1. Air pressure and wind
Chapter 19
Mrs. Schwartz

2. What is Air Pressure?
Air Pressure=the pressure exerted by the weight of air above.
its very important factor in predicting the weather!
Air pressure at sea level is 14.7 lbs per square inch.
Air pressure is exerted in all directions— down, up, and sideways
The air pressure pushing down on an object exactly balances the air pressure pushing up on the object

3. Measuring Air Pressure
Air pressure is measured by using a barometer. 2 TYPES: 1. Aneroid—canister like device, pressure changes on the spring move the arrow

4. Measuring Air Pressure
2. Mercury—liquid device, pressure changes make the liquid go up(high) & go down(low)
As pressure increases the liquid in the barometer rises.
As pressure decreases the liquid lowers in the barometer.

5. What Causes Wind
Wind is the result of horizontal differences in air pressure
Air flows from areas of higher pressure to areas of lower pressure
Wind is nature’s way of balancing out pressure inequalities
The unequal heating of Earth’s surface generates pressure differences
Solar radiation is the ultimate energy source for most wind
If Earth did not rotate, and if there were no friction between moving air and Earth’s surface, air would flow in a straight line from high to low pressure areas
Three factors combine to control wind: pressure differences, the Coriolis effect, and friction

7. 3 factors affecting wind
Pressure differences
Coriolis effect
Friction

8. Pressure differences
Pressure Differences—greater the difference the greater the wind.
Air moves from high to low pressures
Isobars=lines of equal pressure
Closely packed = windy
Widely spaced = not as windy
Pressure Gradient=steep means windy
Pressure Gradient=weak/not steep means Not windy

10. 2. Coriolis Effect—Earth’s rotation affects moving objects --Free moving objects move to the right in the northern hemisphere & to the left in the southern.

11. Friction—only important near Earth.
Friction slows wind & can change wind direction.
JET STREAMS=high up & fast moving!

12. Jet Streams – fast-moving rivers of air that travel between 120 to 240 kilometers per hour in a west-to-east direction
The roughness of the terrain determines the angle of airflow across the isobars; the smoother the terrain, the smaller the angle of airflow
Slower wind speeds caused by friction decrease the Coriolis effect

13. 19.2

14. 19.2 Pressure Centers & Wind
Pressure Centers—help to predict weather
Low (Cyclones) =
air is unstable & rises, cools, condenses, clouds = stormy, cloudy,
air moves counterclockwise in N. hemisphere,
winds move inward
Pressure decreases toward center

15. High (Anti-Cyclones) = air is stable & descends, warms, evaporates,
clear/fair,
air moves clockwise in N. hemisphere,
winds move outward
Pressure increases toward center

16. Global Winds on a Non-Rotating Earth
The underlying cause of wind is the unequal heating of Earth’s surface
The atmosphere balances these differences by acting as a giant heat-transfer system
The system (atmosphere) moves warm air toward high latitudes and cool air toward the equator

17. Global Winds on a Rotating Earth
Trade Winds – two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of subtropical highs

18. Global Winds on a Rotating Earth
Westerlies – dominant west- to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs

19. Global Winds on a Rotating Earth
Polar Easterlies – winds that blow from the polar high toward the subpolar low

20. Global Winds on a Rotating Earth
Polar Front – stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin

21. https://earth. nullschool
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22. 19.3

23. Local Winds
Local Winds—small-scale winds are produced by a locally generated pressure gradient.
Local Winds—are caused by 2 things:
1. by topographic effects (mtns. Etc.)
2. by variations in surface composition
(land or water, etc.)
** Named for where they COME FROM!

24. 4 types of Local winds
1. Sea Breeze—happens during the day, land heats up quicker & causes air to rise allowing cooler air to come from the sea.
2. Land Breeze—happens during the night, land cools of quicker & causes the warmer air over the sea to rise allowing cooler air to come from the land.

25. 3. Valley Breeze—occurs during the day, air rises from valley floors.
4. Mountain Breeze—occurs at night, air sinks from the mountain tops toward the valley floor.

26. Measuring Wind
Wind Direction—wind vane  wind direction is given by the direction the wind is coming FROM, (N,S,E,or W) or sometimes given in degrees!
Wind Speed—anemometer.
Wind Socks—see them at airports measure wind speed & direction!

27. El Nino
At irregular intervals of three to seven years, the warm countercurrents, along the coasts of Peru and Ecuador, become unusually strong and replace normally cold offshore waters with warm equatorial waters – El Niño
The warm waters block the nutrients from reaching the surface waters, causing many fish to die off, and greatly affects the fishing industries of Peru and Ecuador
Some inland areas that are normally arid get an abnormal amount of rain, increasing their crop production
These episodes mostly effect the eastern tropical Pacific, but is a part of the global circulation and affects the weather all over the world

29. La Nina
The opposite of El Niño is an atmospheric phenomenon known as La Niña
Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns
A typical La Niña winter blows colder than normal air over the Pacific Northwest (with more precipitation) and the Northern Great Plains
It warms much of the rest of the United States

31. Global Distribution of Precipitation
Areas dominated by the convergent Trade winds (equatorial low) have mainly rain forests and abundant precipitation
Areas dominated by the subtropical high-pressure cells are regions of extensive deserts
The interiors of large land masses commonly experience decreased precipitation
You will be able to explain much about global precipitation through your knowledge of global winds and pressure systems

32. Global Distribution of Preciptiation