Originally Posted by
Skyyr
It happens when the relative humidity is very high. You see it more commonly with aircraft propellers, as someone else here mentioned.
At any given time, the air can only hold some much humidity before the humidity condenses out as visible water droplets. How much water is in the air is measured as relative humidity. Once relative humidity hits 100%, the air cannot hold any more water and any additional water in the air over the amount that the air can contain condenses out as water droplets. These water droplets are very, very small, and can still stay suspended in the air; however, they become visible as trails, clouds, etc.
Enter aerodynamics.
When a bullet is travelling through the air, it is aerodynamically generating a very, very slight amount of lift (or an area of low pressure, technically). The lift itself is mostly cancelled out by other forces, but the low pressure remains. When the pressure of the surrounding air is lowered, the temperature drops; when the air temperature drops, the air can hold less water due to the lessened energy state of the air, but the amount of absolute water vapor in the air remains the same. This has the effect of raising the relative humidity.
On days when the relative humidity is very, very close to 100%, quickly dropping the air pressure will cause the water vapor to condense out. As mentioned earlier, this is commonly seen with aircraft, but can also happen with bullets, arrows, and anything else that has or creates a lifting surface or area of low pressure as it travels through the air.
On a day like this, the bullet will create a contrail of sorts that will spin with the bullet due to the wake turbulence the bullet is leaving behind. Depending on the atmospheric conditions, the amount of time it's visible is usually very brief, but can vary. Once the bullet has left the immediate area (which is measured in milliseconds given the speed of the projectile), the air pressure will return to normal and the water vapor will evaporate back into the air as the temperature rises back to the ambient temperature.
Given that you were in a marshy area (inherently high relative humidity) on a cool, stable day, this is likely what you saw.
Too bad you didn't get film of it - would've made for some good YouTube.
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