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photo by Jim Laatsch - US ITASE
It's not necessarily the amount of new snow that is a weather severity on Antarctica, but it's sometimes the ferocity of the winds. The wind is responsible for displacing the loose snow and firn to create whiteout conditions and for creating severe wind-chill conditions. The katabatic winds on the continent exist due to the airflow from the high pressure, cold, elevated plains down slope to the relatively warmer lower elevations with less pressure. Looking at the katabatic winds offers an opportunity to explore air pressure and the properties of hot and cold air.
Materials:
| 2 quart size jars | matches |
| tea candle | paper squares |
| ice to fill jar | recording worksheet |
Place a tea candle in the bottom of one jar. Fill the other about 2/3 full of ice. The jars can be set up before the observations so students can rotate through, or use multiple jars for partners.
Have the students place a square of paper first over the candle jar. Observe the reaction of the paper. Record observations on the worksheet. Remind students when a scientist observes "no change," that is indeed a valuable observation.
Repeat the procedure with the ice jar.
Notes:
**Do not use the same papers for repeated trials. The paper warps
over the heat of the candle.
**The students will notice the edges of the paper “lifting”
up over the heat. Discuss the idea of warmer air taking up more
space due to the movement of the molecules. Some students might
say they noticed the paper over ice being “pulled” down.
Although it is unlikely that they could see this, it can still lead
to a discussion of cold air molecules taking up less space.
**Make sure you try this before presenting it to a class. You need to try it with your materials to make sure you will see the intended response. The effect is good, but it can be tricky. Observing the response of the feathers led to a good discussion, but it did not present itself as an experiment suitable for recording like a typical lab write-up. It might best be done as a demonstration activity.
Materials:
| Ice slope (see below) | matches |
| 4 votive candles | small feathers |
| foil | pair of tongs |
Heated surface: Place the votive candles in a closely arranged diamond shape on a sheet of foil. Make sure this is in a draft free location. The idea is to create a heated air space above the candles. Light the candles a few minutes before experimenting to allow the space to heat up.
Ice plain and slope: If there is a difference in pan size, use the larger pan as the plain. Elevate the ice plane on a box or other object and rest the other pan of ice so it is sloping away from the plane at about a 45 degree angle. (It is not important to be precise with this.) Set this up a few minutes before testing to allow the air space to cool.
Experiment:
One at a time,
release a feather above the air space, whether it is the cold or
hot space. Above the heat, release the feather in the middle of
the diamond. Above the ice, release the feather above the point
where the slope meets the plain. The feathers should be released
at about the same height above each to compare the results. Observe
the way the feather moves in the air. Repeat the procedure to find
a consistent pattern.
Notes:
**Have the tongs handy in case the feather does drop into the candles.
**Do not re-use a feather if it falls into the candle or onto the
ice. The weight of the feather changes significantly in either case.
**The students will likely say that the feather tries to get away
from the heat. Guide the discussion to emphasize that the feather
only demonstrates what is happening in the air. In fact, the feather
"floats" in the warmer air, but "sinks" when it is controlled by
the cooler air. Therefore, the feather falls outside of the heated
air space because that is where the cooler air is.
**The feather will occasionally fall into the candles. Usually this
is because a draft has been caused by people moving the heated air
space. Hold your hand above the candles or ice a moment before releasing
to let the air settle.
**Experiment with holding the feather over different areas of the
ice. The best place seems to be where the slope drops away since
the air has room to "fall" there.
**Refer to this site for some good explanations to use during discussions
with the Pull-it-all-together questions on the worksheet. http://hpccsun.unl.edu/nebraska/stuproj/ametf99/wenzl/katabatic.html
Katabatic wind (gravity wind): Downslope wind caused by greater air density on the slope than at some distance, horizontally, from it. The wind is associated with surface cooling of the slope. http://kazan.inf.fu-berlin.de/euromet-zeam/courses/glossary/katabat2.htm
photo - US ITASE
http://hpccsun.unl.edu/nebraska/stuproj/ametf99/wenzl/katabatic.html
Explanation of katabatic winds with illustrations
Firn snow and katabatic wind explanations:
http://www.xrefer.com/entry/614400
http://www.alexski.co.uk/mountainsafety/Weather.htm
The word firn
is derived from the German language. Literally translated means
"of last year". Although frequently used in the alps to
denote a snow condition frequently found in spring, it actually
refers to a snow condition in glaciers.
This condition occurs when overlying snow layers continually melt,
re-freeze and recrystallize the snowflakes. It is currently thought
that this process takes approximately one year to create the firn
condition.
During the spring and summer months the 'firn' layer is often detected
by a thin dusting of frost in the early morning.
After several years and further pressure exacted by the many layers
of snow and ice (45 - 60m), the firn layer will turn to glacial
ice
Pull-it-all-together questions:
Everything is pulled to earth by gravity. Which air do you
think gravity would pull down faster, cold or hot? Why?
We used a feather in our experiment so that we could see what the air was doing. How does what you observed in these experiments help us explain what might happen on a glacial slope?
Would the same thing happen on a desert sand dune? (Why or why not?)
Knowing what you do about the geography of Antarctica, why are there katabatic winds on West Antarctica?
Student Worksheet
Name ________________________
Exploring the conditions
for Katabatic Winds
What are katabatic winds? They are downslope winds that are caused by greater air density on the slope than at some distance, horizontally, from it. The winds are associated with the surface cooling of the slope.
What is density?
Which air is more dense, warm or cold?
Record your observations for the following experiments.
Experiment 1: Paper Covers
Place a paper over the jar with a candle burning inside. Observe. What did you notice about the paper?
Place a paper over the jar with ice inside. Observe. What did you notice about the paper?
Experiment 2: Falling Feathers
Hold a feather a set distance above the cold slope. As you let the feather drop, what do you notice about the way the feather falls? Look at the way the feather moves. How would you describe the path that it takes?
Hold a feather
about the same set distance above the warmed area.
What do you notice about the way this feather falls?