book10geostrophic

book10geostrophic - Weather Observation and Analysis John...

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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 10. GEOSTROPHIC BALANCE 10.1 Wind and Height Gradients You hear it frequently in weather reports, you’ve seen it on weather maps, and it makes intuitive sense: strong pressure gradients are associated with strong winds. That statement is a general one that applies to all classes of fluid motion. But if you have a friend who’s an engineer or physicist and you want to freak them out, tell them about the atmosphere. Sure, strong gradients mean strong winds, but generally the wind doesn’t blow from high pressure toward low pressure. Instead, it blows sideways. This characteristic is observed in its purest form aloft, away from the effects of surface friction. Most of the time, meteorologists use pressure coordinates when considering the distribution of weather elements aloft, and on a pressure surface, gradients of geopotential height have the same meaning and effect as gradients of pressure at a constant altitude. So from here on, we’ll be talking about strong height gradients and the wind not blowing from high heights toward lower heights (on a constant pressure surface such as 500 mb). On a map of a pressure surface, the height information is usually depicted with contours, just like isobars depict pressure information on a sea level map. If you examine any pressure surface, from 850 mb on up, you will find the following general characteristics: 1. Stronger winds tend to occur where the height contours are closer together; weaker winds where the height contours are farther apart. ATMO 251 Chapter 10 page 1 of 28
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2. The winds tend to mostly blow parallel to the height contours. 3. The winds tend to be oriented so that the higher heights are 90 degrees to the right of the direction toward which the wind is blowing. These rules aren’t followed exactly, but they are close enough that if you just had the height contours to go by, you could make a pretty good estimate of the wind speed and direction just about everywhere. A hypothetical wind that follows these rules exactly is called a geostrophic wind . Since the real wind comes close in most cases, it’s fair to say that the actual wind is nearly geostrophic. 10.2 The Geostrophic Equation
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This note was uploaded on 04/02/2008 for the course ATMO 251-501/50 taught by Professor Alcorn during the Fall '07 term at Texas A&M.

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book10geostrophic - Weather Observation and Analysis John...

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