Meteorologists are constantly monitoring “tropical waves,” but what exactly is a tropical wave and why are they watched so closely?
We know from experience tropical waves seem to precipitate tropical cyclones, which is the overall term for anything from a tropical depression to a Category 5 hurricane.
We also know they cross the Atlantic Ocean after moving off the African coast.
Here’s what you should know about tropical waves.
What is a tropical wave?
At its simplest, a tropical wave is an elongated area of clouds and thunderstorms that typically move from east to west. It’s also known as an easterly wave, according to the National Weather Service.
Diving a little deeper, a tropical wave is an area of low pressure in the atmosphere that moves west across the Atlantic basin, which includes the northern Atlantic Ocean, Caribbean Sea and Gulf of Mexico.
Want even more? The National Hurricane Center describes a tropical wave as “a trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.”
Tropical waves are usually seen on weather maps as straight or dashed brown or red lines.
Where do tropical waves come from?
While it may look as if tropical waves come from Africa, they actually originate much farther east, tracing their roots to India, to the Indian Monsoon, which typically lasts from June through September.
Those “seeds for development” for tropical waves originate from disturbances in the upper atmosphere that “rotate around the outer periphery of the Indian Monsoon high pressure area.” They then move west across Africa and into the Atlantic, carried by high winds in the atmosphere.
How often do tropical waves become tropical storms or hurricanes?
According to AccuWeather Senior Meteorologist Dan Kottlowski, 85% of all tropical storms can trace their origins to tropical waves.
“The number of tropical waves emerging from the African coastline tends to intensify as the summer progresses and hurricane season reaches its September peak,” according to AccuWeather.
How does a tropical wave become a tropical cyclone?
Whether tropical waves develop into tropical cyclones depends on environmental conditions in front of them and where they exit the African coast.
The Atlantic hurricane season runs from June 1 through Nov. 30.
“Conditions early in the season are generally unfavorable north of 10 degrees North latitude. Systems that stay farther south, closer to warmer waters near the equator, have a stronger chance of developing early in the season,” according to AccuWeather.
“Waves that pass near and south of the Cape Verde islands off the coast of Africa and develop into hurricanes before reaching the Caribbean are often referred to as Cape Verde hurricanes. Cape Verde storms account for many long-lasting and damaging hurricanes.”
NOAA’s National Ocean Service puts it this way:
The recipe for a hurricane is:
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“A pre-existing weather disturbance: A hurricane often starts out as a tropical wave.
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“Warm water: Water at least 26.5 degrees Celsius — which is 79.7 degrees Fahrenheit — over a depth of 50 meters powers the storm.
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“Thunderstorm activity: Thunderstorms turn ocean heat into hurricane fuel.
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“Low wind shear: A large difference in wind speed and direction around or near the storm can weaken it.”
“Mix it all together, and you’ve got a hurricane — maybe. Even when all these factors come together, a hurricane doesn’t always develop,” NOAA said.
Tropical waves need several conditions to become tropical cyclones
For a tropical wave to organize and become a depression, tropical storm or hurricane depends on several conditions, including:
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Warm water
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Plenty of moisture
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Low wind shear
If those conditions exist in front of a tropical wave, that helps it organize and strengthen as it moves west across the Atlantic basin.
How does a tropical wave become a hurricane?
When tropical waters are over 80 degrees, clusters of thunderstorms can develop. If conditions are right, those clusters can swirl into a tropical wave or even a tropical depression.
If the wave stays over warm water, has plenty of moisture and winds don’t tear it apart, it continues to strengthen.
How do hurricanes form? An inside look at the birth and power of ferocious storms
Once sustained winds reach 39 mph, it becomes a tropical storm and gets a name.
If it continues to strengthen and winds reach 74 mph, the storm becomes a hurricane.
This article originally appeared on Treasure Coast Newspapers: What is a tropical wave? How Atlantic waves become hurricanes
