1. Lecture 6: The Hydrologic Cycle
EarthsClimate_Web_Chapter.pdf, p. 10, 16-17, 21, 31-32, 34
The hydrologic cycle is also called water cycle.
The water cycle is tied intimately to the atmospheric general circulation we have learned from previous lecture.

2. Components of the Hydrologic Cycle
Evaporation, Condensation, Transport, Precipitation, Transpiration, Runoff, Groundwater Flow
Explain the components of the water cycle:
Start from evaporation in the oceans. 80% of all evaporation is from the oceans. Evaporation is that water vapor escapes from the ocean water entering the atmosphere.
When it rises, expands (because of reduction of pressure as you go up), and cools, it condenses to form clouds.
The circulation transports vapor and clouds to the continents.
Precipitation can be in the form of snow, rain, sleet, hail, and so on.
Rainfall fall on the ground can either infiltrate the soil (infiltration) or runs off to streams/rivers.
The streams and rivers all ultimately go to the oceans.
Groundwater (or subterrain) either discharges to streams/springs or flows to the oceans.

3. Water and the Planets
Earth: the only planet where water can exist in three forms.
Water is essential to life.
We are fortunate that Earth is the only planet in the Solar system which can have water in all three forms: ice, liquid and vapor.
Water is a essential for Life.

4. Water is a vital component of the climate cycle.
Where Is Water On Earth?
Atmosphere: 0.001%
Groundwater 0.58%
Surface Water 0.02%
Oceans 97.39%
Ice/Snow 2.0%
Water is a vital component of the climate cycle.
Where does the water store on Earth?
Look at the percent chart.
Look at the stores and fluxes.

5. Movement of Water Between Stores
Hydrologic Cycle
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
First, let’s see what the water cycle is about from another perspective.
Water vapor, despite its small storage in the atmosphere, is dynamic to the climate system: storms, rain, thunders, lightning,..
Waters in rivers, lakes and subterrain are important to humans – because they are fresh water.

6. 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-to-land atmospheric vapor transport balances land-to-ocean runoff.
While the next lecture is about oceans, now let’s discuss more about the role of water vapor in the climate system.
* 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.

7. Cloud Development What causes the air to rise?
Rising air expands, cools, and condenses to form cloud
Causes of a rising air mass
What causes the air to rise?
2. Widespread ascent due to convergence of surface air
1. Surface heating and convection
There are four mechanisms/processes for cloud formation.
Cold Front
Warm Front
4. Uplift along weather fronts
3. Orographic uplift

8. Steps in Making Precipitation
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

9. 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.
Ask the students to give an example of the Rain shadow.
Mention the Great Basin.

10. 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.

11. Global Annual Mean Precipitation
The global distribution of precip shows where it is wet (tropics, rainforest, the Amazon), where it is dry (the subtropics, deserts or semiarid), and midlatitude storm track regions.
Orographic precip: see the State of Washington.

12. Geographic Distribution of Annual P-E (mm)
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.
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.
From Paul Houser

13. Zonal Mean Precipitation

14. Precipitation Seasonality of precip.
Notice the monsoon effects in Asia.

15. Precipitation
Look at Texas.

16. Various Satellites Are Monitoring Earth’s Water Cycle
How can we measure the water cycle globally?
Various Satellites Are Monitoring Earth’s Water Cycle