Watching a thunderstorm build up is interesting, provided you do it from a safe distance.
At first, the sky fills with friendly looking puffy clouds. In the heat of the afternoon, they get increasingly bulky. Bubbles of white expand upward as the air rises.
Soon there’s a dark margin around the cloud bottoms where they are shading the sunlight. At the top, towerlike structures develop.
The storm that’s forming may now be some 20,000 feet tall. The cloud base gets broader.
As the system keeps growing, the top of the thunderstorm cloud is dragged apart by the wind, taking the shape of a baseball hat. At this point, thunder is probably heard, and rain starts falling.
Soon, a full-blown thunderstorm with heavy downpours is in progress.
The physical process is harder to understand than it is to watch. The initial energy that made the early cotton-ball structures expand was heat from the ground.
Pilots of gliders (motorless airplanes) seek them out because air rising underneath provides lift for the plane. By circling around growing storm clouds, a glider can stay aloft for a long time.
Inside the cloud, water vapor, which is water in its invisible gaseous form, turns into liquid. That’s because the air cools as it rises, and loses its ability to hold all the vapor as a high-energy gas.
But heat cannot be “destroyed.” It can only be transferred. So the energy that was stored in the water vapor comes back into the air as heat energy.
It’s like having a credit card that pays a rebate on every dollar you spend, giving back a certain amount of what was expended. This “energy rebate” in the form of heat makes the air rise faster.
In turn, more condensation occurs, and more energy is released by the water vapor. More droplets form, building the highly visible towers that make up the storm cloud.
When the water droplets are packed so densely that they collide, they combine to form raindrops. Very strong heat release inside the cloud takes its top to great heights, reaching 7 or 8 miles up in the most severe storms. Raindrops developing at such height fall with great speed, producing strong downpours.
Overall, it’s heat from the ground that started the process, but the bulk of the energy comes from the moisture contained in the air.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University.