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One day, Melissa was given a weather computer. The packaging claimed that the computer could forecast the weather anywhere and everywhere in the world. Furthermore, the box claimed that it did its forecasting based solely on the laws of physics.
Melissa was skeptical. she knew that it was very easy to get a computer to forecast the weather. For example, one could program a computer to always forecast a high of 70 and a low of 50. It's a forecast, but not very useful. A more accurate forecast would have the computer look up the climatological averages for whatever site it was forecasting for. More accurate than random guessing, but still not very impressive.
Melissa didn't believe that this small computer (with a Pentium 2 processor) was actually going to make forecasts based on the laws of physics, which is how the National Weather Service supercomputers make their forecasts. So she decided to try it out on a simple problem to see if she could figure out how it worked. Choosing something practical, she asked it to forecast the temperature on the 16th green at the TAMU Golf Course. (Melissa was a very good golfer.)
The day Melissa chose had calm winds, so Melissa figured the computer should have a very easy time with the forecast. But it was still much more complicated than she expected.
First, the computer wanted to know what height to forecast the temperature for. Would it be right at the ground, a foot above the ground, four feet above the ground, or twenty feet above the ground? Melissa picked four, figuring that people felt temperatures mostly at chest level.
Then the computer wanted to know what the current temperature at four feet was. Melissa thought this was a reasonable request, it being hard to forecast the future if you don't know what the present is. So she took a thermometer, measured the air temperature, and typed the value in to the computer.
Then the weather computer wanted to know the temperature at one foot and twenty feet. Again, Melissa thought this was reasonable, so she measured the temperature at ankle level, then tied the thermometer to a helium balloon and raised the balloon to a height of 20 feet. She let it sit up there for a while, then lowered it quickly and read off the temperature.
Melissa had just taken her first upper air observation. Admittedly, 20 feet is not exactly upper air, and most weather balloons are released into the air and radio their data back to the ground, but the principle is similar.
This satisfied the computer's desire for initial conditions. The initial conditions are possibly the most important part of the forecast. Consider this analogy: if you want to forecast the position of a baseball, knowledge of the laws of physics (and/or experience) is helpful, but the most important thing is knowing how hard and in which direction the ball was thrown.
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