When does a winter storm become a bomb cyclone?

Blizzards in March, when our thoughts start turning to spring, are never good news. But warnings of “bomb cyclones” take the intensity to a new level. What does this ominous term, and related jargon like “bombogenesis,” tell us about the storm pounding states from Texas to Minnesota this week?

Let’s begin with the easy part: A cyclone – specifically, an extratropical cyclone, to distinguish from its tropical counterpart – is a large weather system with low pressure at the center and precipitation along cold and warm fronts. These storms are very common in autumn, winter and spring in the middle latitudes. The central and eastern United States typically see several over the course of a cool season.

What, then, distinguishes a “bomb” from a run-of-the-mill cyclone? The term was coined by famed meteorologists Fred Sanders and John Gyakum in a 1980 paper, and was inspired by the work of the Swedish meteorological pioneer Tor Bergeron. It describes a cyclone in which the central pressure drops very rapidly – an average of 24 millibars in 24 hours, at Bergeron’s latitude of 60 degrees north (the value becomes a bit smaller at lower latitudes). This is a lot when considering that variations of 10 or 15 millibars are typical over the course of any given week.

“Given their explosive development, it was an easy path to take to just call these systems ‘bombs,’ Gyakum said in an interview last year.

Wind speed is a function of the “pressure gradient” – the magnitude of the change from higher atmospheric pressure outside the cyclone to low pressure at its center, as well as how quickly the pressure changes over time. This means that a storm that rapidly develops an intense low-pressure region will have persistent strong winds.