Pressure is exerted by the weight of the air above.
Sea level pressure is 14.7 pounds per square inch.
Air pressure is exerted in all directions
Air pressure is measured with a barometer.
2. Factors affecting Wind
Wind is the result of differences in air pressure.
Wind blows from high to low pressure.
The unequal heating of the Earth’s surface creates the pressure differences.
The greater the pressure difference, the greater the wind.
Closely spaced isobars indicate a steep pressure gradient and high winds.
Widely spaced isobars indicate a weak pressure gradient and light winds.
Coriolis Effect – all free moving objects are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Friction with the Earth’s surface causes the wind to slow and change direction
3. Lows or Cyclones
Pressure decreases from the outer isobars to the center
Winds blow inward and counterclockwise around a low
Surface convergence causes the air to rise at the center.
Rising air is associated with cloud formation and precipitation.
4. Highs or anticyclones
Pressure increases from the outer isobars to the center
Winds blow outward and clockwise around a high
Surface divergence causes the air to sink at the center.
Sinking air produces clear skies.
5. Global Winds
In tropical regions, more solar radiation is received that radiated back into space. The warm, less dense air rises.
In a non-rotating Earth, one convection cell would be produced. However, a rotating Earth produces six convection cells, three north and three south of the Equator.
Large landmasses affect global wind patterns. Land heats up more rapidly than water, and cools down more quickly. In summer time, the wind pattern would be on-shore and in winter the wind pattern would be off-shore.
In summer, the on-shore wind brings lots of rain to Southeast Asia called the monsoons.
6. Land and Sea Breezes
In coastal areas during the summer, the land is heated more than the water during the daylight areas. The air above the land area heats, expands, and rises creating a lower pressure than over the ocean. A breeze develops from the ocean to the land to replace the rising air.
At nighttime, the land cools off more quickly than the ocean water. The air over the land is cooler and more dense than the air over the warmer water. The breeze flows from the land to the ocean.
7. Valley and Mountain Breezes
During the day, the slopes of the mountains are heated more than the valleys. The heated air on the slopes is warmer, expands, and rises up along the valley walls.
After sunset, the pattern reverses. Rapid cooling causes the slopes to cool and become more dense. Cool dense air sinks down into the valley.
8. Prevailing Winds
In the United States, the Westerlies move weather from west to east across the continent.
9. Wind Speed
Wind speed is measured with an anemometer.
10. El Nino and the Southern Oscillation
At times, the strong westward trade winds weaken or reverse direction. This allows a warm current of water to move east.
El Nino is a warm current that flows southward along the coasts of Ecuador and Peru.
At regular intervals of time of three to seven years, these warm counter currents become unusually strong and replace the normally cold offshore waters with warm equatorial waters.
El Nino causes abnormal weather patters. This brought more rain to the East and winter temperatures were higher in the West.
11. La Nina
Occurs when surface temperatures in the Eastern Pacific are colder than average.
This blows colder than normal air over the pacific, Northwest, and the northern Great Plains. At the same time, it warms much of the rest of the United States. The northwest also experiences greater precipitation. Hurricane damage is much greater during La Nina years.
U.S. Pattern: La Niña events are characterized by drier than normal conditions across the southern U.S. stretching from California to Florida and up into the mid Great Plains and Midwest. And La Niña events are characterized by colder than normal temperatures from California to Washington State and across the northern tier of state to Minnesota and Wisconsin.
12. Global Distribution of Precipitation (page 548)
Tropical lows are the rainiest regions on Earth.
Sub-tropical high pressure cells are regions of desert.
Interiors of large continents have decreased precipitation.