Among the most common features on a weather map are areas designated as pressure centers. Cyclones, or lows, are centers of low pressure, and anticyclones, or highs, are high-pressure centers. As the following figure (▼) illustrates, the pressure decreases from the outer isobars toward the center in a cyclone. In an anticyclone, just the opposite is the case: The values of the isobars increase from the outside toward the center. Knowing just a few basic facts about centers of high and low pressure greatly increases your understanding of current and forthcoming weather.

Cyclonic and Anticyclonic Winds

In the preceding section, you learned that the two most significant factors that affect wind are the pressure gradient force and the Coriolis effect. Winds move from higher pressure toward lower pressure and are deflected to the right or left by Earth’s rotation. When these controls of airflow are applied to pressure centers in the Northern Hemisphere, the result is that winds blow inward and counterclockwise around a low (▼A). Around a high, they blow outward and clockwise (refer to the left side of the map in ▲).

In the Southern Hemisphere, the Coriolis effect deflects the winds to the left; therefore, winds around a low blow clockwise, and winds around a high move counterclockwise (▲B). In either hemisphere, friction causes a net inflow (convergence) around a cyclone and a net outflow (divergence) around an anticyclone.

Weather Generalizations About Highs and Lows

Rising air is associated with cloud formation and precipitation, whereas subsiding air produces clear skies. In this section, we will discuss how the movement of air can itself create pressure change and generate winds. After doing so, we will examine the relationship between horizontal and vertical airflows and the effects of these flows on weather.

Let us first consider a surface low-pressure system where the air is spiraling inward. The net inward transport of air causes a shrinking of the area occupied by the air mass, a process that is termed horizontal convergence. Whenever air converges horizontally, it must pile up—that is, increase in height to allow for the decreased area it now occupies. This generates a “taller” and therefore heavier air column. Yet a surface low can exist only as long as the column of air exerts less pressure than that occurring in surrounding regions. We seem to have encountered a paradox: A low-pressure center causes a net accumulation of air, which increases its pressure. Consequently, a surface cyclone should quickly eradicate itself in a manner not unlike what happens when a vacuum-packed can is opened.

For a surface low to exist for very long, compensation must occur at some layer aloft. For example, surface convergence could be maintained if divergence (spreading out) aloft occurred at a rate equal to the inflow below. The figure below (▼) shows the relationship between surface convergence and divergence aloft that is needed to maintain a low-pressure center.

Divergence aloft may even exceed surface convergence, thereby resulting in intensified surface inflow and accelerated vertical motion. Thus, divergence aloft can intensify storm centers as well as maintain them. On the other hand, inadequate divergence aloft permits surface flow to “fill” and weaken the accompanying cyclone.

Note that surface convergence about a cyclone causes a net upward movement. The rate of this vertical movement is slow, generally less than 1 kilometer (0.6 mile) per day. Nevertheless, because rising air often results in cloud formation and precipitation, a low-pressure center is generally related to unstable conditions and stormy weather (▼A).

As often as not, it is divergence aloft that creates a surface low. Spreading out aloft initiates upflow in the atmosphere directly below, eventually working its way to the surface, where inflow is encouraged.

Like their cyclonic counterparts, anticyclones must be maintained from above. Outflow near the surface is accompanied by convergence aloft and general subsidence of the air column. Because descending air is compressed and warmed, cloud formation and precipitation are unlikely in an anticyclone. Thus, fair weather can usually be expected with the approach of a high-pressure center (▲B).

As discussed previously, it has been common practice to print on household barometers the words “stormy” at the low-pressure end and “fair” on the high-pressure end. By noting whether the pressure is rising, falling, or steady, we have a good indication of what the forthcoming weather will be. Such a determination, called the pressure tendency, or barometric tendency, is a useful aid in short-range weather prediction.

You should now be better able to understand why television weather reporters emphasize the positions and projected paths of cyclones and anticyclones. The “villain” on these weather programs is always the low-pressure center, which produces “bad” weather in any season. Lows move in roughly a west-to-east direction across the United States due to prevailing upper air winds, and require a few days to more than a week for the journey. Because their paths can be somewhat erratic, accurate prediction of their migration is difficult, although it is essential for short-range forecasting.

Meteorologists must also determine whether the flow aloft will intensify a newly-formed storm or act to suppress its development. Because of the close tie between conditions at the surface and those aloft, a great deal of emphasis has been placed on the importance and understanding of the total atmospheric circulation, particularly in the midlatitudes. We will now examine the workings of Earth’s general atmospheric circulation and then again consider the structure of the cyclone in light of this knowledge.

• The two types of pressure centers are (1) cyclones, or lows (centers of low pressure), and (2) anticyclones, or highs (centers of high pressure). In the Northern Hemisphere, winds around a low (cyclone) are counterclockwise and inward. Around a high (anticyclone), they are clockwise and outward. In the Southern Hemisphere, the Coriolis effect causes winds to be clockwise around a low and counterclockwise around a high.

• Because air rises and cools adiabatically in a low-pressure center (a cyclone), cloudy conditions and precipitation are often associated with their passage. In a high-pressure center (an anticyclone), descending air is compressed and warmed; therefore, cloud formation and precipitation are unlikely in an anticyclone, and “fair” weather is usually expected.

• convergence: The condition resulting from a net horizontal inflow of air into an area. Convergence at lower levels is associated with an upward movement of air and is favorable to cloud formation and precipitation.

• divergence: The condition that exists when the distribution of winds in a given area results in a net horizontal outflow of air from the region. In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.

• highs (a.k.a., anticyclones): High-pressure centers characterized by clockwise flow of air in the Northern Hemisphere. Also called anticyclones.

• lows (a.k.a, cyclones): Low-pressure centers characterized by counterclockwise flow of air in the Northern Hemisphere. Also called cyclones.

• pressure tendency (a.k.a., barometric tendency): The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction. Also called barometric tendency.