Sea level pressure in the local area is influenced by migratory high
and low pressure systems during fall, winter, and spring. During
summer, sea level pressure is primarily influenced by westward ridging
from the Bermuda High. The MSL pressure ranges from a high of 1021.0
mb in January to a low in May of 1015.0 mb. The annual mean sea level
pressure is 1018.0 mb.
a. Diurnal Changes: Pressure rise occurs between 0900Z and 1500Z, and
pressure fall occurs between 1500Z and 0300Z. When only diurnal effects
are expected (persistence) with no appreciable change in the weather pattern,
the altimeter setting observed at 09Z will also be the following afternoon's
b. Sea level pressure is converted to field elevation of 30 feet.
c. Pressure Altitude (PA) is the indicated altitude of a pressure altimeter
at an altimeter setting of 29.92 inches of mercury (1013.2 mb). Binders
in the Facility's forecasting spaces contain conversion charts to determine
d. Density Altitude (DA) is the pressure altitude corrected for temperature
and humidity. A quick method for forecasting DA is to first determine
the temperature change during the forecast period, then add/subtract 70
feet per 1 degree F of temperature increase/decrease.
e. When high pressure dominates at the surface and is stationary
over the central U.S. or along the eastern seaboard, diurnal effects
show only a .08 inch fall between the 15Z and 21Z (10L and 16L) timeframe
with a secondary fall between 03Z and 09Z (22L and 04L) of .04 inch. When
the high moves east over the Western Atlantic, expect the 15Z and 21Z fall
to be approximately .10 inch.