Elementary Meteorology Online
Chapter 6 - Humidity, saturation, and stability


  • Text readings
  • Online Readings
  • Adiabatic expansion/compression simulation
  • Cloud formation
  • Fog formation

  • Text reading

  • Review Chapter 4 pp. 90-91 "Phase changes of water"
  • Review Chapter 5 pp. 122-124 "Expansional cooling and compressional warming"
  • Chapter 6: Humidity, saturation, and stability pp. 111-125.

  • Online readings

    Humidity, saturation, and stability

    Please read these sources before tackling your textbook and investigations. Your text explains these well, but as usual is very detailed in its explanations.

    Adiabatic expansion/compression simulation

  • In cloud formation, air temperature is lowered to the dewpoint and to saturation. This is achieved by adiabatic cooling in ascending air parcels. In this simulation, we illustrate adiabatic expansion and compression in an ideal gas. Click here for Java animation of an Ideal Gas. Follow the instructions below to simulate adiabatic expansion and compression.
  • The term adiabatic heating means that temperatures change not because of diabatic transfers of heat (i.e. radiation, conduction, or or convection), but through mechanical work (i.e. moving the walls of the air parcel/vessel).
    • Density: For ideal gas simulation, pump air into vessel until pressure of 1 atmosphere is reached. Releaase excess gas molecules by removing lid on vessel. More gas = denser gas, as molecules are squeezed closer together; There is more mass in the same volume.

    • Pressure Is the force caused by gas molecule collisions with the container walls. Increasing number of molecules (density) increases collisions with walls. Pressure increases. Releasing molecules decreases pressure.
    • Heat the vessel. Temperature increases (gas molecules speed up). This increases collisions with vessel walls and pressure increases also.
    • Ideal gas law: pressure = density x constant x temperature; increasing density and/or increasing temperature will both increase pressure .
    • Expand the volume. This is done on atmospheric air parcels by lowering the ambient (surrounding) pressure. ` Firstly, pressure will decrease because collisions with walls decrease. Secondly, the kinetic energy of the molecules will go into expanding the volume. This will slow down gas molecules, decreasing temperatures. This is called adiabatic expansion or adiabatic cooling.
    • Contract the volume. This is done on atmospheric air parcels by raising the ambient pressure. Pressure increases and temperature increases. This is called adiabatic expansion or adiabatic heating

  • Cloud formation

  • In the real atmosphere, adiabatic cooling occurs when an air parcel is lifted. An air parcel is just a small volume of the atmosphere that does not mix into its surroundings.
  • A lifted parcel expands because the pressure decreases with height allowing it to expand and cool adiabatically (see image at left).
  • The rate of this cooling in the lower troposphre is abut 10 C per km. This rate is know as the adiabatic lapse rate. A lapse rate is just the rate at which the temperature drops off with height. It is very imnportant to note that the air surrounding the parcel usually does not drop off at the same rate. The ambient lapse rate, in other words, is usually lower, not cooling as quickly.

  • An air parcel where the dewpoint and temeprature are equal and relative humidity is 100% is called saturated or "moist"; an air parcel where the dewpoint is lower than the temperature and relative humidity is less than 100% is called unsaturated or dry.
  • A rising, adiabatically cooled air parcel will eventualliy reach its dewpoint, condensing cloud. The higher the humidity of the air parcel, the sooner it will will become saturated and the lower the cloud was.

  • Image at left shows temperature plotted against height on a STUVE diagram. A rising air parcel will reach its dewpoint at the level of condensation or lifting condensation level (LCL) (called Level of Condensation in image).
  • Above the LCL, the condensing water releases latent heat which warms the parcel. As a result, the rising parcel no longer cools at quickly. Within the cloud that it forms, it cools at a rate of about 6 C/km (the moist adiabatic lapse rate).
  • Important! These changes in temperature apply within a lifted air parcel. It is very important to remember that the ambient temperature profile (the temperature of the surrounding atmosphere) is usually very different than the tmperature of the rising parcel!

  • Radiation fog formation

    The COMET module on fog formation illustrates how cooling at the surface of the earth to the dewpoint on clear nights results in condensation in the form of dew and fog. Click here for a more detailed explanation (optional).

    • Radiation fog forms at night along the surface of the earth, especially in valleys on clear nights.
    • The temeprature profile shows a shallow layer of cool air that is saturated with respect to water vapour. This is an inversion because temperature (red line) increases with height in the fog layer.
    • The layer is saturated because the dewpoint (green line) is equal to the temperature in this layer.
    • The key processes radiative cooling, water vapor condensation as dew or deposition as frost, fog initiation by condensation, fog layer deepening, and dissipation by wind changes or radiative heating.

    Radiative Cooling

    • After daytime heating ends, outgoing longwave radiation continues to radiate from the surface of the earth.
    • As the energy escapes, the ground surface cools rapidly and induces cooling of the lowest few meters of the atmosphere.
    • The decrease in temperature (red line) results in a decrease in saturation mixing ratio.
      • The actual mixing ratio and dewpoint (green line) remain constant.
      • The relative humidity increases.
    • If there is enough water vapor in the air and enough cooling at the surface, the low-level air eventually reaches saturation.

    Effect of cloud

    • Clear, dry conditions above hasten radiative cooling. Cloud and humidity can act to hinder fog formation.
    • When skies are overcast, less than 10% of the radiation emitted by the earth escapes to space. Most of the radiation is absorbed and/or reflected by carbon dioxide, water vapor, and cloud droplets in overcast skies.
    • Clear skies allow as much as 20 to 30 percent of the radiation to escape the atmosphere, leading to rapid surface cooling.

    Dew/Frost formation

    • As the temperature reaches the dewpoint near the ground, water vapor begins to condense as dew on solid surfaces (if the surface temperature and dewpoint are below freezing, water vapor is deposited as frost).

    Fog initiation

    • Sensible heating transports heat from the lowest layers of the atmosphere to the surface fo the earth. Weak turbulent diffusion (small currents of air) bring heat to the surface of the earth and conduction transfers the heat to surfaces. This cools the lower layer of the atmosphere to the dewpoint.
    • A fog layer forms as water vapour consenses into water droplets in the this layer.
    • As water condenses, the dewpoint also lowly drops, allowing the temperature to drop further.

    Deepening of fog layer

    • The image at left shows the fog layer deepening as radiational cooling at the top of the fog layer leads to an accelerated condensation of liquid fog droplets.
    • The top of the cloud layer will act as a black body and radiate heat in the same way as the earth's surface.
    • Radiative heat loss cools the top layer of fog, accelerating cooling at this level.
    • This saturates the fog top and encourages the condensation of more fog.
    • Condensation at the fog top is the means by which a radiative fog maintains its depth and/or deepens.
    • The radiative heat loss is maximized when the layer immediately above the fog is relatively dry, the winds are weak, and there are no cloud layers aloft. On a clear night, the rate of fog-top radiational heat loss is much more rapid than that in the lowest few meters of the atmosphere.

    Capping of fog layer by wind

    • Increasing winds at the top of the fog layer will cap the deepening of the fog layer.
    • Stronger winds aloft create turbulent eddies that mix down warmer, less humid air from aloft.
    • The warmer, unsaturated air will evaporate the fog droplets if they move above the fog layer.

    Fog dissipation by sun

    • The main source of radiant heat is the sun. During the daytime, some radiation from the sun is absorbed by the ground, even when there is an intervening layer of fog reflecting away some of the sunlight.
    • As the ground warms, it heats a thin skin of air in contact with the surface through conduction.
    • This heat initiates weak convective mixing, which begins to warm the lowest portion of the fog layer.
    • The relative humidity in this layer begins to decrease, slowing the formation of fog droplets and eventually evaporating existing droplets.
    • As the fog thins, the warming process accelerates, allowing more solar radiation to reach the ground.
    • With moderately strong sunshine, the base of a fog or low cloud layer can lift at a rate of up to several hundred feet per hour.

    Fog dissipation by wind

    • The image at left illustrates the effect of a gust of wind on a fog layer.
    • A gust of wind will set up turbulent eddies that mix in warm, dry air from aloft and heat from the surface of the earth.
    • This will evapourate the cloud droplets at the top and botttom of the fog layer, eroding the fog bank from above and below.
    • This is why radiation fog requires calm winds to form.

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