When you are at a constant elevation a barometer will measure the change in composition and density of the air mass above the barometer. The composition usually changes because of changes in humidity. The density changes usually as a result of a change in temperature of the air mass.
Roughly, falling barometric pressure typically leads bad weather, and rising barometric pressure indicates improving weather. Here are some generalizations about weather conditions that one can make from wind direction and barometric measurement changes from the National Weather Service:
- When the wind sets in from points between south and southeast and the barometer falls steadily, a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours, with wind shifting to the northwest by way of south and southwest.
- When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south of the observer within 12 to 24 hours, with winds shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and amount of the fall in the barometer.
- Winds from the east quadrants and falling barometric pressure usually indicate foul weather, and winds shifting to the west quadrants indicate clearing and fair weather.