Atmospheric water vapor and horsepower
File this one under reasons to keep track of the local weather on race day.
Have you ever noticed that your car runs really well first thing in the morning when it's cool and dry? Have you noticed that performance starts to suffer as the day gets more humid? That's because water vapor in the atmosphere decreases horsepower.
Humidity is a result of water vapor suspended in the air. Any amount of humidity displaces some of the air molecules. Since only about 20% of air is oxygen and oxygen is what is needed for combustion in an engine, anything that displaces air can really decrease the amount of oxygen. Water vapor is not flammable and does not contribute to combustion heat. Any reduction in the amount of air from the suspension of water vapor in the air results in a corresponding reduction in the amount of oxygen and combustion.
Hot vs Cold Air
Generally, when the temperature is cooler, air is denser which leaves less space in the atmosphere for water vapor. As the temperature heats up, air moves around more leaving more space for water vapor to fill in. In certain climates, this is why summer is very humid and winters can be very dry. However this is also why you car's engine performance might suffer at the hottest part of the day.
For example: A nitro racing engine for a low humidity day with an uncorrected barometer of 30 inches HG and a temperature of 80 deg F has about 98.4% of its sea level horsepower from the dry air density. On a high humidity day at the same barometer and temperature, the nitro engine has about 97% of its horsepower from humid air density or a loss of about 1.4% of its power.
You can use this website to track how much water vapor is in the air so you can plan accordingly. We've talked about this before but keep in mind, you do not want to watch the relative humidity variable to track atmospheric water vapor. That is as much a reflection of temperature as it is of water vapor.
The best way to track water vapor is by following the grains of water on the tracks pages. The higher that number, the more water vapor is in the air. On our site, we list the air density with and without water vapor. The bigger the difference between those 2 numbers, the more water vapor is taking up space in the atmosphere. Finally, as a general rule of thumb, the hotter the day, the more space there is for water vapor.
We wrote a blog post discussing what each of the weather variables on our site represent. You can read it here: Measuring the Atmosphere Around a Track.
Reading the weather report: When is it about to rain?
Air Density and Water Vapor
Measuring the Atmosphere Around a Racetrack
Book: 5000 Horsepower on Methanol
Book: Blown Nitro Racing on a Budget
Stack Exchange - How Does Cold Dry Air Affect Horsepower