IDS 102
Evaporation and Boiling
From past IDS groups we have found that there has been some confusion about
evaporation and boiling. Visit the web page below:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html
Where does evaporation happen?
What determines whether a water molecule will remain in the water or evaporate and
become vapor?
Describe the difference between evaporation and boiling.
What determines the boiling point of a liquid?
If you camp at the top of Mt. Rainier, you may discover that it takes longer to boil
potatoes than it would have if you used the same gas stove in a campground at the base of
Mt. Rainier. Explain.
(check with an instructor before you continue)
Cloud Formation
Water vapor is a colorless/clear gas—we cannot see water vapor as a true gas when it is
in a molecular size. For reasons beyond the level of this course, water condenses more
readily if there is something to condense on. The atmosphere also contains compounds
called “Cloud Condensation Nuclei”. These very small particles of water-loving
substances, such as sea salt, provide a spot for water vapor to condense. As water starts to
condense on the CCN, the water droplets get large enough for us to see them due to the
reflection of light.
The winds in the atmosphere are usually strong enough to keep these larger droplets
suspended in the air and as they run into more and more water vapor, the droplets
increase in size. Since this process is happening with many other water droplets, there
tend to be a bunch in one area and we call these clouds. If there is enough moisture in the
cloud and the temperature is low enough for continued condensation to occur, the droplet
may reach a size large enough to fall as a raindrop. See the table below for some values
of water droplets:
Diameter
(micrometers-m)
0.2
20
Terminal velocity
(m/sec)
0.0000001
0.01
Terminal velocity
(ft/sec)
0.0000003
0.03
Type of particle
Condensation nuclei
Typical cloud
droplet
100
0.27
0.9
Large cloud droplet
200
0.7
2.3
Drizzle
1000
4.0
13.1
Small raindrop
2000
6.5
21.4
Typical raindrop
5000
9.0
29.5
Large raindrop
From: Ahrens, 1994, Meteorology Today, Fifth Edition, West Publishing (page 194)
 Determine how many typical cloud droplets there are in a typical raindrop
(the volume of a sphere is 4/3 r3).
 Let’s review something from the pressure module that we recently finished.
When you pumped the air out of the plastic jar, what happened to the
temperature inside the jar?
If air moves up in the atmosphere, it will experience both a reduction in pressure (because
there is less air above it) and lower temperatures.
 So, if air in the atmosphere decreases in temperature, what happens to the
water vapor in the air?
 Let’s think about a parcel of air at sea level in Seattle. The parcel consists of a
collection of gases, including water vapor. As the air moves to the east, the
air rises to go over the Cascade Mountains. What happens to this air? Draw a
picture and explain.