Lesson 7.05 – Pressure Centers & Winds

Standard:  ES5.a Students know how differential heating of Earth results in circulation patterns in the atmosphere and oceans that globally distribute the heat.

ES5.b Students know the relationship between the rotation of Earth and the circular motions of ocean currents and air in pressure centers.

INTRODUCTION

Pressure centers are among the most common features on any weather map. By knowing just a few basic facts about centers of high and low pressure, you can increase your understanding of present and forthcoming weather. You can make some weather generalizations based on pressure centers. For example, centers of low pressure are frequently associated with cloudy conditions and precipitation. By contrast, clear skies and fair weather may be expected when an area is under the influence of high pressure, as shown in Figure 6.

Figure 6 These sunbathers at Cape Henlopen, Delaware, are enjoying weather associated with a high-pressure center.

INSTRUCTION

Highs and Lows

Lows, or cyclones (kyklon = moving in a circle) are centers of low pressure. Highs, or anticyclones, are centers of high pressure. In cyclones, the pressure decreases from the outer isobars toward the center. In anticyclones, just the opposite is the case—the values of the isobars increase from the outside toward the center.

Cyclonic and Anticyclonic Winds

You learned that the two most significant factors that affect wind are the pressure gradient and the Coriolis effect. Winds move from higher pressure to lower pressure and are deflected to the right or left by Earth’s rotation. When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, winds blow counterclockwise around a low. Around a high, they blow clockwise. Notice the wind directions in Figure 7.

Figure 7 This map shows cyclonic and anticyclonic winds in the Northern Hemisphere.

In the Southern Hemisphere, the Coriolis effect deflects the winds to the left. Therefore, winds around a low move clockwise. Winds around a high move counterclockwise. In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow of air outward around an anticyclone.

Weather and Air Pressure

Rising air is associated with cloud formation and precipitation, whereas sinking air produces clear skies.

Imagine a surface low-pressure system where the air is spiraling inward. Here the net inward movement of air causes the area occupied by the air mass to shrink—a process called horizontal convergence. Whenever air converges (or comes together) horizontally, it must increase in height to allow for the decreased area it now occupies. This increase in height produces a taller and heavier air column. A surface low can exist only as long as the column of air above it exerts less pressure than does the air in surrounding regions. This seems to be a paradox—a low-pressure center causes a net accumulation of air, which increases its pressure.

In order for a surface low to exist for very long, converging air at the surface must be balanced by outflows aloft. For example, surface convergence could be maintained if divergence, or the spreading out of air, occurred above the low at a rate equal to the inflow below. Figure 8 shows the relationship between surface convergence (inflow) and divergence (outflow) needed to maintain a low-pressure center. Surface convergence around a cyclone causes a net upward movement. Because rising air often results in cloud formation and precipitation, a low-pressure center is generally related to unstable conditions and stormy weather.

Figure 8 Air spreads out, or diverges, above surface cyclones, and comes together, or converges, above surface anticyclones. Applying Concepts Why is fair weather associated with a high?

Like cyclones, anticyclones also must be maintained from above. Outflow near the surface is accompanied by convergence in the air above and a general sinking of the air column, as shown in Figure 8.

Weather Forecasting

Now you can see why weather reports emphasize the locations and possible paths of cyclones and anticyclones. The villain in these reports is always the low-pressure center, which can produce bad weather in any season. Lows move in roughly a west-to-east direction across the United States, and they require a few days, and sometimes more than a week, for the journey. Their paths can be somewhat unpredictable, making accurate estimation of their movement difficult. Because surface conditions are linked to the conditions of the air above, it is important to understand total atmospheric circulation.

Global Winds

The underlying cause of wind is the unequal heating of Earth’s surface. In tropical regions, more solar radiation is received than is radiated back to space. In regions near the poles the opposite is true—less solar energy is received than is lost. The atmosphere balances these differences by acting as a giant heat-transfer system. This system moves warm air toward high latitudes and cool air toward the equator. On a smaller scale, but for the same reason, ocean currents also contribute to this global heat transfer. Global circulation is very complex, but you can begin to understand it by first thinking about circulation that would occur on a non-rotating Earth.

Non-Rotating Earth Model

On a hypothetical non-rotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form, as shown in Figure 9. The heated air at the equator would rise until it reached the tropopause—the boundary between the troposphere and the stratosphere. The tropopause, acting like a lid, would deflect this air toward the poles. Eventually, the upper-level airflow would reach the poles, sink, spread out in all directions at the surface, and move back toward the equator. Once at the equator, it would be reheated and begin its journey over again. This hypothetical circulation system has upper-level air flowing toward the pole and surface air flowing toward the equator.

Figure 9 Circulation on a Non-Rotating Earth A simple convection system is produced by unequal heating of the atmosphere. Relating Cause And Effect Why would air sink after reaching the poles?

Rotating Earth Model

If the effect of rotation were added to the global circulation model, the two-cell convection system would break down into smaller cells. Figure 10 illustrates the three pairs of cells that would carry on the task of redistributing heat on Earth. The polar and tropical cells retain the characteristics of the thermally generated convection described earlier. The nature of circulation at the middle latitudes, however, is more complex.

Near the equator, rising air produces a pressure zone known as the equatorial low—a region characterized by abundant precipitation. As shown in Figure 10, the upper-level flow from the equatorial low reaches 20 to 30 degrees, north or south latitude, and then sinks back toward the surface. This sinking of air and its associated heating due to compression produce hot, arid conditions. The center of this zone of sinking dry air is the subtropical high, which encircles the globe near 30 degrees north and south latitude. The great deserts of Australia, Arabia, and the Sahara in North Africa exist because of the stable dry conditions associated with the subtropical highs.

Figure 10 Circulation on a Rotating Earth This model of global air circulation proposes three pairs of cells. Interpreting Diagrams Describe the patterns of air circulation at the equatorial and subpolar lows.

At the surface, airflow moves outward from the center of the subtropical high. Some of the air travels toward the equator and is deflected by the Coriolis effect, producing the trade winds. Trade winds are two belts of winds that blow almost constantly from easterly directions. The trade winds are located between the subtropical highs and the equator. The remainder of the air travels toward the poles and is deflected, generating the prevailing westerlies of the middle latitudes. The westerlies make up the dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs. As the westerlies move toward the poles, they encounter the cool polar easterlies in the region of the subpolar low. The polar easterlies are winds that blow from the polar high toward the subpolar low. These winds are not constant winds like the trade winds. In the polar region, cold polar air sinks and spreads toward the equator. The interaction of these warm and cool air masses produces the stormy belt known as the polar front.

This simplified global circulation is dominated by four pressure zones. The subtropical and polar highs are areas of dry subsiding (sinking) air that flows outward at the surface, producing the prevailing winds. The low-pressure zones of the equatorial and subpolar regions are associated with inward and upward airflow accompanied by clouds and precipitation.

Influence of Continents

The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. Here the ocean is uninterrupted by landmasses. At other latitudes, particularly in the Northern Hemisphere where landmasses break up the ocean surface, large seasonal temperature differences disrupt the pressure pattern. Large landmasses, particularly Asia, become cold in the winter when a seasonal high-pressure system develops. From this high-pressure system, surface airflow is directed off the land. In the summer, landmasses are heated and develop low-pressure cells, which permit air to flow onto the land as shown in Figure 11. These seasonal changes in wind direction are known as the monsoons. During warm months, areas such as India experience a flow of warm, water-laden air from the Indian Ocean, which produces the rainy summer monsoon. The winter monsoon is dominated by dry continental air. A similar situation exists to a lesser extent over North America.

Figure 11 Average Surface Pressure and Associated Global Circulation for July. The ITCZ line stands for the Intertropical Convergence Zone.

PRACTICE

1. Take notes on the above information. Go to pages 537-542 of the textbook, Section 19.1 Pressure Centers and Winds, for more information. Use the Reading Focus to guide you through the material.
2. Click here (http://videos.howstuffworks.com/hsw/23247-wonders-of-weather-wind-and-weather-fronts-video.htm) to watch a video about the jet stream and winds.

ASSESSMENT

1. Turn in your notes.
2. Take the 7-05 Quiz