Russell: I know that cloud, it’s a cumulonimbus. Did you know that the cumulonimbus…
Carl: Stayed up all night blowing up balloons, for what?
Russell: …warm air goes by cool air, and the airs go by each other and that’s how we get lightning.
Carl: That’s nice, kid.
– Up
Up may be my favorite Pixar film. You know, along with (almost) every other Pixar film. I love the pairing of lonely, grumpy, elderly Carl with curious, helpful, young Russell as they go off on an adventure to Paradise Falls in a flying house. Near the beginning of the movie, Carl finds Russell a bit annoying, so he tunes out Russell’s chatter. When Russell starts to talk about the cumulonimbus clouds he sees nearby, Carl doesn’t catch the warming.
Clouds are classified and named according to their height above the ground and their appearance. Cumulonimbus clouds get their name from the Latin words cumulus (meaning “heap” or “pile”) and nimbus (meaning “rain”). These heavy, dense clouds tend to form relatively close to the ground, with the base less than 6 kilometers (almost 4 miles) off the ground. Despite their low formations, cumulonimbus clouds form into the shape a mountain or huge towers 10 kilometers (over 6 miles) high off the ground. The top part is often flattened or spread out into the shape of an anvil due to high winds at that height.
Cumulonimbus clouds are known as thunderstorm clouds. As Russell describes to Carl, lightning comes from movement in the clouds. Warm, moist air rises upwards, forming an updraft. The water condenses as the air rises, and the clouds grow as more warm air rises and more water is added. As the water droplets grow larger, they become too heavy to be held up by the rising air, and they fall downward. Meanwhile, cool, dry air sinks through the cloud, creating a downdraft that pulls the water down as rain. The moving air and water creates electrically charged areas, which leads to lightning.
If Carl had been listening to Russell, he could have prepared for the upcoming storm and maybe avoided the chaos. Then again, the story may have played out differently, and isn’t character growth the point of every Pixar story?
Keep calm and science on.
Science On 
