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Spiral Winds

Wind flow along curved isobars. When there is no friction, wind in geostrophic balance will flow along curved contour lines of pressure. We call a wind that flows parallel to curved contours a gradient wind; it differs from the geostrophic wind by the addition of a centrifugal force that causes the wind to flow along a curved rather than straight path. When there are closed contours around low and high pressure centers, the wind will circulate around these centers.

Fig. 1. Wind in geostrophic balance flowing (A) along parallel and (B) curved isobars.

In the diagram below representing the Northern Hemisphere, the green arrows represent the pressure gradient forces for lows and highs (pointing into the lows, pointing out of the highs). For geostrophic balance, the Coriolis forces must point outward from the lows, and inward into the highs. The winds that produce this configuration of Coriolis forces flow counterclockwise around lows and clockwise around highs.

Fig. 2. Frictionless circulation around closed isobars in the Northern Hemisphere.

In the Southern Hemisphere, the Coriolis force acts to the left instead of the right of the wind. So, the wind will have to flow in the opposite direction as in the Northern Hemisphere to get the balancing Coriolis force going in the right direction, since the PGFs go in the same direction in the Southern Hemisphere as in the Northern Hemisphere. Winds flow clockwise around lows and counterclockwise around highs in the Southern Hemisphere.

Fig. 3. Frictionless circulation around closed isobars in the Southern Hemisphere.

Effect of Friction on Wind. So far, we have discussed the behavior of wind when no friction is acting on it. True geostrophic and gradient winds can be expected above the boundary layer, or friction layer, of the earth (above about 1-2 km, up to 950 mb). The height of the boundary layer can vary depending on the type of terrain, wind, and vertical temperature profile.

Within the boundary layer, the turbulent friction that the Earth exerts on the air slows the wind down. This slowing causes the wind to be non-geostrophic, or ageostrophic. This reduction in wind speed automatically reduces the Coriolis force, and the pressure gradient force becomes more dominant. As a result, the total wind deflects slightly towards lower pressure, and crosses parallel isobars rather than following them.

Fig. 4. Friction causing geostrophic wind to cross parallel isobars toward low pressure.

In a similar manner, the friction induced deflection toward low pressure causes wind to cross circular isobars associated with low and high pressure cells near the earth’s surface. As a result, wind spirals into a low at the surface, and spirals out of a high at the surface.

Fig. 5. Friction causing wind in geostrophic balance to cross concentric, closed isobars toward low pressure (away from high pressure).

The flow of air into a low pressure cell is called cyclonic. Cyclonic circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The flow of air into a high pressure cell is called anti-cyclonic. Anticyclonic circulation is clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere.

Fig. 6. (A) Anticyclonic circulation out of a high pressure cell in the Northern Hemisphere (B) Cyclonic circulation into a low pressure cell in the Northern Hemisphere.

Hurricanes are extreme examples of low pressure cells. Now it is clear why wind spirals counterclockwise into hurricanes in the Northern Hemisphere, and clockwise into hurricanes in the Southern Hemisphere! In both cases, the circulation is cyclonic.