Found this article I think many will find interesting.
Knowing the weather important to teams
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by Mickey Schultz
5-24-99
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?
<Picture>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.
<Picture>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 www.altalabinstrument.com.
TERMS
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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