The heat budget

The heat budget. The earth as a whole neither gains nor loses heat over time: the insolation received equals the heat lost. Take insolation at the top of the atmosphere as 100 units:

• 35 units are reflected back to space before reaching the surface: 6 by the atmosphere, 27 from the tops of clouds, 2 from snow and ice. This reflected share is the earth's albedo.
• The other 65 units are absorbed: 14 within the atmosphere and 51 by the earth's surface.

Where the surface's 51 units go: the earth radiates them back as terrestrial radiation:

• 17 units go directly to space.
• 34 units are absorbed by the atmosphere: 6 directly, 9 through convection and turbulence, and 19 through latent heat of condensation.

Balancing: the atmosphere now holds 14 + 34 = 48 units, which it radiates to space. Total returned: 17 + 48 = 65 units, exactly what was absorbed. This is the heat budget or heat balance, and it explains why the earth neither warms nor cools despite huge heat transfers.

Variation by latitude. Between 40° N and 40° S there is a surplus of net radiation; near the poles, a deficit. Surplus heat from the tropics is carried towards the poles, so the tropics do not keep heating up and high latitudes do not freeze for ever.

Example (new): a family's monthly money works the same way: if salary in equals spending and savings out, the balance stays steady. If the earth took in 65 units and gave back only 60, it would warm every year until a new balance was reached.