Page 271 - NS-2 Textbook

Fronts and Storms

Mariners have much to fear when they are threatened by cold front and up to 300 miles wide for a warm front. The
a severe storm. A North Atlantic gale can strain rigging, point where the cold and warm fronts converge is fre-
spring seams, bend plates, smash equipment, and tear quently the center of a low-pressure area.
loose topside equipment, even on aircraft carriers or bulk
petroleum tankers. Winds of 100 knots and waves of FRONTAL ZONES
sixty feet or more are respected by an experienced sea-
man. The prudent mariner will maneuver to stay clear of The world's primary frontal zones are the Intertropical
storms whenever possible. Convergence Zone! Arctic Frontal Zone, and Polar
An experienced mariner should be able to see when Frontal Zone. The convergence of the northeast trade
weather disturbances are coming. One should observe winds of the Northern Hemisphere and the southeast
the sky and sea and carefully assess readings of the me- trade winds of the Southern Hemisphere causes a band
teorological instruments aboard. Also, to day's radio of unstable weather encircling Earth in the doldrums.
communications provide regular v,Teather swrunaries. This is called the Illtertropical COllvergellce ZOlle (lTCZ). It
The mariner at sea will carefully plot such weather infor- varies in position, largely due to the seasons. This is a
mation and compare it with the vessel's position and storm development area, but the storms themselves usu-
where she is heading. ally move pole-ward before they become severe. Brief,
violent windstorms called squalls occur when the warm
DEVELOPMENT OF FRONTS air rises, resulting in sudden, intense rainfall of short du-
ration. There normally is good visibility between these
Fronts develop when air masses of different tempera- squalls. In tropical seas it is often possible to see multiple
tures collide; air masses rarely fuse unless they are very separate rain squalls in progress, and several rainbows,
similar in temperature and moisture content. Fronts are all armmd the horizon.
weather systems that are sometimes called waves, as in TI,e Arctic Frolltal ZOlle develops between the arctic
the term Heold wave." air of the far north and the polar maritime air of the
Along the meeting edge or boundary of two dissim- North Atlantic and Pacific Oceans. This frontal zone may
ilar air masses, a battle for supremacy is fought. Usually disappear as it moves northward during the sununel;
the colder of the two masses, being heaviel; predomi- when it meets similar cold ail'.
nates and forces the warmer air upward. A cold frollt dis- The Polar Frolltal ZOlle is formed by the convergence
places the warm air ahead of it upward, while a war111 of the air that flows toward the equator from the Polar
front moves upward over a retreating cold-air mass. Easterlies and the Prevailing Westerlies-in other words,
When a cold front moves faster than a warm front, it the temperate zones. This polar front is very significant,
overtakes the warm front, forcing the warmest part of the since it greatly influences the weather in the temperate
air mass upward, but leaving the cooler air below. By the zones. The polar fronts move toward the poles during
time the cold front meets or cOllvelges with the cooler the summer and toward the tropics in the winter. This
mass ahead of the warm front, the warm air has been is why people in the temperate zones often experience
pushed above both masses. The convergent frontal mass a series of cold waves or snaps-because the colder
that remains is called an oceluded frollt. Regardless of the polar easterlies often break through the warmer band of
type of front-cold, warm, or occluded-the frontal '\vesterlies.
weather is either unsettled or stormy. Fronts always
bring bad weather. COLD FRONTS
A cold front or warm front may extend for hundreds
of miles. But the area in which frontal weather distur- When a cold front is coming, the first change you notice
bances occur is usually a band 15-50 miles wide for a is a darkening of the horizon to the west and to the north.

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