Hydrologic Cycle: Atmosphere
Atmosphere Review
Think back to what we learned in the Atmosphere Module - specifically, the information about Earth's atmospheric layers.
Earth has five atmospheric layers. From outermost to innermost, they are the Exosphere, Thermosphere, Mesosphere, Stratosphere, and finally the Troposphere.
Troposphere and the Hydrologic Cycle
The troposphere is the bottom layer of Earth's atmosphere. It is closest to Earth, extending roughly 8-15 km (or 5-9 miles) above Earth's surface. The troposphere supports the biosphere and all life on Earth.
The troposphere is where almost all of Earth's weather takes place.
This is because the Troposphere contains 90% of the total mass of the atmosphere - it is the densest of Earth's atmospheric layers, meaning it contains more air molecules than any other layer! (The exosphere, or Earth's outermost layer is the 'thinnest' or least dense.) The majority of all water vapor, clouds, and air pollution takes place within the troposphere.
Atmospheric Water Cycle Processes
There are three main processes in the hydrologic cycle that take place in the atmosphere: Evapotranspiration, Condensation, and Precipitation.
Evapotranspiration
Evaporation is the net movement of free water molecules from wet surfaces into air that is less saturated with water. Higher humidity means the air is more saturated with water, i.e. more water vapor has evaporated into the air. Thus, evaporation will happen more slowly when humidity is high, and more quickly when humidity is low.
Transpiration is the process where water moves into the atmosphere from plants as part of photosynthesis and respiration: plants release water through the stomata of their leaves, which plants open to allow carbon dioxide in and let oxygen out during photosynthesis.
Together, these two processes make up evapotranspiration, or all of the inputs of water from Earth's surface into the atmosphere.
Fun fact... a water molecule spends an average of 10 days in the atmosphere! This plays an important role in regional weather patterns and fluctuations!
Condensation
Condensation is the opposite of evaporation: it is the process where water vapor in the air (i.e. evaporated water) is changed into liquid water. You will learn more about the process of condensation on the Water & Energy page. For now, just remember that condensation is the process responsible for forming clouds.
Precipitation
Precipitation is the moisture supply to Earth's surface in all its forms - rain, snow, sleet, hail, and so on. Precipitation is the primary route for water to return from Earth's surface after it has gone into the atmosphere as water vapor via evaporation.