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Humidity and Your Come-Up: Why It Barely Matters, With Numbers

Humid air is lighter than dry air, so a muggy day needs slightly less elevation. We solved 0% against 100% humidity from 20°F to 100°F: how many feet of density altitude it is worth, and what it moves at 1,000 yards.

4 min readSolved with the Ballista engineUpdated October 2026

Humid air feels heavy, so it’s natural to think it slows the bullet. It’s the other way round. On a 100°F day, going from bone-dry to saturated air raises density altitude by 830 ft and lets a .308 hit 6.0 inches higher at 1,000 yd. On a 60°F day the same change is worth 1.9 inches. Humidity is the smallest of the three weather inputs, and how small depends almost entirely on temperature.

Why humid air is lighter

At a given pressure and temperature, a volume of gas holds the same number of molecules whatever they are. Humidity doesn’t add water to the air so much as swap some of it in. Each water molecule that enters takes the place of a nitrogen or oxygen molecule. Water vapor has a molar mass of about 18 g/mol. Dry air averages about 29. So every swap makes the air a little lighter.

How many swaps happen depends on the saturation vapor pressure, the most water the air can hold, and that rises steeply with temperature. Air at 20°F can’t hold much water at all, so even 100% humidity changes almost nothing. At 100°F the same 100% means many times more vapor. The engine computes it the way the density altitude article describes: dry air and water vapor as two ideal gases sharing the station pressure, with the saturation pressure from Buck’s equation.

Humid air also carries sound slightly faster. That lowers the bullet’s Mach number a little at the same speed. It’s included in the numbers below.

0% against 100% at five temperatures

A few things to take from the table:

• Below about 40°F, ignore humidity. The whole range from dry to saturated is worth 1.0 inch or less at 1,000 yd for either rifle.
• On a hot day it’s worth entering. At 100°F the .308 moves 0.17 mil, about 2 clicks of 0.1 mil. That’s small next to a wind call, but it isn’t zero.
• A guess is good enough. If you leave humidity at 50% and the air is really dry, at 60°F the .308 is off by 0.026 mil at 1,000 yd.

The table is for sea-level pressure. At a 5,000 ft range (24.90 inHg) the same humidity at the same temperature takes the same mass out of each cubic foot of air as at sea level, but a bullet in thin air is less sensitive to a given change in density. For the .308 at 1,000 yd the effect there is 0.80 times the sea-level figure at 60°F and 0.82 times at 100°F.

Humidity next to the errors you actually make

To see where humidity ranks, here are three everyday input errors for the .308 at 60°F and 1,000 yd:

The full swing from dry to saturated air at 60°F is worth about 3°F of temperature error, or 0.15 inHg of pressure error, or 5 fps of velocity. A 5 fps chronograph error matters as much as the whole humidity range on that day.

What to do with humidity

1. Enter it if you have it. It costs nothing, and on a 100°F humid day it’s worth 1 to 2 clicks of 0.1 mil at 1,000 yd.
2. Don’t chase it below 40°F. Dry, 50% and saturated air all give the same come-ups to the nearest click.
3. Spend your attention on station pressure and temperature first. The station pressure article shows a pressure mistake many times larger.
4. Remember the direction: humid air is thinner, so a muggy afternoon needs slightly less elevation than a dry one at the same temperature and pressure.

Ballista’s Air sheet has a Humidity field for every source except Density altitude, because a density-altitude reading from a meter already includes it. The “Get weather” row fills in temperature, pressure and humidity together from Apple Weather. Ballista never saves your location; Get Weather sends it rounded.