Page 263 - NS-2 Textbook

258 NAUTICAL SCIENCES

Fog formation thus requires the presence of moisture, ing the steam fog. You can produce this same effect by
a gentle breeze, and a combination of warm and cold tem- setting a pan of warm water out in freezing cold air. This
peratures. A cool breeze passing over warm waters will type of fog occurs often in the far north, where it is called
create fog, and so will warm air passing over a stretch of Usea smoke." It can be seen in the late fall or winter ,,\Then
sea. The breeze will spread the fog out over the surface, a river or pond "steams" as frigid air cools the water
and it will lie in lower areas such as valleys and swamps. until it begins to form a coating of ice.
Fog is hazardous to aviation because it limits both Radiation Fog. 11,is fog is caused by the heat that the
"ceiling" and visibility. Similarly, fog at sea-near off- Earth radiates. It forms only at night, over a land surface.
shore islands, along coasts, and in bays, inlets, harbors, This is a common type of fog, and it may cover a large
and river mouths-is a continual hazard. Although air- area; but it usually lifts before noon, having been
craft and ships have radar to assist them in foggy condi- "bUTIled" away by the Sun's rays. After slUlSet, the Earth
tions, the eyes of alert pilots and ship lookouts are neces- receives no more heat from the Sun, but the ground con-
sary for safe navigation. Indeed, the nautical "rules of the tinues to radiate heat. The smface begins to cool, and lay-
road" explicitly require that lookouts be stationed aboard ers of air close to the surface are cooled by conduction. If
ship. And many an airplane flight has been delayed, ei- the air is sufficiently moist, it will chill to its dew point
ther in landing or taking off, because of poor visibility. and form fog. 11,is type of fog can be extremely haz-
Fog at Sea. Fog at sea is frequently formed through a ardous for drivers. Fog patches may suddenly develop in
process known as advection (moving forward). When low areas, drastically reducing visibility.
'"arm air that has passed over warm water moves to an Frol1tal Fogs. Although weather fronts are discussed
area of colder water, fog is likely to develop. Because sea- in chapter 5 of this unit, frontal fog should also be men-
water temperatures are fairly uniform within a large tioned here. This fog is caused by the movement of cold-
area, fog often lasts for many days and nights once it de- air masses. It most commonly occurs under the frontal
velops in a given area. surface of the cold-air mass and is caused by the evapo-
The great fog banks of the North Atlantic and those ration of falling precipitation. Such a circumstance is
of the northern Pacific around the Aleutian Islands of common in December or January when a warm front (the
Alaska demonstrate what happens when two adjacent midwinter thaw) is caught between the normal cold
bodies of water have greatly different temperatures. In weather of winter and a new cold front, which pushes
the vicinity of the Grand Banks of Newfoundland, warm the warm air ahead of it over cold ground. In the upper
air that has passed over the warm Gulf Stream quickly Midwest this results in Ii case weather" with very heavy,
tums to fog when it strikes the current of very cold water wet fog dampening the all, melting snow, and causing
that flows southward from the Arctic. Off Alaska, the extremely dreary days. III
same situation exists. The air over the warm Japanese
Current comes in contact with the cold, southward-
flowing waters of the Bering Sea.
Advection Coastal Fogs. Advection fog is the name CRITICAL THINKING
given to air-mass fog produced by air in motion or to fog
1. Why are the fog lights sometimes installed on auto-
formed in one place and transported by wind to another.
mobiles always fairly low to the grotmd and often yel-
These fogs occur when the wind moves warm, moist air
Imv in color?
from a warm ocean surface to a colder land surface-or
vice versa. These fogs will normally dissipate each day,
since the winds carrying the air will change direction
when the Sun rises. Study Guide Questions
Every sailor is fully aware of the fogs that can blanket
1. Of what is a cloud made?
the harbors and coastlines near Newport, Norfolk, New
2. What causes earthbotmd moisture to evaporate?
York, San Diego, Los Angeles, San Francisco, and Puget
3. What are the three basic guidelines used to deter-
Sound. Many a ship has spent hours listening to fog sig-
mine which kinds of clouds are in the sky and how
nals when faced with "pea soup" in harbor. It is even
they may affect weather prediction?
more difficult, howevel, to listen for fog signals when
4. What are the names of the three basic cloud types?
under way. Lookouts covering all quarters peer into gray
5. What are the two means of classifying clouds?
nothingness, while the jlUlior officer of the deck never
6. What are the ranges of altitude for low, middle, and
takes his or her eyes off the radar repeater on the bridge.
high clouds?
Steam Fog. This is a type of advection fog formed by
7. What type of weather is associated with these types
air satuTation. It occurs when cold air moves over ·warm
of clouds?
water. When this happens, water evaporating from the
A. stratus D. cumulus
warm surface easily saturates the cold air, thus produc-