Page 254 - NS-2 Textbook

METEOROLOGY 249

IO~O 31.0 I maritime polar polar
1047 309
1044 30.8
1041
1038 30.7
1035 30.6
1032 aO.5
1029 30.4
1026 303
1023 30,2
1020
1013 MILLIBARS '0.7 30.'
OR .0.4 300
29.92 INCHES -C;0<7.7-!'T- 29.9
AT SEA LEVEL :oo~ 29.8
1005 29.7
1002 29.6
999 29.5 !
996 29.4
993
990 29.3
987 29.2 maritime tropical
(J') 984 29.1 (Pacific)
a:: 981 29.0 (/)
28.9 lLJ
:g 978 28.8 a
:J97S
-1972 2&7 Z The primary air masses that affect the weather of the United States.
l!i 969 26.6 -
966 28.5
963
960 28.4 It is heat, and the transfer of heat, that makes
957 28.3
954 282 weather. This heat, of course, comes from the Sun. Heat
951 28.1 causes the weather changes. Without it there would be
948 2ao
no winds, varying air pressures, storms, rain, or snow.
Inches of mercury and the corresponding millibar scale used for All weather changes are caused by temperature changes
measuring barometric air pressure. in different parts of the atmosphere.
There are some fundamental natural laws that deter-
mine these changes. Warm air is lighter in weight and
When warm- and cold-air masses touch, the bound-
can hold more water vapor than cold air. Cold air is
ary between them is called a front. There will usually be
heavier and has a tendency to flow toward the rising
cloudiness and precipitation in a frontal area. A warm
warm ail; replacing it on the Earth's surface. As this ail'
front is formed ,,\Then a warm-air mass moves over a cold-
moves, wind is created, thus beginning the complex
airmass; when the reverse occurs, it is called a cold front.
forces that cause the changing weather.
When neither mass advances on the other, a stationary
Olli' main source of energy, the StIll, bombards Earth
fran t is said to exist.
with 126 trillion horsepower each second. The Sun's en-
Violent frontal weather systems can be predicted
ergy is transmitted as elech'omagnetic waves, or radia-
from a chart showing atmospheric pressures. Weather
8
tion, traveling at 186,300 miles (3 X 10 meters) per sec-
charts usually illustrate barometric pressures as millibar
ond. The solar radiant energy is referred to as insolation
reading points. The lines in the figure, drawn through
(il1coming solar radiatiol1). About 43 percent of the radia-
points of equal pressure, are called isobars. Isobars never
tion reaching our planet hits the Earth's surface and is
join or cross. Some may run off the chart, but others may
close, forming irregular ovals. Reporting stations send in changed into heat; the rest stays in the atmosphere or is
reflected into space.
their barometric readings to a central weather bureau,
Clouds and other atmospheric influences absorb
where weather charts are made. Isobars also give a rough
indication of the amount of wind in an area. The closer some of the incoming radiation, but they reflect much of
it. A typical cloud reflects back 75 percent of the sunlight
that the isobars are to one another the stronger the wind
striking it. Since Earth's average cloudiness is 52 percent,
in that area.
about 36 percent of the total insolation never reaches our
planet. Dense forests absorb up to 95 percent of the sun-
WHAT MAKES THE WEATHER?
light striking them, and water reflects 60-96 percent, de-
Weather is the condition of the atmosphere. Changes in pending upon the angle at which the light hits the sur-
weather are caused by changes in the air's temperature, face.
pressure, and water vapor content; v-lind causes the Over a long period of time, the Earth's temperature
weather to move. It can be said, therefore, that weather is remains fairly constant, despite the constant inflow of
the condition of the atmosphere, expressed in terms of its solar radiation. TItis tells us that the Earth is also giving
heat, pressure, wind, and moisture. off heat at about the same rate. The Earth's cloud cover