Barometric pressure is air or atmospheric pressure as measured by a barometer.
Because gases in the earth’s atmosphere have mass, they have weight and exert pressure on things inside the atmosphere. The entire column of air above a place has a weight and creates a pressure on the surface corresponding to this weight. The atmosphere is approximately 100 miles (160 kilometers) thick and ranges from near vacuum at the top to a pressure defined as one atmosphere (or about one bar) at sea level.
One atmosphere is 14.7 pounds per square inch pressure (101 kilopascals in metric units). Many early barometers used the weight of a counterbalanced column of liquid mercury metal to measure the pressure. In these terms, the pressure at sea level is 29.9 inches of mercury. This is equal to 760mm, as millimeters of mercury is another widely used measurement for atmospheric pressure.
A millibar is often used in weather reports and forecasts. It is 1/1000 of a bar, which is approximately one atmosphere of pressure.
Instead of mercury, many modern barometers use other, more convenient and safer mechanical (aneroid) or electronic means to measure barometric pressure.
Temperature and humidity can affect barometric readings because they affect the density of the air. Air density affects the weight of a given volume of air and thus the air pressure.
The weight of air above a place also varies because of atmospheric circulation. A weather high is an region where air pressure is higher than normal, often corresponding to clear, dry conditions. A low is a region of lessened air pressure often meaning storms. If barometric pressure is changing to lower pressure, a storm system or front may be near. Changes to higher pressures may mean improving weather. This is how barometric pressure is related to weather and weather forecasts.
As noted above, air pressure changes with altitude, so a barometer can be used to measure altitudes as well. The highest pressure is at sea level, while at 18,000 feet the air pressure is only half as much, at 27,500 feet only a third as much, at 53,000 feet only a tenth as much pressure and so on. Of course there are inaccuracies in using barometers to determine altitude due to the weather and air density, which also affect air pressure.