Tactical Rifles.net: Atmospherics and Long Range Shooting

Tactical Rifles.net: Atmospherics and Long Range Shooting

Tactical Rifles.net's friend and guest contributor, Derek Walters from GIG Long Range Systems, explains how atmospherics affects long range shooting. 

The air is what makes long range shooting challenging.  It moves, it changes composition and it does nothing but restrict the bullet in its flight path.  Understanding it will help to compensate accordingly.  While wind is the most challenging factor to account for, being such that it changes rapidly, the density of the air itself will affect the bullet.  There are three main components that make up the density, pressure, temperature and humidity.  Altitude falls under the “pressure” category. 

Pressure is a measure of how many air molecules there are over a given area.  In a high-pressure situation, the air particles are descending from higher altitudes and compressing closer to the surface, thus cramming more particles into a smaller space.  Low pressure is the opposite.  The air is rising and the molecules are more spread out.  Higher pressure is detrimental to shooting.  Higher pressure equals more drag and lower impact than low pressure.

Temperature is a measure of the speed of the air particles.  At higher temperatures, the air molecules are moving faster.  Also, when a gas is heated up, it expands.  The ideal gas will prove this as will blowing up a balloon and putting it over a warm stovetop.  It will grow.  With colder air, the molecules are moving slower and bunch up more, creating higher density and more resistance for the bullet.  Putting that same balloon in a freezer will show this by it shrinking.  The same number of air particles occupying a smaller space means the air is denser.  Warm air results in less drag and a higher impact, colder air results in more drag and lower impact.  Also the temperature effects on ammunition propellants are a factor to consider.  It is wise to make a note of that in your data book.  Keeping a thermometer and reading your ammo’s temperature is a bit drastic, though, but I would keep a note such as “87 degrees, 82% humidity, air pressure is 29.97 inches of mercury, ammo has been in the sun for 45 minutes before shooting.” Or maybe “ammo covered from sun”

Humidity is the third atmospheric factor in long range shooting.  While for most long range shooters it poses a very small factor, it is important to know about.  Moist air is less dense than dry air.  That seems counterintuitive, yes.  That is because a molecule of water vapor is very big and light compared to the other two main gasses that make up air, nitrogen and oxygen.  Think of it like having a bag full of lacrosse balls and golf balls.  They simulate the oxygen and nitrogen.  The bag will be quite heavy.  If you were to add ping-pong balls to the bag, simulating water vapor, and still fill the same bag, it will be lighter, and less dense.  Certain ballistic programs show conflicting to this, however.  Even then, I worked the programs twice for my 175 grain .308 at 2730fps.  First at 1% humidity, then at 100%, keeping everything else the same.  It resulted in a difference of about 1 MOA at 1000 yards.  To the accuracy minded shooter, that being every one who buys a rifle from Tactical Rifles, that seems like a lot, and it is.  But keep in mind that the humidity wherever you live likely does not vary between 1% and 100%, or even a quarter of that.  Again, keep this in mind and save a spot for it in your data book. 

Although this does not affect the bullet directly, it is important to understand what mirage is.  It is both a valuable tool to read wind, and an evil mistress that messes with your line of sight and makes the target sometimes appear to move.  As stated earlier, warm air and cold air have different densities. For now, think of it like looking through two different thicknesses of glass.  You can tell which glass is thicker and which is thinner based off how they bend and distort the light.  Mirage is when the warm surface of the earth heats the air right above it and that air tries to mix with the cooler air a few yards off the ground.  The air at the ground can be a few degrees warmer than only 10 feet higher.  So when you see mirage, you are seeing varying thicknesses of air. 

Summary:

Low pressure= Higher impact

High temperature= Higher Impact

High Humidity=Higher impact. 

Mirage is looking through different densities of air as it mixes above the surface.