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The many individual tunes which orchestrated as a chorus make up the symphony of "weather" show little sign of harmony when taken each in isolation. Temperature, wind, humidity and rainfall all change in a chaotic way from day to day, and even hour to hour. But if these elements are averaged over a long period, a pattern emerges - a daily rhythm rooted in the motion of the sun.

Air temperature, for example, is at its lowest, on average, a little after dawn, and at its highest early in the afternoon. This affects the other weather elements. Warmth near the surface is conducive to rising, convective currents of air in the atmosphere, and these in turn breed clouds. The afternoon, therefore, tends to be the cloudiest part of the day, and clouds often produce rain - with the result that rainfall, too, tends to peak on average in the afternoon.