1. 19.1

2. Ch 19 Air Pressure & Wind
AIR PRESSURE = the pressure exerted by the weight of the atmosphere above
- equal in all directions
- measured by a barometer, units are millibars
- at sea level, the air pressure is~1kg/cm2
- decreases as you increase altitude
- change in mercury level of a barometer = change in weather

3. Wind Wind is the result of horizontal differences in air pressure
air moves from areas of high pressure
to low pressure
causing wind
These different areas of high and low pressure are caused by:
unequal heating (solar radiation)
of the Earth’s surface

4. Wind speed is measured by an anemometer
Wind direction is measured by a wind vane

5. WIND DIRECTION – CONTROLLED BY 3 THINGS-
Pressure differences
Coriolis Effect
Friction

6. WIND DIRECTION- PRESSURE DIFFERENCES
the greater the difference in air pressure between 2 regions, the greater the wind speed
Specific weather maps indicate pressure differences across a region (Pressure Gradient)
Isobars are lines that connect places of equal air pressure

7. PRESSURE DIFFERENCES cont’d
Closely spaced isobars indicate high wind speeds
Widely spaced isobars indicate light wind speeds

8. WIND DIRECTION – CORIOLIS EFFECT
- Coriolis Effect describes how Earth’s rotation affects moving objects & wind
All free-moving objects are deflected to the RIGHT of their path of motion in the Northern hemisphere and to the LEFT in the Southern hemisphere
affects only wind direction
the stronger the wind speed, the greater the deflection
WIND DIRECTION – CORIOLIS EFFECT

9. Wind Direction Coriolis Effect cont’d

10. Wind Direction - cont’d
Friction
as air travels over rough terrain, the wind slows down
changes wind direction
only important within a few kilometers of the surface
High above friction zone = Jet Stream
= fast moving streams of air that travel between 120 to 240 km/hr
flow West to East in
Northern Hemisphere

11. 19.2

12. Pressure Centers Most weather predictions are based
on locations of high & low air
pressure zones

13. Pressure Centers- LOW
- Low Pressure Zones or Cyclones = cloudy conditions and
precipitation
Pressure decreases toward the center of a low pressure zone
Wind blows inward and counterclockwise around a low
- Because wind blows inward, air must rise to make room for incoming wind = clouds form

14. Pressure Centers cont’d
- High Pressure Zone or Anticyclones = clear skies and fair weather
Pressures increases toward the center
Winds blow away and counterclockwise from a high pressure center
Air sinks in a high pressure zone = no clouds form

15. Global Wind Patterns
Because more solar radiation is received at the equator than at the poles, global wind patterns act to balance heat on Earth = warm air blows toward high latitudes and cool air flows toward equator
Because the Earth rotates, above system is more complex = 3 major wind cells in each hemisphere

17. GLOBAL WIND PATTERNS 4 Major Wind Cells Equatorial Low
Subtropical High
Subpolar Low
Polar High

18. Global Wind Patterns cont’d
EQUATORIAL LOW- air is heated at equator, it rises (producing abundant moisture) until it reaches the tropopause, then air is deflected toward the poles

19. Global Wind Patterns cont’d
SUBTROPICCAL HIGH – the air that was raised at the equator & deflected descends at N & S latitude (subtropics), the dry air cools (at altitude)& sinks
-creating most of the world’s deserts

20. Global Wind Patterns Cont…
SUBTROPICAL HIGH - some air flows toward the tropics (equator)via trade winds BUT some also travels toward the poles (again its deflected by Coriolis Effect)

21. Global Wind Patterns… cont’d
- The wind traveling toward the poles generate the Prevailing Westerlies of the middle latitudes (Our wind in Indiana almost always blows from West to East because of this wind pattern)

22. Global Wind Patterns… cont’d
- POLAR HIGH 90°= cold polar air sinks and flows along the surface toward 60N&S latitude (Subpolar Low), and is again deflected by Coriolis Effect
- creates the Polar Easterlies, that bring cold polar air to Alaska & Canada

23. Global Wind Patterns… cont’d
- SUBPOLAR LOW 60
- 60 latitude is VERY stormy area because of rising air & the collision of warm (prevailing westerlies) and cold air masses (polar easterlies) creating an area known as the POLAR FRONT

24. LOCAL WINDS Local Wind Patterns Two types of local winds:
Land & Sea Breezes
Valley & Mountain Breezes
Local Wind Patterns
local winds are caused either by topographic effects or by variations in surface composition – land and water –in the immediate area

25. SEA BREEZE – develop shortly before noon & reach greatest intensity during mid to later afternoon
Form due to warm air rising over land (lower pressure) causing cooler air over the water (higher pressure) to move in toward the land
Results in moderating temperature on land

26. LAND BREEZE – develop in the evening
Form due to land cooling off more rapidly than water. Cool air over the land sinks (higher pressure) causing air to flow toward warmer waer (lower pressure)
Can occur near oceans or along the shores of large lakes, such as the Great Lakes

27. Local Wind Patterns
Valley & mountain breezes – similar to land & sea breezes but occur in mountainous regions
During the day – air along the slopes of the mountain is heated more intensely than the valley floor
Causing warmer air on slopes to rise creating valley breeze

28. Results in cumulus clouds developing along mountain peaks
After sunset pattern is reversed
Slopes cool more rapidly causing the more dense air to move down into the valley creating a mountain breeze

29. Monsoon
Continents break up ocean surface, so global wind belts are not continuous = large seasonal temperature differences
Large continents like Asia get very cold during winter and create a high pressure zone = air flow goes from continent toward ocean
Winter is extremely dry and cold

30. During summer Asia gets very hot, creates a low pressure zone = air moves from ocean toward continent
This complete seasonal wind reversal is called a Monsoon
summer brings very wet and hot weather from Indian Ocean
Winter is extremely dry and cold

31. El Nino = unusually warm ocean currents (particularly the central and eastern Pacific Ocean) that creates abnormal weather patterns - Reversal of normally cold currents & their direction - causes very wet winters in Gulf Coast & very warm winters in western US & Canada

32. La Nina = opposite of El Nino
- occurs when surface ocean temperatures are significantly colder than average, creates abnormal weather patterns
- Very cold winters to Pacific Northwest & Great Plains, warmer winters for rest of US
- Increases hurricane frequency