Sunspots and cloud formation

Even so, it seems unlikely that so small a change, never amounting to as much as 1 percent, could produce such a big effect. The answer is that the climatic effect is not due to direct heating.

Solar astronomers believe that sunspots are magnetic disturbances. They occur in pairs, with magnetic field lines emerging from one sunspot

Solar constant. The Sun subtends an angle of 0.5' at the Earth's surface. Solar radiation is emitted in all directions, but the Earth is exposed to only a tiny fraction of the total.

The link persists

and entering through its partner. The sunspot cycle is due to turbulence in the photosphere as the Sun rotates about the center of gravity of the solar system, a rotation that takes 11.1 years. When sunspots are present the stream of charged particles leaving the Sun—the solar wind—intensifies. The Sun also becomes brighter and emits more ultraviolet radiation.

Close to the Earth, the Sun's magnetic field and the solar wind combine to deflect the charged particles of cosmic radiation. Solar wind particles are trapped in the Earth's magnetic field and travel along field lines. These draw them down into the upper atmosphere over the magnetic poles and it is collisions between entering solar-wind particles and atmospheric gases that produce auroras.

Cosmic rays have much more energy. They penetrate the magnetic field and enter the atmosphere directly. Collisions between cosmic-ray particles and atmospheric atoms and molecules produce showers of other particles. These particles are charged and they appear to act very efficiently as freezing nuclei—particles onto which water vapor is deposited as ice crystals. Ice crystals then allow further deposition of ice and the process leads to the formation of clouds.

When the solar wind is weak, the amount of cosmic radiation entering the atmosphere increases, and the total amount of cloud increases. When the solar wind is strong, cloudiness decreases.

Cloud has a direct effect on the temperature at the surface and in the atmosphere, but it is complex and not fully understood. High-level clouds are thin and wispy. They allow most of the Sun's radiation to pass, but they trap infrared radiation from below, so they tend to warm the upper air. Clouds at a lower level are thicker and more reflective. They reflect incoming radiation and shade the surface, making it cooler, although this effect is partly offset by the release of latent heat as water vapor condenses. Cosmic rays appear to produce big, heavy, freezing nuclei that trigger the formation of dense, highly reflective cloud.

During sunspot minima the solar wind is very weak, and so Earth is exposed to more intense cosmic-ray bombardment. This triggers increased cloud formation, reducing the temperature at the surface. Increased cloudiness adds to the direct cooling effect that results from the reduction in the solar constant. It also appears that the climate is considerably more sensitive to changes in the intensity of solar radiation than to changes in concentrations of greenhouse gases.

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