Environmental Factors For Development

Below is a list of commonly agreed upon factors for hurricane development.

1. Large amounts of cyclonic positive relative vorticity
2. High enough latitude to get non-zero values of planetary vorticity.
3. Weak vertical shear of the horizontal winds
4. Water temperatures > 26 degrees C (some texts will quote 27 degrees C)
5. Deep conditional instability
6. Large amounts of water vapor.
7. A preexisting cluster of clouds

The first factor says hurricanes need large amounts of relative vorticity. Hold on, Vorticity?! Yep it's a big word but if you like storms and meteorology, get used to it, for without it weather would not be nearly as interesting as it is. Voritcity in its most basic definition is the local spin of the atmosphere. It comes in several flavors but the two you need to know here are relative and planetary. The earth spins and it in turn imparts spin to the air in contact with it. This is planetary vorticity, which causes our old friend the Coriolis force. Relative voriticity is the additional spin of the air relative to the earth. It's cyclonic if it's rotating the same way as the earth is rotating. Cyclonic vorticity is associated with low pressure (Hence the name "cyclone") and Anti-Cyclonic vorticity is associated with high pressure.

Why do you think cyclonic relative vorticity is necessary for tropical storm formations?

The next factor is high enough latitude to get large values of planetary vorticity or the Coriolis force. As we saw earlier in the forces module, Coriolis force is related to latitude and gets larger in value with higher latitude. We also saw in the winds module that the Coriolis force causes parcels of air to deflect to the right along their paths. Here the Coriolis force is needed to balance with the Pressure Gradient Force and the friction force to allow the storm to start spining up. Without the Coriolis force the forming storm would have nothing to cause the air to curve and the forming depression would simply fill up with air coming in towards the low center. You need just enough latitude (about 2.5 degrees) to make the coriolis force strong enough to allow the spin up of the storm to occur, and the faster the spin the faster the air picks up energy.

The third factor important to hurricane development is low values of vertical shear of horizontal winds. This is a wordy statement, see the diagram below to better understand it. Low vertical shear is needed around the center and especially the core of the storm. This allows the upper level features of the storm to be coupled with the low level features of the storm and allows the large thunderstorms needed to form. When there is high shear these parts are separated and the system is literally torn apart. We will explain this a little more in detail later. Shear, like vorticity comes in two flavors: speed (wind speed changes with height, and directional (wind direction changes with height).

The factor that most people think of when considering hurricane development is warm ocean water. Studies tend to show that water with temperatures below 26°C simply cannot support a hurricane. The air saturated at this temperature does not contain enough heat and moisture to keep the system going in the typical tropical atmosphere. It is important to remember though, that although SSTs (sea surface temperatures) are important to hurricanes, they are not the end all, be all of their existence.

Next we reach deep conditional instability. As explained in the sounding module, if a parcel is lifted and is warmer than its environment it is unstable and will continue to rise until it's cooler than its environment. A stable atmosphere is the opposite of this. A conditionally unstable atmosphere is one in which the parcel must be lifted beyond a certain point for it to be warmer than its environment and unstable.

So why is this important?

The last factor we mention here is large amounts of water vapor content. The reason this is important is that it is the release of latent heat energy that is the main source of energy for the hurricane's development and intensification. Latent heat is in essence, the gas of the hurricane engine. Also, if there is plenty of water vapor around, the rain won't evaporate as it falls and create a deadly (to the hurricane) pool of cool air at the surface.

The last important element is the cloud cluster of thunderstorms. Hurricanes don't form out of clear blue sky (Good thing, too!). The challenge of forecasting hurricane formation comes from trying to find cloud clusters that are ripe for this development. To find out about what we know and don't know about hurricane formation, go to this optional page. To cut directly to a summary of the process once you get a cloud cluster go here.

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