Bombs, lows and other cyclones

Earlier this week, we saw reports about a “bomb cyclone” off the east coast of Australia. It’s an exciting name for a dramatic storm, but what does it actually mean – and what does it have to do with other cyclones and storms?

What is a cyclone?

When we say “cyclone”, we’re usually thinking of the intense storms that form in the tropics – with names like Yasi or Tracy. These are technically “tropical cyclones”, and you’ll notice that’s how they’re referred to in news reports and official communications. (In other parts of the world, these storms are called “hurricanes” or “typhoons”.)

In meteorology, a “cyclone” can mean something broader: a rotating mass of air around an area of low pressure. These cyclones can form outside the tropics as well. But these are usually referred to as low-pressure systems in weather reports, to avoid confusion. (High-pressure systems are sometimes called “anticyclones”.)

A “bomb cyclone” is a low-pressure system that forms outside the tropics, and usually forms quickly. It generates strong winds, and often heavy rain or snow.

“We reserve the term ‘tropical cyclones’ for those systems that originate in the tropics over warm waters,” says Joe Courtney, a tropical cyclone meteorologist at the Bureau of Meteorology.

“Tropical cyclones get a lot of energy from the ocean. Warm ocean water evaporates and condenses into clouds, and that releases heat, and helps to drive the cyclone,” explains Courtney.

“By and large, non-tropical cyclones, the ones forming further south, don’t rely upon the ocean water. The water’s obviously colder, so their energy comes from what’s happening well above the surface, in the upper atmosphere.”

Read more: Global warming affects tropical cyclone patterns

So it’s just tropical cyclones and non-tropical cyclones?

Obviously, there isn’t a clear line in the air along the Tropic of Capricorn. All weather systems can move and interact with each other.

“It’s a continuum from the tropics,” says Courtney. Low-pressure systems that form on these borders are sometimes called “hybrid” systems: “They have characteristics of tropical and mid latitude [systems].”

East-coast lows, which can bring heavy wind and rain to the east coast several times a year, are an example of a low-pressure system which has features of both the tropics and mid-latitudes.

This week’s bomb cyclone wasn’t technically an east-coast low as it came from the south, and it hung around for a shorter period than east-coast lows usually do.

Read more: El Niño happening more as climate warms

How is this communicated?

So what’s it like to examine all this complicated data from the atmosphere and then explain to people what to expect from an upcoming storm?

“Standard procedure is to relate it to an historical event,” says Courtney. “For example, we’ll say ‘it’s similar to one that happened in May, but it’s going to be stronger’.”

Explaining the frequency can also give people an idea of how serious a weather event might be.

“Warnings might say ‘this event probably only happens once or twice a year’,” says Courtney. “Or, ‘we haven’t seen an event of this magnitude for at least five years’.”

It’s also important to explain the impact of severe weather events – whether people can expect trees or buildings to survive heavy winds, for example.

“With cyclones, we have a hard time when we say ‘180 kilometres per hour’ – very few people know what that actually means. What’s that going to do to my house? Is my roof going to be safe?” says Courtney.

So, while calling it a “bomb” certainly adds to the drama, it’s most important to figure out how these weather systems will affect us.