What’s a “bomb cyclone" and how bad is it?
If you’ve been reading the news over the last few days, you’ll likely have seen coverage of a #bombcyclone that is due to cause chaos across North America. This “bomb” is an actual meteorological term for a particular type of intense storm in which the pressure falls by at least 24 millibars in 24 hours. Such a “bomb” seems to be developing in North America and is set to bring temperatures and weather conditions that are hazardous in the extreme.
Insight: a millibar (mb) is a unit of air pressure which equates to one thousandth of a bar, or roughly one thousandth of the average air pressure on the surface of the Earth (average air pressure is 1013 mb). For American readers, 1013 mb is 29.91 inches of mercury.
The causes of the current phenomena we are seeing are the presence of adjacent and opposed air pressure systems centred over North America. There is an area of intense high-pressure centred over northern Canada (intense at less than 1049 mb) that extends to the Texas and the Gulf of Mexico, and a similarly intense, but low (around 982 mb and falling), pressure system sat to the East. A high-pressure system is a descending body of air which forces air away from itself (i.e. generates winds) in a clock-wise direction, whereas a low-pressure system is an ascending body of air, drawing air in (i.e. more wind) but in an anti-clockwise direction. The current setup is shown below:
The air pressure status over North America at 1000 GMT on 23 December 2022 (source: windy.com)
Such a setup over North America right now is sucking Arctic air south over the continent, into areas that would not normally experience such extremes of temperature. This situation is also disrupting the high-level Polar Vortex ,and the lower altitude Polar Jetstream (see diagram below). This disruption is drawing the Polar Jetstream, and in turn the Polar Vortex, right down over the Lower 48, and it is precisely this that is bringing the Arctic air masses southward; these would normally be constrained at much higher latitudes.
Where these frigid air masses meet the warmer, moisture-laden air masses from the lower part of the continent, strong winds and precipitation are generated which, at these low temperatures, will bring significant blizzard conditions. Some forecasters are predicting that over the forthcoming Festive Season, a storm unprecedented in size and intensity, may develop, with the US National Weather Service even suggesting a “once-in-a-generation”.
Temperatures may fall to below 0°C (33°F) over much of the continent, even as far south as Houston and New Orleans; Little Rock may experience temperatures of -17°C (0°F) on Friday night and even Florida’s Panhandle will experience freezing temperatures. Snow depths of over a metre (3 ft) are predicted in the northern parts of the Mid-west, although fortunately, such flurries are not expected to make it too far south.
Such conditions, bringing a cold and potentially white Christmas to many, are likely to cause chaos for travellers during December’s festive weekend. Excessive delays are already being experienced over much of the US (see below) and are unlikely to improve anytime soon. Those planning to visit far-flung friends and relatives for Christmas may face disappointment.
Flight delays as of 10.00 GMT on 23 December 2022 (source: flightstats.com)
Is this due to climate change?
This is something which scientists cannot be completely sure of (although most are pretty sure that it is). Such events, whilst rare, have happened before. Readers may remember the extreme winter weather that reached Texas back in February 2021, causing disruption and power outages. What is unique about the current situation however (the “bomb”) is the sheer scale of the event, with most of the US affected (and only Florida getting off lightly). The underlying cause of these events, as has been alluded to, is the abnormally south-reaching Polar Vortex. The vortex is normally kept in check (and much further north) but scientists have begun to notice that it, and the Polar Jetstream which is closely associated with it, have become less stable of late. This has made excursions of extremely cold air so far south, much more common (the UK experienced extreme cold as a result of these same processes, just last week). Such destabilisation, and more frequent cold weather events, have been attributed to our changing climate and to the warmer temperatures associated with it (Cohen et al. 2021).
Over the next few days, let’s hope the infrastructure of US states not used to such extremes, can withstand the inevitable pressure on it, but longer term, we need to be more prepared to see more such events. Climate change is not just a warming of the average global temperature, but it is the knock-on effects that this has on the planet’s entire climate system. We all know that the jury is out in terms of whether or not it’s too late to stop the warming from becoming too intense, but we can all do our bit in the hope that it has an effect, and we all must gear up to an increasingly hazardous atmosphere in the coming decades.