Ridges and troughs
are often mentioned on the weather forecast. A ridge is an elongated
area of relatively high pressure extending from the center of a
high-pressure region. A trough is an elongated area of relatively
low pressure extending from the center of a region of low pressure.
Air in a high
pressure area compresses and warms as it descends. This warming inhibits
the formation of clouds, meaning the sky is normally sunny in high-pressure
areas. But haze and fog still might form. The opposite occurs within
an area of low atmospheric pressure.
measure air pressure with a barometer. Barometers measure the current
air pressure at a particular location in "inches of mercury" or
in "millibars" (mb). A measurement of 29.92
inches of mercury is equivalent to 1013.25 millibars, a more
modern unit or measure of pressure.