The Sun-Weather Connection

The Sun and the weather

The energy that the Earth receives from the Sun is the basic cause of our changing weather. Solar heat warms the huge air masses that comprise large and small weather systems. The day-night and summer-winter cycles in the weather have obvious causes and effects. Are there other, more subtle ways in which the Sun affects weather and climate? Will the future climate - even our survival - depend on sunspots, flares, coronal holes, or other forms of solar activity? If so, can future trends be predicted?

The effects of currently observed changes in the Sun - small variations in light output, the occurrence of solar particle streams and magnetic fields are very small in the Earth's lower atmosphere or troposphere where our weather actually occurs. However, at higher altitudes, the atmosphere reacts strongly to changes in solar activity. The ozone layer, at an altitude of 25 kilometers (16 miles), and the ionosphere, which extends upwards in a series of layers above 60 kilometers (37 miles), are produced by solar ultraviolet light and X-rays which ionize the thin air at these altitudes. Although the visible light of the Sun is stable, large variations in X-ray and ultraviolet radiation accompany solar activity, and these variations on the Sun cause major changes in the ionosphere. Some meteorologists be lieve that the ionospheric changes in turn influence the weather in the lower atmosphere, but the physical mechanism by which this may occur has not been definitely identified. There is much research under way or possible relationships between solar activity and the weather.

A study of short-term weather patterns by Walter Orr Roberts of the University Corporation for Atmospheric Research and Roger H. Olson of NOAA suggests that weather may be affected as the spiral-shaped interplanetary magnetic field rotates past the Earth. They found that about a day after the boundary between inward-pointing and outward-pointing sectors sweeps by, there is a decrease in the number of low pressure weather systems forming in the Pacific Ocean off the western United States and Canada. Because these low pressure systems give rise to most of the storm centers that pass over North America an understanding of this effect may ultimately assist in making weather predictions.

Like most suspected Sun-weather connections, the effect seen by Roberts and Olson is hard to explain. The problem is that the amount of energy present in the weather phenomena themselves far exceeds the energy that apparently is available from the variations in solar activity. In this case, the low pressure storm systems in the Pacific contain far more energy than do the particles and magnetic fields which enter the Earth's magnetosphere from the solar wind. If the Roberts-Olson effect is real, then there must be an amplifier mechanism, whereby the magnetic variations trigger the changes in the weather. But the nature of the amplifier mechanism is currently unknown.

The search for Sun-weather relations is further complicated by the presence of many non-solar influences on both short- and long-term weather patterns. Volcanic eruptions can inject huge amounts of dust and ash into the atmosphere, cutting off some of the Sun's light and heat. Changes in the amount of carbon dioxide in the atmosphere, as a result of volcanic eruptions or the burning of coal and oil, affect the amount of heat absorbed by the atmosphere. Even small variations in the Earth's orbital motion around the Sun from year to year may cause significant changes in the weather. In looking for direct effects of solar activity on the weather we must first disentangle the many non-solar effects that are going on simultaneously. It is a challenging task.

Climate through the ages

Climate is the state of the weather over long periods of time, tens to thousands of years. Long-term effects of the Sun on the Earth's weather are called climate effects.

If the total output of radiant heat and light from the Sun (the solar constant) changed with time, rather thanjust the X-rays, ultraviolet and other fringe effects of solar activity, the variations would affect the lower atmosphere directly and surely would change the Earth's weather and climate. But we still do not know whether the solar constant has changed in the past or even if it is changing today. The necessary measurements are very hard to make with the required accuracy. Because of absorption and scattering of sunlight in the Earth's atmosphere, these measurements are unreliable if made from the ground. Recently, techniques have been developed to measure the solar constant from space vehicles. There are now several instruments in orbit that are measuring the Sun's output with an accuracy that should be sufficient to detect variations capable of changing the climate.

The spacecraft measurements of the solar constant that we are accumulating now will enable us to determine the day-to-day and month-to-month changes in solar output. It should eventually be possible to find out whether the Sun varies, not only during its 11-year sunspot cycle, but perhaps even over longer periods as well.

Our studies with spacecraft are motivated in part by indirect evidence that long-term variations in the Sun's light have actually occurred. Observational records show an almost complete absence of sunspots between the years 1650 and 1715. During this period, named the Maunder Minimum for the English astronomer who first pointed it out, the sunspot cycle apparently ceased to exist. Historical sources attest to the fact that the weather in Europe was particularly cold during these years, a fact which would follow logically if the light from the Sun decreased significantly during years when the sunspot count was low.