Why, in winter do anticyclones cause snow? I thought because anticyclones were formed from descending air clouds did not form but surely clouds are needed for snow???
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geography question. watch
- Thread Starter
- 08-06-2004 18:09
- 08-06-2004 19:17
Not sure about snow, anticyclones do produce frost, fog and mist though.
The cold weather is due to a combination of low rates of insolation and short hours of daylight. In winter the sun is at a low angle in the sky. Therefore, the oblique rays of the sun have to heat up a greater area of the earth's surface than in summer. Even on the clearest of days, the weak winter sun will be shining for less than eight hours in mid-winter. This means that there are 16 hours in every 24 during which surface heat, without any cloud to trap it in, will be lost by radiation into space.
The coldness of the earth's surface, as a result of night radiation, leads to condensation. Condensation occurs because of contact between the cold ground and the moisture-laden air above it. If the ground temperature remains above freezing point, the moisture from the air condenses into dew. In winter, however, a ground temperature below freezing point is more likely, when frost forms. The presence of a slight breeze will stir up the coldness from the ground, leading to condensation around dust particles in the atmosphere. This reduces visibility, and this type of fog is called radiation fog and is the product of the night radiation of heat from the ground when skies are clear. The weak winter sun may not have the power to change the water droplets back into water vapour, so that the fog may persist all day.
In some winters the anticyclone builds up and stays in approximately the same position for days and even weeks. With time, the difference between the low amount of incoming radiation by day and the large amount of outgoing radiation by night becomes more and more pronounced. The high pressure becomes a stagnant mass of cold air. In inland locations, daytime temperatures of -3 to 5 degrees are common and until the anticyclone weakens or drifts away, the sub-zero temperatures persist.
The most severe winter conditions of all arise when frost is stirred up by the breeze to create freezing fog, which makes the weather very cold and damp. A strong surface inversion of temperature is created, preventing any upward movement of air into the atmosphere. Hence, al the waste materials pumped out by vehicles and factories etc. are trapped in its lower layers, raising pollution in large urban areas.