Rain: Signs of approaching rain include wind veers to the northwest, six to 12 hours
a slowly falling barometer (about 0.14 Mb/ of clear and cool weather should follow.
hour); dark clouds with hard, defined out- During this time, you should keep a close
lines; or distant white, watery-looking clouds watch on the barometer and wind. If the
that increase until forming an overcast, barometer begins to fall again, or if the wind
murky vapor. backs toward southwest and upper clouds
Wind: Strong winds and perhaps rain are increase, expect renewed inclement weather
indicated if the barometer is falling or ris- within six to 12 hours.
ing rapidly (1.0 Mb or greater). Hard, more Low-pressure systems may approach from
sharply defined edges on dark, black clouds almost any direction, but in mid-latitudes
signal strong wind. they usually approach from the west, fol-
Change in Wind Direction: High upper- lowing paths determined by the flow of
level clouds moving in a different direction upper-level winds called jet streams. In the
from the lower clouds or from the surface Northern Hemisphere at these latitudes, an
wind signal a change in wind direction. observer south of an approaching low will
These upper clouds can indicate the new experience increasing and clocking or veer-
direction of the surface wind. ing winds—that is, winds that change direc-
Atmospheric Pressure
tion in a clockwise direction. An observer
north of a low’s center will feel increasing
Trends in atmospheric pressure can indi-
and backing (shifting counterclockwise)
cate the strength and stability of a weather
winds. An observer in the path of a west-to-
system and warn of approaching systems.
east-moving low will experience increasing
If the sea -evel barometric pressure is above
winds from a steady, easterly direction.
1020 Mb and is steady or rising, fair weather
Apart from the large changes in barometric
is likely to last for another 24 hours. During
pressure due to the movement of weather
unsettled weather, a rapid rise in barometric
systems, there is also a small diurnal (twice
pressure to 1030 Mb or higher can signal
daily) pressure variation of not more than
improving conditions.
0.5 Mb throughout the world. It’s at its
r
A falling barometer and a consistent, con-
maximum at 1000 and 2200 local time and
tinual change in wind direction and strength
at its minimum at 0400 and 1600. Although
usually indicate the approach of a low-pres-
this daily variation is often masked by larger
e
athe
sure system that may still be six to 12 hours
changes in pressure, it’s worth remembering
W
away from your location. A rapid barometric
when you are interpreting barometric trends.
fall often indicates a strong weather system;
a slow fall, a milder system. However, a slow
Wind
fall to below 990 Mb often indicates a strong Because wind is the single most important
low-pressure system that has stalled or is weather element to mariners, a review of
moving slowly. some fundamentals is appopriate.
A rising barometer often indicates the on- Wind is the movement of air over the
set of fair weather as a high-pressure system earth’s surface caused by differences in atmo-
replaces a low-pressure system. In mid-lati- spheric pressure. These pressure differences
tudes, a rapid rise in barometric pressure can are the result of variations in temperature
indicate that the weather systems are moving and moisture between air masses. The dif-
rapidly, or that the pressure differential be- ference in the atmospheric pressure over a
tween two systems is large, or both. Although specific distance is known as pressure gradi-
fair weather is imminent, upper-air instabil- ent. The steeper the gradient, the stronger
ity can cause strong, gusty winds for a day or the wind. Air also moves vertically in the
so as the high-pressure center approaches. atmosphere, influencing the formation of
It’s wise to remember that some of the stron- high and low pressure systems as well as
gest winds and seas associated with gales can thunderstorms, waterspouts, and other local
come behind a cold front and appear after weather features.
the barometer has begun to rise. Winds that blow from approximately the
In northern mid-latitudes, if the sky clears same direction throughout most of the year
after the passage of a cold front and the in the tropics are called “trade winds”; winds
whose directions change with the sun’s
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