Hands-On, Minds-On Meteorology
Description | Programming | Operation



The Condensation program allows students to view a microscopic volume of moist air and manipulate its temperature and water content to learn more about condensation.

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The primary objective is to discover the relationship between temperature, moisture, and relative humidity. A secondary objective is to use the graphing tool to graph the classic saturation vapor pressure vs. temperature graph.




The theory behind the Condensation tool is to take a very small (but not measured) volume of air and view the dry air molecules, the water vapor molecules, and liquid water found inside. Each particle (particle is used here to include both types of molecules AND the liquid water) has a mass and a velocity. They are then sent off to either collide with the boundary of the volume, or to each other. Each water vapor molecule added helps to calculate the moisture variables. The actual movement depicted here is more for collisions and letting the condensation / evaporation process happen. The code for 3-dimensional collisions is available in 3DCollisions. More information about the parcels themselves can be found in ParcelSet*, so named because there are several versions of parcel controlling programs. The sub-program used here is ParcelSetCond, but they're all similar enough to be discussed in one document.


The Volume:

  • The X direction is from left to right.
  • The Y direction is from bottom to top.
  • The Z direction is from back to front.

Graphics and Visualization:

  • Each individual red 'ball' is one molecule of generic dry atmosphere (Molecular Mass = 29.87)
  • Each individual blue 'ball' is one molecule of water VAPOR (Molecular Mass = 18.00)
  • Each clump of blue 'balls' represents a condensed amount of LIQUID water. (Molecular Mass = 18.00 * #molecules)
  • The clump images are accurate up to clump sizes of ten, with clumps larger than ten looking the same as those with ten.
  • Numbers appear above large clumps (4 molecules or more) of condensed water to help show their numbers.
Condensation Process:
Much of this is discussed in WaterCollisions, but the option to slow down the molecules with the Slower button is unique to Condensation, Lifting Air (Moist), and Evaporation. The primary difference in slow mode is the time between each parcels next movement is slowed, but there is a significant, but more subtly noticed difference as well. It comes in the actual process of condensation. When the temperature or water content is changed such that the RH exceeds 100%, existing water vapor molecules will coalesce with any water type it comes into contact with, thereby reducing the RH until it is back to 100%. Because we are dealing with a small number of water vapor molecules, this process can take too much time to occur in the program, so in the normal mode, every time step that passes with the RH still larger than 100%, the search radius each water vapor molecule uses to try to collide with another piece of water increases by a pixel. The longer this takes, the more a water vapor particle may seem to disappear out of thin air. Generally this doesn't happen, and is so fleeting that it isn't that noticeable. In slower mode, the search radius is not increased, so it takes longer for the equilibrium condition to be satisfied, but this is acceptable, as it is moving slower anyway.


See ParcelSet*


See ParcelSet*



Running the Program

  • Click the link for 'Condensation'.
  • Change temperature and total vapor content by sliding the appropriate scrollbar.
  • Start and Stop the molecules by clicking the appropriate button.
  • The Slower button toggles the speed mode. It is labeling what would happen if you pressed it. (Ex.: If the button says "Slower", the molecules are moving quickly and would move slower if the button was pressed.
  • Use the checkbox to toggle the visibility of the dry air molecules.
  • This program will run better with the dry air molecules invisible.
  • Enable the Graphing Tool by click the appropriate button.

Extra Knowledge



Department of Atmospheric Sciences
University of Illinois at Urbana Champaign

Created by Dan Bramer: Last Modified 07/27/2004
send questions/comments to bramer@atmos.uiuc.edu