Tropical Cyclone Introduction

Hurricane Isabel on September 15, 2003. NASA image.
Hurricane Isabel on September 15, 2003. NASA image.

A tropical cyclone is a warm-core low pressure system, without any "front" attached, that develops over the tropical or subtropical waters and has an organized circulation.

There are several favorable environmental conditions that must be in place before a tropical cyclone can form. They are:

  • Warm ocean waters (at least 80°F / 27°C) throughout a depth of about 150 ft. (46 m).
  • An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection.
  • Relatively moist air near the mid-level of the troposphere (16,000 ft. / 4,900 m).
  • Generally, a minimum distance of at least 300 miles (480 km) from the equator.
  • A pre-existing near-surface disturbance.
  • Low values (less than about 23 mph / 37 km/h) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the change in wind speed with height.

Tropical cyclone formation basins

Global tropical cyclone formation basins - Click for more info.
Global tropical cyclone formation basins - Click for more info.

The need for warm water

What happens with a water temperature of at least 80°F (27°C).

The warm water is one of the most important keys 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.

Therefore, once the eye of the 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.

However, just having warm water alone is not enough for the formation of a tropical cyclone. There also needs to be a disturbance in the atmosphere such as...

  • Easterly Waves: Also called tropical waves, this is an inverted trough of low pressure moving generally westward in the tropical easterlies. A trough is defined as a region of relative low pressure. The majority of tropical cyclones form from easterly waves.
  • West African Disturbance Line (WADL): This is a line of convection (similar to a squall line) which forms over West Africa and moves into the Atlantic Ocean. WADL's usually move faster than tropical waves.
  • TUTT: A TUTT (Tropical Upper Tropospheric Trough) is a trough, or cold core low in the upper atmosphere, which produces convection. On occasion, one of these develops into a warm-core tropical cyclone.
  • Old Frontal Boundary: Remnants of a polar front can become lines of convection and occasionally generate a tropical cyclone. In the Atlantic Ocean storms, this will occur early or late in the hurricane season in the Gulf of Mexico or Caribbean Sea.

Video file

The NASA movie is Hurricane Wilma in October 2005 showing the life of the storm.

The color of the ocean represents sea surface temperature with orange and red colors indicating temperatures of 82°F (28°C) or greater.

As Wilma moves northwest, then eventually northeast, the water temperature decreases (indicated by the change to light blue color) after the storm passes a particular location. This is the result of the heat that is removed from the ocean and provided to the storm.

Therein shows the purpose of tropical cyclones. Their role is to take heat, stored in the ocean, and transfer it to the upper atmosphere where the upper level winds carry that heat to the poles. This keeps the polar regions from being as cold as they could be and helps keep the tropics from overheating.

Once a disturbance forms and sustained convection develops, it can become more organized under certain conditions. 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.

An infrared satellite image of Hurricane Irma, September 12, 2017.
An infrared satellite image of Hurricane Irma, September 12, 2017. This type of image measures temperature of the tops of clouds. The colder the cloud top, the higher it extended into the atmosphere. The colors from greens to reds to black help identify the higher cloud tops with the darkest colors being the tallest clouds.

There are many suggestions for the mitigation of tropical cyclones such as "seeding" storms with chemicals to decrease their intensity, dropping water absorbing material into the storm to soak-up some of the moisture. Some even suggest using nuclear weapons to disrupt their circulation thereby decreasing their intensity.

While well meaning, the ones making the suggestions vastly underestimate the amount of energy generated and released by tropical cyclones.

Even if we could disrupt these storms, it would not be advisable. Since tropical cyclones help regulate the earth's temperature, any decrease in tropical cyclone intensity would mean the oceans retain more heat.

Over time, the build-up of heat could possible enhance subsequent storms and lead to more numerous and/or stronger events.

There has also been much discussion about the abnormally high number of storms for the 2005 Atlantic basin (27 named storms including 15 hurricanes). Compared to the age of the earth, our knowledge about tropical cyclone history is only very recent.

Only since the advent of satellite imagery in the 1960's do we have any real ability to count, track and observe these systems across the vast oceans. Therefore, we will never know the actual record number of tropical cyclones in the Atlantic Oceans.

Read about tropical cyclone myths from the Hurricane Research Division of the Atlantic Oceanographic & Meteorology Laboratory.