In 1610 Galileo Galilei observed the sun and noticed its surface was not uniformly bright. There were dark circles in the lower solar latitudes, and these sunspots have been the subject of a lively interest ever since. Then in the 19th century, it was confirmed that the sun goes through a regular cycle lasting about 11 years, during which the sunspots vary in both size and number.
This "solar cycle" is known to affect our planet because of corresponding changes in the energy output of the sun. For instance, the intensity of the Earth's magnetic field varies with the sunspot cycle, and the aurora borealis, or northern lights, are also more in evidence during periods of high sunspot activity.
But some rather unexpected happenings have also been shown to be related to the sunspot cycle: a case in point is the population of the North American snowshoe hare.
The snowshoe hare is the dominant herbivore of a vast area stretching eastwards from Alaska to Newfoundland. But strangely, the hare population fluctuates in a 10-year cycle, or something like it, with perhaps 50 times as many hares at sunspot maxima as at the minima; it seems to be closely locked to the phase of the sunspot cycle. Moreover, during periods when the sunspot cycle is less pronounced than usual, the hare cycle appears to drift upon its own. The theory is that the population cycle is linked to the food supply of the hares and of their predators, and that this in turn is linked to changes in the pattern of weather.
To meteorologists, the possible effect of sunspots on our weather is a controversial issue. Some researchers have claimed to have found variations in rainfall or temperature that coincide with the solar cycle, but no one really believes them very much.
More recently, researchers at the Scripps Institute of Oceanography in California found that sea surface temperatures across the globe swing up and down in a 10 to 12-year period that lags behind the solar cycle by two to three years. And this, of course, could indeed affect the weather. In fact, it lends some credence to the earlier findings of another team of scientists who suggested that in periods of high sunspot activity the depressions in the North Atlantic seemed to move on a track that was slightly further north than usual.
But even if this relationship between the hares, the sunspots and the weather really does exist, there is still the troublesome question for meteorologists as to how the solar influence might exert itself.