Lecture 6: The Hydrologic Cycle
EarthsClimate_Web_Chapter.pdf
, p. 10, 16-17, 21, 31-32, 34

Components of the Hydrologic Cycle
Evaporation, Condensation, Transport,
Precipitation, Transpiration, Runoff,
Groundwater Flow http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/smry.rxml

Earth: the only planet where water can exist in three forms.
Water is essential to life.

Water is a vital component of the climate cycle.
Ice/Snow 2.0%
Atmosphere: 0.001%
Surface Water 0.02%
Groundwater 0.58%
Oceans 97.39%

Movement of Water Between Stores
1. Most Rapid Movement
In Vapor Phase , in the
Atmosphere
2. Most Water and Energy
Storage
In Liquid Phase , in the
Oceans
3. Most of Water useful to
Humanity in Rivers, Lakes,
Subterranean Water, and
Ice and Snow, as Fresh
Water

Water Vapor
1. Highly variable spatially
• Near 0% over deserts
•3–4% over tropical oceans/jungles
• Decreases rapidly with altitude; most is within a few km of the surface
• Decreases rapidly with latitude; at the equator is 10 times that at the poles
2. Importance to climate and climate change
* Important part of the water cycle; ocean-toland atmospheric vapor transport balances land-to-ocean runoff.
* The most important greenhouse gas: water vapor-temperature feedback.
* Water vapor condenses to form clouds, thus clouds–radiation feedback. Clouds release rainfall, reflect solar radiation , and reduce the infrared radiation emitted by Earth .

 Causes of a rising air mass
What causes the air to rise?
Rising air expands, cools, and condenses to form cloud
1. Surface heating and convection
2. Widespread ascent due to convergence of surface air
3. Orographic uplift
Cold Front Warm Front
4. Uplift along weather fronts

1. Water vapor in air
2. Air with vapor rises, expands, and cools
3. Vapor condensates around nuclei to form droplets (clouds)
4. Droplets suspended by atmospheric upward motion and turbulence
5. Droplets collide and coalesce into drops in warm clouds and droplets diffuse to ice crystals in cold clouds
6. Drops/crystals falls as rain/snow when they are too heavy to be suspended by upward motion

Orographic Precipitation
Winds blowing moist air toward a mountain will experience orographic uplift to an elevation where dew point is reached and clouds are formed.
When the condensed moisture falls as rainfall, the leeward side of the mountain is kept in a rain shadow .

Topographic Controls on Precipitation
Westerly winds blowing moist air from the Pacific Ocean encounter several mountain ranges that create patterns of rising air and precipitation followed by sinking air and warm dry rain shadows.

Global Annual Mean Precipitation http://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.html

• Evaporation excess nearly ubiquitous over sub-tropical oceans, with a sharp contrast at coastal regions.
• Equatorial ocean evaporation minimum.
• Tropical land areas show richest excess in precipitation.
From Paul Houser
• Major desert regions, tundra, and mountainous regions all indicate deficit to marginally-balanced conditions.
• Mid-latitude and boreal coastal/maritime environments exhibit adequate precipitation supply over evaporation.

Zonal Mean Precipitation http://www.atmo.arizona.edu/students/courselinks/spring03/atmo421/prec.html

Precipitation

Precipitation

Various Satellites Are Monitoring Earth’s Water Cycle

• 1. What are three most important elements of the hydrological cycle?
– Evaporation/evapotranspiration, transport, precipitation
• How is water distributed in the earth’s climate system?
– 97% in ocean, 2% in ice, ~0.6% in ground water, 0.02% river/lake,
0.001% in atmosphere.
• How does each form of water (e.g., vapor, liquid and ice) influence climate?
– Vapor: strongest greenhouse gas, liquid/ice forms clouds and precipitation, river and ice/snow, which influence albedo of the earth, sensible and latent heat fluxes.
• How are clouds and rainfall formed?
– Clouds/precipitation are formed by condensation/freeze of the water vapor in rising motion of the atmosphere either due to unstable thermodynamic stratification or due to mechanical lifting by topographic or lower tropospheric wind convergence.