air pressure

In general, the pressure is less for those flying in an aircraft than for those lying on the beach. For the one location, air pressure also varies with time, and these latter changes are very important in studying and forecasting the weather. They are especially valuable in forecasting wind speeds and direction.

Weather charts show the pressure at one altitude or level, such as the mean sea level (MSL). They are drawn with points of equal air pressure joined by lines called isobars (can you spot the two 1028 hectopascal isobars below?).

When drawing charts from air pressures readings taken at different altitudes, the observations are converted to the same level (MSL, for instance) using a mathematical equation (see over the page).

the instrument

This leaves an almost perfect vacuum at the top of the tube, and variations in atmospheric pressure cause the liquid to rise or fall by small amounts.

A more convenient and safer version of the instrument is now commonly used. The aneroid (without fluid) barometer contains a flexible metal cylinder from which most of the air has been evacuated.

When air pressure increases, the sides of the cylinder are pushed closer together. When the pressure falls, the cylinder expands again.

The cylinder is connected to a pointer that moves around a dial. The expansion and contraction of the cylinder is very small but is magnified by a system of springs and levers connected to the pointer.