Unit 12


Unit Overview

This unit examines the process of precipitation, which includes the precursor processes of evaporation, saturation, and condensation, as well as the associated water at the Earth's surface. The main sections are

  • Physical properties of water
  • Measuring water vapour
  • The hydrologic cycle
  • Evaporation
  • Condensation and clouds
  • Precipitation
  • The surface water balance

The mechanisms behind supplying the atmosphere with water vapour are evaporation and transpiration, which are collectively known as evapotranspiration. Various concepts are used to assess the water vapour content of the atmosphere. These include: relative humidity, specific humidity, the mixing ratio, and dew point temperature. Relative humidity and dew point temperature are used to determine how close the atmosphere is to being saturated with water vapour.

Once saturation occurs and there are abundant condensation nuclei, condensation occurs. Condensation in the atmosphere that occurs because of adiabatic cooling can be readily seen in the form of clouds. Processes such as the ice-crystal process and the coalescence process occur within clouds and lead to the formation of precipitation. The resultant precipitation is a vital component of the energy balance all across the globe.

Unit Objectives

  • To discuss the various forms of water and to understand the important heat transfers that accompany changes of these physical states
  • To explain the various measures of atmospheric humidity, how they are related, and the processes responsible for condensation
  • To outline the hydrologic cycle and the relative amounts of water that flow within this cycle
  • To introduce the concept of precipitation
  • To describe the Earth¿s surface water balance and its variations

Glossary of Key Terms

Actual evapotranspiration (AE) The amount of water that can be lost to the atmosphere from a land surface with any particular soil-moisture conditions.
Cirrus clouds The cloud-type category that encompasses thin, wispy, streak-like clouds that consist of ice particles, rather than water droplets; occur only at altitudes higher than 6000 m (20,000 ft).
Cloud A visible mass of suspended, minute water droplets or ice crystals.
Condensation The process by which a substance is transformed from the gaseous to the liquid state.
Condensation nuclei Small airborne particles around which liquid droplets can form when water vapour condenses; almost always present in the atmosphere in the form of dust or salt particles.
Cumulus clouds The cloud-category type that encompasses thick, puffy, billowing masses that often develop to great heights; subclassified according to height.
Dew The fine water droplets that condense on surfaces at and near the ground when saturated air is cooled; the source of this condensate is excess water vapour beyond the saturation level of that air parcel, whose capacity to contain water decreases as its temperature drops.
Dew point The temperature at which air becomes saturated and below which condensation forms.
Evaporation Also known as vaporization, the process by which water changes from the liquid to the gaseous (water vapour) state; it takes 597 cal of heat energy to change the state of 1 g (.04 oz) of water at 0°C (32°P) from a liquid to a gas.
Evapotranspiration The combined processes by which water (1) evaporates from the land surface and (2) passes into the atmosphere through the leaf pores of plants (transpiration).
Freezing The process by which a substance is transformed from the liquid to the solid state.
Hydrologic cycle The complex system of exchange involving water in its various forms as it continually circulates among the atmosphere, lithosphere, hydrosphere, cryosphere, and biosphere.
Latent heat of fusion The heat energy involved melting, the transformation of a solid into a liquid; a similar amount of heat is given off when a liquid freezes into a solid.
Latent heat of vaporization The heat energy involved in the transformation of a liquid into a gas or vice versa.
Melting A change from the solid state to the liquid state; at 0°C (32°F) it takes ca. 80 cal of heat energy to change 1 g (.04 oz) of water from a solid into a liquid.
Potential evapotranspiration (PE) The maximum amount of water that can be lost to the atmosphere from a land surface with abundant available water.
Precipitation Any liquid water or ice that falls to the Earth¿s surface through the atmosphere (rain, snow, sleet, and hail).
Relative humidity The proportion of water vapour present in a parcel of air relative to the maximum amount of water vapour that air could hold at the same temperature.
Runoff The removal - as overland flow via the network of streams and rivers - of the surplus precipitation at the land surface that does not infiltrate the soil or accumulate on the ground through surface detention.
Stratus clouds The cloud-type category that encompasses layered and fairly thin clouds that cover an extensive geographic area; subclassified according to height.
Sublimation The process whereby a solid can change directly into a gas; the reverse process is also called sublimation (or deposition); the heat required to produce these transformations is the sum of latent heats of fusion and vaporization.
Water balance Analogous to an accountant¿s record (which tallies income, expenditures, and the bottom-line balance), the measurement of the inflow (precipitation), outflow (evapotranspiration), and net annual surplus or deficit of water at a given location.
Water vapour The invisible gaseous form of water; the most widely distributed variable gas of the atmosphere.

Unit Outline

  • Physical properties of water
    • Ability to exist as a solid, liquid, or gas
    • Melting: solid to liquid
      • latent heat of fusion
    • Evaporation: liquid to vapour
      • latent heat of vaporization
    • Sublimation: solid to vapour
      • sum of latent heat of fusion and vaporization
    • Condensation: vapour to liquid
    • Freezing: liquid to solid
    • Deposition (Dr also sublimation): vapour to solid
  • Measuring water vapour
    • Vapour pressure is the pressure exerted by molecules of water vapour in the air
    • Saturated air is air holding all possible water vapour molecules at a given temperature
    • Dew is formed by condensation of over-saturated air
      • the dew point is the temperature at which air becomes saturated, and below it condensation will form
    • Relative humidity shows how close a parcel of air is to its dewpoint
      • a proportion of water vapour present in air relative to the maximum amount of water vapour it can hold
    • Specific humidity and the mixing ratio
      • specific humidity is the ratio of the weight of water vapour in the air to the combined weight of the vapour plus air itself
      • mixing ratio is the ratio of the mass of water vapour to the mass of the dry air containing the vapour
        • psychrometer
        • wet-bulb temperature
        • dry-bulb temperature
        • The hydrologic cycle
    • Moves water from the air to the land, plants, oceans, freshwater bodies, and back to air
    • Precipitation is any water or ice that falls from the atmosphere to Earth¿s surface
      • transpiration
      • evapotranspiration
      • runoff
      • advection
  • Evaporation
    • Occurs when heat energy is available at the water surface and the air is not saturated
      • a vapour-pressure gradient exists
    • Evapotranspiration is transfer of moisture from land to the atmosphere by a combination of evaporation and transpiration (photosynthesis)
      • potential evapotranspiration (PE)
      • actual evapotranspiration (AE)
  • Condensation and clouds
    • Clouds are masses of suspended water droplets or ice crystals
    • Cloud formation requires saturated air and a large amount of condensation nuclei
    • Cloud classification
      • stratus
        • nimbostratus
        • cirrostratus
      • cumulus
        • stratocumulus
        • altocumulus
        • cirrocumulus
        • cumulonimbus
      • cirrus
  • Precipitation
    • Formation processes
      • ice-crystal process
      • coalescence process
    • Forms of precipitation
      • rain
      • snow
      • sleet
      • freezing rain (glaze)
      • hail
  • The surface water balance
    • Range of water balance conditions
      • runoff cannot exceed precipitation in a given area
    • Water balance variations and latitude
      • evaporation greatest in subtropical and low latitudes
      • water surplus in upper midlatitudes
      • precipitation, runoff, and evaporation lowest in highest latitudes
    • Population and water balance
      • most people live where there is not a great deficit or surplus of water balance

Review Questions

  1. Define evaporation, condensation, sublimation, and deposition.
  2. Generally describe the global pattern of precipitation using Fig. 12.12 as a guide.