New weather needs new words

We are taking a look back at stories from Cosmos Magazine in print. From heat domes to atmospheric rivers to bombogenesis, news headlines are constantly finding new words for extreme weather events. But how much of this lingo is meaningful and how much is just a muddled message from meteorologists? In March 2024, Nate Byrne told us what works – and what bombs.

The science of weather is both simple and hard – we know the physics, but the vastness of it all means errors abound.  The atmosphere is a huge beast. The bit that most of our weather happens in is just the lowest level, but even that stretches from the surface up to about 10–15 kilometres high, shaped by a complex dance of air of differing pressure, heat and water vapour content.

But perhaps the biggest hurdle of all to understanding weather is the lingo.

Unlike other disciplines, everyone knew about weather long before we decided to figure out how it worked, so the words to describe it were already a part of everyday life. Formalising that language seemed like the only way forward.

I’m talking bland words like “fine” (no rain) and sensual words like “sultry” (hot and humid), ominous terms like “oppressive” (hot with no wind) and the downright curious “haboob” (an intense dust storm). These have been part of the forecaster’s vernacular for as long as anyone can remember.

The atmosphere essentially is an ocean, just one filled with a different fluid.

But new weather needs new words. Now that we’re seeing increasingly odd and often record-breaking weather events in a warmer climate, language is adjusting to try to make sense of it all.

As is the case in every field of science, meteorologists talk in jargon. Weather-affected people (that’s everyone in the world) are hungry to learn more, and meteorologists are keen to explain exactly what’s happening. They are increasingly showing up at press conferences and in public, being pressed for any sort of explanation about the latest extreme weather, so their jargon is increasingly spilling into regular usage.

Like with any good jargon, these new weather words are only really useful to those who already understand them. But that hasn’t stopped journalists from grabbing hold of the most impressive-sounding words and pasting them across headlines in ways that can even leave forecasters scratching their heads!

Some of the relatively new lexicon entrants are primarily heard in the Northern Hemisphere, but they’re making their way down here – sometimes because they’re meaningful, and more frequently because they just sound cool.

I’m here to help. What follows is a brief and necessarily incomplete guide to some of the more unusual and recently popular weather terms you may have spotted in your news feed – along with a few corrections to the record, and a lot of weather nerdery.

Omega Block

First, let’s take a quick run through the basics of weather to set the omega scene.

When you see the weather synoptic chart (the map with highs, lows and isobars), what you’re looking at isn’t actually very representative of the atmosphere, but rather just a thin slice of it near the surface. It’s useful for describing the winds we experience down here on the ground, but move upwards a little bit and things can change very quickly – related, but wildly different.

You might have heard of an “upper disturbance” or jet streams – these terms describe what is happening in the higher reaches of the troposphere, which is the lowest layer of the atmosphere. And while we might not feel them directly, these atmospheric features (and often behemoths) have a direct impact on what’s going on down here on the surface.

As is the case in the ocean – and the atmosphere essentially is an ocean, just one filled with a different fluid – currents, pressures and temperatures change with depth.

In the upper atmosphere (a vague, variable term, because the atmosphere changes in thickness depending on the weather and – most importantly – heat, but on average let’s say it’s between 9km and 14km up) jet streams wrap around each hemisphere, blowing generally from west to east. There are usually two in the north and two in the south, named the subtropical and the polar jets for the rough latitudes they tend to inhabit.

Jets are winds racing at over 100 km/h high in the atmosphere, but at their strongest they can dwarf records here on the ground. While Australia’s Barrow Island recorded the strongest ever wind gust (at 408 km/h) during Cyclone Olivia in 1996, jets reaching similar speeds aren’t unusual.

Like air racing over an aerofoil, jets can create uplift – pulling air from the surface up into the upper reaches of the troposphere – or they can push air downwards. Generally, when air goes up, it encounters colder temperatures and lower pressures, and can hold less water vapour, so it dumps extra out as liquid water – creating clouds. Air moving downwards does the opposite and clears grey skies.

Things get even more interesting when jets get kinky. Because fluids just have to be difficult, nothing is neat and tidy. In their general west-to-east flow, the jets meander sometimes a little north or a little south, strengthen or die off, and sometimes completely split. Each variation can have a direct influence on the weather at the surface.

The omega block is a particular set of these meanderings that is semi-stable, and the diagram (left) shows how it works in the Southern and Northern Hemispheres. As the jet rushes in from the west it gets a kink that sends it southwards (or northwards in the Northern Hemisphere). It then curves in a clockwise direction (anticlockwise), almost completing a circle before kicking out in the opposite direction, ultimately ending up in an inverted omega shape Ʊ (or a right-side-up omega in the Northern Hemisphere Ω), which is where it gets its name.

At the surface, this results in a high-pressure system sitting roughly in the middle of the bowl, flanked on either side by lows. Because it’s semi-stable, that pattern can last for days, or even weeks, as energy from approaching systems is directed around the omega shape and reinforces it.

Near the lows, that can result in persistent rain and flooding. Under the high, it means clear skies and light winds, which can result in a…

Heat Dome

The high sitting in the bowl of an omega block remains in place for an extended period, allowing oppressive heat to build. While the air at the centre of a high generally moves downwards, the lows on either side raise the air, pushing it into the upper atmosphere to complete the loop.

Under clear skies, the sun shining on exposed ground heats up slow-moving surface winds to create hot, dry conditions. This can lead to extended heatwaves that can dry out vegetation and ramp up the fire danger.

In any other situation, a high means fine weather. A heat dome, nestled in an omega block, means unseasonably warm (and often welcome)weather in winter, and in summer can create extreme heat that in Australia is associated with more deaths than any other severe weather event.

Bombogenesis

Not entirely separate, but not necessarily related, this started as an unofficial meteorologists’ term – we use it to talk about a low-pressure system where the central pressure is dropping at an unusually rapid rate. Officially we’d say “explosive cyclogenesis” if the pressure is falling by more than 24 hectopascals (hPa) in 24 hours.

Every low is “cyclonic” even if they aren’t officially a cyclone, and at the surface they are usually associated with wet and windy conditions. The deeper (or lower) the low, the wilder the weather. This can occur in the lows straddling an omega block, resulting in dangerous systems like East Coast Lows that can cause widespread flooding and damaging winds for eastern New South Wales.

Essentially, to get the label of bombogenesis, the pressure in the centre of the low drops “like a bomb”. Add a bit of a penchant for a portmanteau with panache, and “bombogenesis” isn’t much of a promenade.

It’s a useful term to describe something that is rapidly changing, but it has also been misused (often by journalists) to describe a…

Rain Bomb

You will never hear this escape my lips with any sincerity.

Essentially, this is a corrupted term of the “bomb cyclone”, relating to rainfall rather than pressure, and applicable in some circles to anything from a severe storm to a cyclone to a persistent low.

In common parlance, it just means a lot of rain, delivered quickly. It’s the sort of weather that typically results in your local responsible meteorological organisation issuing warnings for heavy or intense rainfall, flash flooding and riverine rises.

Essentially, to get the label, the pressure in the centre of the low drops “like a bomb”.

The thing is, from a meteorologist’s point of view, “a lot of rain” really depends on where it’s falling. In Australia, the tropics can take a hundred millimetres in the gauges during the wet season without blinking, while similar falls in the south could shut down cities. As a result, “rain bomb” doesn’t have much inherent meaning.

It could be talking about a storm with a base just a couple of kilometres across, or a low that spans hundreds.

It sounds good in headlines, though.

Atmospheric River

This is another unofficial term, but one I can fully get behind because it helps to convey the enormity and majesty of the forces that cause our weather, without the tabloid bent.

Water vapour is an invisible gas – if it wasn’t, we’d never see the Sun – but flows in a way that we can spot by analysing the specific wavelengths of infrared light it emits (usually around 6–7 microns).

We all know the water cycle from school:  ocean water evaporates, moves up in the atmosphere, condenses into cloud and rains down, eventually flowing back to the sea and completing the cycle.

In reality, things are far more complicated.

Generally, warm tropical waters are a great source of deep moisture (think of how humid it is in the Top End). When conditions are right, moisture high in the atmosphere can be dragged towards the poles in a long, narrow band, creating a conveyor belt of water vapour. These are “atmospheric rivers”, so named because they sort of look like rivers, and can also carry the same volume of water!

(Fun fact: Atmospheric rivers flowing from the region around Hawaii over the continental US ahead of winter storms are called “The Pineapple Express”.)

In a tricky bit of maths that maintains conservation of angular momentum as the flow changes latitude (analogous to a pirouetting ballerina pulling in her arms and speeding up), polewards flow results in an air mass moving higher in the atmosphere the further it travels.

Atmospheric rivers often also come with strong winds and can result in big rain events. In Australia, a fairly recognisable local example is a northwest to southeast cloud band that ends up with rain on Uluru.

Finally (for now), let’s get out of the tropics, and talk about the…

Polar Vortex

Here, we’re coming full circle. TL;DR: omega block, but the opposite.

If we were to be more official, we’d call this a “circumpolar vortex”, but I’m not bothered about the contraction. It’s a permanent low-pressure system over each of our polar regions that generally keep the cold air in check, surrounded by the polar jet stream.

But we know about jets – they like to wander.

If you take the omega near the polar circle and flip it, putting a broad low-pressure system in the bowl, you end up with a semi-stable system that can bring a deep freeze to anything underneath it.

Typically you’ll hear about them in the Northern Hemisphere, but we get them too. The difference is that the North Pole is in an ocean surrounded by a lot of land (Canada, northern Europe and Russia) whereas the South Pole is on a continent (Antarctica) surrounded by a lot of sea (the Southern Ocean). We don’t talk about them much down this way because there aren’t many people who experience them.

Occasionally the pool of cold air can snap off. When that happens for us, it can cause a low that undergoes explosive cyclogenesis and can bring snow up right up into Queensland. It can also sometimes result in an East Coast Low (see Bombogenesis, above) as the pool of cold air conserves its momentum as it shifts towards the equator (reverse ballerina, see Atmospheric River, above).

It does sound cool (pun intended), but in this case I give headline writers a pass.

Long Story Short

Each and every one of us gazes up into the vastness of the sky at one point or another, but few of us can appreciate the true expanse and interconnectedness of it all – even I don’t, despite my training and forecasting experience.

Stories help us make sense of things that are much greater than us, and that’s what all of these terms are trying to achieve. Each has some inherent vagueness and fuzzy edges, but that’s okay so long as we can appreciate what they are really trying to convey. Hopefully I’ve helped clear some of the fog, but this is far from a complete list.

Pyrocumulonimbus, anyone?