The purpose of this exercise is to give students a feeling for the
concept of "gap winds".  Here is one way to illustrate this using
water.   On a flat surface such as a cookie sheet or plywood build a
wall out
of clay.  It would be ideal if the the shape of the wall would be in a
"U" to look like the area between the Coastal Range, the Okanogan
Highlands and the Rockies.  Put a small depression or gap in the clay
wall at the botton left, representing the Fraser River.
Now pour water behind the clay wall until the water starts to stream
out of the "gap" in the wall.  This is similar to the way the cold air
"dams" up behind the mountain and then flows through the gap.  Also
note that the water flows rapidly through the gap, and that the more
water you have behind the wall, the faster the water will flow through
the gap.  Again, this is the same with air, the greater the pressure
difference between the cold, high pressure air behind the mountains
and the warm, low pressure air by the ocean, the faster the air
streams through the Fraser River valley.
<P>
Additonal activities:
<ol>
<li>do the experiment without and with the gap to show why
the gap changes the weather conditions near the gap.
<li>tilt your flat surface a little so the water flows from top to
bottom toward the bottom of the "U" to simulate the cold air flowing
out of the north.
</ol>
<p>
Questions to ask students:
<nl>
<LI>What does the clay wall represent?  Can you identify mountain(s)
that are similar?
<LI>What does the gap represent?  Can you identify a similar gap in
the mountains you identified in question 1?
<LI>Why does the water flow through the gap?
<p>