Barometers track air pressure, and a rapid fall often means a storm is coming.

The longer version

Air pressure is the weight of the column of atmosphere above a point, and it changes as weather systems pass. A barometer follows those changes, traditionally with a column of liquid or a sealed flexible chamber, while modern versions use a small electronic sensor.

A rapid fall usually means a low-pressure system is deepening nearby. Rising air in such a system removes mass from the column above, and that same rising motion produces cloud and rain. Steady high readings point to sinking air, which suppresses cloud and brings clearer skies. It is the rate of change, more than the value itself, that forecasters watch most closely.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.