Kestrel 4250 (Racing) - Manual (Page 11)

Kestrel ® 4250 Racing Weather ® Tracker
21
GLOSSARY
The below definitions have been greatly simplified in order to keep this section brief. We strongly
recommend that anyone who wishes to make use of these measurements refer to one of the many
excellent weather references available for a more in-depth definition. On the internet, visit www.usatoday.
com or www.noaa.gov. Or, locate the USA Today publication, The Weather Book. Please note that any words
in a definition printed in italics are themselves defined in this glossary.
Altimeter Setting
An aviation term for the local barometric pressure. Same as reference pressure.
Altitude
The distance above sea level. The Kestrel Meter calculates altitude based on the measured station pressure
and the input barometric pressure — or “reference pressure”.
Air Density
The mass of air per unit volume. It is a function of temperature, humidity, and pressure.
Barometric Pressure
The air pressure of your location reduced to sea level. Pressure will change as weather systems move into
your location. Falling pressure indicates the arrival of a low pressure system and expected precipitation or
storm conditions. Steady or rising pressure indicates clear weather. A correct altitude must be input for the
Kestrel Meter to display barometric pressure correctly.
Density Altitude
The altitude at which you would be, given the current air density. Often used by individuals who tune
high performance internal combustion engines, such as race car engines. Also of interest pilots in order to
determine how an aircraft will perform.
Dewpoint
The temperature to which air must be cooled in order for condensation to occur. The difference between
dewpoint and temperature is referred to as the “temperature/dew point spread”. A low dewpoint spread
indicates high relative humidity, while a large dewpoint spread indicates dry conditions.
Heat Index
A practical measure of how hot the current combination of relative humidity and temperature feels to a
human body. Higher relative humidity makes it seem hotter because the body’s ability to cool itself by
evaporating perspiration is reduced.
Moisture Content (Water Grains)
The ratio between the actual mass of water vapor present in moist air — to the mass of the dry air.
Reference Pressure
The local barometric pressure. Input to the altitude screen to provide correct altitude readings. Also known
as the altimeter setting.
Relative Air Density
A proportion of measured air density to standard air density. Standard air density uses standard (fixed)
values for temperature, humidity and pressure.
Relative Humidity
The amount of water vapor actually in the air divided by the maximum amount of water vapor the air could
hold at that temperature, expressed as a percentage.
Station Pressure
The air pressure of your location, NOT reduced to the sea level equivalent.