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 rocket launches, according to Jason Fontenot, the commander who oversees weather monitoring for the mission.
• Elon Musk says his company's rocket and spaceship are designed to withstand lightning, but it wouldn't be "wise" to take the risk.

SpaceX and NASA were forced to delay their first astronaut launch on Wednesday, largely because the sky held too many fluffy clouds.

The high-stakes demonstration flight, called Demo-2, is set to fly people in a commercial spacecraft for the first time ever. If successful, the mission would resurrect the US's capability to launch its own astronauts and kick off a new era of commercial space exploration.

SpaceX, which was founded by Elon Musk in 2002, designed and built its Crew Dragon spaceship with the help of NASA funding to shuttle astronauts to and from the International Space Station. It's currently perched atop SpaceX's Falcon 9 rocket in Cape Canaveral, Florida.

The next launch attempt will be on Saturday at 3:22 p.m. ET. The same weather conditions clouded the skies all of Saturday morning, but forecasters say the clouds may clear before launch time. Mission commanders will watch the weather closely in the hours and minutes leading to liftoff.

Mission commanders scrubbed Wednesday's scheduled launch just 17 minutes before liftoff, since storm clouds created unsafe weather conditions for the rocket and the two NASA astronauts inside the spaceship.

"You have the bubbling clouds out there — those big white puffy clouds that look like big cotton balls — those are cumulus clouds," Jason Fontenot, who oversees the team that monitors and forecasts weather for rocket launches at the 45th Weather Squadron, said in a call with reporters.

"Those provide the biggest threats when it comes to launching rockets," he added.

That's because the clouds' bubbliness comes from vertical development, which creates electric charge inside the cloud.

"So if we launched near that cloud, we could potentially induce or trigger a lightning strike from that cloud," Fontenot said.

That happened to the Apollo 12 mission in 1969. The astronauts inside the rocket felt the lightning strike, and it disabled nine nonessential instrument sensors.

"Falcon/Dragon are designed to withstand multiple lightning strikes, but we don't think it would be wise to take this risk," Musk tweeted on Saturday.

Of the 10 lightning-related weather conditions that could scrub a launch, the presence of cumulus clouds is the most common.

"We had those hard requirements that when something hits, there's nothing we can do about it," Fontenot said of the weather restrictions. "So yes, it was kind of disappointing, but I'd much rather launch during better weather, and hopefully we'll get a chance on Saturday."

The 45th Weather Squadron projects a 50% chance of safe conditions for the upcoming attempt. The main concerns are rain, thunderstorm anvil clouds, and those puffy cumulus clouds.

'Scrubs are part of conducting spaceflight safely and successfully'

If weather doesn't scrub the mission again on Saturday afternoon, the Falcon 9 will launch astronauts Bob Behnken and Doug Hurley to the space station. They will stay there for up to 110 days before the Crew Dragon brings them back to Earth.

"Scrubs are part of conducting spaceflight safely and successfully. During my last mission to [the space station], weather caught us too," Behnken tweeted on Friday. "We're ready for the next launch opportunity!"

The last time the United States launched humans into space from American soil was in 2011, when the last space shuttle made its final voyage into orbit. Since then, NASA has relied on Russian Soyuz rockets to ferry its astronauts to and from the International Space Station, a practice that has become increasingly expensive and limited US access to the station.

But that could all change this weekend.

"It is a pretty surreal experience and I'm extremely grateful to be a part of this big moment in history," Fontenot said.