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Knowing the weather important to teams

by Mickey Schultz

-- Ray Alley

When you watch your local weather forecast, you are normally only concerned with questions like: Will it be sunny?; how hot will it be?; will it rain, sleet or snow? In short, you are concerned with how will the weather effect you: what will I wear?; how warm should I dress?; should I take an umbrella?

The same is true of the JERZEES Top Fuel dragster as it campaigns week in and week out on the NHRA Championship Drag Racing Series. But Ray Alley, the crew chief on the JERZEES dragster, is concerned with more than the details the local weather person reports. He is concerned with terms like: Absolute Barometric Pressure; Absolute Humidity; Dew Point; Grains per Pound; and Relative Humidity.

The following are just a few of the terms and their definitions to better inform you of what a crew chief is saying when he talks about the weather. This glossary is from a Racers' Guide to Weather Terms, copyrighted 1990-1999 by Altalab Instrument. Unlimited permission to copy or use is hereby granted by Altalab Instrument, subject to inclusion of this copyright notice. For additional information regarding copyright or questions regarding terms, you can visit the Altalab Instrument website at

Absolute Barometric Pressure: Expressed in inches of Mercury („Hg). This is not the same as what is reported on weather forecast, which is Sea Level Corrected pressure. Absolute is the actual air pressure at elevation. Barometric pressure is usually measured with an altimeter or barometer. Roughly every thousand feet of elevation reduces the barometric pressure by one inch of mercury. By using absolute, you need not recalibrate for every new location, and you need not know the elevation of the track. The higher the barometric pressure the more oxygen that is available for combustion in a given volume.

Absolute Humidity: This calculated value expresses the actual amount of moister present in the air as a percentage of the volume. This reveals the displacement of O2 molecules by water molecules and has a direct relationship with the power making capabilities of the engine.

Adjusted Altitude: This one number, expressed as a footage is a relative performance altitude compared to STP (Standard Temperature & Pressure). Altalab has found this one number, which relates to observed engine performance, to be the most accurate value for horsepower correction and for predicting vehicle performance.

Air: Atmospheric Air is Moist Air, which is a mixture of Dry Air, Water Vapor and contaminants like smoke or pollen. Dry Air exists when all contaminants and Water Vapor are removed from Atmospheric Air. The amount of Water Vapor in Moist Air (humidity) varies from none (Dry Air) to Saturation (100% Relative Humidity). The most common ways of describing amount of moisture in the air are by Relative Humidity, Absolute Humidity, Grains per Lb., and Dew Point.

Dew: Dew is water that has condensed on objects near the ground, as a result of those objects, like car windshields, getting cooler than the Dew Point temperature.

Dew Point: The Dew Point is the temperature at which the air you are measuring would be saturate (100% Relative Humidity), and condensation (Dew) would begin appearing on surfaces. If the track cools to the Dew Point, condensation will occur on the racing surface. The air and other surfaces may reach the Dew Point before the track does, as the asphalt or concrete can hold heat.

Grains per Pound (GRLB): In drag racing was discovered by Austin Coil, currently crew chief for John Force, the most successful Funny Car driver in history, and Ron Swearingen, currently crew chief for the Matco Tools Funny Car driven by Dean Skuza. In their search for a more meaningful number for humidity (than Relative Humidity), they referred to a Carrier air conditioning and heating psychrometric chart that was made available to the trade and public in huge quantities. Both Coil and Swearingen realized that if you line up the coordinates of temperature and Relative Humidity, you could shoot off the right side of the chart and find this little curious number. It seemed to relate well to various performance changes regardless of the Relative Humidity alone. In this day of scientific and metric values, GRLB seems quaint at best, but it does hit the desired effect on the head. A grain is an ancient measurement which equals one seven thousandths of a pound. Sort of arbitrary, don't you think? Soon, many fuel and alcohol drivers were using this secret weapon. In reality, if you were to plot Absolute Humidity, Dew Point, and GRLB on a graph, they would follow each other in lock-step although the actual values are different. Grains is a mass of water to a mass of Dry Air (Grains per Pound), whereas, the other two are volume numbers.

Humidity: Water Vapor content of the air. This is a big deal in racing, because not only is Moist Air lighter (less dense) than Dry Air, moisture additionally displaces oxygen needed for combustion. The most common ways of describing amount of moisture in the air are by Relative Humidity, Absolute Humidity, Grains per Lb., and Dew Point.

Relative Humidity: Expressed as a percent, this figure is the ratio of the amount of water vapor in a particular temperature air to the maximum amount of water vapor that temperature could hold. 100% Relative Humidity means saturation, or Dew Point at that temperature. (Note: Do not get confused with relative humidity readings at different - - i.e. air at 90 degrees at 50% relative humidity has the same amount of moisture as does air at 70 degrees and 100% relative humidity. The main reason crew chiefs look to other calculated values - - such as Absolute Humidity, Grains per Lb., and Dew Point - -is because Relative Humidity is just that: relative. It is relative to temperature and varies according to the time of day. For example, assuming the same air mass throughout the day, early in the morning it is cool and the Relative Humidity is high. Later in the day it warms up and the Relative Humidity drops. Then, after dark the temperature cools down and the Relative Humidity goes back up. But, even with all those changes: the actual amount of moisture in the air has not changed!

As you can see, a crew chief does more than just turn wrenches; He must be a 'jack of all trades,' including a meteorologist. The weather plays a very important part in the tuning of not only the fuel cars on the NHRA circuit, but of all racing vehicles, no matter the class or the sanctioning body.

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