Insolation is the incoming solar radiation received at the Earth's surface; the heat budget is the long-term balance between the solar energy the Earth receives and the energy it radiates back to space, which keeps global temperature broadly stable.
- Insolation is greatest near the equator, where the Sun's rays are near-vertical, and least toward the poles, where they strike at a low angle and spread over a larger area through more atmosphere.
- Incoming radiation is partly reflected (the albedo, high over ice, snow, and clouds), partly absorbed by the atmosphere, and partly absorbed at the surface.
- The Earth radiates energy back as longwave (infrared) radiation; over the long run incoming equals outgoing, so the planet is in heat balance.
- The tropics receive more energy than they lose (a surplus) and high latitudes lose more than they receive (a deficit); winds and ocean currents transfer heat poleward to even out this imbalance.
- Factors affecting insolation at a place: latitude (angle of the Sun), length of day, transparency of the atmosphere, and the aspect and altitude of the land.
The equator-to-pole gradient, the meaning of albedo, the surplus-versus-deficit zones, and the role of winds and currents in heat transfer underpin the whole of climatology and ocean-current questions.
Insolation (incoming shortwave solar radiation) versus terrestrial radiation (outgoing longwave); albedo is reflectivity (high for snow and clouds); the tropics have a heat surplus and the poles a deficit, balanced by heat transfer.
Incoming solar radiation peaks at the equator; the heat budget balances incoming and outgoing energy, with winds and currents moving surplus heat poleward.