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I found this on the Long Range BPCR site
Humidity
Many shooters believe that humidity (i.e. the amount of moisture in the air) can have a serious impact on accuracy. The theory is that air containing a high degree of moisture must be more dense than dry air. As a result, it must offer more resistance to a bullet in flight than does dry air.
The fact of the matter is that the amount of humidity in the air has virtually no effect on bullet accuracy. That?s because a water molecule weighs less than a dry air molecule. Yes ? that?s what I said. Moist air is actually less dense than dry air.
In fact, when the surrounding air is said to have 100% humidity (i.e. it is holding as much water vapor as it can at its current temperature), it still only contains 4% moisture. Look at the numbers for yourself:
Load Data
? Caliber = .45-90
? Velocity = 1,300 feet per second
? Distance = 1,000 yards
? Ballistic Coefficient = .400
Humidity Level
0% 50% 100%
Bullet Drop (Inches) 1,629.66 1,626.88 1,624.12
Bullet Speed (FPS) 800 801 802
Retained Energy (Foot Pounds) 769 772 774
According to the numbers above, the trajectory of the bullet becomes flatter (with less drop), travels faster (higher speed), and retains more energy upon impact (retained energy) as the air becomes more saturated with moisture.
It is true that a high degree of moisture in the air does help to keep the fouling in the barrel softer than on a hot, dry day. But about the worst thing you have to worry about when the humidity is high is keeping your gear dry and preventing it from rusting!
Humidity
Many shooters believe that humidity (i.e. the amount of moisture in the air) can have a serious impact on accuracy. The theory is that air containing a high degree of moisture must be more dense than dry air. As a result, it must offer more resistance to a bullet in flight than does dry air.
The fact of the matter is that the amount of humidity in the air has virtually no effect on bullet accuracy. That?s because a water molecule weighs less than a dry air molecule. Yes ? that?s what I said. Moist air is actually less dense than dry air.
In fact, when the surrounding air is said to have 100% humidity (i.e. it is holding as much water vapor as it can at its current temperature), it still only contains 4% moisture. Look at the numbers for yourself:
Load Data
? Caliber = .45-90
? Velocity = 1,300 feet per second
? Distance = 1,000 yards
? Ballistic Coefficient = .400
Humidity Level
0% 50% 100%
Bullet Drop (Inches) 1,629.66 1,626.88 1,624.12
Bullet Speed (FPS) 800 801 802
Retained Energy (Foot Pounds) 769 772 774
According to the numbers above, the trajectory of the bullet becomes flatter (with less drop), travels faster (higher speed), and retains more energy upon impact (retained energy) as the air becomes more saturated with moisture.
It is true that a high degree of moisture in the air does help to keep the fouling in the barrel softer than on a hot, dry day. But about the worst thing you have to worry about when the humidity is high is keeping your gear dry and preventing it from rusting!
