Disturbance: The seedlings of tropical cyclones are called disturbances. A few types of common disturbances are:
- Easterly Waves: Easterly waves are also called tropical waves. They are an inverted trough of low pressure moving generally westward in the tropical easterlies. The majority of tropical cyclones form from easterly waves.
- West African Disturbance Line (WADL): WADL is a line of convection (compared to a squall line) which forms over West Africa and moves quickly into and westward across the Atlantic Ocean.
- TUTT: A TUTT (Tropical Upper Tropospheric Trough) is a cold-core low pressure system in the upper atmosphere, which produces convection. Occasionally, it is possible for one of these to develop into a warm-core tropical cyclone.
- Old Frontal Boundary: Remnants of a polar front can become a line of convection and occasionally generate a tropical cyclone. In the Atlantic Basin, this is most likely to occur very early or late in the hurricane season.
Tropical Depression: Once a disturbance forms and sustained convection develops, it can become more organized. If the disturbance moves or stays over warm water (at least 80°F), and upper level winds remain weak, the disturbance can become more organized, forming a depression.
Tropical Storm/Hurricane: Warm water is one of the most important keys for tropical cyclone development as it is water that powers the tropical cyclone. As water vapor (water in the gaseous state) rises, it cools. This cooling causes the water vapor to condense into a liquid we see as clouds. In the process of condensation, heat is released. This heat warms the atmosphere making the air lighter still which then continues to rise into the atmosphere. As it does, more air moves in near the surface to take its place which is the strong wind we feel from these storms.
Weakening Storm: Once the eye of a storm moves over land it will begin to weaken rapidly, not because of friction, but because the storm lacks the moisture and heat sources that the ocean provided. This depletion of moisture and heat hurts the tropical cyclone's ability to produce thunderstorms near the storm center. Without this convection, the storm rapidly diminishes.
(Source: National Weather Service)