JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
associated with these pressure systems. For example, poor weather such as found with fronts and
squall lines are generally associated with troughs and lows, while good weather is associated
with highs and ridges.
Station and Sea Level Pressure
Station pressure is the atmospheric pressure measured directly at an airfield or other weather
station. Sea level pressure is the pressure that would be measured from the existing weather if the
station were at mean sea level (MSL). This can be measured directly at sea level, or calculated if
the station is not at sea level using the standard pressure lapse rate.
Surface analysis charts, such as the one in Figure 1-3, use MSL as the reference level for the
depicted isobars (to provide a common reference), even though the pressure was first measured
at a weather station. This is done so that daily pressure variations associated with weather
systems can be tracked as they move across the country, as mentioned above. If, instead, station
pressures were used, the pressure charts would depict the inverse of the land topography,
reflecting the contour lines of a map. Mountain tops would always have lows over them, and
valleys would have highs. In other words, high altitude stations such as Denver would always
reflect lower pressure than surrounding stations at lower altitudes regardless of the day to day
pressure variations that occur with passing weather systems. Thus, for pressure to be meaningful,
all stations–even those far from the ocean–will report sea level pressure.
ALTITUDE MEASUREMENT
Altitude is defined as the height above a given reference. The instrument that displays altitude in the
cockpit is called an altimeter. The barometric altimeter is an aneroid barometer that is calibrated
to display altitude in feet, as opposed to pressure in inches of mercury (Figure 1-4).
Figure 1-4 — Barometric Altimeter
Since an altitude includes not only the height number, but also the reference, altimeters have a
Kollsman window that shows the reference pressure, known as the altimeter setting. The altimeter
setting is the value to which the scale of the pressure altimeter is set so the altimeter indicates true
altitude at field elevation. It is very nearly equal to the station pressure corrected to mean sea level
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