One of the challenges presented to weather forecasters in the summer is when the temperature from around 5,000 to 10,000 feet above the ground is very warm. This can lead to a condition meteorologists call a “cap.” The key ingredient to any thunderstorm is a rising column of air called a thermal. This column of air rises because a combination of heat and humidity makes it lighter than the rest of the air. As it rises rapidly, it cools, and the humidity condenses into lots of heavy raindrops.
But when the air is very warm at 5,000 to 10,000 feet, the air within a thermal loses its tendency to rise because it is no longer warmer and more humid than the surroundings. This cap keeps the thermals bottled up in the lower atmosphere and keeps thunderstorms from developing unless a pool of cooler air aloft breaks the cap, allowing storms to build rapidly.