People are sometimes kind enough to ask how topics for Weather Eye occur to me with such a monotonous daily regularity. Let me give you an insight into the tortuous workings of a mind preoccupied with recurring and ever-looming deadlines.
The other day I had a cup of coffee in the market-place in Bensheim. They served it with a biscuit, on which its brand-name, Lotus, was embossed, and I was conscious as I munched that I was therefore now a Lotuseater. Obviously it was but a small mental step from there to The Lotos Eaters, which tells the tale of those vagabonds of Greek legend who habitually consumed the plant, enjoying thereby a not unpleasant state of lethargy.
It begins, you may recall,
"Courage!", he said, and pointed towards the land;
"This mounting wave will roll us shoreward soon",
and must surely be the worst of Tennyson's attempts at poetry. Be that as it may, the narrative continues:
In the afternoon they came unto a land,
In which it seemed always afternoon;
All round the coast the languid air did swoon,
Breathing like one that hath a weary dream.
Then it occurred to me that afternoons, in Ireland anyway, are not like that at all. In general the air is neither languid nor inclined to swoon, but tends to blow more freshly than it does at morning-time. Voila! I had a theme for Weather Eye.
Moving air, like any fluid, may be turbulent or smooth. When the flow is smooth, air several hundred feet above the ground moves at a speed dictated by the pressure pattern, but the lower layers are held back by friction with the Earth's surface; indeed, in a very smooth airflow, there is very little wind at all in the lowest five or six feet above the ground.
Turbulence, however, has the effect of mixing the air from various levels. Wind moving aloft at a relatively high speed is entangled with the almost stationary air down below. The net result is to average out the wind-speed, and near the ground where we are this results in a wind significantly stronger than it otherwise might be.
One of the main causes of turbulence in the atmosphere is rising currents of air, brought about by solar heating of the Earth's surface; as the ground gets hotter, bubbles of hot air tend to surge upwards, disrupting an otherwise smooth airflow. Heating by the sun is most effective in the early afternoon, so - all other things being equal - the atmosphere tends to be more turbulent at that time of the day, and therefore, from our perspective, windier.