We talk about "the cloud" as if it were a magical, even invisible, force. But exactly what is it? Even the simplest definition of the cloud - a place for data to be stored remotely instead of locally - doesn't always apply. For Hong Kong in particular, the cloud is anything but remote.
The news that Apple is to begin building a data centre in the city next year - the company's first in Asia, and primarily to fuel the iPhone and iPad's iCloud backup service - is a timely reminder that far from being out of sight, out of mind, the cloud is actually in our backyard.
Each time you update your status on Facebook, chat on Yahoo, or send an e-mail from Hotmail, you're using the cloud. Google, China Mobile and Verizon are just a few big names with data centres in Hong Kong's Tseung Kwan O industrial area, which is fast becoming Asia's main hub for such facilities.
"Hong Kong's data centre sector has grown rapidly in recent years," says Antony Ma, a consultant for North Asia at Verizon Enterprise Solutions. "The demand for data centres in the coming years is also on the rise. The government is committed to positioning Hong Kong as the prime location for data centres in the Asia-Pacific region."
The area's proximity to the mainland is the main attraction, although this is not just a Hong Kong phenomenon. "There has been a rapid demand spike in the data centre market across the Asia-Pacific region and globally in recent years," says Ma.
He's right: the total amount of data stored in the cloud surpassed 1.8 zettabytes last year. That's 1.8 trillion gigabytes, and a growth factor of nine in five years. It's not totally clear where all of that data actually exists.
"The world's largest technology companies tend to dominate the sector, but most are very secretive about the number and location of facilities they have, because they don't want to give away competitive advantages," says Edward Jones, CEO of PMB Holdings, which has recently expanded into the data centre market.
"Most common storage warehouses are in the US and EU," says Greg Mason, a partner at the Forensic Risk Alliance. "Although there has been some growth in Asia and Australia."
As it increases in importance, the cloud has become a potential target for terrorists. It's known that in 2007 al-Qaeda plotted - unsuccessfully - to infiltrate and destroy a major data centre in Europe.
"The widespread use of commodity cloud computing could change the landscape of cyber attacks by activist groups," says Peter Chadha, CEO of DrPete. "In future, actions could be simultaneous cyber and physical attacks at cloud provider sites to maximise damage and publicity."
Physical safety is also a huge issue for the keepers of the cloud. Most endeavour to build their centres away from earthquake zones and even flight paths. But what would be the effect of a major terrorist attack on a data centre?
"Should a key cloud service provider location be subject to an extremist attack or natural disaster, there could be a risk of widespread impact on the economy and social climate of that region," says Peter Allwood, a manager at Deloitte's Enterprise Risk Services practice.
This doesn't mean that the cloud should be avoided, just that data centres need continuity and recovery plans. Backups of backups are needed. All that data needs servers, cooling systems, a constant, uninterrupted supply of electricity and land. Welcome to "dirty data", which is fast becoming a problem.
"Data centres consume a vast amount of power - in fact, it's estimated that 1.5 per cent of all the world's electricity is used by them," says Jones, who claims that much of this energy goes towards preventing equipment from overheating.
A Greenpeace report earlier this year called for data centres to use less coal and nuclear power, criticising Apple, Amazon and Microsoft for relying too much on coal-burning power stations, but praising Google, Yahoo and Facebook for their increased use of renewable energy.
The report revealed that some data centres are so large they can be seen from space. If the cloud were a country, its electricity demand would rank fifth in the world.
As more people around the world use the cloud to store and share photos, videos and documents, the size of the servers behind it are expected to triple in the next eight years alone. The cloud is modern convenience writ large, but we shouldn't overlook its massive physical presence.