Half of the Earth is exposed to the Sun at any time. Radiation from the Sun is the Earth’s main source of energy. This provides huge amounts of both heat and light, without which there would be no life on Earth.
Solar means anything to do with the Sun.
About 41 percent of solar radiation is light; 51 percent is long-wave radiation that our eyes cannot see, such as infrared light. The other 8 percent is shortwave radiation, such as UV rays.
Only 47 percent of the solar radiation that strikes the Earth actually reaches the ground; the rest is soaked up or reflected by the atmosphere. The air is not warmed much by the Sun directly. Instead, it is warmed by heat reflected from the ground.
Solar radiation reaching the ground is called insolation.
The amount of heat reaching the ground depends on the angle of the Sun’s rays. The lower the Sun is in the sky, the more its rays are spread out and therefore give off less heat.
Insolation is at a peak in the tropics and during the summer. It is lowest near the Poles and in winter.
The tropics receive almost two and a half times more heat per day than either the North or South Pole.
Some surfaces reflect the Sun’s heat and warm the air better than others. The percentage they reflect is called the albedo. Snow and ice have an albedo of 85-95 percent and so they stay frozen even as they warm the air. Forests have an albedo of 12 percent, so they soak up a lot of the Sun’s heat.
Sunspots are dark spots on the Sun’s photosphere (surface), 2000°C cooler than the rest of the surface.
The dark center of a sunspot is the umbra, the coolest bit of a sunspot. Around it is the lighter penumbra.
Sunspots appear in groups which seem to move across the Sun over two weeks, as the Sun rotates.
Individual sunspots last less than a day.
The number of sunspots reaches a maximum every 11 years. This is called the solar or sunspot cycle.
The next sunspot maximum will be in the year 2002.
Earth’s weather may be warmer and stormier when sunspots are at their maximum.
Long-term sunspot cycles are 76 and 180 years, and are almost like the Sun breathing in and out.
Observations of the Sun by satellites such as Nimbus-7 showed that less heat reaches the Earth from the Sun when sunspots are at a minimum.
Infrared photographs reveal the dark sunspots that appear on the surface of the Sun.
Solar flares are sudden eruptions on the Sun’s surface. They flare up in just a few minutes, and then take more than half an hour to die away again.
Solar flares reach temperatures of 10 million °C and have the energy of a million atom bombs.
Solar flares not only send out heat and radiation, but also streams of charged particles.
The solar wind is the stream of charged particles that shoots out from the Sun in all directions at speeds of over a million km/h. It reaches the Earth in 21 hours, but also blows far throughout the Solar System.
Every second the solar wind carries away over a million tons of charged particles from the Sun.
Earth is shielded from the lethal effects of the solar wind by its magnetic field (see magnetism).
Solar prominences are gigantic, flame-like tongues of hot hydrogen that sometimes spout out from the Sun.
Solar prominences reach temperatures of 10,000°C.
Coronal mass ejections are gigantic eruptions of charged particles from the Sun, creating gusts in the solar wind which set off magnetic storms on Earth.
Magnetic storms are massive hails of charged particles that hit the Earth every few years or so, setting the atmosphere buzzing with electricity.