What exactly is an atmospheric river? Extreme weather terms defined

Third atmospheric river hitting B.C. District of Hope Mayor Peter Robb speaks about the incoming storm and misinformation being spread.

Barrie — Communities in southwestern B.C. are bracing for more heavy rain forecast to hit the region on Tuesday, as a third atmospheric river moves in.

But what exactly is an atmospheric river?

Here’s a look at some commonly used extreme weather terms.

ATMOSPHERIC RIVERS

According to the U.S. National Oceanic and Atmospheric Administration (NOAA), atmospheric rivers are “relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapours outside of the tropics.”

The NOAA said these vapour columns move with weather systems, and can carry water vapour equivalent to the average flow at the mouth of the Mississippi River.

When they make landfall, the vapour is released as rain or sometimes snow.

Though not always the case, the NOAA said atmospheric rivers can “disrupt travel, induce mudslides and cause catastrophic damage to life and property.”

What’s more, a 2018 study led by researchers with NASA’s Jet Propulsion Laboratory suggests that due to climate change, atmospheric rivers may become slightly less frequent, but will last longer, and become more intense.

THERMAL INVERSION

According to Brittanica.com, thermal inversion -- also known as temperature inversion – occurs when warm air overlays cool air in the lowest atmospheric region, called the troposphere.

In a normal situation without inversion, warm air is closest to the earth’s surface, with cooler air on top.

The NOAA said temperature inversion occurs because air near the ground cools more quickly than air “aloft.”

“This is most likely when the sky is clear and the wind is light/calm,” the NOAA website reads. “Cooling will occur the most readily in low places (such as valleys sheltered from the wind.)”

The problem, the NOAA said, is that because warm air rises, cooler air under the inversion can’t escape, meaning pollution and smoke becomes trapped.

HEAT DOME

According to the American Meteorological Society (AMS), a heat dome is an “exceptionally hot air mass” which develops when high pressure in the air prevents warm air below from rising.

“Thus trapping the air as if it were in a dome,” the AMS said.

According to the NOAA, heat domes occur when high-pressure atmospheric conditions combine with “influences from La Nina, creating vast areas of sweltering heat that gets trapped under the high-pressure dome.”

As heat domes are pushed over land, they can cause heat waves.

BOMB CYCLONE

A bomb cyclone, also known as bombogenesis, is a rapidly intensifying storm.

Meteorologists use millibars – units of air pressure in the metric system – to measure atmospheric pressure.

According to the NOAA, bombogenesis occurs when a “midlatitude cyclone” rapidly intensifies, dropping at least 24 millibars in the span of 24 hours.

The agency said this happens when a cold air mass collides with a warm air mass, “such as air over warm ocean waters.”