Understanding Hurricanes

The reason air deflects to the right in the northern hemisphere and the left in the southern hemisphere is the Coriolis effect.  The Earth rotates on its axis every day, and the air on the Earth’s surface must travel around the world with the spinning Earth.  Air moves faster the closer to the equator it is, so air moving south in the northern hemisphere moves into an area where the Earth literally outruns it, and air moving north literally outruns the Earth, hence the deflection.  This causes air to deflect as it approaches lower pressure, so low pressure have a counter-clockwise rotation in the northern hemisphere.  This same effect causes high pressure systems where air descends from aloft and spreads out to rotate clockwise.

Basically, a hurricane needs rising air.  The reduction of pressure invites other air to spiral in, and it too will rise.  The real fuel is water.  As moist air rises it cools and is subjected to lower pressures, which cause the air to have a reduced capacity of holding water vapor.  When water goes from the gaseous state to the liquid state it releases heat, which expands the rising air and makes it rise even higher, all the while condensing more water and further heating the air.  When the air gets high enough, freezing the liquid water further releases heat and aids in the convection.

The initial thunderstorms can be simply associated with an Intertropical Convergence Zone disturbance, or might be associated with an uplifting of air passing over an island.  The Sahara desert sandstorms put electrically charged sand high in the atmosphere, which attract water and can trigger a thunderstorm cluster.  Or, a warm area of ocean might have air passing over it rise, just like a hot parking lot can send air aloft.  This too can trigger a cluster of thunderstorms.  And, the Gulf Stream is a hotter than the surroundings water area.

If it were not for friction, the inflowing air would soon shut the system down, or the when coupled with the Coriolis effect the air would go in a circle with the system unable to grow to hurricane strength.  Friction slows the air, so the deflection is lessened, but the low pressure attracts air with the same force.  So, the air spirals in, and the system is able to grow.

To understand the path we must consider the global air flow.  Air at the equator rises, and after going to roughly thirty degrees latitude it sinks.  This causes high pressure to form roughly thirty degrees from the equator.  The air flows out from high pressure cells and moves clockwise.  The individual cells often look elliptical, so the storm can be steered along the high pressure cell, first eastward, then slowly tracking more northerly, and finally westerly, giving the appearance of a parabola.  The high pressure cells can split and merge.  Other steering is caused by fronts and anything else that influences air movement.

When the high pressure cell ends at sea the storm will often turn before reaching land.  If the high builds and covers the coast or get close to the coast the storm may well enter the Gulf of Mexico.

The air rotates as the system as a whole moves along.  On the right side, the storm movement and rotation add, so the wind is stronger.  On the left the two movements oppose each other, so the net wind is reduced.

Another problem with the right side of a hurricane is that when a hurricane encounters land the forward right quadrant often triggers tornadoes.

Still another problem is that right of the center the wind blows towards the coast, so a storm surge should be higher.  The wind piles water up at the coast, and the inward flowing air causes a bulge in the ocean in the center of the storm.  These can produce a storm surge which depends on the wind speed and duration, and the contours of the coast.  Normally occurring tides can also contribute to a storm surge.

Hurricanes move slow enough to allow for a long warning period.  While intensity of the system can change, the path often can be forecast with a cone of uncertainty days in advance.  So, for those unfamiliar with hurricanes, visiting a area where they can have an impact should not be a deterrent.  You should have plenty of warning if a problem occurs.

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