Watching a rocket launch is an incredible experience. The enormous burst of flame from the bottom of the rocket pushes it up at a velocity that can put 2-4 G’s of force on the occupants on the low end. On reentry, that can go up to 8+ G’s of force.

That same force is applied to the rocket itself, which means that it needs to be built to withstand incredibly harsh conditions, and that is saying nothing about the hostile environments of space.

With all that in mind, many people are surprised to hear that something as simple as a cloud can cause a launch to be delayed or canceled. In November of 2025, the Blue Origin launch of its New Glenn rocket had to be postponed because of cumulus clouds. They explained in a post on X:

“Today’s NG-2 launch is scrubbed due to weather, specifically the cumulus cloud rule. We’re reviewing opportunities for our next launch attempt based on forecasted weather.”

The cumulus cloud rule is, at least in part, the reason almost all rocket launches have a window of opportunity that covers multiple hours, and even multiple different days or weeks. The people in charge of the launch need to have the flexibility to avoid unwanted weather conditions.

But what is this rule? Well, it is a rule from NASA, and they explain:

“Do not launch through cumulus type clouds with tops higher than the 41 degree F. temperature level. Launch may occur through clouds with tops as cold as 23 degrees F. if the cloud is not producing precipitation, there is a field mill within 2 nautical miles of the cloud, and this field mill and all field mills within 5 nautical miles of the flight path read between -100 volts per meter and +500 volts per meter for the past 15 minutes.”

The rule also says:

“Do not launch through or within 5 nautical miles of the nearest edge of cumulus type clouds with tops higher than the 14 degree F level. Do not launch through or within 10 nautical miles of the nearest edge of cumulus clouds with tops higher than the -4 degrees F. level.”

That is a pretty technical way to say that rockets should not be launched through or near various types of clouds. The reason for this is simple. Lightning.

All rockets going to space are filled with massive amounts of fuel. While the rockets are designed to be able to withstand a lightning strike, this type of event would add some risk of explosion.

When it comes to launching a rocket, even a small additional risk is something that is not worth it. Unlike commercial jets and other planes, rockets are not built with the expectation of an occasional lightning strike.

In addition, fuel makes up a much larger percentage of the overall mass of the rocket than it would on a plane. This would make it much more difficult to build a system that eliminates the risks associated with such a strike.

So, with this in mind, NASA and other space agencies have made it a rule that launches should only take place when the risk of a lightning strike is virtually zero.

If you thought that was interesting, you might like to read about why we should be worried about the leak in the bottom of the ocean.