# Unit 11: Atmospheric Moisture and the Water Balance

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Unit 11: Atmospheric Moisture and the Water Balance
Properties of Water Water Vapor & its Measurements The Hydrological Cycle Evaporation Condensation, Clouds Precipitation Processes Surface Water Balance The hydrological cycle in action.

OBJECTIVES • Discuss the various forms of water and understand the important heat transfers that accompany changes of these physical states Explain the various measures of atmospheric humidity, how they are related, and the processes responsible for condensation • Outline the hydrologic cycle and the relative amounts of water that flow within this cycle. Understand the process of evaporation. Examine the conditions necessary for the formation of clouds. • Introduce the concept of precipitation • Describe the Earth’s surface water balance and its variations

Phase Changes of Water, Latent Heat
Heat is consumed in evaporation, melting; heat is released in condensation, freezing (sublimation). Schematic view of the molecular structure of water in its three physical states and heat-energy exchange among those states. The latent heat-exchange numbers between the arrows are explained in the text (values are for 0°C).

Measurements of water vapor
Vapor pressure is the pressure exerted by water vapor molecules Saturated vapor pressure is maximum pressure of water vapor at that temperature Dew point is the temperature the air must be cooled to reach saturation Relative humidity is ratio of water vapor in air to maximum the air can hold at that temperature. RH is usually highest when daily temp- erature is lowest. Specific humidity is the mass of water Vapor in the air per unit mass of air. Mixing ratio is the mass of water vapor in The air to the mass of dry air containing the Water vapor. Variation of saturation vapor pressure (mb) with temperature (C). The curve is nearly a pure exponential. At temperatures below 0C saturation values over supercooled water are greater than over ice.

Relative Humidity Figure 5.7

Humidity Patterns Figure 5.10

Maximum Specific Humidity
Figure 5.12

Evaporation, Evapotranspiration
The rate of evaporation depends on: temperature, humidity, wind speed and water quality (salinity) Plants lose water to the atmosphere through transpiration Evapotranspiration is the loss of water from the soils and plants to the air Potential evapotranspiration (PE) is the maximum evapotranspiration lost when abundant water is available Actual evapotranspiration (AE) is the amount of water lost in any actual amount of soil moisture AE is usually less than PE, except when there is abundant water available, like a swamp or water surface. In deserts, PE might be very high, while AE is very low.

Fig 12.2 Hydrologic cycle. The numbers attached to the stages express each value as the volume of water divided by Earth surface area. Thus the values shown represent the depth of water (centimeters per year) associated with each mass transfer. All can be directly compared to the global average precipitation rate, which is about 100 cm/year.

Water in the Hydrosphere
Most of the water is salt water in the ocean Most of the fresh water is locked up in ice sheets and glaciers Most of the liquid fresh water is in the ground Fig 12.3 Distribution of water in the hydrosphere. The middle and lower bars show the percentage distribution of the 2.8 percent of total hydrospheric water that is fresh. Of that freshwater component, only about one-tenth is easily available to humans.

Water Usage in the United States, 2005
Which states use the most water? Total water withdrawals (millions gallons per day) for the United States in 2005.

Condensation and Clouds
For water vapor to condense into liquid, there must be -condensation nuclei for water molecules to condense upon -sufficient water vapor to reach saturation by either cooling or through evaporation of more water vapor Condensation at the ground is dew or frost (if temperature is below freezing) Condensation near the ground can form fog, if the cloud is in contact with the surface Condensation above the ground can form clouds

Condensation near the ground forms fog, a cloud in contact with the ground.
As the ground and surface air cools to the dewpoint, water vapor condenses into a radiation fog (cooling by longwave radiation overnight) here in East Africa.

Advection Fog Figure 5.20

Evaporation Fog Figure 5.21

Valley Fog Figure 5.25 Figure 5.22

Radiation Fog Figure 5.23

Cloud Types Schematic diagram of the different cloud types – stratus, cumulus, and cirrus, arranged by their typical altitude.

Precipitation Processes
The Ice-Crystal Process-requires the coexistence of ice and super-cooled water droplets in the cloud. Ice grows at the expense of water droplets that evaporate water molecules which adhere to the ice until they are large enough to fall as snow. If the air is warm enough, the snow melts and rain occurs. The Coalescence Process-requires different sizes of water droplets within warm clouds. Larger droplets grow by falling faster and sweeping up smaller droplets by coalescing until they are large enough to fall as rain. Source:

FIGURE 7. 6 The distribution of ice and water
in a cumulonimbus cloud.

FIGURE 7.9 The ice-crystal (Bergeron) process.
(1) The greater number of water vapor molecules around the liquid droplet causes water molecules to diffuse from the liquid droplet toward the ice crystal. (2) The ice crystal absorbs the water vapor and grows larger, while (3) the water droplet grows smaller.

FIGURE 7. 4 Collision and coalescence. (a) In a warm cloud
composed only of small cloud droplets of uniform size, the droplets are less likely to collide as they all fall very slowly at about the same speed. Those droplets that do collide, frequently do not coalesce because of the strong surface tension that holds together each tiny droplet. (b) In a cloud composed of different size droplets, larger droplets fall faster than smaller droplets. Although some tiny droplets are swept aside, some collect on the larger droplet’s forward edge, while others (captured in the wake of the larger droplet) coalesce on the droplet’s backside.

Types of Precipitation
Besides rain and snow, there is: Sleet-melting ice that refreezes before reaching ground Freezing rain-melting ice that freezes on contact with a frozen surface Hail-ice particles that grow within clouds that have strong updrafts Graupel-soft, partially melted hail Source: Four major forms of precipitation. (A) A rainstorm douses the Ponderosa Pine Forest near Flagstaff, Arizona. (B) Falling snow accumulates in south-central Alaska. (C) Freezing rain forms an icy coating on pine needles on a golf course in Wawona, near Yosemite National Park, California. (D) Golf-ball-sized hailstones litter the countryside following a storm in northern Texas.

FIGURE 7. 22 A heavy coating of freezing rain (glaze) covers Syracuse,
New York, during January, 1998, causing tree limbs to break and power lines to sag.

FIGURE 7. 28 This giant hailstone — the largest ever reported in
the United States with a diameter of 17.8 cm (7 in.) — fell on Aurora, Nebraska, during June, 2003.

Four Forms of Precipitation
B A A-rainstorm B-snow C-freezing rain D-hail C D

The Water Balance Based on methods devised by climatologist C. Warren Thornthwaite, calculates inputs and outputs of water at the Earth’s surface based on simple formulae using monthly temperatures and precipitation of a station. When PE is greater than P (precipitation), there is water loss When P is greater than PE, there is a water gain By calculating monthly PE values and comparing with P, one can calculate amounts of surplus (runoff) and deficit (not enough soil moisture) in surface water. Different climates exhibit different water balances

Water Balance for Different Climates
Range of water balance conditions found at the surface of Earth. (A) Baghdad, Iraq, experiences a constant deficit because potential evapotranspiration normally exceeds precipitation. (B) At Tokyo, Japan, the situation is reversed, and a constant water surplus is recorded. (C) At Faro, Portugal, the intermediate situation occurs, with a combination of surplus and deficit at different times of the year.

Water Balance Averages by Latitude
Average annual latitudinal distribution of precipitation, evapotranspiration, and runoff in cm per year. The arrows show the direction of the water vapor flux by the atmospheric circulation.

Global Distribution of Annual Evaporation, Evapotranspiration
Fig 12.10? Global distribution of annual evaporation and evapotranspiration in centimeters, with land elevations adjusted to sea level. Red isolines show the pattern over land; blue isolines over the oceans.

Global Distribution of Annual Precipitation
Source: Global distribution of annual precipitation in millimeters/day.

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