Heating and cooling the air

The earth, heated by insolation, passes heat to the air near it in long-wave form. The atmosphere gains and loses heat in four ways:

• Conduction: when two bodies at different temperatures touch, heat flows from the warmer to the cooler until they are equal or the contact breaks. Air touching the warm land is heated slowly, and passes heat to the layers above. Conduction mainly heats the lowest layers.
• Convection: air heated at the surface rises in currents, carrying heat upward. This vertical transfer is confined to the troposphere.
• Advection: heat carried by horizontal movement of air. It is relatively more important than vertical movement. In middle latitudes, most day-to-day weather change comes from advection. The hot summer wind of north India, the 'loo', is an outcome of advection.
• Terrestrial radiation: the heated earth becomes a radiating body and gives off long-wave energy. This is absorbed by atmospheric gases, especially carbon dioxide and other greenhouse gases, so the atmosphere is heated indirectly, from below. The atmosphere in turn radiates heat to space, keeping temperatures steady.

Two laws the book mentions: Planck's law: the hotter a body, the more energy it radiates and the shorter the wavelength. Specific heat: the energy needed to raise the temperature of one gram of a substance by 1°C.

Example (new): sitting near a bonfire you feel heat on your face from radiation; your hands warm up by conduction if you hold a warm cup; the smoke and hot air rising above the fire are convection; and the warm air a breeze pushes towards people standing downwind is advection.