What Are They?

“A microburst is a localized column of sinking air (downdraft) within a thunderstorm and is usually less than or equal to 2.5 miles in diameter,” according to a National Weather Service website. “A wet microburst is accompanied by heavy precipitation at the surface. Dry microbursts, common in places like the high plains and the intermountain west, occur with little or no precipitation reaching the ground,” said the National Severe Storms Laboratory website.

You may also see the term “macroburst,” which conceptually is similar to a microburst but has a diameter larger than 2.5 miles in diameter. Macrobursts, dry microbursts, and wet microbursts are all types of downbursts. So what causes such powerful winds to “burst” from thunderstorms so quickly?

How Do They Form?

“There are a handful of factors that cause microbursts to develop,” according to NWS. The list includes:

• Dry air entering the thunderstorm within the middle of the atmosphere.
• Evaporation of precipitation and associated cooling below cloud base.
• Sublimation, if the base of the cloud is above the freezing level — think of this as ice crystals changing directly to water vapor.
• The sheer load of rain or hail falling from the thunderstorm.

"Hybrid microbursts possess characteristics of both wet and dry microbursts…. Forced in the mid-levels by dry air entrainment and/or precipitation loading and in the low-levels by cooling beneath the cloud base and/or sublimation, wrote NWS Amarillo’s website.

Where Did The Term “Rain Bomb” Come From?

The Nova episode has stimulated questions about the term “rain bomb.” In full disclosure, I am one of the experts in the show. I can confirm that the episode is not inventing a new weather process. We are talking about “wet microbursts.” Where did the term come from?

A 2017 study published in the journal Weather traced the terms “water bomb” or ‘bomba d’acqua.’ Around 2014 the term became popular in Italy to describe cloud bursts, which were abrupt and damaging downpours. Andrew J. L. Harris and Massimo Lanfranco were the author the scholarly paper. They emphasized that the evolution of the term “water bomb” could cause confusion because it is similar to other terms like “bomb cyclone," a term introduced nearly fifty years ago by Fred Sanders and John Gyakum to describe explosive development of low pressure systems.

There are reports that New Zealand weather forecasters deliberately used the term “weather bomb” in the early 2000’s to convey significant rainfall and severe weather risks from a destructive storm. A 2022 article in The Guardian noted that Australian politicians referred to extreme rainfall from an atmospheric river as a “rain bomb. That storm produced over 20 inches of rainfall in Brisbane, according to NASA.

The term “rain bomb” used in the Nova episode is likely a further evolution of “water bomb." In a 2016 You Tube video, a wet microburst is referred to as a “rain bomb.” The same year, a Washington Post Capital Weather Gang writer posted a scathing rebuke of the term. A year later, the same outlet used the term to describe a wet microburst over Dallas, Texas in its “Pic of the Week” section.

It is certainly reasonable to question the evolution of the term when wet microburst terminology exists. In the current media and social media landscape, the term “rain bomb” likely grabs attention in a manner that wet microburst does not. We see it all of the time. Atmospheric river or “Pineapple Express” may resonate (or not) with the average person better than “narrow plume of water vapor originating in the tropics.”