To the average person, the chief importance of weather is reference to it as an introduction to general conversation. At sea and in the air, advance knowledge of what the weather will do is a matter of great concern

to all hands. We plan or cancel operations on the basis of weather predictions. Accurate weather forecasts are made only after a great deal of information has been collected by many observers located over a wide area.

One of the instruments used in gathering weather data is the barometer, which measures air pressure. Remember, the air is pressing on you all the time. Normal atmospheric pressure is 14.7 psi. As the weather changes, the air pressure may be greater or less than normal. Air from high-pressure areas always moves toward low-pressure areas, and moving airor wind-is one of the main causes of weather changes. In general, as air moves into a low-pressure area, it causes wind, rain, and storms. A high-pressure area usually enjoys clear weather. Ships use two types of barometers to measure air pressure: aneroid and mercurial. 

Figure 9-6.Diaphragm pressure gauge.