Page 102 - ภาษาอังกฤษสำหรับสายวิทยาการอุตุนิยมวิทยา

97

a. Except in periods of overall clear weather, the three types of visibility that affect
aviation function independently of each other. Horizontal surface visibility may be good at a
time when cloud condi- tions 1imit air-to-air and air-to-ground visibility. At other times,
horizontal surface visibility and air-to-air visibility may be good in an area where cloud layers
limit air-to-ground visibility. A particular airport may be closed so that aircraft do not arrive or
depart because of bad horizontal surface visibility, while air-to-air visibility may be excellent a
short distance above the surface.
b. The stability of air largely determines the type and intensity of restrictions to visibility
near the ground. Stable air, which resists vertical movement, doesn't break up and spread
out re to visibility. However, unstable air produces vertical currents which vertically and
horizontally. d to break up and separate fog, and to spread haze and smoke Precipitation in
stable air tends to without stopping, precipitation in unstable air ally cover large areas, nor
does it usually continue a long t can say, stable air will have a characteristic of poor visi -
bility, and unstable air, good visibility.
c. As the earth and lower layers of air become warm during , air that was stable during
the early morning hours ma unstable. For this reason visibility usually improves as
temperatures rise. If cloud layers aloft keep the sun's heat from reaching the ground, visibility
improvement is usually slow.

2. Fog
a. Fog is a cloud with its base at the earth's surface. f Fog forms by an atmospheric
process that affects surface air in one or both of the following ways:
(1) Cooling the air to its dewpoint or to saturation by:
(a) the ground being cooled by nighttime loss of heat, which then cools
the air contacting it
(b) the movement of moist air over cold ground..
(c) moving air being cooled as it is forced to rise.up sloping terrain

(2) Raising the dewpoint to that of the air temperature. Normally happens when
the evaporation

of warm rain adds water vapor to the air. This air is then cooled by
surrounding cold air.

b. The conditions favorable to fog formation are: light winds of 10 knots or
less, and a small difference in the temperature and the dewpoint