The Atmosphere
Atmosphere Thin envelope of gases that surrounds the planet. Weather Climate The state of the atmosphere at a given place and time Weather conditions at a locality averaged over a specified time period
Atmosphere is divided into four distinct zones of contrasting temperature due to differential absorption of solar energy. There is little mixing between layers. As you move higher in altitude the atmosphere thins out.
Troposphere contains about 75% of the mass the earth’s air, but is only about 17km thick. Most weather events occur here. It’s composition is uniform due to mixing caused by winds. Temperatures drop with altitude. The sharp boundary in temperature at the tropopause limits mixing with upper layers
Stratosphere extends to about 50km thick. Similar in composition to the troposphere except in two ways. It contains1000 times less water and is 1000 times higher in ozone. Ozone is produced by lightning and solar irradiation of oxygen molecules. The Stratosphere is relatively calm, volcanic ash or human caused pollution can remain in suspension in the stratosphere for many years.
Past and Present Composition The composition of the Earth’s atmosphere has changed since the formation of the Earth. The Earth’s first atmosphere was mainly helium and hydrogen. Volcanic emissions later added carbon dioxide, nitrogen, water, sulfur dioxide and other elements.
Large amounts of carbon dioxide from volcanoes caused the Earth’s past climate to be warmer than today’s. Without carbon dioxide no life could be present on earth. Primitive bacteria and algae in oceans could now photosynthesis.
But no life on land due to solar radiation from the sun. But life in oceans is protected from radiation and can develop due to presence of carbon-dioxide
Sunlight + CO2 + H2O O2 (g) + C6H12O6 Free oxygen gas was absent in the earth’s early atmosphere. Any oxygen produced by photosynthesis reacted with other chemicals and was trapped in the form of oxide compounds in rock, Fe2O3 . It took millions of years before oxygen was present as a gas in the atmosphere Nearly all the molecular oxygen in the atmosphere today was produced by photosynthesis. Sunlight + CO2 + H2O O2 (g) + C6H12O6 Living organisms are responsible for the evolution of our atmosphere. Free oxygen made it possible for life to evolve on land. Before large amounts of free oxygen were present in the atmosphere, lethal intensities of solar radiation flooded the Earth’s surface. Free oxygen reacts in the stratosphere to form ozone (O3). This ozone layer shields the Earth’s Surface from lethal radiation.
Oxygen produced by photosynthesis builds up in the atmosphere Oxygen produced by photosynthesis builds up in the atmosphere. This turns into ozone due to solar radiation.
Present composition of the lower Atmosphere Water vapor varies depending on the location. From 0.01% to 5%
Suspended particles within the Atmosphere are called aerosols. Aerosols can be both solid and liquid. Some are too small to see, other are clustered together and can be seen as clouds. Volcanoes are one major source of natural aerosols
How does the Earth keep itself warm? The average radiant energy from the sun falling on the surface at about 343 watts /m2. As it passes through the atmosphere, 6% is scattered back into space by atmospheric molecules.10% is reflected back into space from land and ocean surface. The remaining 84% actually heats up the surface. To balance this the Earth itself needs to radiate the same amount of energy back into space. The Earth emits long-wave length radiation in the form of infrared radiation. (this depends on the temperature of the surface and the type of surface) But the amount of energy is not balanced.
Nitrogen and Oxygen gas can neither absorb or emit thermal radiation Nitrogen and Oxygen gas can neither absorb or emit thermal radiation. It is Water vapor, carbon dioxide, and some other minor gases which absorb long-wave thermal radiation leaving the surface. This is why the average surface temperature is 15C instead of -6C which it should be it balanced. 1. Solar radiation 2. Radiation from greenhouse gases 3. Radiation scattered by atmosphere.
Some wavelengths of radiation from the Earth can escape into space if there are no clouds. Other wavelengths are absorbed by gases and re-emitted into space or back to the surface. Graph of radiation emitted from Earth’s surface that is absorbed by the Atmosphere.
Absorption of solar energy by the atmosphere is selective Absorption of solar energy by the atmosphere is selective. Visible light passes through, ultraviolet is absorbed mostly by ozone in the stratosphere. Infrared is absorbed mostly by carbon dioxide and water in the troposphere.
Homework: Read chapter 14 Answer Q 2, 3, 4, 5, 6
Solar Radiation and the Atmosphere