Hurricane air flows

This week marks the beginning of hurricane season in the northern Atlantic, which lasts until November 30th. Scientifically, tropical cyclone season would be a better name but, I suppose, hurricane season makes better headlines. The definition of a tropical cyclone from Wikipedia is:

A tropical cyclone is a storm system characterized by a low-pressure center and a spiral arrangement of thunderstorms that produce strong winds and heavy rain.

We classify tropical cyclones by wind speed such that those with wind speeds of:

  • less than 39 mph are called tropical depressions,
  • between 39 and 74 mph are called tropical storms, and
  • greater than 74 mph are called hurricanes.


We then further classify hurricanes into categories one to five at wind speed break points of 74, 96, 111, 131, and 155 mph. Wind speeds for category five hurricanes occasionally top 200 mph.

Due to the influence of the earth’s rotation (the Coriolis Effect), storms in the Northern Hemisphere spiral in a counter clockwise direction and those in the Southern Hemisphere spin clockwise. The first draft of this column included a lengthy exposition on the physics of the Coriolis Effect, which I decided you might not really want to read. What I do need to convey is that as you approach the equator, the Coriolis Effect becomes neutral and will not cause storm systems to rotate at all. Therefore, tropical cyclones cannot be formed near to the equator.

The driving force of tropical cyclones is the evaporation and condensation of water, which sounds rather mundane for such awesomely powerful phenomena. In order for there to be a sufficient rate of evaporation for a tropical cyclone to begin, the surface of the ocean needs to be at least 79.7 °F. This temperature limitation explains why our hurricane season does not get started until June when water temperatures have risen sufficiently. As the water vapor rises it will eventually encounter cold air and condense into rain drops.

OK, here comes the most technical part of the column. When water evaporates it removes heat from its surroundings. This is the mechanism by which sweating cools your body. The heat from the surface of your skin is carried away in the water evaporating from the surface. When water vapor condenses back into a liquid, the same amount of heat that it absorbed when it evaporated is released back into the air, warming the air up a bit. This is called the heat of condensation. It you have a pocket of air in which many water droplets are condensing, this zone of the air will warm up compared to the air outside of the condensation zone. Now you have adjacent zones of air with different temperatures and, thus, different densities. Mother Nature does not abide these density differences and works rapidly to even them out in a process we call wind.

When zones of cold upper air form over warm ocean water, a situation arises where you have both rapid evaporation of water from the surface and rapid condensation of water droplets in the air above. If this situation persists for long enough, air flow patterns like those shown in the graphic at the top of the page begin to develop. This is what weather forecasters are referring to when they talk about a storm system becoming organized.

If the storm remains organized, then the rate of downward flow of air on the outside edge of the storm system will increase. This will in turn increase the horizontal flow of air along the surface of the water back towards the eye of the storm. Increased horizontal air flow drives faster water evaporation which results in faster rates of condensation at the top of the storm which creates more wind so the system reinforces itself and the storm becomes stronger.

The track of the storm is driven by large scale wind streams such as the Trade Winds and the Jet Stream, as well as by the Coriolis Effect. Storms in the North Atlantic are pushed to the west when they are within 30° of the equator – about the latitude of Jacksonville, Florida – and pushed to the east north of that point. During this time the Coriolis Effect, in addition to causing the system to spiral, pulls the storm to the north. All together these influences result in our familiar North Atlantic storm tracks in which tropical depressions form in the middle of the Atlantic, gather strength, head towards the Caribbean and Florida, and then are finally pushed northward and then out to sea again.

Given that the driving force of tropical cyclones is the evaporation and condensation of water, the storm systems rapidly lose energy when they move over land, although there can still be strong winds and heavy rains. Storms also eventually dissipate in the Northern Atlantic when the water becomes too cold to sustain sufficient rates of evaporation.

Systems are given a name once they reach the tropical storm stage. The process was initiated in 1953, at which time only female names were used. In keeping with the times, a coed naming system was adopted in 1978. The second storm system of 2013 will be named Barry. Those of us at WHCL and are looking forward to having a little fun with WCHL President Barry Leffler as his storm approaches. Let the bad puns begin.

The energy released from category five hurricanes can reach 200 exajoules (1018) per day – more than 70 times total world energy consumption – which can cause serious and life threatening damage. That’s where I will pick up the story next week.

Have a comment or question? Use the interface below or send me an email at