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Knowing some of the important factors in hurricane development we can now move on to the first phase of a hurricane's life (formation). So as a first guess how do hurricanes start?
They appear magically?
Somehow we get clouds rotating?
Let's see...
All hurricanes form from preexisting cloud clusters. Somehow though we have to get these clusters to start spining. More importantly however, where do the clouds come from? Well, the clouds for the most part form along a line of convergence known as the Intertropical Convergence Zone, or ITCZ. Here the air converges and is lifted up to form clouds. You may have heard of the ITCZ referred to as the monsoon trough. The monsoon trough is only one type of ITCZ though, there are two types, a trade wind and a monsoon ITCZ. Besides these troughs thunderstorm clusters can form in a multitude of other ways, such as the easterly waves that come off Africa, and along the southern ends of stalled midlatitude fronts.
The easterly waves that come from Africa form along the ITCZ there and then migrate off the coast working their way across the Atlantic and then into the Pacific. Along the way they may form into a hurricane in either ocean basin.
The ITCZ migrates north and south throughout the year roughly between 20 degrees north and 20 south, although its average position is 5 degrees north. The trough is found in the summer hemisphere and serves as a region of convergence. Those basins that were mentioned to have two peaks of formation have those because the trough makes two passes through those regions, once heading north, the other heading south.
To form a hurricane these cloud clusters must persist for a long period of time, generally serveral days. Those systems that persist for long periods of time tend to generate a MCV (mesoscale convective vortex). This is a warm core vortex that forms in the altostratus deck of the system somewhere around 300 to 700 millibars. Below this vortex the system often has a cold core. Remember, cold here is relative, by cold we may mean the core is 30 degrees C while the surroundings are 31 degrees C or something to that effect. Now, somehow in hurricane development this vortex migrates to the surface and turns the surface into a warm core system. How this transition is made is one of the major unknown areas of hurricane formation. However, once the core is at the surface it is now in a position to tap into the oceanic latent heat energy.
Not all MCV's turn into hurricanes and much research still goes into determining what differentiates forming from non-forming MCV's. The ITCZ is also a source of spin up. Winds approach it from different directions at the surface and these differences help to start cloud cluster rotation. This spin mechanism is also found at the ends of stalled fronts and in easterly waves.
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