Unit 16

Evaporation — physical-meteorological processes and observations

This unit provides one of the links between the water in all three phases that exists in the atmosphere and the water in reservoirs like ocean, lakes and rivers and soil water.

#Bowen ratio method
Schematic view of the setup of the Bowen ratio method. From: https://civildigital.com/bowen-ratio-method-pan-evaporation-explained/

Goals

The goal of this unit is to describe one of the processes that adds water vapor to the atmosphere against the gravity force.

After successful completion of this unit students will be able to

  • Calculate the evaporation from open water surfaces and  bare soil
  • Determine the energy required for evaporation
  • Explain various  methods to perform evaporation measurements

Students’ tasks

  1. Read Dingman’s Chapters  3.3 to 3.3.2 and 6 to 6.4 (included)
  2. Watch this movie by  Waikato Earth and Ocean Sciences master’s student Tehani Kuske
  3. Watch this video of a worked problem

  4. Watch this video of a worked problem

  5. Fill out the questionnaire prior to Thursday 2359 Alaska time
  6. Solve the tasks assigned at your class level in this Unit 16 Applications  sheet and submit the scanned results by Thursday 2359

FAQ

Q: The book said that evaporation from a large pan is less than from a small pan. Why?

A: Watch this video.

Q:  I can’t get the image of the explanation in pp.115, “However, in tiny spherical cloud droplets, there are more molecules in the surface than in the next layer below. Because of this, the intermolecular hydrogen-bond forces acting on the surface molecules are weaker than for a horizontal surface. Thus the vapor pressure at the surface of a very small droplet is higher than given by equation (3.9a) for a horizontal surface, and there is a correspondingly greater tendency for the molecules to evaporate.”

A:   Take a look at the schematic view of a water droplet below. The  red and blue atoms are H and O, respectively. Count the molecules that make up each ring in the cross section. More energy is needed to create the surface of the small than large droplet. The ratio of the surface (A=4Ï€r2) to volume (V=4/3 Ï€ r3) is smaller for a large than small drop.

Schematic view of a droplet composed of water molecules. The red and blue atoms are H and O, respectively. The values at the end of the green lines give the amount of molecules on the respective cross section of the sphere.

Acknowledgements

The source of the movie material by Tehani Kuske is the University of Waikato Earth and Ocean Sciences master’s student Tehani Kuske.   All rights on this movie are  The University of Waikato.

© 2019 Nicole Mölders | All rights reserved