Weather forecasting

How does the weather app know it will rain tomorrow? Behind every forecast lies a lot of work: all over the world stations, balloons and satellites measure how warm the air is, how humid it is and from which direction the wind blows. From this data big computers work out how the weather will develop.

High and low

The weather depends strongly on air pressure, that is on how heavily the column of air above you weighs. It is measured in hectopascals, hPa for short: at sea level it averages about 1013 hPa. In an area of high pressure the air sinks downwards, clouds dissolve and mostly the sun shines. In an area of low pressure humid air rises, cools down and forms clouds that often bring rain. On the weather map a large H marks the high and an L marks the low. If the pressure at your place drops noticeably within a few hours, a low with wind and rain is usually approaching.

Warm front and cold front

Where warm and cold air masses meet, a front is formed. At a warm front warm air slides slowly over the cold air, the sky clouds over and gentle steady rain falls for hours. At a cold front heavy cold air pushes under the warm air and lifts it strongly, giving short, heavy showers and sometimes a thunderstorm. After the cold front it becomes clearer and cooler.

Why no forecast is perfect

The atmosphere is huge and constantly on the move. Even a tiny inaccuracy in the measurements can grow over time into a big difference. That is why a forecast for tomorrow is usually very good, for the day after a little less so, and for next week only rough tendencies can be given.