Sunspots, as we noted yesterday in Weather Eye, are dark areas on the solar disc that increase and decrease in number in a rhythmic cycle of about 11 years. There may be perhaps 20 or 30 of them on any given day, or more, or less, depending on our progress through this solar cycle. In size they are anything from 1,000 to 20,000 miles across, and their lifetime varies from an hour or two to several months.
It is known that when there are many sunspots to be seen, slightly more energy than usual is emitted by the sun, and there has therefore long been an ambition to relate our weather in some way to these indicators of variation in solar activity. Many years ago, for example, an otherwise eminent climatologist discovered that the level of the water in Lake Victoria in Africa varied directly with the sunspot numbers. His study was of the years from 1890 to 1920, and within that period the rise and fall of the lake water did indeed match the 11-year sunspot cycle - but further research showed that outside these years, there was no relationship whatsoever. Indeed, all attempts to find an 11-year cycle in the weather have been unconvincing.
A few years ago, however, two Danish scientists attempted something different. Rather than try to relate the weather of a particular year to sunspot numbers, they concentrated instead on the length of the sunspot cycle - something no one had ever focused on before. They had noted that the sunspot cycle is not exactly 11 years, but varies between 10 and 12, and sometimes even more widely; they hypothesised that a long cycle might be indicative of a "weak" sun, and that a short cycle might coincide with high values of the solar constant.
The Danes examined the varying lengths of 12 solar cycles over 130 years or so, and when they matched these figures to the average global temperature, they found that there was very close agreement. The cycle was almost 12 years long in the early 1890s when global temperatures were low; it gradually shortened to 10 years during the first four decades of the new century - while the earth was getting warmer; and since then the pattern of solar cycle length has coincided very closely with that of global temperature.
More recently, temperature readings at Armagh Observatory from 1844 until the present day have been subjected to a similar analysis, and very much the same result obtained. It remains to be seen, however, whether all this will ever allow sunspot numbers to be harnessed as predictive tools.