When we extend tons per mile to include the effects of humidity, it gets more complicated since the amount of water vapor that a cubic foot of air can contain increases with temperature, but I found a calculator to do this.
Air Density Calculator
At sea level, 70 degrees F, and at 0% RH, I get 3.94 tons/mile
The same at 100% RH is 3.90 tons/mile
For a change of 0.04 tons/mile or 80 lbs/mile
At sea level, 100 degrees F, 0% RH, it is 3.73 tone/mile
The same at 100% RH is 3.64 tons/mile
A change of 0.09 tons/mile or 180 lbs/mile
At sea level, 0 degrees F, 0% RH it is 4.54 tons/mile
The same at 100% RH is 4.54 tons/mile
The value is unchanged since the air holds almost no water vapor at this temperature.
According to the calculator, you have to get above 5 degrees F before the air holds enough water vapor for the relative humidity to start affecting the air density.
Neil. you can repost the numbers.