Atmospheric Pressure, Continued
Figure 10-4 shows comparative
readings on the inch and millibar
The aneroid barometers normally
can be read no closer than 0.01
Aneroid barometers are the standard
aboard ship and the type of
barometers that Quartermasters will
encounter most frequently.
Barometers should be calibrated
yearly in accordance with PMS.
Figure 10-4. Inches and millibars.
A chart of the atmospheric pressure over a large area of Earth as surface
at any given time tells you which way different air masses (an air mass
is a large body of air that has common temperature and humidity
characteristics) are moving. Some air masses originate in the cold polar
regions; some, in the tropics. By the time they reach you, these air
masses, called maritime air masses, have moved from vast bodies of
water. Others, called continental air masses, have grown up over more
or less dry land. Air masses carry along with them the temperature and
humidity characteristics of the areas they crossed. Where distinctly
different air masses touch, the boundary between them is called a front
and is marked by cloudiness and precipitation.