It is made of an airtight metal box that has had most of the air removed. A needle on a spring is attached to the box. A rise in air pressure presses the box and spring and causes the needle to go up. A fall in air pressure relaxes the spring and the needle falls. The weather of the United States is dominated by large weather systems called Highs and Lows.
Highs are usually very large, fair weather systems.
Lows, on the other hand, are smaller and show changeable and stormy conditions. Either system can last for several days or even as long as a week. However, Highs generally persist for longer periods of time than do Lows. It gradually takes on the temperature and humidity of this surface. This horizontal movement makes for relatively calm conditions with little wind.