Flytec Windwatch Pro - Manual (Page 8)

8 Barometer
Barometer
By tracking air pressure trends at a given location it is possible to make predictions
about approaching weather conditions. The barograph on the WWP Barometer main
screen represents the air pressure progression over the past 3, 6, 12 or 24 hours (user
selectable where each bar represents 1/12
th of the period selected). The graph runs from
left to right so that the value on the left is the oldest and the value on the right is the most
recent. Each segment in a bar corresponds to a pressure change of 2 hPa. The WWP
measures and updates the digital air pressure display every 7.5 minutes.
If the diagram is ascending from left to right, it signifies an increasing barometric
pressure trend, which is generally associated with dry weather conditions, sunshine and
blue skies. Conversely, if the graph is decreasing from left to right it indicates decreasing
barometric pressure, generally associated with a warm or cold front. For alpinists, hikers,
pilots and other outdoor sportsmen this is a warning sign for the possibility of bad
weather and the need to watch for worsening conditions. These front systems very often
produce a change in the weather, combined with low clouds and the possibility of
persistent rains or snow.
Barometer Pressure Display QNH / QFE
Because air pressure decreases with altitude, it is necessary to relate air pressure at a
given elevation to the air pressure at sea level. In meteorology this value is referred to as
QNH
. The average pressure at sea level during one year over medium latitude is
1013.25 hPa; this was set as standard pressure by the ICAO for the flying sector in
1928.
To measure air pressure at an altitude other than sea level, you must enter the altitude
of your current position into the barometer. In the WWP this height value is entered in
the set-mode for the Barometer screen. This also means that if the instrument is moved
to a location with a different elevation, the new reference elevation must be entered. If
the altitude of the WWP changes by more than 50 meters within a short period of time,
the barometric graph is automatically cleared and restarted.
If the reference altitude is set to zero, the displayed pressure is the actual air pressure at
that elevation and is referred to as QFE (Absolute Pressure).