Desert Dust goes On Tour...

Wow. Weren't those surreal scenes in Sydney yesterday, as swirls of red Outback dust enshrouded the city? It was like some special effects from an Armageddon sci-fi blockbuster. I almost expected The Terminator to appear from the gloom.

It's not the first time Australian cities have experienced this sort of dramatic weather. Back in February 1983, it was Melbourne's turn to choke in the orange gloom of dust, blown-in from the parched interior of Victoria. Much like this week's event, the prevailing weather was one of prolonged drought in parts of Australia: devastating wildfires followed the dust storm and again, much like now, a wide-scale El Niño event was underway in the Pacific Ocean, bringing profound change to the region's climate.

The smaller-scale mechanism bringing such drama varies. Sometimes, it's the passage of a 'dry' cold front, carrying the dust aloft as it moves across the landscape; at other times, the urban sandblasting comes courtesy of thunderstorms, as powerful outflow of gust front winds whips-up the parched landscape into an ominous approaching wall of dust.

Here in Britain, the only time we see tend to see our streets as red as Sydney's is when Man Utd parade yet another premiership trophy (i.e., almost annually in recent years).

But we also get our share of desert dust, too. And it's quite likely you've seen it.

In fact, I can think of a number of occasions in the past decade when - often after a spell of overnight summer showers - - I've discovered my car windscreen, roof and bonnet covered in a fine film of curious fine dust, ranging in colour from a light grey-yellow to almost ochre-red.

It's been carried here all the way from the Sahara - originating in parched dustbowl regions, such as southern Algeria, Mauritania, the Bodélé depression in Chad, and elsewhere.

Sometimes, this all gets blown westward off the Sahara and out over the Atlantic, where vast quantities - estimated at around 500 million tonnes a year - are an important source of nutrients for huge plankton blooms. Some of it journeys even further afield. Around 40 million tonnes of dust blows out of Chad each year, all the way across to the Amazon where it provides important minerals for trees and plants.

Once over the Atlantic, the Saharan dust can get caught-up in the train of depressions running back towards the British Isles, blowing it all eastwards again to be later deposited onto our streets.

More often however, the dust is carried fairly directly - on a warm southerly flow, straight up from Northwest Africa, across Spain and France at about 10,000ft altitude, and then washed-out over the British landscape in showers. If the showers are especially heavy however, the evidence tends to be washed-away as soon as it settles.

But cleaning it off our cars is a mild repercussion of this amazing process. It could deliver far more potent effects to our shores.

Some scientists hypothesized that the costly outbreak of Foot and Mouth in the UK during February 2001 started after a mass of infected dust was blown out of North Africa. Not proven, but a viable theory, for sure: the potential for nasty micro-organisms to be transported across vast distances in swirling dust is certainly very real.

Maybe Sydney's Armageddon scenes aren't quite so removed from flights of Hollywood sci-fi fantasy after all...