If there are viruses on the ground and viruses in the water, one might expect there are viruses in the sky as well.

And boy howdy are there ever. Scientists have just found that hundreds of millions of viruses per day are deposited above the lowest layer of the atmosphere.

This could explain a curious phenomenon - how almost identical viruses end up at wildly distant geographical locations and varying environments.

Of all the microbes on the planet, viruses are the most abundant, with an estimated nonillion (10^30) in the ocean alone. And of course we know viruses can be airborne - that is one of their major transmission methods.

Previously, the USDA Forest Service established that over a trillion viruses per square metre rains down every year.

That, as it turns out, is a conservative figure.

"Every day, more than 800 million viruses are deposited per square metre above the planetary boundary layer - that's 25 viruses for each person in Canada," said University of British Columbia virologist Curtis Suttle.

He's one of the senior authors of a new study that, for the first time, quantifies the number of viruses being swept up into the free troposphere above the lowest layer of the atmosphere - the planetary boundary layer where all the weather happens, but below the stratosphere, where planes fly.

"Roughly 20 years ago we began finding genetically similar viruses occurring in very different environments around the globe," he said.

"This preponderance of long-residence viruses travelling the atmosphere likely explains why - it's quite conceivable to have a virus swept up into the atmosphere on one continent and deposited on another."

The aerosolisation mechanisms of viruses - how they get airborne - are not well understood, but studies have suggested that, at least in some cases, they are swept up into the atmosphere mixed with dust and sea spray. We know bacteria are dispersed this way, so it makes sense that viruses can be, too.

Suttle and his team wanted to know exactly how many viruses were being transported to the altitude of 2,500 to 3,000 kilometres (1,550 to 1,860 miles).

They installed two collectors on platforms above the planetary boundary layer in Spain, in the Sierra Nevada Mountains, a region under the influence of a global dust belt.

They found that there were millions of bacteria and billions of viruses being deposited per square metre per day in the free troposphere.

The deposition rates for viruses were 9 to 461 times higher than the deposition rates for bacteria.

That doesn't mean the situation is dire - obviously we've been living with it just fine, and whether or not a virus can survive in a new ecosystem depends on whether there's a suitable host.

However, they can survive atmospheric transport, so there is a possibility that they can have an effect on a new ecosystem.

Viruses also aren't just pathogens. Recent evidence suggests they play a key role in the ocean's carbon cycle. There are also viruses called bacteriophages that help humans by killing harmful bacteria.

Dispersing in the atmosphere and staying there for a long time, the team writes in their paper, provides a mechanism of preserving the diversity of viruses, much like a sort of "seed bank."

"Significant downward fluxes of bacteria and viruses from the atmosphere may have effects on the structure and function of recipient ecosystems," they wrote.

"Rather than being a negative consequence, this deposition provides a seed bank that should allow ecosystems to rapidly adapt to environmental changes."

Their research has been published in the International Society for Microbial Ecology Journal.