I want to buy a drone to use in the mountains, i have been looking at the new Mavic Air and the Phantom 4 Pro. Which of them is better at flying in -15°c and is it a good idea to fly a drone in cold weather.

I have also heard there will be a new drone coming in march, should i wait and see what that is or buy now?

Welcome aboard ! the importan thing is pre-heat the battery and motors before to fly on these hard conditions. About new releases , i have no idea , im a user also ;)

I can only comment on my experience with the P4P in cold temps.  I have the P4P and live at 8,000 ft elevation in Colorado.  I have flown the P4P several hours at temps below -5 F (-21 C ) with no problems.  I return when the low battery alert triggers at 30%  and get typical flight times of  20-22 minutes. Coldest temp I've flown is -18F (-28C) and had no problems.  DJI doesn't recommend flying at those temps but summers are short here in the Colorado mountains and I wouldn't get to fly much at sunrise if I followed the official recommendation.  I have seen others report problems so search the forums and make your own decision. But I was pleasantly surpised to see the P4P do so well in such cold conditions without even a hiccup.

Very helpful, thanks!

Oh, I have heard people say the iOS devices don't hold up well in the cold.  I have the 'Plus' controller and it has done well, but the pros & cons of that unit are well discussed in other threads. The other thing I should add is the equipment holds up better than my fingers. I found using a stylus for the touchscreen allows me to keep better gloves on & not have to remove them.

The spec is 32F My Obsidian loses GPS at 34F and any colder it will not leave the ground and when a bit warmer it will disconnect from the wind chill. Others have had the same issue and then there are others that have no issues. Luck of the Draw.

Hi there, it is not recommended to fly the drone at a low temperature which may cause permanent damage to the battery.

..and do not forget the dew-point temperature: the electronics inside the copter can harmed!

https://en.wikipedia.org/wiki/Dew_point

I flew Phantom 3 Pro in -20 quite a few time and had absolutely no problems, however i also flew P4S in -15 and it almost fell out of the sky.. I had a fresh charge on the battery and when battery reached 45% suddenly it went red and went critical warning at 0% in the matter of split second..., thankfully I was already back about 100 ft away from my take off position so I landed immediately.
From then on I do not trust P4 flying in the cold. I also flew P3P right down to 0% at one point and it still flew for 1.5 minutes, P3P was an amazing drone.
I am thinking about buying one again and keep it  on stock firmware for long range flights...