Glossary of Weather Terms
This glossary is a living document.
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)
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)
Adjusted Altitude (AA)
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)
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.
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.
Density Altitude (DA)
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 Point (DP)
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
Grains per Pound (grlb)
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.
IR Radiation (Heat)
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.
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.
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.
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?
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.
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.
Relative Humidity (rh)
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.
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.
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.
Vapor Pressure (VP)
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.