The air that surrounds us weighs about one-thirteenth of a pound per
cubic foot and exerts a pressure, at sea level, of nearly fifteen
pounds per square inch. Its temperature varies from 30° below to 100°
above the Fahrenheit zero. The pressure of the air decreases about
one-half pound for each thousand feet of altitude; at the top of Mt.
Blanc it would be, therefore, only about six pounds per square inch.
The temperature also decreases with the altitude. The weight of a cubic
foot, or _density_, which, as has been stated, is one-thirteenth of a
pound ordinarily, varies with the pressure and with the temperature.
The variation with pressure may be described by saying that the
_quotient_ of the pressure by the density is constant: one varies in
the same ratio as the other. Thus, at the top of Mt. Blanc (if the
temperature were the same as at sea level), the density of air would be
about 6/15 × 1/13 = 2/65: less than half what it is at sea level. As to
temperature, if we call our Fahrenheit zero 460°, and correspondingly
describe other temperatures--for instance, say that water boils at
672°--then (pressure being unchanged) the _product_ of the density
and the temperature is constant. If the density at sea level and zero
temperature is one-thirteenth pound, then that at sea level and 460°
Fahrenheit would be
(0 + 460)/(460 + 460) × 1/13 = 1/26.
These relations are particularly important in the design of all
balloons, and in computations relating to aeroplane flight at
high altitudes. We shall be prepared to appreciate some of their
applications presently.
Generally speaking, the atmosphere is always in motion, and moving air
is called wind. Our meteorologists first studied winds near the surface
of the ground: it is only of late years that high altitude measurements
have been considered practically desirable. Now, records are obtained
by the aid of kites up to a height of nearly four miles: estimates of
cloud movements have given data on wind velocities at heights above six
miles: and much greater heights have been obtained by free balloons
equipped with instruments for recording temperatures, pressures,
altitude, time, and other data.
When the Eiffel Tower was completed, it was found that the average wind
velocity at its summit was about four times that at the base. Since
that time, much attention has been given to the contrasting conditions
of surface and upper breezes as to direction and velocity.
Air is easily impeded in its movement, and the well-known uncertainties
of the weather are closely related to local variations in atmospheric
pressure and temperature. When near the surface of the ground,
impingement against irregularities therein--hills, cliffs, and
buildings--makes the atmospheric currents turbulent and irregular.
Where there are no surface irregularities, as on a smooth plain or
over water, the friction of the air particles passing over the surface
still results in a stratification of velocities. Even on a mountain
top, the direction and speed of the wind are less steady than in the
open where measured by a captive balloon. The stronger the wind,
the greater, relatively, is the irregularity produced by surface
conditions. Further, the earth's surface and its features form a vast
sponge for sun heat, which they transfer in turn to the air in an
irregular way, producing those convectional currents peculiar to low
altitudes, the upper limit of which is marked by the elevation of the
cumulus clouds. Near the surface, therefore, wind velocities are lowest
in the early morning, rising to a maximum in the afternoon.
[Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS
(From Rotch's _The Conquest of the Air_)]
Every locality has its so-called "prevailing winds." Considering the
compass as having eight points, one of those points may describe
as many as 40% of all the winds at a given place. The direction of
prevalence varies with the season. The range of wind velocities is
also a matter of local peculiarity. In Paris, the wind speed exceeds
thirty-four miles per hour on only sixty-eight days in the average
year, and exceeds fifty-four miles on only fifteen days. Observations
at Boston show that the velocity of the wind exceeds twenty miles per
hour on half the days in winter and on only one-sixth the days in
summer. Our largest present dirigible balloons have independent speeds
of about thirty-four miles per hour and are therefore available (at
some degree of effectiveness) for nearly ten months of the year, in the
vicinity of Paris. In a region of low wind velocities--like western
Washington, in this country--they would be available a much greater
proportion of the time. To make the dirigible able to at least move
nearly every day in the average year--in Paris--it must be given a
speed of about fifty-five miles per hour.
Figures as to wind velocity mean little to one unaccustomed to using
them. A five-mile breeze is just "pleasant." Twelve miles means a brisk
gale. Thirty miles is a high wind: fifty miles a serious storm (these
are the winds the aviator constantly meets): one hundred miles is
perhaps about the maximum hurricane velocity.
As we ascend from the surface of the earth, the wind velocity steadily
increases; and the excess velocity of winter winds over summer winds
is as steadily augmented. Thus, Professor Rotch found the following
variations:
Altitude in Feet Annual Average
Wind Velocity, Feet per Second
656 23.15
1,800 32.10
3,280 35.
8,190 41.
11,440 50.8
17,680 81.7
20,970 89.
31,100 117.5
Average Wind Velocities,
Altitude in Feet Feet per Second
Summer Winter
656 to 3,280 24.55 28.80
3,280 to 9,810 26.85 48.17
9,810 to 16,400 34.65 71.00
16,400 to 22,950 62.60 161.5
22,950 to 29,500 77.00 177.0
[Illustration: DIURNAL TEMPERATURES AT DIFFERENT HEIGHTS]
These results are shown in a more striking way by the chart. At a five
or six mile height, double-barreled hurricanes at speeds exceeding 200
miles per hour are not merely possible; they are part of the regular
order of things, during the winter months.
The winds of the upper air, though vastly more powerful, are far less
irregular than those near the surface: and the directions of prevailing
winds are changed. If 50% of the winds, at a given location on the
surface, are from the southwest, then at as moderate an elevation
as even 1000 feet, the prevailing direction will cease to be from
southwest; it may become from west-southwest; and the proportion of
total winds coming from this direction will not be 50%. These factors
are represented in meteorological papers by what is known as the _wind
rose_. From the samples shown, we may note that 40% of the surface
winds at Mount Weather are from the northwest; while at some elevation
not stated the most prevalent of the winds (22% of the total) are
westerly. The direction of prevalence has changed through one-eighth
of the possible circle, and in a _counter-clockwise_ direction. This
is contrary to the usual variation described by the so-called Broun's
Law, which asserts that as we ascend the direction of prevalence
rotates around the circle like the hands of a watch; being, say, from
northwest at the surface, from north at some elevation, from northeast
at a still higher elevation, and so on. At a great height, the change
in direction may become total: that is, the high altitude winds blow
in the exactly opposite direction to that of the surface winds. In
the temperate regions, most of the high altitude winds are from the
west: in the tropics, the surface winds blow _toward_ the west and
toward the equator; being northeasterly in the northern hemisphere
and southeasterly in the southern: and there are undoubtedly equally
prevalent high-altitude counter-trades.
[Illustration: THE WIND ROSE FOR MOUNT WEATHER, VA.
(From the _Bulletin_ of the Mount Weather Observatory, II, 6)]
The best flying height for an aeroplane over a flat field out in the
country is perhaps quite low--200 or 300 feet: but for cross-country
trips, where hills, rivers, and buildings disturb the air currents, a
much higher elevation is necessary; perhaps 2000 or 3000 feet, but in
no case more than a mile. The same altitude is suitable for dirigible
balloons. At these elevations we have the conditions of reasonable
warmth, dryness, and moderate wind velocities.