20 What does the teacher havethe children do when there isa fire drill? Listen andnumber. Then say, A FIRE DRILLA walk to the main doorB wait quietlyC walk outside in a lineD go to the meeting point1 The teqcher has the children walk toThe main door​

Earth is surrounded by air membrane - atmosphere. it is a mechanical mixture of gases. the atmosphere has several layers. most important for a person is the bottom layer - the troposphere. there's clouds are formed, born thunderstorms, downpours and snowfalls. here is all the water vapor. its height ranges from 8 to 12 km. a characteristic feature of the troposphere is to reduce the height of the air pressure and decreasing temperature. the reason is that sunlight tropospheric air almost transparent and it is heated and cooled mainly on the ground.the layer of air that separates the troposphere from the layer that lies above is called tropopause (thickness from tens to hundreds of meters).above at a height of 8-17 to 50-55 km is the stratosphere. it has poor water vapor. therefore, there is no rapid process of cloud formation and precipitation. the temperature in the stratosphere increases with altitude. the air in it is heated by sunlight. here is the ozone layer, the air is enriched triatomic oxygen. it protects all life on earth from harmful effects of ultraviolet solar radiation hard.more stratosphere is the mesosphere above, where the temperature drops with height.thermosphere borders between the heights of 80 and 800 km. on the stratosphere and thermosphere we know very little.the bulk of the atmosphere contained in a relatively thin layer,in which the density of air decreases rapidly with altitude.because 1 m3 of air at sea level weighs 1033 grams and at an altitude of 40 km - only 4 hwe live in the lower layer of the atmosphere. so you can see how clouds are formed, feel pody winds, falling on our heads precipitation. with the atmosphere associated with such extreme natural phenomena such as snow, dust storms, hurricanes and tornadoes, floods and more.recently -meteorolohy scientists are sounding the alarm: today the earth's atmosphere is warming much faster than ever in the past. this is mainly due to human activities: burning large quantities of coal, oil, gas, and commissioning of nuclear power plants; burning fossil fuels and deforestation as a result of accumulated carbon dioxide atmosphere. over the past hundred years the concentration of carbon dioxide in the air has increased by 17%. the earth's atmosphere it effect like glass in a greenhouse or a greenhouse, it freely passes solar rays to the earth's surface, but keeps it warm. this causes heating of the atmosphere known as the greenhouse effect, which could cause a global environmental crisis.as already mentioned, at altitudes of 20-50 km air contains increased amounts of ozone, which is formed by ordinary diatomic oxygen absorbing ultraviolet solar radiation. as a result of photochemical reactions of ultraviolet radiation energy is spent on the formation of ozone from oxygen. recently, the amount of ozone in the ozone layer rapidly decreases, which is extremely alarmed scientists in the world. if the ozone layer disappears completely, then at least it threatens destruction of the earth's biota (environment).established that the ozone layer contribute to some chemicals, including products and technological activity. modern industry makes extensive use of so-called freon as a refrigerant in household refrigerators and refryzhektorah as aerosol rozbryzhuvachi in cans with paint, perfume and so on. each year the world produced millions of tons of cfcs. for a man freon vapor is not harmful, and they are extremely dangerous to the ozone layer. once in the stratosphere by ultraviolet radiation they decompose releasing chlorine. this substance acts on ozone as a strong catalyst, expanding it to oxygen molecules. now take measures on preservation of the ozone layer: in 1985 the montreal protocol on protection of atmospheric ozone, which is based on an agreement to reduce the consumption of cfcs with subsequent change of safer chemicals.