Many of us don't have refrigerators for these 50 liter kegs and the CO2 tank. With due care not to expose the CO2 tank in direct sunlight, to what temperatures will these pressurized kegs be fine with 4% alcohol cider inside them ? How about 7% ?
I don't have all the math in front of me but a typical beer freezes in the mid 20's Fahrenheit. Lower alcohol might be 27F and higher might be 24F. There are a lot of different factors like the volume of liquid and wind and the alcohol. I would think that if you left a full keg out overnight and the temperatures dipped into the upper teens, you'd probably be ok with it not freezing completely through and busting the keg open. The larger the volume of liquid the longer the liquid will take a while to get to freezing. Long term storage outside is not recommended because of the swing in temperatures is not good for the quality. I store beer and wine outside all the time during the winter months.
Here is a good discussion about the freezing point of beer and it applies here.
There's a formula used for determining the freezing point of a given beer in the brewing industry, taken from this book:
Freezing point(°C) = -(0.42 * ABW% + 0.04 * OG[°P] + 0.2)
You can easily convert ABV to ABW by:
ABW% = ABV% * 0.82
Or you can get a formula for which you only need OG and FG, by:
ABW% = ~ 0.42 * (OG - FG[°P])