The atmosphere. Earth’s climate zones
The atmosphere is the air shell around the Earth; it is 78 % nitrogen, 21 % oxygen and 1 % other gases (A. Lavoisier determined its gas composition in 1774). Its layers are: the troposphere (17 km thick at the equator and 8–9 km at the poles), the stratosphere (up to 50–55 km, with the ozone layer in it), the mesosphere (up to 80–85 km), then the thermosphere and exosphere. About 80 % of the mass of air is in the troposphere, and weather happens there; with height, air temperature falls by an average of 0.6 °C every 100 m. Climate is shaped by three factors: geographic latitude (solar heat), atmospheric pressure and air masses, and the surface (relief, distance from the ocean). The air masses are equatorial (hot, humid), tropical (hot, dry), temperate (mild, humid) and polar — Arctic and Antarctic (cold, dry). There are 7 main climate zones (equatorial, two tropical, two temperate, two polar) and 6 transitional zones (two subequatorial, two subtropical, two subpolar); in transitional zones air masses change with the seasons.
“Ladder of the sky”: draw a ladder on the wall, write each layer’s name and height on a rung, then students decide in which layer an aeroplane flies.