Lesson 3 Activity 2: In the Rain Shadow

• 45 minutes
• By recording the annual precipitation (rain and snow) for cities on the east and west sides of the Cascade Mountains, students will discover that volcanic mountains do not have to erupt to affect the atmosphere.

Key teaching points

• The Cascade Range comprises a 1,130-kilometers (about 700 miles) long chain of volcanoes lying about 160 to 240 kilometers (100 to 150 miles) inland from the coast of the Pacific Ocean. Their location affects the climate of the Pacific Northwest region.

• Because the Cascades act as a geographic barrier to moisture-laden masses of air arriving from the Pacific Ocean, cities on the west side of the mountain receive more precipitation annually than those on the east side. The cities on the east side are in the "rain shadow" created by the mountains.

Materials

• Activity Sheets 3.2 a - b
• Wall map of the United States
• Glass of ice water

Procedures

1. Using a large wall map, locate the Cascade Range. Remind students that Mount St. Helens is one of the volcanic mountains that make up the Cascade Range.

2. Tell students that in the Pacific Northwest region, the prevailing winds blow from west to east. That means that most of the weather that affects the region forms over the Pacific Ocean where it picks up a great deal of moisture. Given this fact, ask students if they think annual precipitation is greater or lesser on the west or east side of the mountains.

3. Distribute Activity Sheets 3.2 a-b.

   • After the students have completed their activity sheets, discuss with them the reasons why cities on the west side of the Cascade Range have a greater precipitation than those on the east side of the mountains.

   • The mountains act as a barrier: Air must rise to get over the mountains. As the air rises, the temperature of the air falls and moisture in the air condenses. As the moisture condenses, it falls as rain or snow. By the time the air reaches the top of the mountain, most of the moisture has been lost as rain or snow.

   • As the air descends on the other side of the mountain, most of the moisture that remains is lost through evaporation instead of falling as precipitation.

   • That is why cities on the western, or windward, side of the Cascades receive a higher annual precipitation than those on the eastern, or leeward, side of the Cascades. The mountains have produced a "rain shadow" on the leeward side (fig. 4).

Extension