They talk about weather issues in this video at 3:40.

http://www.youtube.com/watch?v=QkYFIhJ3rnY

In summary: They can handle darkness and rain, but not snow.

If you're interested, Sebastian Thrun has an intro to the topic as this course (you can preview all videos and problems)

http://www.udacity.com/overview/Course/cs373/CourseRev/apr20...