When a barometer rises or falls it means that the atmospheric pressure is rising or falling.
A barometer is a device which measures atmospheric pressure. There are two types of barometer, mercury and aneroid, and the words rising and falling in relation to a barometer derive mainly from the former. This is because it consists essentially of a column of mercury, maintained at 0 degrees Celsius.
Average atmospheric pressure is considered to be that which is able to push the column of mercury up by a distance of 29.92 inches, or 760 mms. If the ambient air pressure rises, the column of mercury will be pushed higher, and conversely it will be lowered by a fall in atmospheric pressure. Hence people speak of a barometer rising or falling.
An aneroid barometer is made up of a metal box, inside which there is virtually no air. When the atmospheric pressure outside the box increases, it pushes in the box’s flexible sides, and conversely, when the pressure outside decreases, the sides move outwards. The sides of the box are connected by a spring to a needle on a calibrated dial, so that all movements of the box can be recorded.
Aneroid barometers are often calibrated in inches, although atmospheric pressure is more often measured in millibars, the average pressure being 1013.25 mbs, or 1.01325 bars.
To understand what it truly means when a barometer rises or falls, it is necessary to understand the significance of rising and falling pressure.
An area of relatively high atmospheric pressure is known as an anticyclone. This relates to the fact that a cyclone is an area of very low pressure. In an anticyclone the atmosphere will be falling, thus causing the pressure at ground level to be higher than normal. As clouds are formed in rising air, within falling air there will be no cloud, and this produces weather which could have a wide diurnal (daily) temperature range, with uninterrupted solar radiation by day and uninterrupted heat loss by radiation at night.
Low pressure areas, often known as depressions, or in their more intense forms, cyclones or hurricanes, have the opposite features. They occur where air is rising and cooling and therefore where clouds easily form. There is usually a considerable generation of winds (moving air) around low pressure areas as air is drawn in from areas of relatively higher pressure.
Clouds and winds are associated with storms, and this is the generally accepted weather type associated with low pressure areas.
Therefore a rising barometer suggests that the atmospheric pressure is rising, and that the air is therefore beginning to fall and become stable with little or no cloud, and little or no wind. Whereas a falling barometer suggests that the pressure is falling, the air is beginning to rise, causing clouds and possible rain with associated winds.