In Either Hemisphere Friction causes a net inflow (convergence ) around a cyclone and a net outflow (divergence ) around an anticyclone .
Buy’s Ballot’s Law (1857) In the Northern Hemisphere, if you stand with your back to the wind , low pressure will be found to your lef, and high pressure will be found to your right . (The opposite situation holds in the Southern Hemisphere.) Wind direction is directly linked to the prevailing pressure pattern. Buy’s Ballot’s Law is appropriate for the air aloft . At the surface , if you stand with your back to the wind, then turn clockwise about 30 0 , low pressure will be to your left and high pressure to your right .
How Winds Generate Vertical Air Motion Vertical airflow is associated with cyclones and anticyclones. Cyclones (Low Pressure Systems) Air spiraling inward , causing a shrinking of the area occupied by the air mass. (This is horizontal convergence .) Thus the air that is converging horizontally starts to pile up , increasing in height to allow for the decreased area it now occupies. Thus air would tend to pile up , and the air column would get heavier, wiping out the low pressure. Instead, for a low pressure system to remain in existence, surface convergence is maintained if : Divergence (spreading out) alof occurs at a rate equal to the inflow below.
Divergence alof can sometimes exceed surface convergence, thereby resulting in intensified surface inflow and accelerated vertical motion . Divergence aloft, in so doing, may intensify storm centers as well as maintain them. Inadequate divergence aloft permits surface flow to “fill” and weaken the accompanying cyclone. A Low Pressure Center Is generally related to unstable conditions and stormy weather, because rising air ofen results in cloud formation and precipitation. Surface convergence about a cyclone causes a net upward movement . The rate of vertical movement is slow, generally less than 1 km/day
Often divergence aloft creates a surface low . Spreading out aloft initiates up flow in the atmosphere directly below , eventually working its way to the surface , where inflow is encouraged .
Anticyclones Anticyclones are also maintained from aloft . Outflow near the surface is accompanied by convergence alof t and general subsidence of the air column . (Without upper air convergence, the high at the surface would dissipate from divergence .) Because descending air is compressed and warmed , cloud formation and precipitation are unlikely in an anticyclone. Fair weather can be expected with the approach of a high-pressure system. Pressure Tendency of Barometric Tendency The pressure trend (rising, falling, steady), gives a good indicator of forthcoming weather. The pressure tendency is quite useful for short range weather prediction .
A close tie exists between the surface conditions and those alof .
- Fall '16
- Mark Smith
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