The general circulation (Chapter 12) is the sum of the synoptic-scale winds (Chapter 13), averaged over space and time, and these in turn contain local surface winds, which are the topic of this chapter. The flow of these local winds around surface irregularities generates swirls which eventually cause frictional heat, i.e. increased vibration of the air molecules. So there is a chain of descending scale and increasing irregularity of motion, as the Sun's radiation (which fundamentally energises all the various circulations) degenerates to the least useful form of power: low-temperature heat.

There are four main factors which create a surface wind. They are (i) the synoptic-scale wind, which is usually close to the gradient wind (Note 12.D) in speed and direction, (ii) a horizontal difference of temperature in the PBL (Section 14.2), (iii) the topography, which makes cold air flow downhill and warm air upwards (Section 14.3), and (iv) storms, especially thunderstorms. If none of these operates, the air is 'calm', i.e. it has a speed below 1.5 m/s (5.4 km/h) and standard anemometers become less accurate.

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