Chapt - 35.pdf - po ang - Januszek66

CHAPTER 34

WEATHER ELEMENTS

GENERAL DESCRIPTION OF THE ATMOSPHERE

3400. Introduction

latitudes to about 10 miles at low latitudes.

The standard atmosphere is a conventional vertical

structure of the atmosphere characterized by a standard sea-

level pressure of 1013.25 hectopascals of mercury (29.92

inches) and a sea-level air temperature of 15

Weather is the state of the Earth’s atmosphere with

respect to temperature, humidity, precipitation, visibility,

cloudiness, and other factors. Climate refers to the average

long-term meteorological conditions of a place or region.

All weather may be traced to the effect of the Sun on

the Earth. Most changes in weather involve large-scale

horizontal motion of air. Air in motion is called wind . This

motion is produced by differences of atmospheric pressure,

which are attributable both to differences of temperature

and the nature of the motion itself.

Weather is of vital importance to the mariner. The

wind and state of the sea affect dead reckoning. Reduced

visibility limits piloting. The state of the atmosphere affects

electronic navigation and radio communication. If the skies

are overcast, celestial observations are not available; and

under certain conditions refraction and dip are disturbed.

When wind was the primary motive power, knowledge of

the areas of favorable winds was of great importance.

Modern vessels are still affected considerably by wind and

sea.

F).

The temperature decreases with height at the standard

lapse rate , a uniform 2

C (59

F) per thousand feet to 11

kilometers (36,089 feet), and above that remains constant at

–56.5

C (3.6

F).

The jet stream refers to relatively strong (greater than

60 knots) quasi-horizontal winds, usually concentrated

within a restricted layer of the atmosphere. Research has

indicated that the jet stream is important in relation to the

sequence of weather. There are two commonly known jet

streams. The sub-tropical jet stream (STJ) occurs in the

region of 30

C (-69.7

N during the northern hemisphere winter,

decreasing in summer. The core of highest winds in the STJ

is found at about 12km altitude (40,000 feet) in the region

E, although considerable

variability is common. The polar frontal jet stream (PFJ)

is found in middle to upper-middle latitudes and is discon-

tinuous and variable. Maximum jet stream winds have been

measured by weather balloons at 291 knots.

and

150

3401. The Atmosphere

3402. General Circulation Of The Atmosphere

The atmosphere is a relatively thin shell of air, water

vapor, and suspended particulates surrounding the Earth.

Air is a mixture of gases and, like any gas, is elastic and

highly compressible. Although extremely light, it has a

definite weight which can be measured. A cubic foot of air

at standard sea-level temperature and pressure weighs 1.22

ounces, or about 1 / 817 th the weight of an equal volume of

water. Because of this weight, the atmosphere exerts a

pressure upon the surface of the Earth of about 15 pounds

per square inch.

As altitude increases, air pressure decreases due to the

decreased weight of air above. With less pressure, the

density decreases. More than three-fourths of the air is

concentrated within a layer averaging about 7 statute miles

thick, called the troposphere . This is the region of most

“weather,” as the term is commonly understood.

The top of the troposphere is marked by a thin

transition zone called the tropopause , immediately above

which is the stratosphere . Beyond this lie several other

layers having distinctive characteristics. The average height

of the tropopause ranges from about 5 miles or less at high

The heat required to warm the air is supplied originally

by the Sun. As radiant energy from the Sun arrives at the

Earth, about 29 percent is reflected back into space by the

Earth and its atmosphere, 19 percent is absorbed by the

atmosphere, and the remaining 52 percent is absorbed by

the surface of the Earth. Much of the Earth’s absorbed heat

is radiated back into space. Earth’s radiation is in compara-

tively long waves relative to the short-wave radiation from

the Sun because it emanates from a cooler body. Long-

wave radiation, readily absorbed by the water vapor in the

air, is primarily responsible for the warmth of the

atmosphere near the Earth’s surface. Thus, the atmosphere

acts much like the glass on the roof of a greenhouse. It

allows part of the incoming solar radiation to reach the

surface of the Earth but is heated by the terrestrial radiation

passing outward. Over the entire Earth and for long periods

of time, the total outgoing energy must be equivalent to the

incoming energy (minus any converted to another form and

retained), or the temperature of the Earth and its atmosphere

would steadily increase or decrease. In local areas, or over

481

482

WEATHER ELEMENTS

relatively short periods of time, such a balance is not

required, and in fact does not exist, resulting in changes

such as those occurring from one year to another, in

different seasons and in different parts of the day.

The more nearly perpendicular the rays of the Sun

strike the surface of the Earth, the more heat energy per unit

area is received at that place. Physical measurements show

that in the tropics, more heat per unit area is received than

is radiated away, and that in polar regions, the opposite is

true. Unless there were some process to transfer heat from

the tropics to polar regions, the tropics would be much

warmer than they are, and the polar regions would be much

colder. Atmospheric motions bring about the required

transfer of heat. The oceans also participate in the process,

but to a lesser degree.

If the Earth had a uniform surface and did not rotate on

its axis, with the Sun following its normal path across the

sky (solar heating increasing with decreasing latitude), a

simple circulation would result, as shown in Figure 3402a .

However, the surface of the Earth is far from uniform, being

covered with an irregular distribution of land and water.

Additionally, the Earth rotates about its axis so that the

portion heated by the Sun continually changes. In addition,

the axis of rotation is tilted so that as the Earth moves along

its orbit about the Sun, seasonal changes occur in the

exposure of specific areas to the Sun’s rays, resulting in

variations in the heat balance of these areas. These factors,

coupled with others, result in constantly changing large-

scale movements of air. For example, the rotation of the

Earth exerts an apparent force, known as Coriolis force ,

which diverts the air from a direct path between high and

low pressure areas. The diversion of the air is toward the

right in the Northern Hemisphere and toward the left in the

Southern Hemisphere. At some distance above the surface

of the Earth, the wind tends to blow along lines connecting

points of equal pressure called isobars . The wind is called

a geostrophic wind if it blows parallel to the isobars. This

normally occurs when the isobars are straight (great

circles). However, isobars curve around highs and lows,

and the air is not generally able to maintain itself parallel to

these. The resulting cross-isobar flow is called a gradient

wind . Near the surface of the Earth, friction tends to divert

the wind from the isobars toward the center of low pressure.

At sea, where friction is less than on land, the wind follows

the isobars more closely.

A simplified diagram of the general circulation pattern

is shown in Figure 3402b. Figure 3402c and Figure 3402d

give a generalized picture of the world’s pressure distri-

bution and wind systems as actually observed.

A change in pressure with horizontal distance is called

a pressure gradient . It is maximum along a normal

(perpendicular) to the isobars. A force results which is

called pressure gradient force and is always directed from

high to low pressure. Speed of the wind is approximately

proportional to this pressure gradient.

Figure 3402a. Ideal atmospheric circulation for a uniform and non-rotating Earth.

WEATHER ELEMENTS

483

Figure 3402b. Simplified diagram of the general circulation of the atmosphere.

Figure 3402c. Generalized pattern of actual surface winds in January and February.

484

WEATHER ELEMENTS

Figure 3402d. Generalized pattern of actual surface winds in July and August. (See key with Figure 3402c .)

MAJOR WIND PATTERNS

3403. The Doldrums

deflected toward the west. Therefore, the trade winds in the

Northern Hemisphere are from the northeast and are called

the northeast trades , while those in the Southern

Hemisphere are from the southeast and are called the

southeast

A belt of low pressure at the Earth’s surface near the

equator known as the doldrums occupies a position approxi-

mately midway between high pressure belts at about latitude

trades .

The

trade-wind

directions

are

best

defined over eastern ocean areas.

The trade winds are generally considered among the

most constant of winds, blowing for days or even weeks

with little change of direction or speed. However, at times

they weaken or shift direction, and there are regions where

the general pattern is disrupted. A notable example is found

in the island groups of the South Pacific, where the trades

are practically nonexistent during January and February.

Their best development is attained in the South Atlantic and

in the South Indian Ocean. In general, they are stronger

during the winter than during the summer season.

In July and August, when the belt of equatorial low

pressure moves to a position some distance north of the

equator, the southeast trades blow across the equator, into

the Northern Hemisphere, where the Earth’s rotation

diverts them toward the right, causing them to be southerly

and southwesterly winds. The “southwest monsoons” of the

African and Central American coasts originate partly in

these diverted southeast trades.

Cyclones from the middle latitudes rarely enter the

regions of the trade winds, although tropical cyclones

originate within these areas.

on each side. Except for significant intradiurnal

changes, the atmospheric pressure along the equatorial low is

almost uniform. With minimal pressure gradient, wind speeds

are light and directions are variable. Hot, sultry days are

common. The sky is often overcast, and showers and thunder-

showers are relatively frequent. In these atmospherically

unstable areas, brief periods of strong wind occur.

The doldrums occupy a thin belt near the equator, the

eastern part in both the Atlantic and Pacific being wider than

the western part. However, both the position and extent of the

belt vary with longitude and season. During all seasons in the

Northern Hemisphere, the belt is centered in the eastern

Atlantic and Pacific; however, there are wide excursions of

the

to 35

doldrum

regions

longitudes

with

considerable

landmass. On the average, the position is at 5

N, frequently

called the meteorological equator .

3404. The Trade Winds

The trade winds at the surface blow from the belts of

high pressure toward the equatorial belts of low pressure.

Because of the rotation of the Earth, the moving air is

WEATHER ELEMENTS

485

3407. Polar Winds

3405. The Horse Latitudes

Partly because of the low temperatures near the

geographical poles of the Earth, the surface pressure

tends to remain higher than in surrounding regions, since

cold air is more dense than warm air. Consequently, the

winds blow outward from the poles, and are deflected

westward by the rotation of the Earth, to become

northeasterlies in the Arctic, and southeasterlies in the

Antarctic. Where the polar easterlies meet the prevailing

westerlies, near 50

Along the poleward side of each trade-wind belt, and

corresponding approximately with the belt of high pressure in

each hemisphere, is another region with weak pressure

gradients and correspondingly light, variable winds. These

are called the horse latitudes , apparently so named because

becalmed sailing ships threw horses overboard in this region

when water supplies ran short. The weather is generally good

although low clouds are common. Compared to the doldrums,

periods of stagnation in the horse latitudes are less persistent.

The difference is due primarily to the rising currents of warm

air in the equatorial low, which carry large amounts of

moisture. This moisture condenses as the air cools at higher

levels, while in the horse latitudes the air is apparently

descending and becoming less humid as it is warmed at lower

heights.

S on the average, a discon-

tinuity in temperature and wind exists. This

discontinuity is called the polar front . Here the warmer

low-latitude air ascends over the colder polar air creating

a zone of cloudiness and precipitation.

In the Arctic, the general circulation is greatly

modified by surrounding landmasses. Winds over the

Arctic Ocean are somewhat variable, and strong surface

winds are rarely encountered.

In the Antarctic, on the other hand, a high central

landmass is surrounded by water, a condition which

augments, rather than diminishes, the general

circulation. The high pressure, although weaker than in

the horse latitudes, is stronger than in the Arctic, and of

great persistence especially in eastern Antarctica. The

cold air from the plateau areas moves outward and

downward toward the sea and is deflected toward the

west by the Earth’s rotation. The winds remain strong

throughout the year, frequently attaining hurricane force

near the base of the mountains. These are some of the

strongest surface winds encountered anywhere in the

world, with the possible exception of those in well-

developed tropical cyclones.

N and 50

3406. The Prevailing Westerlies

On the poleward side of the high pressure belt in each

hemisphere, the atmospheric pressure again diminishes.

The currents of air set in motion along these gradients

toward the poles are diverted by the Earth’s rotation toward

the east, becoming southwesterly winds in the Northern

Hemisphere and northwesterly in the Southern

Hemisphere. These two wind systems are known as the

prevailing westerlies of the temperate zones.

In the Northern Hemisphere this relatively simple

pattern is distorted considerably by secondary wind

circulations, due primarily to the presence of large

landmasses. In the North Atlantic, between latitudes 40

and 50

, winds blow from some direction between south

and northwest during 74 percent of the time, being

somewhat more persistent in winter than in summer. They

are stronger in winter, too, averaging about 25 knots

(Beaufort 6) as compared with 14 knots (Beaufort 4) in the

summer.

In the Southern Hemisphere the westerlies blow

throughout the year with a steadiness approaching that of

the trade winds. The speed, though variable, is generally

between 17 and 27 knots (Beaufort 5 and 6). Latitudes 40

3408. Modifications of the General Circulation

The general circulation of the atmosphere is greatly

modified by various conditions. The high pressure in the

horse latitudes is not uniformly distributed around the belts,

but tends to be accentuated at several points, as shown in

Figure 3402c and Figure 3402d . These semi-permanent

highs

remain

about

the

same

places

with

great

persistence.

Semi-permanent lows also occur in various places, the

most prominent ones being west of Iceland, and over the

Aleutians (winter only) in the Northern Hemisphere, and in

the Ross Sea and Weddell Sea in the Antarctic areas. The

regions occupied by these semi-permanent lows are

sometimes called the graveyards of the lows, since many lows

move directly into these areas and lose their identity as they

merge with and reinforce the semi-permanent lows. The low

pressure in these areas is maintained largely by the migratory

lows which stall there, with topography also important,

especially in Antarctica.

Another modifying influence is land, which undergoes

greater temperature changes than does the sea. During the

to 50

S, where these boisterous winds occur, are called the

roaring forties . These winds are strongest at about latitude

The greater speed and persistence of the westerlies in

the Southern Hemisphere are due to the difference in the

atmospheric pressure pattern, and its variations, from the

Northern Hemisphere. In the comparatively landless

Southern Hemisphere, the average yearly atmospheric

pressure diminishes much more rapidly on the poleward

side of the high pressure belt, and has fewer irregularities

due

continental

interference,

than

the

Northern

Hemisphere.

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