From SKYbrary Wiki
The Stratosphere is characterised by a temperature which is steady or increases with height. Towards the top of the layer, the average temperature is only a little below 0°C. The relatively high temperatures in this region of the atmosphere is due to the presence of small quantities of Ozone.
Flight Safety Issues
Generally, flights conducted in the Stratosphere will not encounter weather except where particularly vigorous thunderstorms extend above the Tropopause. Other flight safety issues associated with flight in the Stratosphere include: