19.2 Pressure Centers & Wind

Highs and Lows

• Cyclones-Low pressure regions where pressure decreases to the center

• Anticyclones- High pressure regions where pressure increases to the center

Cyclonic and Anti-cyclonic Winds: Effects of the Pressure Gradient

and the Coriolis Effect

• Northern Hemisphere: Winds blow counter clockwise around a low (cyclone) and clockwise around a high (anticyclone)

• Friction causes a net flow of air inward toward a low and outward away from a high

Weather and Air Pressure

• Rising Air (rises over a low pressure zone and raises relative humidity) = Cloud formation & Precipitation

• Sinking Air (sinks over a high pressure zone and lowers relative humidity) = Clear Skies

Pair share: What happens to the rising air that increases relative humidity?

Global Winds

• Unequal heating of the Earth’s surface causes wind

• The atmosphere balances this unequal heating by moving warm air toward the poles and cool air toward the equator

AND ……

Effects of a Rotating Earth on Winds: Coriolis Effect

• The space between the Equator and Pole now divides into 3 wind “cells” instead of one, with wind directions alternating

• This creates the “Westerlies” (global winds that travel from the west (for the US this is the prevailing wind direction, carrying our weather) and the Trade winds which travel from the east across Central America

Effects of Land and Water

• Land adds further differences: heats up and cools down faster than water

19.3 Regional Wind Systems

Local Winds

• Caused by topographical effects or the surface being either water or land

• Land and Sea Breezes - Land heats up and hot air rises, moving air in from the ocean during the day (sea breeze) and moving air out to the ocean at night (land breeze)

Valley and Mountain Breezes

• Valley Breeze : daytime heating causes less dense, hot air to rise from the valley floor

• Mountain Breeze : nighttime cooling causes more dense, cold air to sink from the mountain toward the valley floor

El Nino• Every 3 to 7 years normal cold offshore South

American eastern Pacific ocean currents yield to warm currents from the equator, causing poor fishing and above average rainfall in other regions.

• Pair share: Compare the water off the cost of South America in the top picture to the bottom. How are they different?

Normal year

El Nino year

La Nina

Atmospheric conditions that occur when surface temperatures of the eastern Pacific are colder than normal, resulting in colder than normal air over the northern US states and warmer than normal air in the middle and southern states

Global Precipitation Pattern

• Pair share: What 4 regions of the Earth get the most rain?

White Board Review

1. What type of weather is rising air associated with?

2. How does the atmosphere balance unequal heating of the Earth’s surface?

3. What type of local winds can form in the Grand Canyon at night?

4. From which direction does a SE wind blow?