Why the Air Itself Is a Variable
Every ballistic table is calculated for some assumed set of atmospheric conditions, because drag depends directly on how dense the air is. Temperature, barometric pressure, altitude, and humidity all change air density, and therefore all change how quickly a projectile slows down. Reading a chart intelligently means knowing which atmosphere it assumes.
The Standard Atmosphere
A standard atmosphere is an internationally agreed reference describing temperature, pressure, and density at sea level and how they change with altitude. Ballistic data is frequently computed against such a baseline so that published figures from different sources can be compared on common ground. The standard is a convention, not a description of any particular day's weather.
How Density Depends on Conditions
Air grows thinner, meaning less dense, as temperature rises, as barometric pressure falls, and as altitude increases. Humidity has a smaller effect, but moist air is actually slightly less dense than dry air because water molecules are lighter than the nitrogen and oxygen they displace. Each of these shifts changes the drag a projectile encounters.
Density Altitude as a Single Number
Density altitude combines all the atmospheric variables into one figure: the altitude in the standard atmosphere at which the air would have the same density as the current conditions. A hot day at moderate elevation can produce a density altitude thousands of feet higher than the physical elevation, telling the analyst the air is effectively thin.
How Thin Air Flattens the Path
Because thinner air produces less drag, a projectile in high density altitude retains velocity longer, drops less, and drifts less than the same projectile in dense, cold, low air. Ignoring these effects can make a chart calculated for one atmosphere noticeably wrong when applied in very different conditions at long distance.
Worked Example
Consider a projectile whose chart was computed at sea level on a cold 40-degree day. Fire it at 6,000 feet elevation on a 90-degree afternoon and the density altitude might exceed 9,000 feet. In that thin air the projectile could drop several inches less at 500 yards than the cold sea-level chart predicts, because it met far less drag along the way.
A Common Misconception
People often assume only physical elevation matters, so a chart used at low ground will always behave as printed. In reality a hot, dry, low-pressure day at modest elevation can create a density altitude far above the ground height, thinning the air as if the shot were taken on a mountaintop. Temperature and pressure frequently matter more than raw altitude.