Climate Conversations: The latent heat of evaporation

Clyde Burnett

Question: We all know about the liquid water evaporation-precipitation cycle, but how is energy involved?

Answer: Best to first do the experiment featured in the middle school text, “Science and Technology: Patterns of Change”, edited by my daughter as Project Director at Biology Sciences Curriculum Study in Colorado Springs.

Start with 4 ice cubes in a saucepan over a constant source of heat. Monitor the times elapsed for the ice to melt, the water to warm from freezing to boiling, and the time for all the water to evaporate. The elapsed time for each process is a measure of the energy being absorbed. Stir the water continuously and make a continuous measurement of temperature. The melting time will be almost as great as the time it takes to warm from freezing to boiling and the time for evaporation will be more than 5 times the warming time. The temperature will remain constant at 0oC for melting, and will remain constant at 100oC (at sea level) for the evaporation, and will increase gradually during the warming from 0 to 100.

The experiment is not expected to be especially accurate because of energy transfer to and from the surroundings, but will be a good indication of the energy behavior. The accurate values are 100 calories per gram (by definition of calorie) to warm the 100 Celsius degrees change, 79 calories per gram at constant temperature for melting, and 539 calories per gram at constant temperature for evaporation.

With global warming, we see that 90% of the trapped energy from the greenhouse effect is being absorbed by the oceans on 70% of the earth’s surface and each water molecule that escapes from liquid into the atmosphere as vapor carries with it that large latent heat of evaporation. It doesn’t take a rocket scientist to understand that this energy will be released upon condensation as rain or snow.

Solar radiation absorbed on the ground warms the surface air; that air expands and is buoyed upward due to the reduced gravity. As that air parcel moves upward into reduced pressure, it cools and eventually we have condensation into cloud droplets. The released latent heat warms this parcel and increases its buoyancy. If we also have cool air transported into the higher elevations, this process of condensation and energy release is encouraged and continues so long as the source of water vapor is transported from the Gulf of Mexico. This atmospheric circulation may also result in wind shear with elevation. So, global warming results in the extra supercells that may produce tornadoes like the 29 observed on March 6, 2017.

Question: But why is the ocean getting warmer?

Answer: Water vapor is a major player in the greenhouse effect. Water vapor molecules have the unique mechanical and electrical structure that absorbs the infrared photons radiated upward from the Earth’s surface. (The nitrogen and oxygen molecules have similar mechanical structures, but their electrical structure does not permit infrared absorption.) This energy can be reradiated; the downward energy warms the Earth, the upward energy is absorbed in the next upper water vapor layer. The compressible air density decreases with altitude so that at about 20,000 ft., at a temperature of –18oC, some of the upward radiation escapes into space. This is the effective radiation surface of our planet. This layer of water vapor in the lower troposphere is the blanket causing the greenhouse effect. This layer is becoming thicker due to the increasing CO2 control of ocean evaporation. Now the effective radiation surface moves into the cold high atmosphere where there is less infrared radiation into space. We have an increasing energy imbalance that makes the oceans warmer.