An air mass that’s colder than the surface to gradually shift more to the southwest.
below is called a cold air mass. The air rises as From June to August the high pressure pat-
it’s heated by the warmer surface, and the ris- tern is fairly constant. In late September
ing (convective) currents bring instability to and October, the subtropical high migrates
the area that can translate into vertically de- farther south in the North Atlantic as heat is
veloped cumulus cloud formations and gusty lost from the Northern Hemisphere, and this
wind at the surface. This instability is often shift allows low-pressure systems to intrude
accentuated during afternoon hours of maxi- more frequently.
mum surface warmth and is diminished dur-
Weather Along Fronts
ing relatively cold nights and early mornings.
In contrast, stability often characterizes the
The cold fronts formed at the leading edge
air in a warm air mass, in which the surface
of an advancing cold air mass can bring some
is colder than the air. The surface cools the
dramatic, but usually quickly passing, condi-
air just above it, and vertical currents are not
tions. The pressure is higher on the cold-air
likely to form. When the air contains a high
side of the front because the air there is more
amount of moisture, low stratus-type clouds
dense and heavy. The steeply sloped wedge of
with horizontal development are likely to
heavier cold air undercuts the warm air and
form, possibly accompanied by rain. As in
lifts it up. If the rising warm air is stable, the
cold-air masses, the daily heating and cool-
result can be moderate vertical cloud forma-
ing of land beneath a warm air mass can af-
tion and steady precipitation. If the lifted
fect its weather, as well. Because water heats
warm air is unstable, though, squalls and
and cools more slowly than land, the impact
thunderstorms can punctuate the precipita-
of daily temperature oscillations is less pro-
tion along the front. Squall lines can also form
nounced over offshore waters.
some 50 to 300 miles ahead of a fast-moving
cold front. Conditions after a cold front
Prevailing Weather Systems
passes also vary with stability, i.e., the stabil-
Two semi-permanent surface features con- ity of the cold air. When the advancing cold
trol North Atlantic weather—the Icelandic air is stable, clear skies are likely to follow the r
Low and the widespread subtropical high front’s passage. When it’s unstable, numerous
known as the Bermuda or Azores High. clouds and scattered showers are possible.
The Icelandic Low is a region of low aver- The weather along warm fronts behaves
e
athe
age atmospheric pressure (997 Mb) that is in a different way. The advancing warm air
W
centered near Iceland for most of the year. Its rides up over the retreating cold air, but
pressure is lowest in winter when its center climbs up a slope that is much more gradual
lies to the southwest of Iceland and its circu- than the cold front wedge. The cloud and
lation is large enough to embrace the coast precipitation pattern can spread out ahead
r
of northwestern Europe. of the front for over 500 miles, in contrast to
In summer, the Icelandic Low is relatively the relatively narrow band of special weather
e
athe
weak and its pressure is higher—around along cold fronts. Mariners should become
1009 Mb. When this happens, the Azores/ adept at reading and understanding weather
Bermuda High—a dominant feature in the maps, particularly the symbols for warm and
summer months in the North Atlantic— cold fronts and the wind arrows, or station
strengthens and spreads. The center of this plots, displaying local wind speed and direc-
subtropical high remains to the south or tion (see Figure 2).
southwest of the Azores throughout the year,
Recognizing Cold Fronts
r
standing W
but its average central pressure is somewhat
higher in July (1025 Mb) than in January
A cold front introduces a high-pressure area nde
(1020 Mb). The pressure on its north side
and its associated cooler, drier air. During the
U
also rises in summer when the Icelandic Low
passage of a cold front, the barometric pres-
is weak.
sure can fall dramatically. Once the front has
During spring (April and May), a ridge
passed, the pressure will often rise rapidly.
of high pressure often builds and extends
In the Northern Hemisphere, the wind
northeast from the Azores in response to sea-
will shift clockwise around the compass,
sonal heating of the Northern Hemisphere.
from east-southeast to south-southwest,
This process causes the prevailing west wind
before a cold front. Strong, gusty north-
northwest winds will often follow behind
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