Latent heat is cool

How does latent heat factor into our daily lives? In more ways than you may think. The effects of latent heat are not confined just to fueling thunderstorms.

As an example, suppose we pour ourselves a bowl of very hot soup. The first thing you may notice is a wisp of steam rising from the surface of the soup. Because of the heat stored in the soup, the molecules of water within the soup are very active and some of those at the surface of the soup will escape into the atmosphere as vapor. As they do, they release latent heat stored in the soup, cooling it. Assuming the air surrounding the soup bowl is calm, eventually enough water vapor molecules accumulate just above the surface of the soup to begin to slow further escape of water vapor. When you blow on the soup to cool it, you dissipate this layer of water vapor above the soup and the soup resumes cooling again.

The same thing happens with sweat on your skin. As the sweat evaporates, it dissipates heat from your skin resulting in cooling. The rate of evaporation of the sweat is dependent on the relative humidity of the air next to your skin. If the relative humidity is high, i.e. the surrounding air is relatively saturated, the rate of evaporation is much slower. Sweat accumulates on the skin surface, which results in the sticky, sweaty feeling we're all familiar with on muggy days. A breeze dissipates the water vapor adjacent to your skin, resulting in cooling. Conversely, a day of low relative humidity allows molecules of water to more easily escape. Add a nice breeze, and the sweat will evaporate easily, without the sweaty feeling.