AIR IN MOTION
In high-pressure area, the air at the center the air at the center flows outward. In a low, the air flows inward. This flow is not, strictly outward or inward. The Earth's rotation deflects the air, so that in reality it flows more or less tangent to the isobars. In the northern hemisphere this almost circular movement of the air is clockwise and away from the center of a high, but counterclockwise and toward the center of a low. In the southern hemisphere, the reverse movement occurs.
The little symbols that cross the isobars indicate wind direction and velocity. They're like arrows except that they have no head and only half a tail. The long arm of each symbol points, like an arrow, in the direction of the wind flow. Some of the symbols have one tail feather, some two, and some three. Each long feather represents 10 knots of wind; each short feather, 5 knots. A arrow with one long and one short feather indicates wind velocity of 15 knots; an arrow with four long feathers indicates 40 knots of wind.
The flow of air is influenced not only by the pressure and the Earth's rotation but also by friction against the Earth's surface. This friction, which slows down air motion, is greatest over land areas, especially where there is abrupt mountainous terrain. As the air within the low or the high rotates the whole circulation of air also moves. Consider the weather charts for several days in a row. On the first day a low may appear over the Pacific Coast region. The chart for the next day probably shows it somewhere in the Rocky Mountain region. A day or two later it may be over Arkansas. If it has not broken up by this time, it moves on eastward and northward, and eventually dissipates over the North Atlantic. All lows in the United States do not follow this same track.
