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. Barometric pressure is caused by the gravitational pull on
the 'column of air' lying directly above the place you are measuring.
Absolute Humidity (AH) This calculated value expresses the actual amount of moisture present in the air as a percentage of volume.
Air Atmospheric air is Moist Air, which is a mixture of Dry Air, Water Vapour, and contaminants like smoke or pollen.
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 vapour 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 90'sF 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 vapour present in a quantities 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.
Humidity Water Vapour 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.
Relative Humidity (rh)
Expressed as a percent, this figure is the ratio of the amount of water
vapour in a particular temperature air to the maximum amount of water vapour
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.
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