Weather and Shot Patterns

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Atmospheric effects on shot patterns can indeed be significant.

Air density exerts a drag force on any object moving through it. The higher the density, the more drag is exerted. This drag serves to slow down the object moving through it.

A smooth or nearly smooth object such as a bullet will normally fly in a relatively straight path when subjected to this air drag.

However if the object has side, top, or backspin, its path will curve in response to the air drag force. Lead pellets fired from a shotgun end up being far from smooth spheres. Those toward the rear of the load are subjected to setback forces when the load is fired, which deforms them and creates flat spots. Aerodynamic forces acting on these flat spots move the pellet transversely off track, as well as create spin, which does the same thing.

The main point here is: These random aerodynamic forces open up (disperse) shot patterns. This effect increases with increasing air density.

So what atmospheric factors affect shot dispersion?

The answer is:
Anything that affects air density, such as air pressure, air temperature, and humidity. Of these, air temperature has the greatest range and gives the greatest changes in air density. The higher the temperature, the lower the density.

In absolute terms, air temperature changes from summer to winter can cause air density variations of up to 25%, with corresponding changes in shot pattern dispersion (tighter patterns when it's hot, more open when it's cold).

Within a given day, normal air temperature variations can cause air density, and thus shot pattern, variations of as much as 8%. Normal air pressure changes within a single day can change patterns as much as 3 or 4%.

Humidity effects amount to no more than 1 or 2%, even in the summer. Contrary to what most people think, the more humid the air, the lighter (less dense) it is.

One other atmospheric factor, atmospheric turbulence, can cause shot pattern changes from hour to hour within a given day. When wind blows over the ground, it creates random eddies of different sizes, some quite small, within which the air is moving in different directions. On a hot day, there are also thermal eddies caused by hot air near the ground rising. This combination of mechanical and thermal eddies makes up what is caused atmospheric turbulence. When a pellet ensemble (or pattern) traveling through the air encounters atmospheric turbulence, different portions of the pattern can fly through different turbulent eddies of air moving in random directions. These can impart a small transverse movement to the various pellets, which has the overall effect of opening up the pattern.

~Source: Warren Johnson, Shotgun Report, October 31, 2005

Tip: Yes, weather conditions affect your shot pattern to some degree. But shooting skill is what affects your shooting accuracy. When pheasant hunting in Kansas you can't control the weather but you can control your shooting skills. Practice, Practice, Practice..

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