Disclaimer: This is not affiliated with the National Hurricane Center, Hurricane Hunters, Storm Prediction Center, or National Weather Service. ALL forecasts herein are the result of my analysis, (to which you will see me at times, insert excerpts from various agencies due to the nature of the importance of the information) and I am solely responsible for the content. As ALWAYS, follow the National Hurricane Center, National Weather Service, and your local Emergency Management officials for emergency decisions. In addition, this is strictly a FORECAST OFFICE. I CANNOT make decisions regarding travel plans, etc. My purpose, is to provide you the information, based solely on information I analyze, and the accuracy of the information at hand of the time of analysis, so you may make informed decisions.
(T. F. “Storm” Walsh)
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This tutorial will focus on hurricane formation. I was able to find one half decent video explaining how tropical storms and hurricanes develop. It’s a little long at 14 minutes, and although the presenter seems to ramble on, the video contains some decent information. The video is posted after the Hurricane Science graphic.
Development of a tropical storm or hurricane depends more or less on the following basic criteria. The following is from a NWS training page:
There are six widely accepted conditions for hurricane development:
1. The first condition is that ocean waters must be above 26 degrees Celsius (79 degrees Fahrenheit). Below this threshold temperature, hurricanes will not form or will weaken rapidly once they move over water below this threshold. Ocean temperatures in the tropical East Pacific and the tropical Atlantic routinely surpass this threshold.
2. The second ingredient is distance from the equator. Without the spin of the earth and the resulting Coriolis force, hurricanes would not form. Since the Coriolis force is at a maximum at the poles and a minimum at the equator, hurricanes can not form within 5 degrees latitude of the equator. The Coriolis force generates a counterclockwise spin to low pressure in the Northern Hemisphere and a clockwise spin to low pressure in the Southern Hemisphere.
3. The third ingredient is that of a saturated lapse rate gradient near the center of rotation of the storm. A saturated lapse rate insures latent heat will be released at a maximum rate. Hurricanes are warm core storms. The heat hurricanes generate is from the condensation of water vapor as it convectively rises around the eye wall. The lapse rate must be unstable around the eyewall to insure rising parcels of air will continue to rise and condense water vapor.
4. The fourth and one of the most important ingredients is that of a low vertical wind shear, especially in the upper level of the atmosphere. Wind shear is a change in wind speed with height. Strong upper level winds destroy the storms structure by displacing the warm temperatures above the eye and limiting the vertical accent of air parcels. Hurricanes will not form when the upper level winds are too strong.
5. The fifth ingredient is high relative humidity values from the surface to the mid levels of the atmosphere. Dry air in the mid levels of the atmosphere impedes hurricane development in two ways. First, dry air causes evaporation of liquid water. Since evaporation is a cooling process, it reduces the warm core structure of the hurricane and limits vertical development of convection. Second, dry air in the mid levels can create what is known as a trade wind inversion. This inversion is similar to sinking air in a high pressure system. The trade wind inversion produces a layer of warm temperatures and dryness in the mid levels of the atmosphere due to the sinking and adiabatic warming of the mid level air. This inhibits deep convection and produces a stable lapse rate.
6. The sixth ingredient is that of a tropical wave. Often hurricanes in the Atlantic begin as a thunderstorm complex that moves off the coast of Africa. It becomes what is known as a mid-tropospheric wave. If this wave encounters favorable conditions such as stated in the first five ingredients, it will amplify and evolve into a tropical storm or hurricane. Hurricanes in the East Pacific can develop by a mid-tropospheric wave or by what is known as a monsoonal trough.
I want to touch on a couple items on the conditions. Almost every season, we do see development over waters of less than 26.0 – 26.5 degrees Celsius. This can occur, provided the ocean is warmer than the surrounding atmosphere (this provides a steep enough lapse rate for clouds and thunderstorms to develop).
Though it is rare, cyclones CAN develop less than 5 degrees, north or south of the equator. The general rule is for a tropical system to develop, it must be at least 5 degrees latitude from the Equator for the Coriolis effect to take hold. The following list is from a Wikipedia article:
https://en.wikipedia.org/wiki/List_of_tropical_cyclones_near_the_Equator
Regarding wind shear, we know from previous hurricane seasons strong shear can totally displace convection away from the LLC (Low Level Circulation). Shear also spreads the heat or heat energy over a larger area, therefore not focusing the energy in a vertical column for the storm to maintain itself or strengthen. In addition, shear “tilts” a storm, not allowing for vorticity to be focused. Think of a child’s large spinning top…as the top begins to tilt as it slows, it begins to wobble.
The following graphic is linked to a text tutorial of the basics of hurricane formation. A little lengthy, however it contains various topics for you to click on in the far left hand column, starting with Hurricane Structure. When you mouse over a topic, the link will change to white.
HURRICANE SCIENCE
HURRICANE DEVELOPMENT (CLICK GRAPHIC FOR VIDEO)
IF you’ve been a follower of my site for a while, and if you read my entire forecasts carefully, you’ve probably heard me speak of “Hot Towers” A hot tower is basically a cumulonimbus cloud that shoots up near or around a hurricanes eyewall. These clouds can extend almost to the base of the stratosphere (some 6 miles above sea level). These towers allow for warm, moist air to shoot up through the hurricane. When this phenomenon occurs, and hot towers are detected, it is usually a sign that the hurricane is getting ready to intensify.
The following video from NASA explains the formation of Hot Towers, and their effect on hurricanes:
HOT TOWER VIDEO (LINKED TO VIDEO)
My next tutorial will feature short videos produced by the NHC in Miami. These videos are from older seasons, which you’ll recognize by older storm names, and give a basic overall look at items such as storm surge, wind, inland flooding, etc, followed by videos of how to board up your windows in preparation for the arrival of a storm.
You may direct any questions by contacting me personally, ANYTIME, at: twalsh22000@yahoo.com
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
WEST CENTRAL FLORIDA HURRICANE and SEVERE WEATHER FORECAST CENTER 