The phase changes of water are governed by the flow of heat energy between water molecules and their surroundings. When water molecules in the atmosphere absorb or release enough heat energy, a phase change occurs that strongly influences the type of weather we experience. The phases of water we observe with weather are: the solid phase, ice (sometimes seen as snow, sleet, or hail); the liquid phase, water; and the gaseous phase, water vapor.  

If water in the atmosphere is only absorbing energy, we will not see any cloud formation or precipitation nearby. Clouds form when water vapor releases energy to the atmosphere, changing state to ice crystals and liquid water droplets, which we then observe as clouds. When water is absorbing and then releasing a lot of energy in the atmosphere, a storm may soon develop.

The perfect conditions for a monster storm occur when a massive and continual supply of heat energy gets carried into the atmosphere by water vapor on convection currents from the surface of the ocean or land. This upward-moving warm air collides with cooler air, which allows the water vapor to transfer its energy into the atmosphere and form storm clouds. With a continuing supply of energy-laden water vapor moving up, the upper-level air parcel is forced to circulate downward to complete the convection. If this process occurs over a large geographic area, it can form tropical storms and hurricanes over the open ocean, or thunderstorms, supercells, or tornadoes over land.

 

The energy that drives these phase changes originates from the sun. Through the transfer of energy, Earth is kept warm by the sun, and this input of energy drives our water cycle and weather. Natural energy transfer processes on Earth therefore involve the absorption of energy from the sun, the retention of energy with the help of water and other greenhouse gases, and the eventual dissipation of energy back into space. It is the constant inflow of new energy from the sun that keeps driving the water cycle and other atmospheric processes, and keeps Earth livable for us.

The next time you look skyward, you will not actually see the transfer of energy happening, but you can certainly see the results. Watch the clouds—or even the lack of clouds—and you will have a pretty good indicator of the atmospheric processes happening far above you. As you watch clouds form or dissipate, imagine the phase changes of water that are happening in the air and try to follow the transfer of energy going on in the atmosphere.