Acadia National Park - Atmosphere

Temperature T he atmosphere forms a protective layer around the Earth. Air can contains many gases including water vapor. The amount of water vapor in the air depends on the temperature. The relative humidity of the air in a given spot is the balance (ratio) between how much water vapor is in the air, and how much water vapor the air can hold at its current temperature. Water vapor is lighter than the nitrogen and oxygen in the air, so rises in the atmosphere. When it reaches a cooler temperature, the air can hold less water vapor, causing some to condense around particles of dust or spores in the air, forming floating droplets of water. Water vapor in the atmosphere condenses to form clouds, which cover roughly 40% of the Earth. Clouds move through the atmosphere in bands more or less parallel to the equator.

Temperature differences within clouds cause air currents that move the water droplets around, sometimes carrying them into higher, cooler regions, where the rate of condensation increases, causing the droplets to grow larger. Droplets can rise and fall within a cloud until the air can no longer hold them up, and they fall.

When those droplets grow large and heavy enough, they fall toward Earth as precipitation. Some precipitation may evaporate on the way down, some may reach the surface as rain, snow, sleet, fog, or perhaps hail. Water vapor in the atmosphere holds heat beneath it, influencing the climate, warming and stabilizing the surface temperature of the Earth. Much of the energy that drives movements of air comes from the process of condensation of water vapor inside clouds. When liquid water evaporates from the oceans, it takes energy out of the water, lowering its temperature. Condensation, the opposite process by which water vapor forms molecules of liquid water, gives off heat. In any portion of the atmosphere, evaporation and condensation take place at the same time, some gaseous water (water vapor) forming drops of water and giving off heat, while some liquid water is evaporating, absorbing heat. The overall balance between evaporation and condensation determines whether the air is being heated or cooled, getting lighter or heavier.