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Cliff, river cliff
Any very steep slope formed
by natural processes. Cliffs are too steep for soil and are always bare rock.
Cliffs can be created by rivers as they undercut their banks. These cliffs, called river cliffs, are on
the outside of the curving section (meander) of a river and may be between a few meters and hundreds of meters high.
Condensation
Water vapor condenses into liquid water when the temperature of the air falls. One way air cools is when it comes into contact with a cold surface. That is why, for example,
a glass containing ice cubes soon gathers a coating of condensation. Warm air flowing over a cold ocean has the same effect; in this case contact with condensation takes place through the cooler air and results in fog.
Air can also cool if it rises
in the atmosphere. That is the reason clouds form. In this case condensation forms on tiny dust
 Condensation—Condensation on the inside of a cold windowpane in a warm kitchen.
and salt particles suspended in the air. If the air is very cold, then water vapor changes directly into ice crystals. This is called sublimation.
Confluence
A place where two rivers meet. Confluences have often been the site of important towns and
cities because of their good natural route advantages. Cities built on confluences include Pittsburgh
(a confluence of two tributaries
 Confluence—This is Pittsburgh, where the Allegheny and the Monongahela Rivers meet
at the Golden Triangle, close to the city center. You can see the location of the original fort (the white lines just behind the fountain) close to the confluence, showing the strategic advantage of a confluence site.
of the Ohio River) and Koblenz (confluence of the Rhine and Mosel, Germany).
Continental divide
The line that separates areas of land that drain into one ocean from areas draining into another ocean.
Crater lake
A lake that fills the top of a volcano.
Crater lakes form when a volcano explodes so violently
that it leaves the magma chamber below it empty. At the same time, the enormous explosion causes many earthquakes in the rocks above, and they begin to crack and weaken. The top of the volcano then collapses down into the magma chamber, leaving a huge circular pit known as a caldera.
In time the caldera fills with water to make a crater lake.
Crater lake
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