NAME: _________________________________________________ QUIZ SECTION: ____________
ATMS 101 Autumn 2008
Homework #7 (Due Wednesday, November 26)
Please show all work and write neatly in the space provided.
This homework is much shorter than normal. We expect you’ll spend some extra time
learning how to identify clouds and how the different types form. We’ll provide some
tools to help you learn about clouds.
1. A cumulonimbus cloud contains supercooled cloud drops and ice particles in the
layer where the temperature is between 0 and -40 degrees C. How can a hailstone
the size of a golf ball develop in this part of the cumulonimbus, but never in the 0
to -40 degrees C layer of a nimbostratus?
A nimbostratus has slow rising air. A strong updraft, like that of a cumulonimbus, is needed to
suspend a heavy hailstone and allow it to grow.
2. Discuss why fog forms in these two situations, AND name the type of fog:
a. In Minnesota, a warm wind is blowing across a lake filled with very cold
water (hint: this is different than “steam fog”; consult your textbook).
Advection Fog: warm air that blows across the cold water is cooled to its dewpoint temperature,
forming fog.
b. It's a cold clear night in Tennessee.
Radiation Fog: the surface radiates energy (via infrared radiation) out to space, cooling off the
ground. The air just above the ground is cooled to its dewpoint temperature, forming fog.
3. Explain why a cumulonimbus cloud looks bumpy in its lower section but its top
portion is smooth and in the shape of an anvil.
Rapidly rising parcels of air condense into liquid water, forming regions (“bumps”) of cloud. Strong
winds at the top of the cumulonimbus blow ice crystals away from the cloud, forming a smooth sheet
extending away from the main portion of the thunderstorm.
4. The Matterhorn in Switzerland often has a banner cloud attached to and streaming
downwind of its peak.
a. Explain why this happens. A well labeled sketch may help, but you still
must write a description.
The sharp peak of the matterhorn splits the air flow near the top of the mountain, causing a small
region of lower pressure (and therefore lower temperature --> the air can reach its dewpoint and
form a cloud) in the lee of the peak. This also causes air to rise up the lee slope and cause more air to
reach its dewpoint temperature.
b. Why did Mt. Saint Helens in Southern Washington sometimes have
banner clouds before its eruption on May 18, 1980 but not today?
Mt. St. Helens used to have a sharp peak – which is required for banner clouds to form – but now it
does not.
Links for Cloud Identification review:

Online Tutorial for cloud identification (for this link, click “play through” at the
bottom of the screen)
o http://asd-www.larc.nasa.gov/SCOOL/tutorial/clouds/cloudtypes.html

Houze Cloud Atlas (includes formation and example pictures)
o http://www.atmos.washington.edu/gcg/Atlas/