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



The Evaporation application is designed to the allow the students to see the process of evaporation and condensation between the ocean and the atmosphere.

click for whole shot



The primary objective of the Evaporation program is to give the students a sense of the never-ending equilibrium condition of water vapor. The speed at which humidify is transferred from one system to the other is graphically displayed.




The Evaporation program allows students to view a microscopic volume that is right at the air-ocean interface. The bottom part of the volume is in the ocean, while the upper part is in the air. In the air, the water vapor and liquid water molecules are shown, but unlike other programs where this is the case, dry air molecules are never shown. Red-colored particles are shown in this program, but they represent molecules that have just changed from the ocean to the air or vice-versa. Also, water in the ocean is represented by dark blue particles.

Students begin the experiment by choosing an ocean temperature and an air temperature. Upon clicking the start button, ocean water molecules will begin to move into the air, turning red for a few time steps, and then turning blue -- the normal color for water vapor in this program. Some of these vapor molecules may condense into liquid water, some may return to the ocean, turning red again, then the dark blue color that represents ocean water. Ultimately, the Relative humidity will escalate into the 90-100% range, but how quickly depends on the initial settings of the ocean and air.


See ParcelSet*

Graphics and Visualization

Ocean water is differentiated from other forms of water by making it a little darker (dkbluesm.gif). Red particles in Evaporation do not represent dry air molecules, they represent molecules that have recently changed phase.

Once condensation has occurred, it is often difficult to determine how many water molecules are included in the clump, so when a condensed liquid droplet has accumulated 4 or more water molecules, a small number is placed above it.

Also, at the moment of condensation, a small quick explosion-looking image is shown and quickly disappears. These flashes represent the release of latent heat for that individual condensation occurrence.

Also See Condensation
Condensation Process

See Condensation and Water Collisions.

Changing a parcel form ocean to atmosphere and back

The calculations made to determine this process are arbitrary. If a liquid water molecule in the ocean comes into contact with the ocean-atmosphere interface at a speed greater than 800 m/s, it is allowed to pass into the atmosphere. If a water molecule (liquid or vapor) in the atmosphere comes into contact with the ocean-atmosphere interface at a speed less than 250 m/s, it is allowed to pass into the ocean. If a condensed water particle strikes the interface with a slow enough speed, it will give one water vapor molecule to the ocean, not the entire particle.

For ease of viewing, parcels entering the atmosphere from the ocean always do so on the right side of the cube, and atmospheric parcels entering the ocean do so on the left side of the cube, regardless of where they hit the interface.


See ParcelSet*


See ParcelSet*



Running the Program

  • Click the link for Evap.
  • Change the ocean temperature by adjusting the Ocean Temperature scrollbar.
  • Change the atmospheric temperature by adjusting the Atmospheric Temperature scrollbar.
  • Click the 'Stop' button to pause the simulation. The button's label will change to 'Start'.
  • Click the 'Start' button to resume the simulation. The button's label will change to 'Stop'.
  • Click the 'Reset' button to reset the simulation. This will remove all water vapor from the atmosphere. Click 'Start' to then begin a new simulation.
  • Uncheck the '3D View' checkbox to view the volume head-on. This view helps to distinguish the red particles that represent both new atmosphere and new ocean water molecules.

Extra Knowledge

Any other information


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