How do hurricanes form?

Hurricanes usually form in the summer or early autumn when several key atmospheric ingredients come together. Contrary to common belief, many factors other than warm ocean water cause hurricanes to form. In fact, the water is always warm enough in the deep tropics for hurricanes to form all year long. During the winter and spring, upper air winds are hostile, and usually do not allow tropical cyclone development. So, what does it take for a hurricane to form?

• 1. Pre-existing Disturbance or low pressure area must have formed in the low levels of the atmosphere to start winds converging and uplift.



• 2. Warm Water to a sufficient depth to support the energy that a hurricane will use. The threshold temperature for tropical cyclone development needs to be about 26.5º Celsius or 80º Fahrenheit. A water column depth of about 50 meters or 150 feet deep is required.



• 3. Low Stability will allow deep convection or cumulonimbus clouds to build to great heights in the atmosphere. A stable air mass will inhibit cloud development and not
  allow for significant cloud growth to support the deep convection needed for a hurricane to develop.



• 4. Coriolis Force The disturbed area of weather needs to be at least 4-5º away from the equator. This is the approximate distance from the equator for the Coriolis force to achieve a gradient wind balance to sustain the low pressure area.



• 5. Moist Mid Level of the atmosphere. If there is dry air aloft it will weaken or choke off the updrafts in the cumulus clouds.



• 6. Low Vertical Wind Shear from the surface to upper troposphere. This allows for the thunderstorm clouds to build to great heights. If the wind speed increases or changes direction with height, the cumulonimbus clouds get deformed and can not sustain the hurricane heat engine.



• 7. Divergence in the upper Atmosphere This allows for the transport of mass away from the hurricane that is brought in at the low levels of the atmosphere.

Rich is a Certified Broadcast Meteorologist with 25 years of weather forecasting and television experience. Visit his website for more on expert hurricane forecasts and travel resources. Hurricane Forecasts & Travel - Hurricane & Tropical Weather Quizzes

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