Why Drying Tracks After Rain Takes So Long

Why does it takes so long for a track to dry after the rain?  Why does humid weather make track drying take even longer? Here’s the physics behind rain delays.

Air and Water Vapor

Air is a mix of gas molecules:  mostly (78%) nitrogen, about 21% oxygen, the rest misc. gases.  The composition is pretty uniform with the exception of how much water is in the air.   The absolute humidity is the amount of water in some chosen volume of air. For example, how much water vapor is in one cubic meter of air.  Air can only hold so much water vapor and that amount depends on the temperature and pressure.  Dry air would be no ounces of water in a cubic foot of air.  If the air saturates at 86 F, there could be up to three one-hundredths of an ounce of water per cubic foot.

The mechanisms we use to get rid of rain on the track are evaporation and possibly boiling. 

Evaporation vs. Boiling

We use evaporation to dry dishes, or even ourselves when we sun dry.  Evaporation is simply changing a liquid into a gas. 

Boiling also changes a liquid to a gas, but there’s a difference.  Evaporation happens at the surface of a water drop.  Only the outermost few water molecules change from liquid to gas.  Boiling affects the bulk of the water drop.

Regardless of whether we’re talking evaporation or boiling, the rain on the track doesn’t exist in a vacuum.  There’s that water vapor in the air.

Why Relative Humidity Matters

Nature likes equilibrium.  Equilibrium is when things are equal and concentration is one property that can be equal.  If you pour red dye into a fish tank, the dye molecules will uniformly distribute themselves throughout the fish tank.  (Don’t try this if there are fish in the tank, please…)

We have water molecules in the raindrop – a lot of water molecules – and water molecules in the air.  The concentration of water molecules in the air is smaller than the concentration in the raindrop, but varies depending how humid it is. 

The picture below shows three situations with increasing amounts of water vapor in the air surrounding the water drop. The darker the green, the higher the concentration of water molecules.

Nature likes equilibrium, so it would like to have the same concentration of water molecules everywhere.  The rate at which water molecules can move from the raindrop to the air is proportional to the concentration difference.

Really dry air creates a big difference in concentrations, and the water from the droplet moves into the air faster.  Have you ever hung your swimsuit out to dry on the balcony of a Florida hotel in July?  It takes forever to dry because the air is so moist. 

Less humidity makes it easier for water molecules to move from the track into the air. 100% relative humidity means that you absolutely can’t put any more water vapor in the air. There’s nowhere for the raindrops to go.

Rain Delays and Jet Dryers

NASCAR track uses jet dryers — literally jet engines — to speed up evaporation of rain on the track.  The fuel combusts around 1100 degrees F, but it cools pretty quickly as it leaves the dryer. That’s why the jets are so close to the track surface.  Let’s look at eight jet dryers, each operating for 50 minutes on 175 gallons of fuel. If it takes 150 minutes to dry the track…

= \frac {\text{8 dryers}}{} \times \left( \frac {\text {150 minutes}} {} \right)\times \left( \frac {\text {175 gallons}} {\text {50 minutes}} \right)

…which gives you 4200 gallons of jet fuel.

In my next post, I’ll explain how the Air Titan system and why it’s a huge improvement over jet dryers.

This is a major revision of a post that originally appeared on the now-defunct stockcarscience.com on 4/18/10.