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The Highs and Lows of Weather: Part 2 -- The Low

© Keith C. Heidorn

Low pressure cells or Lows are characterized in many ways beyond the obvious pressure difference in direct opposition to the High discussed in Part 1. A low pressure cell is characterized by converging air flow at the surface, ascending air currents at its core where the converging currents meet, and regions of divergent air flow above where air currents flow away from the rising core. When the combination of these processes removes air faster from the surface than it can be replaced by the converging inflow, the temporary mass deficit in the air column lowers its weight, and thus reduces the surface pressure, causing it to become lower than the surrounding atmosphere.

To replace the ascending air at the Low's core, surface air must flow inward toward the cell's centre. But as with the circulation in a high pressure cell, the Coriolis effect prevents a direct route. Thus, the geostrophic balance around a low pressure cell produces a wind field that flows around the Low in a counterclockwise direction in the Northern Hemisphere. This we call cyclonic flow. When surface friction affects the flow, the resulting gradient wind has an inward component toward lowest pressure.

Low pressure cells that travel long distances across the earth are called cyclones, or storm systems due to their attendant stormy weather. When a quasi-circular region of low pressure elongates to a long and narrow band, it is referred to as a trough of low pressure or just a trough (as opposed to a high pressure ridge) by weather analysts.

Ascending currents within a Low push surface air toward higher altitudes. Air moving upward cools by expansion, usually reaching its condensation level along the way. Clouds form at the condensation level and extend upward in the atmosphere. If conditions are right, precipitation develops as more and more water vapour is transformed to larger liquid water or ice crystals that eventually fall earthward. Thus, the presence or approach of a low pressure cell signals cloudy and wet weather.

Preferred Locations For Low Formation

The formation of a low pressure cell, called cyclogenesis, can be a complicated process. Some lows form in specific geographical locations. Others form through the clash of contrasting polar and tropical air masses and influence of high speed air currents aloft that start a twist in the mobile polar front zone between the air masses.

The leeward sides of mountains are a common low pressure spawning ground. Over North America, cold-season storms such as the Colorado Lows and Alberta Clippers emerge from the lee of the Rocky Mountain chain. During the summer above desert regions, extreme solar heating may form large, quasi-stationary

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