Air in the lower atmosphere is always on the move. Moving air masses are notable weather makers. A change in atmospheric pressure is also a factor affecting the circulation of air, causing weather to change. When warm and cold air meets, it marks changes in weather causing rainfall, thunderstorms etc.
The boundary between air masses of different temperatures is called front. When warm air replaces cold air it is called warm front. When this happens, the warm air rides up over the cooler air, and its moisture begins to condense. High wispy clouds are the first signs of advancing warm front. Gradually clouds lower and thicken and a long steady rainfall follows. In contrast a cold front occurs when cold air wedges beneath the warmer air. Showers and thunderstorms, followed by shifting winds, falling temperatures and a clearing sky mark the cold front’s passage.
Now the weather forecast relies on thousands of weather stations located both on the land and sea. At each station, the weather is monitored at least four times a day. Air pressure, wind speed, cloud cover, precipitation, temperature measurements are done. Upper level observations are done by weather balloons and satellites. They send continuous flow of photograph back to earth. Finally all this information is sent to national weather bureaus, where it is plotted on graphs and charts and analyzed by meteorologists.
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