to hot days. In winter it might lead to dull or foggy conditions.
If the centre lies farther north, off the western coasts, mixed weather would be the
result, with bright intervals, scattered showers, winds north to north-west and perhaps
temperatures below average for the time of the year. If the centre is to the south of the
British Isles, then the weather is likely to be mild and humid, with cloudy skies. There
are periods when neither depressions nor anticyclones prevail. This is called unstable
weather, with local developments of threatening skies, rain and perhaps thunder. In
winter, unstable weather might result in cold north winds, sleet and snow.
In weather reports, among other terms which are used, there are fronts and troughs. At a warm front associated with a depression the warmer air is forced upwards and over the underlying cold air. As the warm air rises, it cools and some of its water vapour forms into clouds producing a wide belt of rain which moves ahead of the front. In the case of a cold front, the heavier cold air meets and pushes under the warmer light air. This action leads to the formation of towering clouds, showers of rain and sometimes thunderstorms, which arrive not ahead of but at the same time as the front. There are times when by the action of the heavier cold air the lighter warm air is lifted completely off the ground. The front is then called occluded, and since there is no warm air at ground level, there will be no rise in temperature as it passes. Many of the depressions moving over the British Isles are partially or completely occluded. Sometimes one will see on a weather map that, instead of being drawn in a rounded pattern, the isobars (lines of barometric pressure) are joined at sharper angles. These represent a trough, which is a V-shaped depression. Troughs may have warm, cold or occluded fronts. A warm front brings cloudy warm wet weather; cold or occluded fronts, heavy rain followed by better weather.