What is the Atmosphere made of?

• What is condensation? This is the reverse of evaporation and happens when water vapor becomes liquid water. Dew formation is an example of this process.

• What is transpiration? You can think of this as plants sweating. Plants will have water close to the tops of their leaves. As the sun warms the leaves, they sweat water, which then evaporates naturally into the air. This water is released by the leaves as part of their water transport processes.

• What is sublimation? Sublimation is the process where ice becomes gaseous water vapor while skipping the liquid phase. This is seen when your ice cubes give off steam or when those in your freezer shrink over time.

Water enters the atmosphere in one of 3 ways: evaporation, transpiration, and sublimation. Among these 3, evaporation is the largest contributor by far. In terms of transfer of water from the atmosphere to the earth and vice versa, you have evaporation and precipitation as the major players. You should know that over a year's time, there is very little change in the amount of water in both the atmosphere and on the ground. Certainly, things can get off balance as the years progress.

How long do you think any given water molecule takes from the time it is evaporated up into the atmosphere until it ends back on earth again? It takes an average of 11 days for this to occur. The water molecule will need to condense into a cloud droplet and then fall as precipitation from a cloud.

Water that is in a liquid or solid form on earth lasts much longer than a gaseous water molecule in the atmosphere. Any given water molecule will last for 2800 years somewhere on the earth before it evaporates. Imagine how long a water molecule in a glacier must stay before evaporating. It can be tens of thousands of years, depending on the climate and the size of the glacier.

Both evaporation and condensation rely on the kinetic energy of water in its various phases. What happens in evaporation, is that surface water gains enough kinetic energy to essentially break away from its liquid surface and enter the air. There are several factors that can control the rate of how this happens. First, you need to have some surface area on the water's liquid surface so that the molecules can come in contact with the surrounding air. Next, you need some heat energy. You do not need the energy it