Most people have now heard of the atmospheric river called the “Pineapple Express” and the heavy rain and snowfall it caused in California.

The Pineapple Express is one of several atmospheric rivers, or ARs — regions in the atmosphere that range from 250 miles to 375 miles wide and are responsible for most of the horizontal transport of water vapor from the tropics toward the poles.

Atmospheric rivers flow in the lowest part of the atmosphere only half a mile to a mile above the ground. A strong AR can carry an amount of water vapor 15 times the average flow of water at the mouth of the Mississippi River.

When they make landfall, they release this water vapor in the form of rain or snow, as recent severe flooding in California and heavy snowfall in the Sierra Nevada attest. But not all ARs cause damage; most are weak and simply produce beneficial rain or snow that is crucial to water supply, providing up to 50% of rainfall along the U.S. Pacific coast.

ARs result from wind associated with midlatitude storms that concentrates moisture into a narrow region just ahead of a cold front.

In the eastern Pacific there is a semipermanent high-pressure region (the subtropical high) around which air circulates in a clockwise gyre. It moves north and south with the changing seasons and is also influenced by the unpredictable jet stream.

This is the source of the tradewinds that characterize Hawaii’s weather.

The Pacific is so large that the subtropical high cannot sustain circulation throughout the entire ocean basin.

On the western side of the Pacific, midlatitude weather is controlled by low- pressure systems that form around 60 degrees latitude and move southeastward.

Air circulation around low-pressure systems is counterclockwise. The movement of the low- pressure system, like the subtropical high, is influenced by the jet stream. They form at more southerly latitudes during the winter.

As the low-pressure center gets caught in the jet stream, it eventually encounters warmer air circulating through the tropics from the clockwise gyre of the subtropical high.

A front or shear line forms when this warm, moist tropical air meets colder, drier polar air. The cold air tunnels under the warm air, lifting it and causing clouds to form.

The intensity of frontal storms depends on the difference of temperature and humidity between the two air masses, which is greatest during early winter months.

Even during the summer months when the polar and subtropical air masses have similar temperature and humidity, the convergence of clockwise and counterclockwise gyres causes a persistent northeasterly flow of air along a shear line that centers on the Hawaiian Islands.

Hurricane and tropical storm tracks reflect this as they nearly always make a right turn to the north near the Hawaiian Islands. In 1982 Hurricane Iniki got caught in that northeasterly flow and made a drastic northerly turn to hit Kauai head-on.

National Weather Service forecasters can now identify ARs, use them in numerical forecast models and monitor polar orbiter satellite imagery to provide advanced warning of the presence and movement of ARs in the Pacific. This allows for advanced warning of the Pineapple Express and its potential heavy rain five to seven days in advance along the Pacific coast from California to Washington.

The Pineapple Express is not just a catchphrase. It is one of the more significant weather phenomena that are just beginning to be understood.

Don’t blame it on Hawaii. We just happen to lie at the convergence of the great weather systems of the Pacific Ocean.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.