During this recent deluge, the media headlines were all trumpeting bomb cyclones and atmospheric rivers, weather terms that have become popular in recent years to describe certain types of storms. The terminology is used to hype or sensationalize these weather events — events that are neither new nor unusual.

At the recent annual meeting in Denver of the American Meteorological Society, this exact topic was discussed. As these terms enter our everyday language they are now being overused, to the point that the definitions are being stretched or misconstrued by the public when they talk about the weather.

It reminds me of the scene in the Wizard of Oz, “Lions and tigers and bears, oh my!” Now it’s “bomb cyclones and atmospheric rivers and polar vortexes. Oh my!”

Given that, I’ll explain what these trendy weather terms really mean.

Bomb cyclones

A cyclone in the atmosphere is basically a closed circulation around a low-pressure center. In the Northern Hemisphere, this would be a counterclockwise rotation. As opposed to an anticyclone, which is a clockwise circulation around high-pressure.

In short, a cyclone is a storm. A bomb cyclone is a rapidly developing storm, with what is described as an explosive deepening, averaging 1 millibar of pressure fall each hour for 24 hours. That is, the central pressure of the storm must decrease 24 millibars within 24 hours.

There have been plenty of storms throughout history that would have qualified as bomb cyclones. However, the term did not become popular until about 2017, when the moniker was used to describe powerful Nor’easters, out in the Atlantic Ocean. There have been many reported as bomb cyclones since then.

In late October 2021 a bomb cyclone formed off the Oregon and northern California coast that dumped up to 10 inches of rain in some locations, triggering mudslides and flooding.

January ’23 bomb

One of those storms this January qualified as a bomb cyclone, on Jan. 4 and 5, as it developed out in the Pacific and moved toward the West Coast. That storm was bigger in size and lower in pressure than most hurricanes.

This was just one in a series of storms that followed. None of the others became bombs, but all of them carried copious Pacific moisture to the West Coast. While these did produce excessive precipitation in California, here in western Wyoming all we got were the leftovers, as the storms dissipated, tracked north along the coast or south across Utah and Colorado.

Some bombs that develop out in the Pacific are also accompanied by atmospheric rivers, but ARs, as they are now popularly abbreviated, have strict criteria to meet the scientific definition in meteorology.

Atmospheric rivers

Let’s simplify it: The whole atmosphere is a river. It carries streams and swirls of moisture all around the globe, and distributes that moisture in various places on any given day.

A true atmospheric river is a long, narrow, transient corridor of deep moisture that is transported in a steady stream, 250 to 375 miles wide and up to 1,000 miles long or longer. The copious water vapor in an AR is supplied from a tropical or sub-tropical source.

The term “atmospheric river” first appeared in scientific literature in 1994. Since 2010 more research has been done to further define a true AR event, using the latest weather satellite technology and aircraft dropping instrument soundings along their paths.

It has only been within the last 10 years that the term atmospheric river came into vogue. Back in the day, we used to refer to this same phenomenon as a Pineapple Express, now considered an old school catchy weather phrase.

The bottom line is that there needs to be a tropical or sub-tropical connection to truly qualify as an AR. Not every stream of moisture that rammed the West Coast this December and January had that connection, nor did they all meet the criteria for width, length or depth of moisture to qualify as an atmospheric river, in the scientific sense of the term. But that didn’t stop the media from saturating the news with that term.

I am out of space this week, so the term “polar vortex,” another overused and often misused term, will have to wait for another column.

Just a final observation, though, from my 40 years as a meteorologist: It is only the names that have changed, not the frequency or strength of these storms.