Weekly variability and the weather

on daily and weekly timescales, the main mechanism that causes variations in the temperature and wind is simply the familiar weather. The mechanism that gives rise to weather resides in the atmosphere, and it is the consequence of a fluid instability called baroclinic instability. This instability can be thought of as a type of con-vective instability in which if a fluid is heated from below it expands, becomes lighter than its surroundings, and therefore rises. Baroclinic instability has a similar origin, but it also involves the lateral motion of fluid parcels, with a typical scale of up to a few thousand kilometers and a typical velocity of about 10 m s-1. It takes a parcel moving at this speed a little more than a day to travel 1,000 km, hence accounting for the typical timescale of days to weeks. In midlatitudes, the average temperature difference between two lines of latitude that are 1,000 km apart is about 5°C, and because weather is stirring the air on these spacescales, a typical weather system can lead to typical temperature variations of up to 5°c on timescales of a few days.

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