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ATMO 251 Chapter 2 page 1 of 21 Weather Observation and Analysis John Nielsen-Gammon Course Notes These course notes are copyrighted. If you are presently registered for ATMO 251 at Texas A&M University, permission is hereby granted to download and print these course notes for your personal use. If you are not registered for ATMO 251, you may view these course notes, but you may not download or print them without the permission of the author. Redistribution of these course notes, whether done freely or for profit, is explicitly prohibited without the written permission of the author. Chapter 2. POINT OBSERVATIONS 2.1 Introduction Surface observations are not the world’s first meteorological observations. I imagine that cloud-watching takes that honor. Similarly, they were not the first observations you made as a human being. That honor goes to, “Hey, the top of my head is cold. Stop pushing!” The beginning of meteorology as a quantitative science came about with the invention of instruments such as the thermometer (which measures temperature), the anemometer (which measures wind speed), and the barometer (which measures air pressure). Scientists discovered that they could make measurements of the atmosphere and record its current and past conditions numerically. The commencement of such observations was an essential step toward understanding how the atmosphere worked. 2.2 Accuracy and Representativeness Today, possibly millions of surface observations are made on any given day. A significant fraction of them are made because of curiosity about the atmosphere. People who put up a weather station in their backyard are curious about the weather or want to learn about it. Schools install weather stations on the school roof for similar reasons. Most observations like that don’t make it past the TV weathercast. To be meteorologically useful, the observations must be both accurate and representative. Accurate means that whatever the instruments record is
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ATMO 251 Chapter 2 page 2 of 21 very close to what actually was happening at the site of the instruments. If the temperature is 75F and your thermometer reports 65F, that’s not a very accurate temperature measurement. Even if your thermometer reports 73F, it’s still not accurate enough. People are arguing about whether the temperature’s gone up 1 or 2 degrees this century, and you want to make that big a mistake in one day? Even if your instrument is perfectly accurate, or at least close enough for government work, the weather conditions it measures must have some known, predictable relationship to the weather in the vicinity of the weather station. The federal and worldwide standards call for a thermometer to be sheltered from the sun, ventilated, and placed 1.5 m to 2.0 m above ground level in the middle of a grassy field, assuming grass can grow there. If instruments are sited according to the guidelines, temperatures from place to place can easily be compared. If the official instruments say that the temperature is three degrees warmer in Berkeley
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