NYSE closed and stopped even electronic trading when Hurricane Sandy hit New York. Many questioned why the electronic trading has to stop. Well, I guess they don't really understand about computer systems.
I believe nowadays, people have been spoiled, no thanks to Google because it seems that regardless of rain or shine, access to Google just seem to work. No one bothered to question why so they just assumed that it should be easy, since Google is "free to use".
I do not know the real reasons behind why NYSE closed but based on my experience in the technology industry, I can hazard some guesses.
Power - I believe power should be a major concern and one of the key factors in closing the electronic trading because multiple scenarios could happen when Sandy hits New York. Yes, most data centres have backup generators, and backup UPS but just think about this for a moment. How long does UPS last? Maybe a couple of hours, but UPS is just meant to be there so that you can safely shutdown the systems. How about backup generators you might ask? I will then ask you what does backup generators get their power from? Fuel? Connection to another powergrid? What happens if Sandy cuts both of them?
If there is no guarantee on consistent flow of power to the systems, proper risk management dictates that you should shutdown the systems because you will
not want to encounter data corruption in your system.
Connectivity - Even if you can assure yourself of the availability of power, you also need to think of the connectivity to the systems itself. How do people connect to systems you might ask? Well, the same way you connect to the Internet at your house, through wire (yes, it is much more efficient than any mobile connectivity).
No one can guarantee that the connectivity will be available when Sandy hits New York. If the flooding is bad, the salt water may reach the connectivity lines and corrode it, causing corruption to the data that is being transmitted. This can be at the data centre end, or at the customers' end. No one will know, and it is extremely difficult to pinpoint where's the problem. Again, it's data corruption.
Service Level Agreement - Now with all being said, do you think NYSE is able to guarantee the service level availability of the system? If anything goes wrong, do you think that they will be able to marshal the resources to get people to fix it in the agreed timeframe? Do you know that the penalty for not meeting the service level agreement can be quite severe?
From the risk management perspective, it is definitely safer to shutdown all the systems and allow trading to resume when Sandy passes by New York, which was what they did. I think many people do not really understand the amount of effort done by engineers at the backend, ensuring that everything is working as intended.
That's why people say that ignorance is bliss.