Weather: Why our winters are unavoidably delayed
Monday 05 January 1998
We all know that it's cold in winter because the tilt in the earth's axis of rotation points us away from the sun at this time of year. We can see the sun lower in the sky, and the dark mornings and early sunsets confirm that we are receiving less solar warmth. So far, so good; but if the shortest day of the year - which must be when we receive the least solar energy - falls a few days before Christmas, why does the weather continue to get colder for the next couple of months?
The way the sun provides warmth is not as simple as it seems. Since the air is a poor conductor of heat, most of the warmth we feel round us is radiation from the Earth itself. The sun heats up the Earth, which then warms up the air by radiation. This explains why the higher you go, the colder it gets. The top of a mountain may be nearer to the sun, but that counts for nothing compared with the fact that it is farther away from the nice warm ground.
What makes the real difference, however, is the difference between the land and the sea. Water is far more efficient than land at storing heat. Indeed, it may be said that the oceans are a reservoir not only of water, but also of the world's heat. Not only are the waters more efficient at storing heat, but it takes more heat energy to warm them up in the first place. This results in the oceans providing a sort of temporal thermal lagging around the continents. Water temperatures will rise and fall more slowly than those of nearby land.
Both the land and the sea warm up during the periods of maximum sunshine, then gradually cool down, radiating their warmth into the air. When the sun is less generous with its energy, the world cools down, and needs some time to warm up again before we begin to feel the benefit. We are living on a massive storage heater, simultaneously operating on two different tariffs: the quick-release land storage and the slow-release ocean storage.
Over land that is far from the sea, there is about a one-month time-lag between the shortest day and the coldest day (or the longest day and the hottest day). Over the oceans it is about two months. In Britain, where our weather is dominated by the seas around us, the hottest day of the year generally falls in the middle of August (it was 10 August last year) - two months after the summer solstice.
One aspect of all this, however, remains perplexing: if there is such a time-lag between our receiving solar energy and experiencing its effects, why is long-range weather-forecasting still such an inaccurate science? One might naively think that with all the solar energy stored up a month or two in advance, we ought to be able to predict what will happen when it is released. Yet it does not seem to work like that. The diffusion of heat between various sea depths, and the vagaries of heat transfer by ocean currents, seem to make predictions impossibly difficult.
As with so many aspects of the weather, the key to successful long-term forecasting is a better understanding of the oceans.
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