Map assignment

LECTURE 7: THE HYDROLOGIC CYCLE

The hydrologic cycle is the system, which describes the movement of water from the ocean to

the atmosphere, and back to the ocean through runoff. The hydrologic cycle consists of four

stages, see page 227 (9th ed) or page 220 (10th ed);

1. EVAPORATION: describes the change of liquid water to vapor, we measure evaporation rates as humidity in the atmosphere.

2. CONDENSATION: describes the change of water from the vapor stage to a liquid or solid. This cooling is caused by the upward movement of air in the atmosphere, cooled by the average

lapse rate.

3. PRECIPITATION: describes the growth of raindrops and ice crystals to a size large enough to be acted upon by gravity.

4. RUNOFF: describes the movement of water, snow and condensation back to the ocean. Runoff includes surface water (lakes, rivers and reservoirs) and subsurface water (aquifers, water

tables…) moved by gravity back to the ocean. Humans attempt to slow down the runoff process,

using as much water as possible before it heads back to the oceans to begin the cycle over again.

This image very good to show you just how little precipitation actually occurs over the

continents, also, p. 171 and 172 (9th ed) and page 159 (10th ed), are good illustrations of the

phase changes which occur as water is carried through the hydrologic cycle.

Now, let's break down each stage of the hydrologic cycle to understand how and where it occurs,

as well as specific criteria, which must occur before we move to the next stage in the cycle.

1. EVAPORATION

In discussing evaporation (also see :http://www.usatoday.com/weather/wwater0.htm), there are

four terms that we need to understand regarding humidity.

a.) Specific humidity: the actual amount of vapor in the atmosphere, measured in grams at a

specific time. Important in describing the moisture content of air masses.

b.) Absolute humidity: the maximum amount of water the air can hold, this varies according to

temperature. All the graphs of humidity show the same relationship; as temperature of an air mass

increases, the more vapor it can hold.

c.) Relative humidity: the amount of vapor in an air mass, measured as a percentage of the

absolute amount of vapor an air mass could hold at a specific temperature. On page 174 (9th ed)

or page 162-163 (10th ed), these two figures show that relative humidity increases as

temperature falls, and relative humidity decreases as temperature increases. Most important is

the concept of 100% relative humidity. This means that the air parcel is carrying the absolute

amount of vapor it can hold, and this lead

to condensation. 100% relative humidity is also known as saturation, the dew point and

maximum specific humidity.

d.) Saturation: also known as 100% relative humidity. This occurs as the atmosphere surrounding

an air mass cools, and relative humidity increases. In California, we experience greater

precipitation at night due to the escape of LW radiation, cooling vapor which exists in the air.

Such cooling also occurs when an air mass is forced to rise in the atmosphere and is cooled by

the average lapse rate (3.5 degrees F/1000ft).

WHERE EVAPORATION OCCURS: we can locate patterns of evaporation by examining

areas of low pressure (warm air) which dominate warm water regions. See page 20-21 in the

Goode’s World Atlas to locate warm water areas, and correlate this pattern with the pressure

maps on page 18-19 Goode’s World Atlas. From these maps, we expect great amounts of

evaporation occurring over the Kuroshio current and the Gulf Stream in January, and over the

ITCZ throughout areas affected by the Equatorial countercurrent. But we will not find as much

evaporation in areas dominated by low pressure near cold water currents, such as the California

Current or the Peru Current. Since the water is cool, it will take much more energy to raise the

water to a temperature where vaporization can occur. Also-evaporation will not occur in an area

dominated by high pressure. Cool air holds relatively little vapor, and it is considered

adiabatically stable, cool air does not rise in the atmosphere.

2. CONDENSATION: once 100% relative humidity takes place warm cools from a vapor to a liquid. We can see this in the atmosphere as condensation, fog and clouds. Condensation occurs

if there are particles present which allow water to coalesce around them, and the air mass rises in

the atmosphere, either forced to rise (running into mountains or cooler air masses) or comes into

contact with a cold surface (such as warm air moving over a cold surface, also known as

advection fog). Particles needed to stimulate condensation are known as condensation nuclei.

Examples are: dust, pollen, pollution, nuclear particles, salt, and ash.

WHERE CONDENSATION OCCURS: examples are demonstrated in the form of fog and

clouds. See page 185 (9th ed) or page 173 (10th ed), the fog incidence map for North America.

On this map we can locate a variety of fog types. The first is advection fog.

1. Advection fog occurs when warm, moist air travels over a cool surface. This would occur in the SF Bay on a daily basis. Other locations would be along the Eastern seaboard as warm air

along the Gulf stream contacts the cool continent, see Halifax and areas around Concord.

2. Orographic fog occurs when warm, moist air is forced to rise and cool in the atmosphere due to collision with mountain systems. We can also see this in Portola Valley and Yosemite

throughout the year. Good examples from the Fog map, p. 185, would include the Sierras, the

Cascades, and the Appalachians.

Do you know why very few days of fog exist in a similar pattern along the Rockies?

3. Radiation fog occurs due to the loss of LW radiation at night. As the surface cools, air is chilled to its dew point and condenses. A prime example of this type of fog is tule, or valley fog

experienced in the Central Valley,. Inland areas experience this type of fog, as the surface

cools more rapidly than oceanic regions.

4. Evaporation fog occurs as cold air (with relatively low humidity) flows over a warm water surface. The warmth of the water stimulates convection of air, and the air mass absorbs more

vapor, leading to 100% relative humidity. Places where evaporation fog occurs would be in

Quebec (near the Gulf Stream) and Labrador, where cool air from inland (40 degrees F below

zero) flow over the cool, but relatively warm Labrador current.

Clouds also illustrate the amount of humidity in the air. The most significant types of clouds are: cirrus, cumulous, and stratus. Cirrus are wispy clouds found high in the atmosphere, and

hold very little moisture. Stratus clouds are clouds which blanket the lower atmosphere, having

little vertical development. And cumulous clouds are clouds with vertical development, caused

by the updraft of warm air. Cumulous clouds are clouds which carry much moisture. The vertical

development of cumulous clouds allow us to predict precipitation. Any cloud can be a nimbus

cloud, this simply means it will precipitate. Cumulonimbus clouds are the most feared,

stimulating hurricanes, thunderstorms, tornadoes and other strong storms.

3. PRECIPITATION: Once condensation occurs, precipitation is not far behind. In the

precipitation process raindrops and ice-crystals must grow in size to become heavy enough to be

pulled to earth by gravity. We say that rain falls, but it would be more scientifically correct to say

that rain is pulled to earth. The process of raindrop and ice-crystal growth are discussed in the

text. This is where the presence of condensation nuclei becomes significant for raindrop growth.

The collision-coalescence model occurs in warm climates, and lower portions of the atmosphere,

while the ice-crystal model takes place in higher latitudes, and high in the atmosphere where the

atmospheric temperature is extremely low causing sublimation (the phase change from vapor to

ice).

Three types of precipitation exist (addressed as lifting mechanisms). These types are

differentiated according to the lifting forces which cause warm, saturated air to rise in the

atmosphere and precipitate.

1. Convergence: this type of precipitation occurs in the tropics. Here, warm, moist air is evaporated on a daily basis. The upward convection of such air causes the vertical development

of cumulous clouds, and by 4 or 5 PM each evening, as the atmosphere cools, precipitation

occurs. Convergent precipitation is limited to areas dominated by the ITCZ, and creates the most

significant amounts of precipitation, see p. 20-21 atlas, illustrating tropical areas like the

Amazon, experiencing over 150 inches of rain per year.

2. Orographic Lifting: this type of precipitation occurs in mountainous regions. As air collides with major mountain systems, such as the Sierras and the Himalayas, water vapor is forced to

rise, cool, condense and precipitate. Associated with orographic precipitation is the rainshadow

effect. On p.197 (9th ed) or p. 188 (10th ed), you can see that the windward side of the mountain

experiences increased levels of precipitation. As the mountain rings out the moisture in the air

mass, the downslope side, or leeward side, of the mountain experiences descending, drying air

as the heat caused by friction with the surface stimulates evaporation. The rainshadow effect is

used to

explain patterns of aridity and precipitation. Examples are the Atlas Mountains and the Sahara

desert, the Sierras and Nevada (called the Great Basin), the Himalayas and the Gobi desert, as

well as the Western Ranges, and the Great Victoria Desert in Australia. See atlas p. 4-5 and p.

20-21 for specific examples. Examine Washington State examples on p. 198 (9th ed) and 189

(10th ed). Still want to move to Seattle?

3. Frontal Lifting: this type of precipitation is caused by air mass interaction (discussed in detail next lecture), where air masses move out of their source regions and interact due to the

movement of the Jet Stream and the poleward movement of warm air and the equatorial

movement of cold air to balance global temperatures. In either case, precipitation occurs when

relatively cool air forces warm, moist air masses to rise higher in the atmosphere. This causes

cooling, condensation and precipitation. Frontal lifting only occurs in the mid-latitudes, between

40-60 degrees north and south.

4. RUNOFF: the last stage of the hydrologic cycle. Runoff is simply the amount of precipitation which is stored by the earth in surface or subsurface locations, until it returns to the

ocean. Runoff is significant because it determines our local water budgets, or the amount of

water which is available for use in a specific location. If you look at p. 233 (9th ed) or 226 (10th

ed), you will notice that Berkeley, California, Kingsport, Tennessee and Phoenix, Arizona

illustrate very different local water budgets.. Underground water can be extracted for

consumption through digging wells to the water table, yet, this too may become depleted over

time. Too much groundwater extraction in coastal locations can lead to subsidence (Old Mill in

Mountain View) and salt-water intrusion along coastlines.

In conclusion, each stage in the hydrologic cycle are dependent upon the previous stage.

Humans have learned the workings of the hydrologic cycle as a means of altering local water

budgets. Examples of such modification are the Hetch Hetchy Aquaduct and Crystal Springs

Reservoir, the California Aquaduct (see page 118 in Atlas for detailed illustration), cloud seeding

and desalinization programs (such as those in the Middle East). The problem is that by altering

the location of precipitation, such as cloud seeding, we also alter the pattern of precipitation,

which naturally exists. This can also impact the health of soils (salinization) and greater amounts

of evapotranspiration (discussed next two lectures) which may alter microclimates.

There are many web sites applicable to this lecture.

To examine current weather, climate and historical data or weather topics.

 The Weather Channel - Home Page

 http://www.noaa.gov