Oklahoma Climatological Survey


Cloud Development





Clouds are masses of condensed water vapor which are visible signs of atmospheric processes at work. Clouds help regulate the earth's energy balance by reflecting and scattering solar radiation and by absorbing the earth's infrared radiation. In addition, clouds help redistribute surplus heat from the equator toward the poles and return water (in the form of precipitation) to the oceans and land masses across the globe.



The Importance of Clouds

Clouds are essential to the earth-atmosphere system. Clouds complete the following functions:

  1. Clouds help regulate Earth's energy balance by reflecting and scattering solar radiation and by absorbing Earth's infrared energy.
  2. Clouds are required for precipitation to occur and, hence are an essential part of the hydrologic cycle.
  3. Clouds indicate what type of atmospheric processes are occurring (e.g., cumulus clouds indicate surface heating and atmospheric turbulence).
  4. Clouds help redistribute extra heat from the equator toward the poles.



Making a Cloud

  • Clouds are masses of condensed water vapor.


  • Clouds are formed when water vapor is condensed into liquid water (cloud droplets).


  • There are three basic requirements for clouds to occur:
    1. Water vapor must be present in sufficient amounts so that saturation can be reached by some means.
    2. Cloud condensation nuclei (CCN) must be present to provide a surface on which water will condense. (Examples of CCN include dust in the air from the earth's surface, salt particles from the sea, combustion products, and volcanic or meteorite dust.)
    3. Cooling mechanism is required to cool the air temperature to the dewpoint temperature.

    The requirements for sufficient water vapor and CCN are rarely the limiting factors for cloud development. Typically, the limiting factor is a cooling mechanism. Thus, the air temperature needs to be lowered to the dewpoint temperature for a cloud to form. If a cooling mechanism is not present, clouds will not form.



Common Cooling Mechanisms

The following are common mechanisms in the atmosphere that cause cooling and, hence, may be associated with cloud formation:

  • Radiative cooling from the ground
  • Evaporative cooling
  • Cooling by lifting (as air rises in the troposphere, it cools)
  • Mechanical lifting
    • Lifting near a front
    • Lifting in low pressure systems
    • Local circulations
    • Thermal instability
  • Cooling by horizontal motion (warm, moist air moves over cool surface or cool air moves over warm, moist surface)



Atmospheric Stability


  • Warm air is less dense than cold air at the same pressure.


  • Moist air is less dense than dry air at the same pressure and temperature.


  • If the air is stable, an air parcel which is moved upward or downward will return to its original position. If the air is unstable, an parcel which is moved upward or downward will continue to move in the direction of initial motion and will accelerate in that direction until stopped by an opposing force.

    An air "parcel" refers to an imaginary blob of air to which may be assigned certain properties such as a temperature, pressure, water vapor content, etc. Generally thought of as the size of a beach ball.

  • As an air parcel rises, it will expand (because the air pressure decreases with increasing height) and cool. The environmental temperature may decrease at the same rate as the expanding air parcel, but in general the environmental temperature decreases (or even increases) at a rate different from that of the rising air parcel.

    The "environment" refers to the average properties of the air nearby the air parcel.

  • Stability can tell us whether or not clouds will form, where they will form, when they will form and what type of clouds they will be.





OK-FIRST Project, Oklahoma Climatological Survey, 100 East Boyd Street, Suite 1210, Norman, OK 73019.
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