THE NATURAL ENVIRONMENT
Geography 101 Online
The forces discussed in the previous section act on air simultaneously to produce the horizontal resultant wind direction as shown in the diagram below for the northern hemisphere. The pressure gradient force (PGF) sets air in motion from High toward low pressure. Coriolis deflects that moving air to the right of the direction of motion. Imagine standing on the High center, facing the Low center. Now hold out your right hand, which will point in the direction of Coriolis deflection. In the upper atmosphere, PGF and Coriolis combine to cause wind to blow parallel to the isobars, as shown in the middle panel below.
At the surface however, friction counteracts the Coriolis force somewhat making the resultant wind (the actual wind direction) flow slightly toward the Low and away from the High as shown in the right-hand panel. With this simple relationship, you can fairly accurately map the surface wind pattern from an isobaric map. Just remember: the wind flows almost parallel to the isobars, slightly toward lower pressure.
Look at the resultant wind diagram above (right-hand panel). Imagine the wind blowing in a circular path around the High pressure center: it would flow in a clockwise direction. Similarly, if you imagine the wind blowing in a circle around the Low center, it would flow in a counterclockwise direction. That is true of the Northern Hemisphere, and, of course, the opposite is true in the Southern Hemisphere.
For example, look at a portion of the Pacific weather chart from the previous section, Forces. Blue arrows show wind direction (arrowheads have been added for clarity). The wind blows clockwise and nearly parallel to the isobars, but at a small angle away from the High pressure center, exactly as shown in the resultant wind diagram. This is typical Hawaiian weather: High pressure to the north causing winds from the east-northeast in the vicinity of Hawai'i. Notice that the winds form a circular pattern around the High pressure center; winds to the north of the High blow in the opposite direction of winds to the south.
It is important to understand the basic relationship between air pressure and the wind direction. Practice guessing whether the winds would blow clockwise or counterclockwise for the scenarios given in the Wind Direction Simulator. Just click on High and Low for the Northern and Southern Hemisphere. Note whether the arrows are slightly TOWARD or AWAY from the pressure center. Also note that the animations include an extra wind arrow near the center of the Low, but not the High. This reflects the fact that wind near the center of Highs is often calm or nonexistent, while the center of Lows may experience strong winds.
An old adage for mariners, called Buys-Ballot's law, states that, "In the northern hemisphere, if you stand with your back to the wind; the low pressure area will be on your left." This helped sailors avoid low pressure storms like hurricanes. Can you see why this law is always valid? How would it be stated for the southern hemisphere and for High pressure systems?
Vertical Air Motion
Another important aspect of the airflow around High and Low pressure centers is the vertical movement of the air (usually at much lower speed than horizontal wind). At Low pressure centers, air rises. At High pressure centers, air sinks toward the surface.
At surface Highs (also called ridges), wind spirals slightly outward, away from the center. This outward flowing air must be replaced by air from above, causing descending, or sinking, motion near High centers.
The opposite is true of Low pressure areas (also called troughs). Wind spirals slightly toward the center at the surface and then vents upward, producing ascending, or rising, motion.
The rising or sinking motion is inter-linked with the underlying cause of the pressure system itself, often surface temperature. Over warm surface areas, the air heats, expands, and rises. This can often spawn a large Low pressure trough. The opposite is true of Highs. Over cold surfaces the air cools, contracts, and draws overlying air downward to form a High pressure ridge. Over land areas this relationship is especially obvious. Midlatitude continental areas, such as central Asia and North America, become very cold in winter. This can raise air pressure to 1050 mb or more, a very strong High, which dominates the winter weather of these areas. The opposite is true in summer when surface warming causes air to expand and form Low pressure centers. Over Asia, this seasonal change in surface air pressure drives the monsoon wind system.
Surface temperature does not always determine the overlying air pressure, however. The Hawaiian High pressure system for example, commonly present northeast of the Islands, is caused by the overall circulation of the atmosphere, which we will discuss in the next section.
The diagrams above summarize vertical motion of air over surface pressure systems and typical sky conditions for each.
Look at the Low pressure center. Air spirals in toward the center and rises. As air rises, it cools, clouds form, and rain is possible. This diagram generally describes all low pressure systems, including thunderstorms, hurricanes, and midlatitude cyclones. This relationship is discussed further in Chapter 5.
The opposite occurs at High pressure ridges, where air sinks toward the surface and flows outward from the center. When air sinks, few clouds form and mostly clear skies prevail. The Hawaiian Islands lie in an area usually dominated by High pressure, and thus skies are generally clear with only scattered clouds. In the next section, we look at overall wind and pressure patterns for the entire Earth.
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