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.
If the pressure pattern is steady, the wind waxes and wanes in much the same way; it is at its lightest around dawn, and strongest on average in the afternoon. Relative humidity, on the other hand, varies in the opposite direction. As we know, the lower the temperature, the higher the relative humidity for a given amount of moisture in the air; relative humidity, therefore, peaks just after sunrise when the temperature is at a minimum, and is at its lowest shortly after the sun has passed its zenith .
But atmospheric pressure produces some surprises. As we know from tapping the barometer, pressure rises and falls from day to day irregularly, but superimposed on the larger pressure changes caused by weather systems, is another daily rhythm - a cyclical twice- daily rise and fall. Everywhere in the world, a pressure peak occurs when the local time is about 10 a.m. and 10 p.m.; corresponding troughs are apparent 12 hours later in the afternoon, and very early in the morning.
These pressure waves, too, can be traced to solar heating of the atmosphere, and to the rotation of the Earth beneath the sun. But why a double daily cycle, rather than, as in the case of other elements, a 24-hour one? The complex answer is related to the fact that the atmosphere has a natural "resonance" with a period of about 12 hours.
In other words, if you were to give it a slight push, like a wobbly jelly, and leave it to its own devices, it would sway back and forth at that frequency - a predilection which causes the daily thermal stimulus originating from the sun to manifest itself as a double 12-hour "wave".