Air Density Online

Reading the weather report: When is it about to rain?

How many times have you heard the weatherman say it's going to be a bright sunny day only to get caught in a sudden rainstorm? Conversely, how often have you looked outside thinking there is about to be a downpour only to have the clouds part and the sun appear? There is a lot of science behind weather forecasting but it is complicated and not easy to get right all the time. If you learn how to interpret a few of the variables, it will help you to be better prepared for sudden weather changes.

A little bit of science

The whole explanation is too complicated to sum up in a short blog post. In simplistic terms, temperature is a measure of the amount that atmospheric molecules are moving around. The faster they move, the hotter it gets. The more the molecules move, the thinner (less dense) the air is in the atmosphere. The less they are moving, the cooler it gets, the less space there is and the more dense the air is.

When the amount of moisture in the atmosphere reaches a saturation point, dew, fog, rain, or snow can result. However that saturation point does not occur just because there is a lot of moisture in the atmosphere. If it is a cooler day where there isn't a lot of extra space in the atmosphere, it might not take a lot of moisture to reach that saturation point.

Pressure systems can bring more moisture and can change the composition of the atmosphere so there is more room for that moisture to fill the space. Weather reports gauge how these pressure systems will affect the local atmosphere. When you see the weather man say a low pressure system is coming in and to expect rain, they are forecasting that the atmospheric pressure will lower to the point that moisture will come in and saturate the air to the point that rain will occur. However the low pressure system might move in a different direction than expected or it might dissipate as it moves in. That's when the weather report can be wrong.

Do not gauge the relative humidity

Relative humidity is not a good measure of the chance of rain. It is a percentage of the current atmosphere that is taken up by moisture. As mentioned above, when the atmosphere is heating up, it causes molecules to move around more which thins the air. When the atmosphere cools, air molecules slow down and tend to take up more space. The amount of moisture in the air might remain the same but the change in other factors will change the percentage of humidity.

Look at the dew point

A better way to gauge the chance of rain is to compare the dew point to the current temperature. Dew point is the temperature at which the amount of moisture in the atmosphere will reach that saturation point causing dew, rain, or snow. The closer the current temperature is to the dew point, the greater the chance of rain.

Look at the saturation pressure and the vapor pressure

On this site, you can also look at the saturation pressure and the vapor pressure. Saturation pressure is similar to the barometric pressure reading. It is the pressure point that the moisture has saturated the air and rain will likely occur. Vapor pressure is the current pressure of the moisture in the atmosphere. The closer the vapor pressure is to the saturation pressure, the more likely it is to rain. Check out vapor pressure at various tracks on the tracks page or by using the free calcs page.


Tracking the weather not only helps to tune your engine but it also helps to make sure you are well prepared for your day at the track. With a little knowledge, you don't have to worry about the irony of rain on your racing day.

Learn more from previous blog posts on this site:
Air Density and Water Vapor
Measuring the Atmosphere Around a Racetrack

Outside resources:
Wikibooks: High School Earth Science/Weather and Atmospheric Water
Dew point map at Weather Underground
Wikipedia - Saturation Vapor Pressure
NOAA Glossary > Dew Point
NOAA Glossary > Relative Humidity

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By Jennifer on March 20th, 2017 in Info and Education, Main News