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Lecture 8

Lecture 8 notes.pdf

5 Pages
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Department
Environmental Science
Course Code
EESA09H3
Professor
Tanzina Mohsin

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EESA09H WIND Lecture 8 Notes Outline of this lecture • Part I Measuring Wind • Instrumentation – Anemometers • Measures – Wind Chill • Recording Wind – Anemograph – Wind rose • Sound of wind • Part II Wind Strength Scales • Beaufort • Fujita • Saffir-Simpson • Part III Special Focus: Mach Speeds and Sonic Booms Part 1 Measuring Wind 1.1 Instrumentation 1.1.1 Anemometers Anemometers are instruments used to measure wind speed. Weather vanes or wind vanes are used to measure wind direction. There are at least seven types of anemometers. These anemometers are primarily used to measure wind at or near the surface of the earth. Higher level winds are measured using radiosonde (weather balloons) or deduced from pressure distribution. Upper level winds are geostrophic and thus the knowledge or the pressure gradient enables us to calculate the wind speed and direction. a) Deflection The first anemometer was invented by Leon Battista Alberti in Italy in 1450. It was a deflection anemometer. He linked wind speed to the deflection of a horizontal plate. He hooked the plate to a spring and measured the compression of the spring. Leonardo da Vinci created a similar anemometer a few decades later. A century later, Robert Hooke of England created the widely used Hooke anemometer. b) Pressure In 1774 James Lind developed a tube anemometer. It was a U tube in which wind blows into and causes liquid to move up and down in a tube. It measures the force of the wind relative to the force of gravity; the higher the wind speed the greater amount of displaced liquid. It works on the same principle (hydrostatic balance) as the barometer. A chief advantage of this device was its low maintenance requirements. A commonly used pressure anemometer was invented in 1892 by William Henry Dines of England. c) Cup The cup anemometer was inoented in 1846 by Thomas R. Robinson of England. It consisted of four cups (90 apart). The cup speed (rotation rate) was approximately 1/3 of the wind speed. This fraction varied with cup shape and arm length. An advantage of the cup anemometer is that it doesn’t need to be reoriented as the wind direction changes. The three cup anemometer was created in 1926 by Canadian John Patterson. It proved to be more accurate, usually within 3% up to 100 km/h. d) Wind mill The windmill anemometer uses a propeller rather than a cup to gauge the wind speed. An aerovane is used to insure the anemometer is pointing into the wind. It is commonly used today. e) Thermoelectric In a thermoelectric anemometer a wire is heated to above the ambient temperature. The rate of cooling of this wire is proportional to the wind speed. Higher speed winds cool objects more quickly. The instrument has the advantage of no moving parts. f) Laser Radar The laser radar anemometer uses the Doppler principle to measure wind speed. The Doppler principle explains how sound waves from object moving towards us have higher pitch (higher frequency) than sound waves from objects moving away from us (lower frequency). This explains the apparent change in pitch of train whistles and emergency vehicle sirens as the vehicles travel past. Laser radar anemometers use the same physics is employed to measure wind speed. A fraction of the laser light reflects off of nearby air molecules; some of this is returned to the anemometer (backscatter). The frequency of the backscattered light indicates the speed of the air molecules (and therefore the wind). g) Sonic Sonic anemometers were invented by Dr. Andreas Pflitsch, a geologist, in 1994. They measure wind speed by how sound waves are modified by moving air. 1.1.2 Measuring Upper Level winds Upper level winds are determined through measurements from radiosondes (weather balloons). Radiosondes are launched from meteorological stations around the world simultaneously twice each day, at 0:00 and 12:00 UTC (Universal Coordinated Time). This provides what meteorologists call a synoptic observation of the atmosphere (synaptic in this sense meaning simultaneous). Radiosonde information can be supplemented by dropsondes, which are instrument packages dropped from airplanes. Radiosondes travel up through the troposphere and much of the stratosphere, generally bursting at around 30 km in altitude. They are often capable of measuring the wind speed directly, but more importantly they record changes in pressure at specific altitudes. Knowledge of the horizontal pressure gradients between radiosonde locations allows meteorologists to calculate the strength and direction of upper level winds. Upper level winds are geostrophic (refer to Lecture 2); the effect of friction on these winds is negligible. This means that knowledge of the pressure gradient together with latitude (which is important for the Coriolis force) is sufficient information to calculate the strength of the wind; and it always flows 90 to the right of the pressure gradient force. 1.2 Wind Chill How cold people feel depends on both temperature and wind speed. The air is colder than a person’s body, and so it carries away head from conduction. In warm weather it is common to get relief from the heat by using a fan. How cold we feel in cold weather depends on the same principle and is called wind chill. Wind chill is a measure of how cold it feels and is in essence a measure of heat loss. It is a combination of temperature and wind speed and is often reported as an equivalent temperature. The temperature does not decrease because of the wind, but rather the wind increases the heat loss. Thus the equivalent temperature is the temperature at which we would experience the sa
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