PRESSURE and WIND

Acts in all directions because the atmosphere is a gas. Example: the weight of air on the roof of a typical house is about 2,000,000 pounds, however the roof does not collapse because the same force acts on both the top and bottom of the roof.

In the atmosphere as air is heated it expands. Because it expands it becomes less dense and therefore, rises. This creates an area of low pressure at the surface. As the warm air rises it begins to cool, eventually causing it to sink back to the surface creating an area of high pressure. In general, air flows towards areas of low pressure and away from areas of high pressure.

Measurement of Pressure

1. Mercury barometer - the pressure air exerts on a column of mercury. At sea level this pressure averages 29.92 inches.
2. Aneroid barometer - uses a partial vacuum that expands or contracts as a function of changing atmospheric pressure. Same device as an altimeter in airplanes.

Pressure is usually given in inches of mercury by television weathermen. But the National Weather Service reports pressure in millibars. For those scientists out there one millibar = 10 newtons.

Wind

Caused by pressure gradients. Wind is an attempt to equalize the pressure differential. This differential is the result of unequal heating of different portions of the Earth's surface.

Pressure Gradient - the change in air pressure with distance.(Figure)

Winds start blowing perpendicular to the pressure gradient, but the Coriolis effect deflects the wind to the right in the Northern Hemisphere. Results in a spiral-like effect in which the winds end up blowing parallel to the pressure isobars.

Wind direction is generally given as the direction from which the wind is blowing. Therefore a westerly wind would be one that blows from west to east.

Types of Winds

• Cyclone - low pressure system, wind blows in a counterclockwise direction
• Anticyclone - high pressure system, clockwise wind circulation
• Land-Sea Breeze - (Figure) result from differences in temperatures of the land surface and ocean. During the daytime the land heats up more rapidly than the ocean, the warm air rises and cool air blows in from the ocean to take its place. At night the opposite occurs. The land cools quickly while the ocean remains warmer. The wind direction reverses itself and blows offshore as the warm ocean air rises and the cooler air from the land moves in to take the place of the rising air.

• Monsoons - seasonal land-sea breezes. During the winter cool, dry air flows offshore to the ocean. In the summer cooler, moist air from the ocean flows landward. If it is pushed upward by a mountain range it looses its moisture as it rises and cools.
• Chinook Winds - warm, dry winds that flow downslope. Occurs when a low pressure system is on the lee side of a mountain range. Air rushes downslope toward the low and is heated by compression. Example, Southern California Santa Ana Winds are caused when air moves down through passes from the high desert to the coastline. It is heated by compression due to the pressure increase at sea level relative to the typical 3000 foot altitude of the high desert.

Global Wind Circulation (Figure)

1. At the equator the Earth receives the maximum amount of thermal energy from the Sun. This causes equatorial air to rise, losing its precipitation in the process.

2. Steady surface winds are absent in this area (doldrums), but aloft the winds diverge to flow northward or southward toward the poles. As the wind flows to the north or south the Coriolis effect deflects it. At about 30 degrees N or S the winds are flowing due east.

3. As the air moves northward it cools. By 30 degrees north latitude it is cool enough it begins to sink to the surface creating the subtropical high pressure areas. It warms as it cools and since it lost most of its moisture at the equator it is very dry (note that most desert are located 30 degrees N or S of the equator)(Figure). Since the air movement is vertical there is an absence of surface wind in the "horse latitudes".

4. The descending air splits with some flowing back toward the equator and some continuing poleward. Surface winds blowing toward the equator are deflected until they are blowing from the northeast (in the Northern Hemisphere) to give us the "Trade Winds".

5. The winds flowing toward the poles are also deflected by the Coriolis effect to give us the prevailing "Westerlies" of the middle latitudes.

6. At the poles the cold air sinks and flows toward the lower latitudes. It is deflected to give the polar "Easterlies". These converge with the prevailing Westerlies at the Polar Front at 60 degrees N or S of the equator.

This global circulation model is termed the three-cell model. Keep in mind, this is only a general model and local geography as well as seasonal changes have considerable effect on patterns of wind circulation.

Air Masses

Large volumes of air that have uniform characteristics at any given latitude. Air masses are named for where the originate:

• Arctic/Antarctic (A) - at the poles
• Polar (P) - to the south (or north) of the polar front
• Tropical (T) - form in the region from 30 to 60 degrees north or south of the equator
• Equatorial (E) - form near the equator

Sub-classified as:

• Continental (c) - form over continents (dry)
• Maritime (m) - form over oceans (moist)