It was raining that morning but the rain stopped by the time AF One arrived in Dallas.flown-the-coop said:
That is my understanding as well. But as Guitersoup pointed out, the bubble was there primarily for weather and if memory serves there were thoughts that there may be sprinkles and JFK wanted to go topless and take his chances.
I do not believe the bubble was part of any conspiracy, just another example of all the pieces having to come together for a successful assassination.