What does 'tropical moisture' mean?
Tropical air masses have a much higher water vapor content than air masses originating from other parts of the globe.
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As hurricane season gets underway, meteorologists will increasingly use the term "tropical moisture" when referring to weather systems having to do with hurricanes or tropical storms, but what exactly is it?
It's essentially an adjective used to describe a characteristic of an air mass that originates from the tropics – the regions of Earth that are immediately around the equator and extend from there to approximately 23.5 degrees latitude north and south of the equator.
Tropical air masses have a much higher water vapor content than air masses originating from other parts of the globe. This is why tropical regions are often associated with daily rain, high humidity and rainforests.
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This graphic depicts a seven-day loop of moisture in the air, showing that most of the Earth's moisture is concentrated in the tropics.
(FOX Weather / FOX Weather)
The amount of water vapor in an air mass is measured in precipitable water – the amount of water vapor contained in a column of air if it were to be condensed and collected. It’s expressed in inches.
One of the main reasons tropical air masses have so much more water is the tilt of the Earth's axis with respect to the Sun. The tilt allows the tropics to receive the most sunlight on average of any point on the planet, and absorb more heat than the region radiates back into the atmosphere.
All that heat keeps the weather in tropical regions consistently hot and humid, and the waters warm.
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A pedestrian walks through a flooded street caused by a deluge of rain from a tropical rain storm passing through the area on June 4, 2022 in Miami, Florida.
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This warm water, specifically water over 80 degrees, is what's needed to sustain cyclones such as tropical storms and hurricanes. Warm water allows for continuous cloud development and increases the amount of precipitable water as compared to drier areas. That leads to abundant, soaking rains and more heat trapped in the region.
While tropical systems always bring heavy rain and high humidity, it is possible for non-tropical systems to take on tropical moisture by interacting with atmospheric circulation patterns that allow that moisture to flow farther north out of the tropics. That's why areas along the Gulf and East coasts of the U.S. occasionally see flooding rain from systems that don't originate in the tropics. Tropically infused systems are much less common along the West Coast of the U.S.