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?
The ingredients for hurricane formation
- 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.
Hurricane Formation
If the conditions above are met sufficiently, showers and storms will start to gain organization as low level winds converge toward the center of the low pressure area. The cluster of convection will start to form bands. As the convection increases, the warm air near the surface rises and cools. As it cools the water vapor condenses. There is a tremendous amount of heat produced from the condensation of water vapor. Some of this heat warms the center region of the low. As the temperature of the air near the core rises it produces lower pressure. In response to this lower pressure, winds increase in intensity. A tropical storm forms when the surface winds reach sustained winds of 39-73 mph. A hurricane officially classified as the winds reach sustained values of 74 mph. Even if all of the ingredients of a hurricane are in place, it does not guarantee that a hurricane will form. Many of the factors in hurricane formation exist in the tropics, especially in the heart of hurricane season. Despite this, very few disturbances actually develop into a hurricane.
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.
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Hurricanes are also formed by warm moist air rising over the ocean. Hurricanes begin to form off the Atlantic coast of Africa, and move, due to prevailing winds, toward the Southern United States and the Caribbean. Hurricanes are long lived, and grow in intensity as they move over warm ocean water. Hurricanes can take one of three paths. Some draw near the Atlantic coast of the US and move northward, dying out over the ocean before they ever reach land. Some can strike the southern US coast, dying out as they move inland. Others move through the warm Caribbean waters and pick up intensity, moving over the Caribbean Islands and striking Mexico, Central America, or any of the Gulf Coast States. Hurricane categories are determined by the maximum intensity of the winds in the eye wall of the storm. Numbers from 1 to 5 are used to distinguish the intensity of hurricanes, with 1 having the lowest winds and damage, to 5 having the most intense winds, rain and damage.