Storing Air Tanks Outside??

In the shade where it will remain cool. Heat builds pressure. Maybe up off of the ground, too.

Be advised that big temp swings will change the pressure of an organic gas (air) substantially, maybe as much as a couple/few hundred psi in a 4500psi tank,, from in the sun at 95° to a cool 40° night, or even sitting in the snow at below freezing. The above tip by FF to store in the shade and up off the ground are good ones,

I'll add, might want to make a rack to secure/strap them down firmly in a horizontally position with the valve protected/pointed out of harms way just in case something falls on it. That way it can't tip over and fall down maybe hitting/damaging the valve. Unlike many DOT approved HP gas storage tanks that have big caps that screw over the valve to protect the valve, ours do not, so keep that in mind when you choose where to store them...Are these cf scba or scuba steel/alum tanks..?

Sunlight even if indirect will degrade the resin that holds the fiberglass and carbon fibers in place. Also temperature changes will cause dew point effect on tank and it will get wet. Tanks need to be kept in a cool dry steady temperature for the best service. If you have to store them outside I would make a well insulated box for them that is water proof from rain. There is an insulation called Armaflex that is a rubbery foam that is used in industrial pipe installations that would work great not sure of cost though. MM

Great answers and info above!

To add to what Metalmaniac mentioned... 
Sunlight [UV] can and will degrade the top coat / material that binds and protects carbon fiber [tanks in this case].
When I owned/operated my custom body shops, I used to refinish carbon fiber hoods, spoilers, etc. all the time.
The only exception would be IF the manufacturer used a high grade UV resistant clear / top coat.

I highly doubt that CF tanks would be any more resistant than auto parts that were meant to be outside / under the sun..

So, that said, use precaution!

These tanks very in pressure anyways - effecting their size. They'll expand and contract ever so slightly, 
and with that movement, the materials are disrupted.. Trying to keep them as consistent as possible is the best.

Sam -

I didn't even think to add this before, so I will now..
I used to do a LOT of TIG welding, and have several large argon and specialty mixed gas tanks out in my shop..
Here in NY, the weather and temperatures fluctuate pretty drastically, and I always notice a bit of difference 
in the pressures of the tanks when I check on them from time to time.
Temps will range from 100+F in the summers to -20F in the winter. Other than that, they're well protected..
I still don't like it, and wish that I could keep them all at a constant temp.

Sam -

So storing them in a garage that is kept above freezing would be ok? How about in the hot garage in the summer as long as its out of sunlight

I have a huge 6000 psi steel Nitrogen tank that stays outside strapped to one of the supports of my metal carport. Been using it this way since 2014 with no problems. In the summer, I top off my 4500 psi SCBA tanks as soon as the fresh nitrogen tank is changed out. That way I have a buffer if the Nitrogen pressure expands from the heat? I keep my SCBA tanks inside though!

You can actually tell the pressure fluctuates on cold days verse hot days when filling my SCBA tanks from it. It usually lasts me 8 months before I have to change out for a freshly filled tank.