A cyclone’s strength depends on its air pressure. The lower the pressure, the stronger the storm. Air pressure is the weight of the atmosphere. In a storm, air is rising, so the pressure is lower.
Typical surface-air pressure tends toward 1010 millibars. That’s how we measure how much air is sitting over us. Most of the big storm systems that sweep rain and snow across the United States clock in around 995 or 990. But for a storm to rank a “bomb,” it must rapidly intensify — it has to drop at least 24 millibars in 24 hours.
The storm expected to ride up the East Coast and strike New England looks as if it will be a classic bomb cyclone, with the expectation of a 50-millibar drop in about 24 hours.
When a storm strengthens this quickly, it’s a signal of how much air is being drawn into the storm’s circulation. It then spirals inward toward the center, rises and exits through the top. If more air is leaving the storm than is sucked inward, the pressure falls even more and the system will continue to grow.
It’s not rare, but bombogenesis is still a sight to behold from a meteorological perspective. It is most common in nor’easters, the fierce gales that spin up off the East Coast in the late fall and winter. They feed off the temperature contrast between the cold land and adjacent Atlantic waters still holding on to heat left over from the summertime.
As the bomb storms strengthen — the process called “bombogenesis” — many of these storms are accompanied by very heavy rain or snow, coastal flooding and hurricane-force wind gusts.
One overachieving low-pressure system in March 2014 dropped more than 40 millibars in 24 hours, easily earning the title of “bomb.” While most of the snow stayed offshore, the large circulation surrounding the system racked Cape Cod and Nantucket with 80-mph wind gusts.
Even more astonishing was the legendary Dec. 9, 2005, “Snowicane.” This quick-hitting but furious system hit Connecticut, Rhode Island and Massachusetts with an epic assortment of wild weather. Winds gusted to over 100 mph on outer Cape Cod while waterspouts danced off the waters nearby.
Farther west, more than a foot of snow fell, accompanied by whiteout conditions and thundersnow. Lightning even struck a plane as it landed at Boston Logan International Airport. This came as a result of pressure dropping an unbelievable 13 millibars in three hours, quadrupling the typical rate found during bombogenesis. As a result, an “eye” appeared in the center for a brief time, with clear skies and outer winds betraying what would soon return.