book03advection - 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 3. SPACE, TIME, AND MOTION 3.1 Wind Observations We can divide weather elements into scalars and vectors. Anything that can be represented as a single value is a scalar. Almost all observed atmospheric quantities are scalars. Temperature, dewpoint, pressure, rainfall, infrared radiation, concentration . .. all of these are things whose value is specified by a single number. The one important exception to the ubiquity of scalars is wind. Wind, like all vectors, has a magnitude and a direction. The wind is defined as the motion of the air at a particular location, averaged over some period such as two or ten minutes. According to normal convention, the wind is a two-dimensional vector. Strictly speaking, air moves in three dimensions (east-west, north-south, and up-down), and sometimes the velocity vector is taken to be the full three-dimensional wind, but it is more common in normal use to work with the vector horizontal wind and treat the vertical component of air motion as a separate scalar. Thus, an air parcel might be said to have a wind vector of 16 m/s from 130 degrees and also be ascending at 2.4 cm/s. Remember that wind directions are expressed as the direction the wind is coming from, not the direction it is going toward. As with all vectors, the wind can be described in terms of its components as well as a speed and direction. There’s no law that requires it, but you are probably accustomed to the three cartesian coordinates being designated as x , y , and z . By similar inviolate convention, the three components of air motion (toward the x , y , and z directions respectively) are represented as u , v , and w . When the air motion was described as 16 m/s from 130 degrees and ascending at 2.4 cm/s, that 2.4 cm/s was the w ATMO 251 Chapter 3 page 1 of 29
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component of the air velocity, the component of motion in the positive z direction. You could compute the u and v components of motion from the horizontal speed and direction using trigonometry. That technique will be reviewed in Chapter 12. Whether you choose to express the horizontal wind as separate components or as a speed and direction, you need to use two numbers. Thus, as you might imagine, the measurement of the wind generally
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book03advection - Weather Observation and Analysis John...

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