On weather maps, pressure is recorded as a "sea-level" pressure and is coded on the upper right of the station circle as the last three digits of millibar pressure. For example, 1013.2 mb would be coded as 132. 994.8 mb would be coded as 948.
To decode pressure data given on a station circle one must place a decimal in between the 2nd and 3rd digits and add a "9" or a "10" in front of the three digits, whichever brings the pressure closest to 1000.0 mb. For example, 159 = 1015.9 mb. 804 = 980.4 mb.
When drawing isobars, each isobar represents a pressure interval of 4 mb with the standard values being a multiple of 4 mb above or below 1000.0 mb. For example, 992, 996, 1000, 1004, 1008, 1012, etc.
Density is important because it is what determines whether a parcel of air will rise or sink in the atmosphere. If a parcel of air is MORE DENSE than its surroundings it will SINK. If a parcel of air is LESS DENSE than its surroundings it will RISE.
Since cooler air has less kinetic energy and therefore less motion than warmer air it has more molecules (more mass) in any given volume so is more dense. Warmer air is less dense because it has more energy and more motion and therefore has fewer molecules (less mass) in a given volume.
As air becomes MORE HUMID (meaning more water vapor molecules) it becomes LESS DENSE. DRY AIR is MORE DENSE than humid air at the same temperature. Water vapor molecules have a lower molecular mass (18 per molecule) than dry air molecules (29 per "molecule.")
In summary, a cool, dry air parcel is more dense than a warm, humid air parcel.
Helpful Links:
USA Today Guide to Density
Lifting by Convection
Station Model Reporting
How Turbochargers Work
