Puffy 'cotton ball' clouds are a rocket launch's most common nightmare. Here's why they delayed SpaceX's historic flight.

· SpaceX's first launch of NASA astronauts was scrubbed on Wednesday because of poor weather. The next attempt is scheduled for Saturday.
· Rockets launching through puffy cumulus clouds can trigger lightning strikes, endangering the astronauts they carry.
· Those "cotton ball" clouds are weather's "biggest threats" to...