As I mentioned in Part 1, hurricanes start with a pressure differential.  Air moves from high pressure to low pressure.  Let us see if we can unpack that a little more.

Weather can be tricky because air, for the most part, does not absorb heat from sunlight.  The energy from sunlight passes right through the air to the ground.  It is the earth that absorbs the heat and earth is referred to by scientists as a “black body”, because it captures 100 percent of the sun’s heat.  That is why mountains have snow caps during the summer.  The way that air gets hot is through conduction with the ground.

Sometimes the sun will heat up some parts of earth more than others, and when that happens, the air expands as any gas would, and starts to rise into the atmosphere due to its reduced density – bouyancy.  Hot air can hold much gaseous water compared to cold air.  So, as the air starts to rise, it gets colder; and water is precipitated – usually as small droplets (or ice) that form clouds.

hi low.png

When hot air rises, it creates a vacuum, low pressure, and this is indicated by “L” in the diagram above.  Note that although the ground below is hot, the sun is not out over a low pressure system; rather, clouds.  As the hot air continues to rise, the air around the low pressure is now the high pressure.  Air starts to flow from high to low pressure – high pressure is indicated by “H”.  As air moves from high pressure to low pressure, cold dense air from high above starts to fill the void over the high pressure.  The heavy, dense air helps to make the high pressure, in fact, high pressure.  On the high pressure side, there is sun shine; because the cold air hits the ground, gets hot, and thus can hold more moisture in the form of water gas – no cloud formation.

Hurricanes that strike the Gulf Coast usually start in Africa.  The trade winds blow hot air from the Sahara desert onto the Atlantic Ocean.  The hot air absorbs tons of moisture from the ocean and rises up into the atmosphere, and forms rain clouds.  A rotation is started.  Hot air is rising, while cold air is falling; and air is flowing from high pressure to low pressure.  This cycle will intensify so long as the ocean temperature is 79F or higher.  The trade winds will also continue to blow the cycle towards our coast.

The question is how does the hurricane form from this?  I will get into those details next time when I discuss pseudo forces.


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