Understanding Wind

Review: What is air pressure? Air pressure: the force of the weight of air on a surface – exerted in all directions

How is air pressure measured? Barometer: a device used to measure air pressure baro = _____ meter = _____ – when air pressure ↑, mercury in the tube ↑ (and vice versa) – pressure is measured in millibars (mb) (Sea-level pressure = mb) Mercury Aneroid How a barometer works

High v. low air pressure High air pressure = more downward force, air sinks, clears the air of clouds! Low air pressure = less downward force, air rises, causes clouds, rain, bad weather High pressure sky Low pressure sky

What causes wind? The sun is the ultimate source for most wind, creating temperature and pressure differences Temperature differences:Pressure differences:

Mr. Fetch’s Earth Science Classroom Go with the (air) flow Air flows from areas of higher pressure to areas of lower pressure. This causes air to move, creating WIND ! YouTube: Indoor Skydiving

What controls wind movement? Three factors combine to control wind: – pressure differences (aka. pressure gradient) – the Coriolis effect – friction

How do air pressure differences affect wind speed? Wind is caused by differences in air pressure The greater the difference in pressure, the greater the wind speed (= stronger wind)

What are isobars? Isobars: lines on a map that connect places of equal air pressure

What is a pressure gradient? Pressure gradient: the spacing between isobars – (remember that wind moves from higher pressure to lower pressure) – Closely spaced isobars = steep pressure gradient and strong winds – Widely spaced isobars = weak pressure gradient and light winds

What is the Coriolis effect? The Coriolis effect describes how Earth’s rotation deflects all free-moving objects or fluids, including wind and water Air does not move in a straight line because of Earth’s rotation. Coriolis Effect

How does Coriolis effect change wind? Changes only wind direction, not wind speed – stronger wind = greater deflection – strongest at poles, weakens toward the equator

YouTube: Coriolis

Coriolis effect Northern Hemisphere Southern Hemisphere

How does friction affect wind? “Bumpy” surfaces such as hills and mountains increase friction, which can slow wind and change its direction – If friction ↑, wind speed ↓ – If friction ↓, wind speed ↑

Global Winds 60°S 30°S 0° 30°N 60°N Prevailing Westerlies - From 30°N to 60°N - Deflected to right Easterly Trade Winds - From 30°N to O° - Deflected to right Polar Easterlies - From 90°N to 60°N - Deflected to right Easterly Trade Winds - From 30°S to 0° - Deflected to left Prevailing Westerlies - From 30°S to 60°S - Deflected to left Polar Easterlies - From 90°S to 60°S - Deflected to left Doldrums Global winds move in 6 giant convection currents called cells (e.g., trade winds)

What is a cyclone? Cyclone = a center of low pressure Pressure decreases from the outer isobars towards the center Moves counterclockwise in Northern Hemisphere What type of weather would you expect?

What is an anticyclone? Anticyclone = center of high pressure pressure increases from the outer isobars towards the center Clockwise in Northern Hemisphere What type of weather would you expect?

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