Introduction

Water is an essential ingredient for life to exist on earth. Water vapor is highly variable in the atmosphere, ranging from .2 percent in cold dry climates to as such as 5 percent in the humid tropics. These numbers seem small but for every minute of the day nearly 1 billion tons of water is given up to the atmosphere. 80 percent of the water present in the atmosphere comes from evaporation from oceans. The geography of water plays a crucial role in shaping the earth, the distribution of soils, vegetation, and animals. In this lecture we'll look at the phase changes that water goes through, the products of condensation, and major cloud types.

The Hydrologic Cycle

Click image to enlarge
(Redrawn after Gabler et. al., 1999)

• Model of the cycle of water through the earth system

• Cycle of energy through the earth system

Phases of Water in the Atmosphere

SOLID: Ice

• molecules arranged in a particular order.

• molecules do not circulate.

Liquid: Water

• molecules freely circulate

Gas: Water Vapor

• Molecules freely circulate.

Transfer of Water into and out of the Air

Evaporation

• Definition : Change of liquid water to a gas.

• Latent heat of vaporization = 600 cal/gm

• Requirements for Evaporation:

  • Available water

    • 88% of evaporated water originates from oceans between 60 N and 60 S latitude.

  • Energy

    • Highest rates of evaporation occur where there is ample available energy (tropical oceans).

  • Unsaturated atmosphere (a vertical gradient of moisture)

  • Wind (not necessarily required)

    • removes water vapor from the evaporating surface

Transpiration

• Definition : Water loss from plants through leaf pores (stomata).

• Depends on:

  • Energy: Approximately 600 cal/gm of energy is required to transpire water from leaves.

  • Water availability

  • Unsaturated atmosphere

• Transpiration is a cooling process

• How much water do plants contribute?

  • Mature Oak Tree = 100 gal/day

  • Acre of Corn = 4000 gal/day

• Evapotranspiration: Combined transfer of water to the atmosphere by evaporation and transpiration.

Moisture in the Atmosphere

• Humidity: the water vapor content of the air.

• Measures of Humidity:

  • Absolute humidity

  • Specific humidity

  • Mixing ratio

  • Vapor Pressure

  • Saturation vapor pressure

    • See "Focus on the Concept of Saturation"

  • Dew point *

  • Relative humidity

    • Why does cold air feel drier than warm air at 100% RH?

      Continental Polar air: Ave. Temp. = 5 C (41 F)

      • Water holding capacity = 6 gm/kg at saturation

      • Relative Humidity Continental Polar at 100%
        RH = 6 gm/kg (in the air)/6 gm/kg (capacity)

      Maritime Tropical Air : Ave. Temp. = 22 C (71.6 F)

      • Water holding capacity = 16 gm/kg at saturation

      • Relative Humidity Maritime Tropical at 100%
        RH =16 gm/kg (in the air) / 16 gm/kg (capacity)

        Therefore, polar air is drier at 100 % relative humidity because it can't "hold" as much moisture.

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The Condensation Process

• Condensation (Definition): Phase change from water vapor to a liquid. 600 cal/gm is lost from water vapor

• Requirements:

  • Condensation nuclei

  • Air must be at, or near, saturation - achieved through cooling or humidification

  • Cooling mechanisms:

    • Contact cooling - Warm air comes in contact with cold surface.

    • Expansional cooling - Caused by uplift of the air. (Adiabatic Cooling)

    • Uplift Mechanisms

      • Orographic uplift - air forced upward by mountains. Air expands and cools as it rises.

• Convective uplift of air is warmed at the surface and rises due to a change in density. As the air rises it expands and cools.

• Frontal uplift - Uplift of air along a weather front where different types of air masses collide.

• Convergent uplift - convergence of air into a region of low pressure.

Products of Condensation

• Dew

• Fog - a cloud near the ground

  • A few types

    • Advection Fog

    • Steam Fog

    • Radiation Fog

      • See: "Focus on Radiation Fog"

  • Geographic Distribution (See "Focus on Fogs")

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