Low barometric pressure is caused by warm air from the planet's surface rising, decreasing the amount of air pulled downward by Earth's gravity. This rising heat can lead to adverse weather conditions.
When the air on the surface of the planet is warmer than the air higher in the atmosphere, the hotter air moves upward. When air is heated, it causes the molecules to become excited and expand. Since the heated molecules are more spread out than in cold air, the hotter air rises up above the heavier colder air. This rising hot air and its lower density is the cause of lowered downward pressure from the atmosphere. Normal barometric pressure at sea level is 1013.2 millibars (force per square meter of surface area). Overall, science has established a normal range of barometric pressure on Earth is from 980 millibars to 1050 millibars.
Barometric pressure is measured with a mercury or aneroid barometer. Barometric pressure can vary across the planet and even in a single location. This is due to uneven heating of the planet's surface and the movement of winds. In general, low or falling barometric pressure is an indication of bad weather to come. High or rising barometric pressure indicates the likelihood of pleasant weather.