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Glossary of Weather Terms

This glossary is a living document.
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Absolute Barometric Pressure, expressed in inches of Mercury ("Hg), is not the same as what is reported on weather forecasts, which is Sea Level Corrected pressure. Absolute is the actual air pressure at elevation. Roughly every thousand feet of elevation reduces barometric pressure by one inch of mercury. For instance, if you were in Colorado at 6000', the absolute pressure would be around 24 "Hg, while the Sea Level Corrected reading would be around 30 inches. By using absolute, you need not recalibrate for every new location, and you need not know the elevation of the track.

When the barometric pressure is higher there is more oxygen available for combustion in a given volume. During a typical 12 hour period the barometric pressure will change only 1 to 2 tenths of an inch of Mercury (for example from 27.24 to 27.40). An approaching front may bring in air that is higher or lower by 1"Hg (28.95" to 27.95").

Barometric pressure is caused by the gravitational pull on the 'column of air' lying directly above the place you are measuring. It is usually measured with an altimeter or barometer. Creative racers (with way too much time on their hands) may wish to experiment with a hypsometer, which indicates pressure by monitoring the boiling point of water.

Absolute Humidity (AH)
This calculated value expresses the actual amount of moisture present in the air as a percentage of volume. This reveals the displacement of O2 molecules by water molecules, and has a direct relationship with the power making capabilities of your motor.

What a wonderful word, and as with statistics, the numbers used can be made to say almost anything. When you say an instrument has 1% accuracy do you mean it is 99% inaccurate? No, accuracy is usually measured in terms of inaccuracy but expressed as accuracy, and as a percentage of the full-scale range. Two things to keep in mind when evaluating claims of accuracy are:
1) The plus/minus statement which can make a gauge look twice as good as it tests out to be. For example, a thermometer that claims + or - 1% accuracy full scale is really 2% accurate, not 1%.

2) On the lighter side, remember that you will seldom use a gauge to both ends of its scale. So a 2% full scale temperature sensor (measuring from 0 to 125 degrees) that you only use from 45 to 100 degrees is for all practical purposes better than 1%, (or + or - 4/10 of a degree).

ACE Factor (ACE)
The Air-Cooled-Engine Factor describes the cooling ability of the air. Moist air cools more efficiently than dry air. Developed by Altalab to help Junior Dragsters predict ET in situations where using the AA alone is not sufficient, the ACE Factor can also help Kart racers avoid costly engine destruction.

Adjusted Altitude (AA)
This one number, expressed as a footage, is a relative performance altitude compared to STP (Standard Temperature and Pressure which is 60 oF, 0% RH, and 29.924 "Hg). We have found this one number, which relates directly to observed engine performance, to be the most accurate value for horsepower correction and predicting vehicle performance, and our software for ET and TS prediction uses AA. All Altalab weatherstations calculate AA. (The Alta Series also provides . Density Altitude.)

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 composition of Dry Air by volume is nitrogen, 78.084; oxygen, 20.9476; argon, 0.934; carbon dioxide, 0.314;neon, 0.001818; helium, 0.000524; methane, 0.00015; sulphur dioxide, 0 to 0.0001; hydrogen, 0.00005; with krypton, xenon and ozone at 0.0002.

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 the amount of moisture in the air are by Relative Humidity, Absolute Humidity, Grains per Lb., and Dew Point.

Air Density Ratio (ADR)
Similar to the information from an air density gauge, ADR is calculated using the added effect of water vapor displacement of oxygen. ADR is expressed as a percentage, or ratio and is usually used when tuning maintain an ideal air/fuel ratio in changing conditions.

An instrument which determines altitude, or physical elevation. Usually pressure altitude can also be shown. A good reconditioned aircraft quality altimeter can be very accurate, but more expensive than a comparable quality barometer.

Characterizes instruments whose output varies continuously and smoothly, as opposed to digital instruments whose output switches immediately from one level to another. (You know, the difference between a dial type and digital watch.)

An instrument that measures the velocity of the wind. Wind velocity is a vector term, and describes both speed and direction. Common types include the low tech handheld ball and vane type, the contact, and the cup anemometer.

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An instrument that measures the pressure of the atmosphere. The two most common analog barometers are the aneroid (or dial type); and mercurial.

Poor quality aneroid barometers are subject to creeping, or sluggish response to a large, sudden change in air pressure; and also to hysteresis, the tendency of the gauge not to return completely to a previous value.

Barometic Pressure
See Absolute Barometic Pressure

A theoretical body which absorbs all incident radiation (direct solar and IR), independent of wavelength and direction.See IR Heat.

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Calculated Values
Altalab's calculated values have been extensively tested. They are ideal for predicting performance changes, and for evaluating engine performance on the track or with a dyno. These are calculated from the measured readings of the sensors, and can provide a more definitive analysis of air quality and moisture content than the measured readings alone.

Which of these additional calculated values proves most helpful to you is a combination of observation, previous experience, fuel type and other vehicle characteristics. We place special emphasis on evaluating the amount of moisture in the air, and provide several different calculated values to help monitor this important component.

In addition to the measured values of Temperature, Relative Humidity, and Absolute Barometric Pressure, the standard weatherstations include Adjusted Altitude, Vapor Pressure, Dew Point, Grains per Pound, and Absolute Humidity. Density Altitude and ADR are also included with AltaCom II and AltaCom TDS. The Delta ACE replaces vapor pressure and grains/lb with Air Density Ratio and the ACE Factor.

Cold Front
When a mass of colder air arrives, it sinks below and displaces the warmer air.

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Density Altitude (DA)
Originally developed from formulas used by aircraft pilots to calculate lift, this relative performance altitude when calculated for racing includes additional compensation for the effects of humidity on engine performance.

Altalab prefers Adjusted Altitude as an indicator of the oxygen available for combustion, but our Alta Series weatherstation also include DA. Our version of this number matches the most commonly calculated versions used for ET and TS prediction. See STP.

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 (DP)
The Dew Point is the temperature at which the air you are measuring would be saturated (100% RH), and condensation (dew) would begin appearing on surfaces. As air cools it contracts, leaving less room for moisture. If the track cools to the Dew Point, condensation will occur on the racing surface. The air and other surfaces may reach DP before the track does, as the asphalt can hold heat.

Dew Point temperature is a good indicator of water vapor quantities and is used frequently by the meteorological folks on their weather maps. A typical summer day, with sub-tropical air flowing north from the Gulf of Mexico, may have dew points ranging well into the seventies. The mid-day air temps could easily be into the ninties producing extremely uncomfortable conditions for outdoor activities like racing. These conditions are also ripe for afternoon thunderstorms.

At the other extreme, a nice brisk, blue sky day in early spring can have dew points way down in the teens or twenties. In both cases, the dew point is indicating the total amount of water vapor present in a quantitive sense. When dew points are in the seventies, absolute humidity will be above 3%, when dew points are extremely low, AH will be less than one percent.

Dry Bulb Temperature
The temperature of the air, especially for comparison to Wet Bulb temperature.

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Altalab pioneered fan-aspiration for motorsports weatherstations. The fan pulls air across the sensors so they can measure the ambient air, and the fan also prevents the case and sensors from overheating in direct sunlight. All our weatherstations have the fan as standard or optional equipment. See IR Heat for more information.

A form of precipitation made up of water droplets so small they can remain suspended in the atmosphere indefinitely.

The boundary, or transition zone between 2 air masses.

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Grains per Pound (grlb)
This value of this interesting yet archaic number was discovered early on in drag racing by Austin Coil and Ronnie Swearingen, both sucessful crewchiefs for sucessful drivers. In their search for a more meaningful number for humidity than RH, they refered to a Carrier air conditioning and heating psychrometric chart that was made available to the trade and public in huge quantities. Both realized that if you line up the coordinates of temperature and RH, you could shoot off to the right side of the chart and find this little curious number. It seemed to relate well to various performance changes regardless of the RH alone.

In this day of scientific and metric values, grains per pound seems quaint at best, but it does hit the desired effect on the head. A grain is an ancient measurement which is equal to one seven thousandths of a pound. Sort of arbitrary, don't you think? Soon, many fuel and alky racers were using this secret weapon. In reality, if you were to plot Absolute Humidity, Dew Point, and Grains per Pound on a graph, they would follow each other in lock-step although the actual values are different. Grains is a mass of water (grains) to a mass of dry air (pound), whereas, the other two are volume to volume numbers.

Altalab Instrument weatherstations provide gr/lb automatically. In the past, one had to line up values on what is essentially a three axis chart . Using a chart is tedious and there is room for error. Not only that, but the charts are made for sealevel standard pressure. There is no explaination on the chart of how to make a correction for actual pressure, and so no one until Altalab was doing it.

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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. Humidity can be expressed as relative humidity, absolute humidity, grains/lb, and also Dew Point.

An instrument that measures relative humidity. Analog hygrometers (dial type gauge) use a hair or synthetic filament which shrinks and expands in length with changes in RH. A synthetic filament is more accurate full scale. Digital rh sensors may be thin film silicon.

Attracting or absorbing moisture. Salt and alcohol fuel are examples.

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IR Radiation (Heat)
One of the biggest problems getting accurate and reliable information at the track is IR Heat. Objects in the sun can absorb heat and re-radiate it to your weatherstation. What you're trying to measure is air temperature, and that extra heat has NOTHING to do with actual air temperature. Even if you shade your weatherstation from direct sunlight, it can gain heat radiating from the asphalt, black trailer walls, hot cars, etc. If your weatherstation registers extra heat in this way, the Temperature will read higher and the Relative Humidity will be artificially lower than actual air conditions. Radiation effects can be minimized by shielding and by fan aspiration.

First, you should select a weatherstation that offers some immunity from IR Heat through its design. For example, in our weatherstations we put our temperature and humidity sensors inside the case, not sticking out. This shields them from direct sunlight. We chose a light colored case, which is slower to gain solar radiation than a dark case. (See blackbody.) Both our portable DeltaLite and Delta ACE have screened air vents to encourage maximum air flow and minimum radiation gain. If you have fan-aspiration, the movement of air will both prevent the sensors and case from heating up, and promote best response to ambient air changes.

Second, stay aware of the location of your weatherstation and provide additional shielding if necessary. For example, if your weatherstation is not fan-aspirated it must stay in the shade. You can also place corrugated cardboard between your gauges and objects that may be radiating IR Heat. Stand where your weatherstation is and look around to find any sources of IR Heat. If you can see it, so can your gauges. For example, gauges under an awning will usually benefit from cardboard placed across their face. Placing a weatherstation under the gooseneck of a trailer is usually a good place, as is the wheel well. (You might want to hang a "remove before flight" tag on it to prevent the obvious.)

If you are a drag racer who wants to take the weatherstation to the lanes you should read about microclimates.

Third, provide air flow. Place your weatherstation in a lawn chair. If you use a pit fan place the weatherstation where it will get the benefits of air movement. Don't hang the weatherstation too close to the awning. Even a light colored awning will heat up in the sun. In addition, since hot air rises there is likely to be a pool of warmer air just under the roof, particularly if the awning has side walls that trap air by preventing air flow.

If your weatherstation is fan-aspirated so much the better. Our DeltaLite/f and Delta ACE/f can be operated in direct sunlight without picking up extra heat, and obviously the fan-aspirated remote sensor housing on the Alta and AltaCom weatherstations is designed to withstand direct solar radiation. But keep in mind that even a fan-aspirated weatherstation must be protected from IR Heat. A classic mistake would be to have the fan running full tilt with the weatherstation sitting on a 120 degree car hood. The fan will pull that hot air right across the sensors. That's why our Alta remote sensor housing is supposed to be mounted at least 4 feet away from the trailer roof or walls.

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The output of a linear instrument in response to change of a certain amount is the same at all ends of its range. In contrast, a non linear instrument might show smaller changes in output at one end of its range than the other. A non linear dial type gauge would have a graduated scale to compensate, with smaller steps between values at one end. A non linear digital sensor would need software or circuit compensation.

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In racing, microclimates refer to the difference in the air between two nearby locations. A macro example is how air temperature in a city is usually several degrees warmer than a few miles outside town. This is because city buildings and asphalt gain ir heat. In the racing environment, drag racers should realize that the air near the head of staging may not be the same as the air downtrack. In staging you have asphalt and cars which absorb ir heat and block airflow. In other forms of racing the difference between the track and the sidelines may be less severe.

If you take measurements at the track sometimes, and at the trailer other times, you may be observing and recording the difference between microclimates instead of the difference in ambient air between one time and another. Altalab has solved this problem with AltaCom wireless weatherstations, which provide the ease of portable information from sensors located in the same place all the time.

A unit of measuring pressure. In the United States pressure is usually expressed as inches of Hg (mercury). Our weatherstations use inches Hg, and we have a conversion chart available for racers in other countries which has millibars, inches Hg and mm Hg. Call 304 497 2756 or email us for this free chart.

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About three-quarters of the Earth's atmosphere is nitrogen. For our purposes, it does not readily combine with other substances. Therefore, it has negligible effect on the internal combustion process. It is interesting to note that compounds of nitrogen make up most manufactured explosives.

One compound of racing interest is nitromethane, the fueler's go juice. Nitro is very stable under normal pressures and temperatures, however, if it exceeds ~600 degrees and ~900 psi it will self-detonate with awesome results. When this explosion is controlled and harnessed, it can produce the 800 horsepower per 62 cubic inch cylinder in a Top Fuel motor, or around 13 horsepower per cubic inch. The average HP per cubic inch for your average race car is only one or two. Hopefully, this perspective should indicate a more manageable task in producing and predicting performance in the relatively sedate race motor.

Noise in a digital instrument is any signal that isn't supposed to be there. It can be caused by tiny electrical sparks when a switch is activated, or from electrical, magnetic or radio signals leaking from one circuit to another, or by poor quality components. In a well designed instrument, noise can be minimized or eliminated.

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This is the gas we want to know about. It takes up more percentage by weight than volume because oxygen is a relatively heavy molecule for a gas. It s molecular mass is 32.00 compared to 28.01 for nitrogen, 18.02 for water vapor, and only 2.01 for hydrogen. You'd think that this heavy molecule could settle down in calm valleys as to enrich the oxygen content, but atmospheric mixing forces prevents this from occurring.

Oxygen readily combines with many substances. When fuels such as oil, wood, or racing gas are burned (oxidized), the oxygen combines with the hydrogen and carbon in the fuel to form carbon dioxide, water, and, of course, the desired release of energy. The earth's vegetation keeps our oxygen at a stable percentage.

Since the whole point of a motorsports weatherstation is to measure oxygen why doesn't Altalab use an O2 sensor?
A: Altalab pioneered the use of the O2 sensor when we released the 02 ALTA in the Fall of 1994, but gave it up after 1 year because it had more problems than could be justified by the value of information gained. The four main reasons for our decision are listed here.

1) Current technology 02 sensors function like a battery, and slowly drain down over their lifespan. They need annual calibration in order to remain accurate, particularly if the racer wants to build upon previous performance. Also, they have non-linear output, so to compensate we calibrated each sensor individually and generated from 6 to 10 calibration points per sensor. Most racers do not want to be bothered with an annual calibration, and their ability to relate current information to past references thus degrades over time.

2) The current technology O2 sensor contains an electrolye, and poor temperature compensation. When overheated heated, such as would happen on the dashboard of a car, the readings could go haywire and could take HOURS to settle. Altalab does not consider this acceptable in a portable instrument designed for motorsports. Our 02 Alta (like our current Altas), was trailer-based with fan aspirated sensors.

3) The O2 sensor currently available has a 1% accuracy rating which sounds pretty good until you realize that this means 1% from 0-100%, not 1% of 20.94 (standard O2 %). You could have an arbitrary fluctuation from 20.54% to 21.54% and still be within spec. This is NOT useful or reliable information.

4) Perhaps the most important reason - In all the testing and weatherlogging with the 02 sensor over a 2 yr. period, and after studying data from more than 20 different weatherstations we did not find significant correlation between 02 measurement changes and performance changes. The biggest changes in 02 measurement were caused by temperature fluctuations.

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An instrument used to calculate relative humidity. The temperature indicated by a wet bulb thermometer is compared to a dry bulb thermometer. The difference indicates the amount of moisture in the air.

A sling psychrometer can be very accurate, but in the field care must be taken to use distilled water, and to carefully repeat the length of time spent swinging. We have seen racers wet the wick with spit or pepsi! This leaves residue in the wick which leads to inaccuracy.

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Relative Humidity (rh)
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 air could hold.

100% RH means saturation, or dew point at that temperature. Do not get confused with relative humidity readings at different temperatures. For instance, air at 90 degrees at 50% RH has the same amount of moisture as does air at 70 degrees and 100% RH.

One reason Altalab offers racers several different calculated values that describe moisture in the air, is because RH is relative, it is relative to temperature. For example, early in the morning it is cool and the RH is high. Later in the day it warms up and the RH drops. Then, after dark the temperature cools and the RH goes up again, but the actual amount of moisture in the air has not changed! (Unless a different air mass has moved in.) Some of the other calculated values, like Absolute Humidity, Grains per Pound, Vapor Pressure, and Dew Point indicate the quantity of moisture in the air in such a way that can be more useful to the racer trying to tune or predict.

Remote Sensing
Altalab pioneered remote sensing in motorsports. In our ALTA Series weatherstations the temperature and RH sensors are located inside an aluminum, fan-aspirated housing that is connected to the console by a cable. This provides the important benefit of elevating the sensors away from generator exhaust or warm air trapped under an awning. It also means that readings are coming from the same location all the time. AltaCom further add wireless transmission from the weatherstation to a pager. Readings on the pager are unaffected by its location.

Usually measured as non-repeatability, but expressed as repeatability, as a percentage of full scale range. This measure of sensor quality describes the degree to which an instrument gives the same measurement when placed in the same conditions at different times.

The smallest change in the environment that will cause a response in the instrument.

Response Time
The length of time it takes an instrument to register changes in what is being measured. Sometimes called "settling time". If you bring a weatherstation from one environment to another, (such as from the air-conditioned motor home to the track) it will take some time for the instrument to correctly read new conditions.

The larger and heavier the instrument, the longer it will take. Typically a small digital instrument will settle faster than a bulky analog weatherstation. A fan-aspirated weatherstation will respond quickest, as the fan will both provide a sample of new air to the sensors and help the components of the weatherstation to come to the new temperature.

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When the air is at 100% RH. Dew Point is the saturation temperature of a particular air sample.

Settling Time
The length of time it takes an instrument to register changes in what is being measured. See Response Time.

Station Pressure
The same as Absolute Barometic Pressure. This term is used at airports.

Standard Temperature and Pressure, accepted by international agreement to be:
29.9231 "Hg (or 760mm or 1013.25 mb)
60 degrees F (some scales use 59 degrees)
0% RH ( or dry air)
Some values of a weatherstation which are calculated from all three of these measurements directly relate to STP. At STP the Adjusted Altitude and Density Altitude are equal to 0 feet, and Air Density Ratio is equal to 1.

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Temperature(degrees F)
This is the air temperature in degrees Fahrenheit, measured with a thermometer or temperature sensor.

There are several common types of digital temperature sensors. The RTD, or Resistance Temperature Detector; the Semiconductor; and the Thermistor, which boasts of great accuracy in the lab, but is non-linear, and not often properly temperature compensated. Altalab uses solid state semiconductors which are highly linear and fast responding.

A typical analog thermometer is a bimetallic coil, two thin metal strips that expand and contract differently brazed together into a coil that swings an indicator in response to temperature changes.

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Vapor Pressure (VP)
This is the part of atmospheric pressure (absolute barometric pressure) due to water vapor, and is expressed as inches of mercury.
Abs - vp = dry pressure, or the partial pressure of dry air.

Vapor Pressure is often monitored by racers of alcohol fueled vehicles, who have noticed vehicle performance changes at some observed vapor pressure. For example, a sportsman drag racer might rely on the Adjusted Altitude for predictions unless the Vapor Pressure is over a certain point that the racer has observed decreases performance.

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Search our Site
Search Help
Racers' Guide to Weather Monitoring
File and Document Downloads
Get Tech Help
Contact Altalab
More Winners Use the Altalab Weatherstation