Weather changes before and after dark
During a typical day at the races, the weather can fluctuate quite a bit. As the temperature heats up in the afternoon, you can feel the heat radiate from the pavement and the moisture in the air. Then as the day wears on, the night becomes cool and calm. Not only does this influence whether you wear shorts or pants but it also influences how your engine performs.
This is especially an issue in places with extreme temperature fluctuations like in coastal areas. It's not as much of an issue in places where the temperature stays near a median. While you will want to tune your engine according to the local weather, in these places you do not necessarily need to check multiple times per day.
The sun heats the atmosphere
When the sun rises, it starts heating the atmosphere. This accumulates until approximately mid-afternoon when the hottest part of the day is reached. Hotter temperatures mean that the molecules in the atmosphere are moving faster and, therefore, there is more space in the air for other molecules like moisture. This is why the air sometimes feels soupy in the summer. The atmosphere is filled with moisture.
The sun heating the atmosphere is also heating the earth's surface. In a place like a racetrack, this means that the pavement gets hot and tires have a harder time hooking up. In tracks that use VHT, the surface gets gummy with high temperature (130 deg. F). Spraying water on a drag strip starting line, for example, can cool the pavement enough for the tires to better hookup.
With sunset, the air begins to cool
As the sun begins to set and there is less heat beating down, the molecules in the atmosphere do not move around as much. This doesn't necessarily mean the amount of moisture in the air becomes less, but it does bring the moisture down to the surface, and pavement can get moist.
You might notice that the percentage of relative humidity changes even though the amount of moisture in the air remains the same. This is because humidity is a measurement of the amount of moisture in the air relative to the amount of other molecules in the air. As the temperature cools and air molecules are not moving around as much, the amount of air molecules increase which changes the ratio of air to moisture.
Because of this, relative humidity is not a total measure of the amount of moisture in the air. However, the number of grains of water is a better measure. This is an absolute measurement of how much water vapor is in the air. You can learn more about this and other weather variables in our first article Measuring Racetrack Atmosphere.
As the temperature cools, the amount of oxygen in the atmosphere increases. Usually this helps engines to run better. However, you should monitor the weather to ensure you are maintaining the best air fuel ratio for your engine. The night before race day, you should check the weather report so you can plan accordingly. For mechanical fuel injection or carburetors, you can plan your jetting ahead of time so you have the necessary nozzle sizes with you in the pits.
On this site, you can check the forecast for various tracks. If you would like assistance setting up your mechanical fuel injection engine jetting so you can tune for the weather, check out our ProTuning Package for assistance in getting set up.
Fuel Injection Racing Secrets