Editor’s note: Don Lincoln is a senior scientist at the Fermi National Accelerator Laboratory. He is the author of “The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind.” He also produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are solely his.
Say what you will about weather being mundane small-talk, but it has a flair for inspiring dramatic ways to describe it. The term snowmageddon seems to show up every year these days, but there’s also snOMG, snowpocalypse, polar vortex and now the bomb cyclone.
Of course, this begs the question: Is the bomb cyclone a real thing, or just a bit of clickbait hyperbole? The foreboding moniker became famous last winter when the Northeast was hit with a powerful snowstorm and has re-emerged in the last few days as another storm bears down on the middle of the country. As those who have witnessed these tempests can attest, a bomb cyclone can indeed be a severe weather event. It usually comes with incredibly strong winds and very substantial precipitation. If one’s heading your way, it is completely reasonable to make appropriate preparations. Just take a look at some of the footage from the Denver airport.
But just what is a bomb cyclone and how are they made? Well, they’re not new and they’re probably not quite as dangerous as the name sounds because, well, “bomb” sounds super scary. Typically, a bomb cyclone occurs when cold winds blowing over land collide with warm ocean breezes. This causes a very fast drop in pressure and the result is kind of a cold hurricane.
The term “bomb” is a colorful one, originating from the speed at which the atmospheric pressure changes. On average, air pressure at sea level is about a thousand millibars (1,013.25 for the science wonks). Pretty much all weather originates from the changing of pressure, which can stem from temperature fluctuations, among other things. Changes of 12 millibars over 24 hours are relatively common. However, in order to be a bomb, the pressure must change by one millibar or more per hour for at least 24 hours. The recent storm, which experienced a change of 33 millibars from Tuesday to Wednesday, certainly satisfied that criterion.
When pressure changes so rapidly, weather conditions develop very quickly. In fact, the term that is sometimes used is “explosive,” which is of course from where the term bomb arose.
To give a sense of context, this change in pressure was accompanied by winds approaching 100 mph. By any judgment, this storm is rather severe. Were this weather event a hurricane, it would have satisfied many of the criteria for a category 2 storm.
However, the recent storm didn’t have the classic origins of a bomb cyclone. In this instance, a deep trough moved over the Rockies and encountered two jet streaks. A trough is an area of low pressure that dipped down from farther north. Jet streaks are places in the jet stream with especially high wind speed.
This chance confluence of events caused air in the upper atmosphere to move away from a central point called a divergence. Because the air was leaving, air closer to the Earth was sucked upward, resulting in very low pressure near the ground. And that quick drop in air pressure is what caused the bomb cyclone, with the damaging winds and precipitation. It’s an unusual origin for a bomb cyclone, which more often relies more directly on the interplay of warm winds over water and cold winds over land.
Dr. Philippe Papin, an atmospheric scientist at the Naval Research Laboratory, has tweeted some fascinating animations of the storm’s development, which reveal pretty impressive wind patterns with an enormous counterclockwise movement.
As I write this, I’m sitting in my office, just outside Chicago, looking west from my 11th-floor window and watching the clouds roll in. The storm is already weakening but according to the US Weather Service, we’re in for some heavy rain and wind gusts that could hit 40 mph. And, just to make things even more inconvenient, we’re looking at an upcoming temperature drop of about 30 degrees. In short, it’s a typical Chicago spring, albeit perhaps a little more dramatic than usual.
This storm isn’t done with America yet. It will continue to bring rain and snow to much of the middle part of the country, with flooding in the North and the potential for tornadoes in the South.
It’s said that March weather has a personality, “In like a lion and out like a lamb.” If that’s true, I think that this year we can look forward to a glorious April.
