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Earth Science- Ms. GillWeather Lab

Lab: Orographic Effect on Climate

Introduction: Mountain ranges acts as barriers to the flow of air across the surface of the earth. They act to squeeze moisture out of the air. When a parcel of warm air reaches a mountain range, it is lifted up the mountain slope, cooling as it rises. This process is known as orographic lifting and the cooling of the air often results in large clouds, precipitation, and even thunderstorms.

As a parcel of air rises up the windward side of a mountain range, much of its moisture is depleted. Thus, when the air begins to descend the leeward side of the mountain, it is dry. As the cool air descends, it compresses and warms, reducing its possibility of precipitation. This is known as the rain shadow effect and is the primary cause of leeward deserts of mountain ranges, such as California's Death Valley. Orographic lifting is a fascinating process that keeps the windward sides of mountain ranges moist and filled with vegetation but the leeward sides dry and barren.

Objectives:

The students should understand the influence of a large mountain range on average precipitation amounts (measured as rain) and on average temperature.

Procedure 1: Mapping

1)Examine the map below of Washington State. Find the following cities and color them with colored pencil: Yakima: Red Mt. Rainer: Green Oympia: Yellow

2)Color the Strait of Juan de Fuca, the Strait of Georgia and the Pacific Ocean blue.

3)Find the Cascade Mountain Range and shade it orange.

Part 2: Graphing:

Yakima, Washington Period of Record Monthly Climate Summary: 1/3/1931 to 7/31/2000

Jan / Feb / Mar / Apr / May / Jun / Jul / Aug / Sep / Oct / Nov / Dec / Total
Average Temperature
Total (˚F) / 44.7 / 50.1 / 53.4 / 59 / 66.7 / 71 / 76.9 / 76.3 / 71 / 61 / 52 / 47 / 60.8
Average Total in. of
Precipitation / 1.42 / 1.03 / .84 / .7 / .98 / 1.1 / .33 / .54 / .6 / .8 / 1.5 / 1.7 / 11.4

Mt Rainer, Washington Period of Record Monthly Climate Summary: 10/26/1950 to 4/30/1994

Jan / Feb / Mar / Apr / May / Jun / Jul / Aug / Sep / Oct / Nov / Dec / Total
Average Temperature
Total (˚F) / 23.75 / 27 / 30.1 / 35 / 42 / 49 / 55.6 / 55.6 / 50 / 41 / 30 / 25 / 38.7
Average Total in. of
Precipitation / 13.29 / 9.23 / 7.3 / 5 / 3.63 / 3.2 / 1.36 / 2 / 4 / 7.2 / 12 / 13 / 81.54

Olympia, Washington Period of Record Monthly Climate Summary: 6/2/1948 to 7/31/1983

Jan / Feb / Mar / Apr / May / Jun / Jul / Aug / Sep / Oct / Nov / Dec / Total
Average Temperature
Total (˚F) / 39.45 / 43.6 / 45.8 / 51 / 56.9 / 62 / 66 / 66 / 62 / 54 / 46 / 42 / 52.75
Average Total in. of
Precipitation / 4.95 / 4.23 / 3.52 / 2.3 / 1.5 / 1.5 / .96 / 1.08 / 1.9 / 3.2 / 4.9 / 5.8 / 35.86

Part 3: In the boxes below sketch a cross section of the state through the weather stations. Label each city and draw an arrow to show the prevailing wind direction.

Part 4: Conclusion

1)Analyze your graphs and write a paragraph in which you describe and account for the temperature differences at each station.

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2)Analyze your graphs and write a paragraph in which you describe and account for the precipitation differences at each station.

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3)Write a sentence or two summarizing the orographic effect on the climate of this region.

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4)Label this picture (windward side, leeward side, moist air, dry air, desert, condensation, peak, cooling due to expansion, warming due to compression)

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