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EESA09H3 (185)

Lecture. 8

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University of Toronto Scarborough
Environmental Science
Tanzina Mohsin

Lecture #8: Measuring Wind Musical Weather Vane Song  Tannis Simmons and Lewis Melville Anemometers: – Deflection anemometers: ○ First anemometer ○ 1450 Leon Battista Alberti in Italy ○ Horizontal plate pushes by wind ○ connected to a spring ○ compression of the spring proportional to wind speed ○ Not good for light winds because you need strong winds to push the plate ○ Hooke’s anemometer ○ Da Vinci deflection Anemometer – Pressure anemometers: ○ James Lind in 1774 developed a tube anemometer ○ U tube  wind blows into tube and pushes liquid in the tube ○ Dines anemometer 1892 (William Henry Dines) – Cup anemometer: ○ Thomas R. Robinson in 1846 ○ Found in most weather stations ○ 4 cups ○ Cup speed is 1/3 of the wind speed ○ UTSC’s 3 cup anemometer:  1926 by Canadian John Patterson  More consistent behaviour  < 3% error up to 100 km/h – Windmill anemometer: ○ Uses a propeller to measure wind speed ○ Aerovane is used to insure anemometer is pointing into the wind – Thermoelectric anemometer: ○ A wire is heated to above the ambient temperature ○ Rate of cooling of wire is proportional to wind speed ○ In ancient time, hair was used – Laser Radar Anemometer: ○ Same principle as Doppler radar  Shift in freq. e.g., train whistle at railway crossings, sirens on emergency vehicles • Train whistle  when the train is coming towards the platform, the sound increases and when the train starts to move further from the you, the sound decreases = doppler radar ○ Laser beams are backscattered by moving air molecules  Air molecules have a role in how strong or weak the sound is heard ○ Doppler shifted frequency of the backscatter indicates wind speed – Sonic anemometer: ○ Invented in 1994 by Dr. Andreas Pflitsch (geologist) ○ Measures how sound waves are modified by the wind (deduces wind speed)  Changes in speed of sound by wind Upper Level Winds: – Radiosonde drift: ○ weather balloon ○ Twice daily – universal coordinated time ○ Temp. pressure, humidity, wind ○ Generally “pop” around 30 km – halfway through the stratosphere ○ 34 in Canada, 1500 world wide – Dropsonde: ○ Instruments dropped from an airplane ○ Same mechanism as radiosonde, weather balloon – Aircraft – Deduced upper level wind speeds from pressure structure ○ Geostrophic winds – Upper wind calculations: ○ Upper level winds are geostrophic winds (above 1 km from the surface) ○ Knowledge of pressure distribution (gradients) enable calculation of winds from geostrophic balance ○ Pressure distribution can be obtained from radiosonde data – Geostrophic wind: ○ Pressure Gradient force pulls from high to low pressure ○ Coriolis force pulls moving objects to the right in northern hemisphere, left in SH ○ Strength of CF is proportional to the speed of an object ○ Friction is negligible in upper level winds ○ Balance between PGF and CF Wind Chill: – Measured by combination of temp. and wind speed – Higher wind speeds feels colder – grater heat loss – **Temp. does not decrease because of the wind, but rather the wind increases the heat loss – Change the skin’s epiclimate** ○ Epiclimate is a very small scale climate surrounding an object ○ For the skin it forms a small insulating layer  Heat transfer by molecular diffusion (slow process) ○ Wind disrupts this epiclimate and removes protective layer allowing heat to be lost at a much faster rate  Wind is changing the temperature of our ski
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