The Lifetime of a Sunspot Group
Unlike the Earth, no surface feature on the Sun lasts for any appreciable length of time. The most easily observed feature on the solar disc is the sunspot group, and most of these last for only a few days. The longest lasting sunspot groups are observable for no more than about 100 days, but these are very few and far between. Other solar features, such as prominences, have similar lifetimes to sunspot groups.
The graph below was drawn from Royal Greenwich Observatory observations of over 3000 sunspot groups in the first half of the 20th century.
The graph may be interpreted by noting that the vertical direction shows the percentage of sunspot groups that have lifetimes equal to or in excess of the interval shown on the horizontal axis. We can therefore see that fully 50% of all groups have lifetimes of less than 2 days, and only 10% of all groups last for more than 11 days.
It is interesting to note that on the average, larger sunspot groups last longer. The famous solar scientist Waldmeir derived a simple rule of thumb relating the lifetime of a sunspot group to the maximum area that it attains during its life:
T = 0.1 Amax
where T is the group lifetime in days and Amax is the maximum area in millionths of the solar hemisphere (typically areas range from 1 to 5000 millionths). This can be useful in predicting the lifetime of a sunspot group as it will typically grow to maximum area much faster than it decays into oblivion. The formula, however, is only a rough guide, as wide individual variations occur.
Material prepared by John Kennwell