1. Using the concepts of evaporation and condensation, explain what happens when it rains in Portland, Oregon. 2.  What might be the effect on Portland's climate if the Cascade Range was to the...

1. Using the concepts of evaporation and condensation, explain what happens when it rains in Portland, Oregon.

2.  What might be the effect on Portland's climate if the Cascade Range was to the west rather than to the east of Portland? Be sure to consider the prevailing pattern of winds in the United States and the concept of a rain shadow in answer.

Asked on by marley112

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ncchemist | eNotes Employee

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First let's define a couple of terms.  Evaporation is when a liquid chemical enters the gaseous state.  Condensation is when a gaseous chemical enters the liquid state.  So they are opposing processes in the water cycle, which tracks water as it changes its chemical state over time (solid, liquid, gas).  Portland, Oregon lies on the northwestern part of Oregon, not right on the coast but close to it.  Sunlight warms the surface of the water in the Pacific ocean, thus causing water to evaporate into the air.  Prevailing easterly winds bring this moist air off the ocean and over land.  As the air rises and cools over land, the moisture is lost from the air as the water vapor condenses into raindrops that fall from the sky.  Warm air holds a larger percentage of water vapor than cool air.  The rainfall on the land makes its way back into the ocean via rivers, so the water is reclaimed by the ocean to start the cycle over again.

The Cascade mountain range lies to the east of Portland.  As the wind carries moist air off the ocean and brings it east over land, it eventually hits the Cascades.  This causes the air to rise quickly, thus cooling it quickly and causing the rain to fall on the western side of the mountain chain.  This traps all of the moisture on the windward (western) side of the mountain chain.  As a result, the air on the leeward (eastern) side of the chain stays dry and the climate more arid than the damp windward side.  This phenomenon is called a rain shadow.  Since Portland is on the western side of the Cascades, it stays damp with rainfall.  If Portland were on the eastern side of the Cascades, it would sit in the rain shadow and have a much drier, more arid climate.

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