Insolation

The earth gets almost all its energy from the sun and radiates the same amount back to space, so over time it neither warms up nor cools down. But different parts receive different amounts of heat; these differences create pressure differences, and winds carry heat from one region to another.

Insolation is incoming solar radiation, received mostly in short wavelengths. Because the earth is round, the rays strike the top of the atmosphere at a slant and the earth intercepts only a tiny part of the sun's energy: on average 1.94 calories per sq cm per minute at the top of the atmosphere.

• On 4 July the earth is farthest from the sun (152 million km): aphelion.
• On 3 January it is nearest (147 million km): perihelion. So slightly more energy arrives in January, but land-sea distribution and atmospheric circulation mask this, so it has little effect on daily weather.

What makes insolation vary during a day, a season and a year:

• the earth's rotation on its axis;
• the angle of the sun's rays (which depends on latitude); the earth's axis makes an angle of 66½° with the plane of its orbit, which strongly affects how much each latitude receives;
• the length of the day;
• the transparency of the atmosphere;
• the land's aspect (the direction a slope faces). The last two matter less.
• Slant rays spread the same energy over a larger area and pass through more atmosphere, so less energy reaches each square metre.

Passage through the atmosphere. The atmosphere is largely transparent to short-wave radiation. In the troposphere, water vapour, ozone and other gases absorb much of the near infrared. Tiny particles scatter visible light: this gives the blue sky and the red colour of the rising and setting sun.

Distribution at the surface: from about 320 W/m² in the tropics to about 70 W/m² at the poles. The maximum is over the subtropical deserts, where there are fewest clouds; the equator gets less than the tropics. At the same latitude, continents receive more than oceans. Middle and high latitudes get less in winter than in summer.

Example (new): shine a torch straight down onto a table and you get a small, bright circle; tilt it and the same light spreads into a long, dim oval. The midday sun in the tropics is like the straight torch; the low sun of high latitudes is like the tilted one.