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Temperature and Barometric Pressure
Air temperature directly relates to the average velocity that air molecules move about. Cold air molecules move slowly, resulting in a high density of air molecules and a high barometric pressure. Warm air molecules move quickly, resulting in a low density of air molecules and a low barometric pressure. Therefore, in general the air over an ice cap has a higher pressure than the air over a hot desert.
High Pressure
A high pressure system or "high" indicates an area where the barometric pressure is higher than the surrounding air. Highs descend due to their higher density of air molecules, resulting in warming, drier air that inhibits the formation of clouds. Therefore, high pressure is generally associated with fair weather. Weather watchers look at a rising barometric pressure as a sign of good weather to come.
Low Pressure
A low indicates an area where the barometric pressure is lower than the surrounding air. Lows rise due to their lower density of air molecules. As air rises it cools; its humidity condenses into water droplets, or if it's cold enough, ice crystals. If the clouds accumulate enough water or ice, the precipitation falls as rain or snow. Low barometric pressure indicates inclement weather and a falling barometer signals stormy weather ahead.
Wind and Barometric Pressure
Observing the wind along with the barometric pressure can help you predict the weather. As a general guideline, when the wind comes from the south or southeast combined with a steadily falling barometer, a storm is approaching from the west or northwest within 12 to 24 hours. When the wind comes from the east or northeast combined with a steadily falling barometer, a storm is approaching from the south or southwest within 12 to 24 hours. Winds from the east combined with a falling barometric pressure indicate inclement weather. Winds shifting to the west indicate clearing, fair weather.