Evaporation & Condensation

evaporation diagram, caption follows
Imagine water evaporating from your skin on a hot day. For the process of evaporation, molecules of liquid water need energy to break their bonds to other water molecules. Each molecule gains the needed energy by absorbing it from your skin. The heat carried off by the water vapor helps to reduce your body heat and cool you off. As the vapor molecule moves away from your body, it takes this energy – called latent heat – with it. This heat doesn’t disappear; it has been converted from body heat into the vapor molecules added energy. The illustration shows the composition of a water molecule: 2 parts hydrogen and 1 part oxygen.
Credit: The Asian Monsoon CD, GSFC, NASA

condensation diagram, caption follows
When the molecules of water vapor condense to form liquid water droplets, they release latent heat to the surroundings. Imagine the molecule of water that evaporated from your skin rise to join other water molecules they condense to form the tiny drops of a cloud. As vapor condenses to liquid, each molecule releases the latent heat carried from your skin, helping to warm the air in the growing cloud. This release of latent heat might not seem like much, but multiply it by the number of molecules in a drop of water (about 1000 billion, billion, or 10E21), and multiply that by the number of droplets in an average cloud (another very large number) and you see that it is the major source of energy for hurricanes and monsoons.

The release of latent heat energy in the upper atmosphere is an important source of energy driving global circulation, which in turn redistributes the atmospheric moisture. The Asian monsoon region is one of the most important latent heat sources and moisture sinks. The pouring rain during the monsoon season not only provides an important water resource for agriculture in many Asian countries, but is also the source of tremendous latent heat energy that is released into the atmosphere. Every year, between April and September, the monsoon is largely responsible for global circulation.
Credit: The Asian Monsoon CD, GSFC, NASA