Animation: Water Vapour Circulation on Earth

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Animation: Mountain Weather
Background
On average, the tropics receive more solar radiation annually than zones located at higher latitudes. But
solar radiation is only one of several variables that determine the mean annual temperature of a given
region. For example, Quito, Ecuador is on average 10°C (18°F) cooler than Miami, Florida, despite sitting
much closer to the equator. The main reason for this discrepancy is elevation. While Quito sits 2,800
meters (9,200 feet) above sea level in the Andes Mountains, Miami is a mere 3 meters (9 feet) above sea
level.
Ecuador is located on the west coast of South America. This means that the Pacific Ocean lies to the west
and the Amazon rainforest lies to the east, allowing the prevailing winds to pick up moisture from
whichever direction they are blowing. These moisture-laden air masses are then driven upward by the
slopes of the Ecuadorian Andes.
A rising air mass experiences decreasing atmospheric pressure the higher in altitude it goes. This decrease
in pressure allows the air mass to expand, which in turn causes it to cool. On average, air temperature
decreases 10°C for every 1,000-meter (18°F for every 3,280-foot) increase in altitude. This cooling also
results in precipitation. If an air mass continues to rise, it eventually cools to its condensation
temperature and forms clouds. When water droplets become large enough, they fall to Earth as
precipitation. This example demonstrates how topography, moisture sources, and air masses interact with
solar radiation to create local climate.
Discussion
 What do the dots in the interactive activity represent? What do the different colors represent?
 If you walk around the base of the mountain, will your ears pop because of the pressure? Explain.
 If you walk up the mountain, will your ears pop because of the pressure? Explain.
 Describe why a decrease in air pressure causes air to cool.
 Describe why the windward side of a mountain is usually wetter than the leeward side.
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