Why do large areas of low and high pressure spin?
The Earth’s rotation has major implications for the big picture weather patterns we observe. Here I am talking about general wind patterns tied to areas of low and high pressure often referenced on media weather coverage. Weather enthusiasts likely know that “Lows” tend to be linked to unsettled weather with opportunity for precipitation, while “Highs” are indicative of a stable weather pattern and diminished precipitation chances. By considering that air flow movement is fundamentally directed from high to low pressure, the rationale behind the general weather conditions expected in either case start to makes sense.
For example, Lows have air mass converging towards and into a central low pressure area. The net gain in air mass must go somewhere, hence atmospheric vertical lift is supported around the center of the Low. Rising motion and, therefore, subsequent cooling and potential condensation of available water vapor to create clouds become possible.
Now let’s look at Highs. The air mass would be spreading outward away from the central area of highest pressure. This outward spreading motion does a couple important things to preclude the formation of clouds. First, spreading motion at the surface is not a source of atmospheric lift. In fact, diverging air at the surface will force downward motion of air from above. Meteorologists call sinking air “subsidence”. Sinking air leads to air in the atmospheric column to compress and heat. Remember to generate clouds you need air to cool and condense!
So, why are Lows and Highs ultimately spinning? Enter the Coriolis force! Earth’s rotation induces a force that deflects air wanting to simply travel from high to low pressure. Specifically, a right deflection in the Northern Hemisphere and leftwards in the Southern Hemisphere.
Focusing on the Northern Hemisphere and with an astronaut perspective looking down, air trajectories attempting to converge on a point of lowest pressure from all sides being deflected rightwards generate a counterclockwise circulation, while diverging air leaving a high pressure center appears to flow outward in a clockwise manner. Lows and Highs will spin opposite in the Southern Hemisphere.
Now let’s look at Highs. The air mass would be spreading outward away from the central area of highest pressure. This outward spreading motion does a couple important things to preclude the formation of clouds. First, spreading motion at the surface is not a source of atmospheric lift. In fact, diverging air at the surface will force downward motion of air from above. Meteorologists call sinking air “subsidence”. Sinking air leads to air in the atmospheric column to compress and heat. Remember to generate clouds you need air to cool and condense!
So, why are Lows and Highs ultimately spinning? Enter the Coriolis force! Earth’s rotation induces a force that deflects air wanting to simply travel from high to low pressure. Specifically, a right deflection in the Northern Hemisphere and leftwards in the Southern Hemisphere.
Focusing on the Northern Hemisphere and with an astronaut perspective looking down, air trajectories attempting to converge on a point of lowest pressure from all sides being deflected rightwards generate a counterclockwise circulation, while diverging air leaving a high pressure center appears to flow outward in a clockwise manner. Lows and Highs will spin opposite in the Southern Hemisphere.
Data Source: NOAA NWS Weather & Hazards Data Viewer
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