The values shown on the screen when it opens are the ICAO Standard Conditions. These are an altitude of 0 (sea-level), temperature of 59 degrees Fahrenheit, pressure of 29.92 Hg (sea-level) and relative humidity of 0%.
Select Units for Environmental Settings – This is a Drop-Down Menu. The available options are:
- Hg, DegF, Ft – (Inches of Mercury, Degrees Fahrenheit and Feet)
- mbar, DegC, M – (milliBars, Degrees Celsius, and Meters)
The selected units will be reflected in the input prompts below.
Atmospheric Pressure: Be careful here. The Atmospheric Pressure as reported by the NWS (National Weather Service) is always corrected back to its equivalent at sea-level. For this to be the most relevant, you must be able to measure local station pressure directly. The ICAO atmospheric standard pressure can be set by using the Altitude set Pres button below or converting the reported pressure from sea-level back to local or station pressure by using the NWS to local Pres button.
Temperature: Enter the current temperature in the units selected above. Valid entries are from -200 to +200. The atmospheric standard temperature can be set by using the Altitude sets Temp button below.
- Altitude: Enter the current altitude in the the units selected above. Valid entries are from -200 to 32000.
Altitude set Pres – The altitude setting will be used to compute the standard atmospheric pressure at that altitude. This is the recommended setting unless you have access to an absolute atmospheric pressure gauge. The rule of thumb here is that every 1000 feet of elevation reduces the pressure by 1 inch of mercury. By the rule of thumb, 2000 foot elevation would have a pressure of 29.92 – 2 = 27.92, not far off from the calculated value of 27.87.
Altitude set Temp – The altitude setting will be used to compute the standard atmospheric Temperature at that altitude. The current measured temperature is the recommended setting.
NWS to local Pres – The altitude setting will be used to convert the reported pressure from sea-level back to the local (station) pressure. Be careful and only hit this button ONCE. Each time the button is selected the current value is corrected. If you do inadvertently press it more than once, you can start over with the Reset to Standard button. Here is a tip on how to get a bit closer in estimating local station pressure. Let’s say the closest town to your shooting location reports a sea-level corrected pressure of 29.4 Hg even though it is at 3000 feet elevation. If you will be shooting 1500 feet higher than the town’s elevation, enter 4500 for the altitude, 29.4 for the pressure and select the NWS to local Pres button. It will update the entered pressure to a value of 24.87 Hg which will be closer than the Altitude set Pres value of 25.31 unless you are far away from the town reporting the pressure.
Show Example Result – The drag correction computed from the environmental inputs will be applied to the ballistic coefficient currently in use.
Reset to Standard – The values for Pressure, Temperature, Relative Humidity and Altitude will be reset to their standard ICAO values.
Done Button – Press when your are done. It will return you to your previous screen.
Note: Relative Humidity does affect bullet drag, but its effect is quite small. The humidity affects the Atmospheric pressure when water molecules (hydrogen and oxygen) displace the heavier nitrogen molecules. The difference in bullet drop (0% humidity versus 100% humidity) for a 150 gr 308 round with a 2800 fps muzzle velocity is 0.1 inches at 500 yards and 2.1 inches at 1000 yards when using a 250 yard zero. There is more drag at 0% humidity than at 100% humidity.