Subject: What is the avg. (and max.) wind speed in the stratosphere?

I am designing a low power aircraft which will achieve an altitude of 
70,000 ft on solar power.  I would like to position the aircraft over a 
point on land, but I don't have much information about the nature of the
stratosphere.  Are there clouds at 70,000 ft?  Avg. temp? Any help or
direction would be much appreciated

Re: What is the avg. (and max.) wind speed in the stratosphere?