Tropical Cyclones just don’t blow in unannounced.  What eventually becomes a named meteorological event begins as a tropical wave (Bowditch calls it an “easterly wave,” elsewhere it is known as an “African easterly wave”) and progresses from this stage to become a tropical disturbance. If conditions are encouraging, it evolves into a tropical depression and the isobars close. If really disturbed it morphs into a tropical storm.  And if not stopped, what emerges is a Betty or Bruce, Camille, Florence, Gustav, Harvey, Hugo, Katrina or Maria.

This week we will look at the first stage.

Before getting disturbed, depressed, storming and going cyclo, a Harvey began as a tropical wave.The tropical or easterly wave is a trough of low pressure that begins over the sub-Saharan desert in North Africa and migrates westward to the Atlantic Ocean. As many as sixty waves can be generated during the hurricane season of which, on average, only one in ten matures into a full-blown tropical cyclone. The waves form around a north-south axis as a “Λ”-shape and are embedded in the prevailing easterly winds where they get a push from the Bermuda-Azores High. Fair weather with cumulus clouds running northeast to southwest usually precedes the tropical wave. Often the only indicator of its arrival is a pressure drop, a drop that invites the more complicated atmospheric turbulence that can follow.

The Approach of a Tropical Wave

A tropical wave is usually preceded by ________________.
A. tropical storms
B. good weather
C. heavy rain and cloudiness
D. heavy seas
Answer: B

The Passage of a Tropical Wave

What kind of weather would you expect to accompany the passage of a tropical wave?
A. Heavy rain and cloudiness
B. Good weather
C. Tropical storm
D. Dense fog
Answer: A

The Characteristics of a Tropical Wave

Low pressure disturbances, which travel along the intertropical convergence zone, are called __________.
A. permanent waves
B. tropical storms
C. tropical waves
D. tidal waves
Answer: C
What kind of pressure systems travel in tropical waves?
A. Subsurface pressure
B. Terrastatic pressure
C. High pressure
D. Low pressure
Answer: D
In the Northern Hemisphere, what type of cloud formations would you expect to see to the West of an approaching tropical wave?
A. Cumulus clouds lined up in rows from Northeast to Southwest
B. High altostratus clouds in the morning hours
C. Cirrostratus clouds lined up in rows extending in a Northeast to Southwest direction
D. Cirrostratus clouds lined up in a North to South direction
Answer: A  (Tropical waves are lows so clouds would have vertical development.)

The Behavior of a Tropical Wave

A tropical wave is located 200 miles due West of your position, which is North of the Equator. Where will the wave be in 24 hours?
A. Farther away to the West
B. Farther away to the East
C. In the same place
D. Closer and to the West
Answer: B (Easterly waves move west, bringing with them cloud formations consistent with low pressure systems.)
You are in the Northern Hemisphere and a tropical wave is located 200 miles due East of your position. Where will the wave be located 12 hours later?
A. Farther away to the East
B. Nearby to the East
C. In the same position
D. Farther away to the West
Answer: B (Easterly waves move West.)

The following mesmerizing video illustrates, through a series of high-resolution global simulations, African easterly waves and the formation of Hurricane Helene in 2006.