Ever notice how in many parts of the world, puffy, cauliflower-shaped cumulus clouds are more common in the summer? There’s a reason for this: thermal convection. In winter, the sun has less time to heat the surface and cause instability in the atmosphere. But during the summer, heat from the sun warms the land surfaces so much that pockets of hot air—scientists call them thermals—bubble upward much like steam in a pot of boiling water. As the hot air rises, the water vapor trapped within condenses into microscopic cloud droplets. If the air is humid enough, rapidly changing cumulus clouds puff up in the atmosphere, sometimes bulging to heights above 39,000 feet. Watch in the visualizations below—based on a climate model that simulated cloud formation during a Southern Hemisphere summer—how cumulus clouds pop up over the forests of Africa and South America.