Any liquid that has a temperature below its
normal freezing point but still exists in the liquid
state is known by what term?
What distinguishes drizzle from rain?
What type of hydrometeor is composed of hard
grains of ice formed from freezing rain or the
refreezing of snowflakes?
If rain is accumulating at a rate of 0.25 inches
per hour, how should the intensity be classified?
What does the abbreviation LTGCA mean?
When is a thunderstorm considered to have
LEARNING OBJECTIVES: Recognize the
importance of an accurate pressure
observation. Describe atmospheric pressure,
barometric pressure, and station pressure.
Explain sea-level pressure and altimeter
setting. Define pressure tendency.
Pressure is an important weather analysis and
forecasting item used by agriculturalists, pilots, and
weather forecasters. Many years ago, farmers
discovered that falling atmospheric pressure is
associated with poor, unsettled weather and that fair
weather is associated with rising atmospheric pressure.
Today, most farmers rely on scientific forecasts to
regulate their activity. Overland, at and below 18,000
feet, pilots fly aircraft at their assigned flight levels
based on the altimeter setting provided by local
In forecasting, pressure is used to analyze the
isobar patterns, or lines of equal pressure. From the
isobar patterns, analysts can determine wind speeds,
centers of high and low pressure, and other critical
information. By tracking the movement of high- and
low-pressure centers, forecasters may anticipate future
movements of the centers, and their associated weather
For pressure values to be meaningful to pilots,
analysts, and forecasters, the reported readings must be
accurate. An error in a reported sea-level pressure may
cause an analysis to be in error, especially over data-
sparse areas, such as the oceans. But an error in an
altimeter setting can be disastrous for a pilot. The
responsibility for observing, calculating, and reporting
pressures accurately rests solely on you, the weather
In this section, we cover the different types of
pressure that must be observed, associated pressure
terms, and the pressure values that must be calculated
for an observation.
The standard units used to measure and report
pressure values are inches of mercury and hectopascals.
The term hectopascals (hPa) replaced the term
millibars (mb) several years ago. A hectopascal is
exactly equal to one millibar. See Appendix II for
conversions between inches of mercury and
Atmospheric pressure refers to the pressure exerted
by the column of air on any point on the earths surface.
The term is not specific as to where the point in question
is located. The vagueness of the term causes some
confusion in military weather because the observer can
never be sure if the person asking for atmospheric
pressure wants station pressure, sea-level pressure, or
even an altimeter setting.
Barometric pressure is the pressure read directly
from a precision aneroid barometer or a tactical aneroid
On the ML-448/UM precision aneroid
barometer, this value may be read in inches or in
millibars. Readings in millibars can be converted
directly to hectopascals; for example, 978.7 millibars
equals 978.7 hPa.
Station Pressure is the pressure value read on the
barometer (barometric pressure in inches or
hectopascals) corrected for the difference between the
height of the barometer and the station elevation. The
correction that is added to the barometric pressure may
be an instrument correction, a removal correction, and a
The station elevation is the height of the highest
point on the runway above mean sea level (MSL). This
is the height that is found published in the Flight
Information Publications. Aboard naval ships, the
station elevation is considered to be the height of the
barometer above the water line, not the height of the