Climate and Weather
Answer to question posed in 5.1:
What energy source drives the cyclone? Latent heat. In fact, latent heat provides fuel for cyclones, storms and hurricanes,
Latent heat is the energy given up or taken up by the air or other substances as water changes phase, such as vapor condensing into liquid. It's a key to understanding weather because latent heat is a major source of energy for thunderstorms and hurricanes as well as being a factor in the transport of heat from low to high latitudes.
Let’s consider what happens during evaporation – as water changes phase from liquid to vapor. Fast-moving air molecules in warm air collide with liquid water molecules. The water could be falling rain drops or in a pond or a bowl. The collision transfers energy from the air molecules to the water molecules. This gives some of the water molecules enough energy to speed up, break away from the liquid and go into the air as water vapor. Since energy can't be created or lost, the air molecules lose energy and slow down. This lowers their temperatures; and the air cools. Perspiration evaporating from our bodies cools us by taking heat energy from our skin in a similar way.
Heat is added to the air when water condenses from vapor into the liquid phase. This occurs the same way air is cooled by evaporating water, except that energy is transferred from the water molecules to the air. Energy added to the air by condensation heats the air, making it lighter. The lighter air rises, which is what powers thunderstorms and hurricanes.
The main principle behind latent heat is the first law of thermodynamics, commonly known as conservation of energy. We say energy is conserved because it isn't created or lost; it changes form. During evaporation some of the air's heat energy is converted into kinetic energy of the water molecules that speed up and become vapor.
