October 11, 2010

The Adiabatic Process

The Adiabatic process is one of the more fascinating topics we have covered in meteorology class so far.  The following is a VERY watered down description of how it works, but hopefully you will get the idea.

One of the first concepts we learned was how the sun heats the atmosphere, which in turn heats the earth, and that the earth's surface can only hold so much latent heat before it has to release/radiate it back into the atmosphere. Now, as the earth radiates this heat into the air, the air absorbs some of this heat. The air gets warmer, and warm air wants to rise, as it rises, the decreasing pressure on the air allows it to expand. Remember, air pressure decreases as we increase altitude because there is less atmosphere pressing down on it.  Air will expand and cool at a rate of ten degrees Celsius per 1000 meters risen. We are assuming that this particular parcel of air is dry.  The numbers and effects change if the air is saturated. So our parcel of air will rise, cool, and expand until it reaches the same temperature as the air surrounding it, and it will stay there, assuming no other forces act upon the air to change it's temperature.  It is possible for our air parcel to go a little bit too high, and become cooler than the air around it.  At that point our parcel will being to sink down toward the earth and get compressed and warm back up. The same rule applies for sinking air as it does for rising air(as long as the parcel stays dry).  Temperature will increase ten degrees Celsius for each 1000 meters lost.  So, we learn from this that the air in our atmosphere is constantly in motion, warming and cooling, rising and falling, changing pressure with altitude.   This process is called the dry adiabatic process. Changing this process by doing something as simple as adding water vapor to the dry air changes the results dramatically.  We'll touch on those results in the next article. Don't forget your umbrella!


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