Geography - More Suggestions On Lessons ( Originally Published
1915 ) THERE are great rivers of water in the ocean called ocean
currents, such as the Equatorial Current, Gulf Stream, and Japan
Current, that are continuously moving in fairly definite
courses. So, too, in the atmosphere, there are great rivers of
air called wind currents or winds that also move in more or less
definite courses. The air is very unstable and is therefore
easily moved by outside forces. The principal forces that
influence its movements are heat, moisture, and the rotation of
the earth. Heat, as was pointed out in Form II, causes air to
expand and thus makes it lighter in 'weight ; moisture also
makes it lighter; the rotation of the earth influences its
direction. The lighter air rises as the cooler and heavier air
at the sides flows under it and lifts it up. When the air
becomes lighter than usual, its pressure is said to be "low".
Hence high pressure means a heavy atmosphere ; and low pressure,
a light atmosphere. These condition