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EPS 102 lecture 15

EPS 102 lecture 15 - EPS 102 lecture Thursday March 12th...

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EPS 102 lecture Thursday March 12 th , 2009 Surface and climate and Greenhouse effect: we talked about the sun which is the starting point for thinking about what environments around the sun will be like. Greenhouse effect is a good thing because without it our planet would be unbearably cold. It is the difference in the temperature if we had no atmosphere vs. what we do have. With no atmosphere it would be -19 degrees and with an atmosphere our surface is about 14 degrees. The greenhouse effect is about the atmosphere. Our atmosphere is unusual: the presence of oxygen at 21% means there is something funny going on here=life. Things that contribute to the greenhouse effect has little to do with oxygen and nitrogen; its moreso about CO2, water vapor, ozone, and methane which are in trace amounts. What would the surface temperature be with an atmosphere? 14 degrees. Going up means pressure goes down and consequently temperature goes down. Water vapor condenses and doesn’t get higher than the top of the troposphere which is good so it cannot be attacked by UV light. Amount of solar radiation coming into the atmosphere, the composition of the atmosphere (make up of it), and these are all things that enter into the temperature effect as well as thinking about the greenhouse effect. Convection: why is it that the temperature decreases as we go from surface to the upper atmosphere? When material rises, the pressure goes down and the material expands and cools. In dry air, we get 9.5 degrees that the air will cool per kilometer due to its expansion. This also depends on humidity. When the earth receives sunlight, different parts warm up. sunlight gets preferentially absorbed in dark asphalt due to albedo quality and not very much light gets reflected. It is absorbed and then radiated as a black body. If you warm up the air above the surface of the asphalt, it will be less dense than its surroundings and it rises. Then it cools and expands. So it started off hot but then it rises up. This cold air comes down and gets compressed again. The point is that you get large convective cells in the troposphere that mix and homogenize the atmosphere. There are warm temperatures in the surface, well mixed in the troposphere and then decreased temperature above. If we start with a balloon and tape a thermometer inside and pump it up with air and take it with you up in the plane, then you have air pressure in the balloon at the very top but as you come back down the pressure on the atmosphere will increase closer to the surface so as it comes down the pressure of the atmosphere makes it compress and get smaller and smaller. The atmosphere has done work on the balloon so it gets more energy and the temperature goes back up. We start at the surface, it rises up and the atmospheric pressure is lower but the balloon expands and does work on the atmosphere and consequently it’s temperature goes down.
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