Temperature — Page 62, Lesson 54

I want to walk you through the four processes that heat the troposphere — the lowest layer of the atmosphere where weather happens and where we fly. These are terrestrial radiation, conduction, convection, and condensation. Let's take them one by one. First, terrestrial radiation. The Earth radiates heat at all times — day and night. This is relatively long-wave radiation, with a wavelength range of 4 to 80 microns, and it peaks at 10 microns. That long-wave radiation is absorbed by what we call the greenhouse gases — principally water vapour, carbon dioxide, and methane. This absorption is what gives rise to the lapse rate in the troposphere, which we'll define shortly. The increase in carbon dioxide in the troposphere is one factor contributing to global warming, though the book notes that the global warming phenomenon is much more complex than just that. Next, conduction. During the day, air that lies in direct contact with the Earth's surface gets heated by conduction from the warm ground. At night, the opposite happens — air in contact with the surface gets cooled by conduction. Now here's the key point: air is a poor conductor of heat. So the air just above the surface — at a higher level — stays at the same temperature it had during the day, while the surface air cools. That creates a situation called an inversion, where temperature increases with height instead of decreasing. Third, convection. Air that gets heated by conduction becomes less dense than the surrounding air, so it rises. These rising currents are called thermals or convection currents. They carry warm air up to higher levels in the troposphere. Together with terrestrial radiation, convection is one of the two main processes that heat the troposphere. Fourth, condensation. As the rising air from convection goes higher, it cools by what's called the adiabatic process — that means it cools because it expands, without exchanging heat with the surrounding air. Eventually the water vapour in that air condenses out as visible droplets, forming cloud. When that condensation happens, latent heat is released by the water vapour. That released heat adds to the heating of the troposphere. Now let's move to temperature variation with height. Even though our ultimate source of heat is the sun, the troposphere is virtually transparent to incoming solar radiation — called insolation. So the sun's energy passes through the air and warms the ground directly. The troposphere is actually heated from the surface upwards, by that long-wave infrared radiation the Earth gives off. So as we move further away from the surface, the heating effects diminish — temperature drops. That brings us to the lapse rate. The lapse rate is the rate at which temperature falls with an increase in height. In an ideal uniform atmosphere, the lapse rate would be constant — like the ISA, the International Standard Atmosphere. The ISA lapse rate is 0.65°C per 100 metres, which is approximately 1.98°C — often rounded to 2°C — per 1000 feet. Finally, an isotherm or isothermal layer. If temperature remains constant with height — it doesn't change as you go up — that layer is called an isothermal layer. So to summarise: the troposphere is heated from below by terrestrial radiation and convection as the two main processes, with conduction and condensation also playing roles. Temperature normally decreases with height at the lapse rate, but inversions and isothermal layers are important exceptions we'll encounter.