Before proceeding to an examination of this connected atmospheric
machinery, and an investigation of the particular ocean from which our
rivers return, it may be well to look at the form in which they appear to
return, that we may have a clear understanding of terms.
They seem to return in the form of clouds, and in storms and showers,
although, in truth, they return in regular, uniform, ordinarily invisible
currents, and the storms and showers are but condensations in, and
discharges from portions of those currents, aided by the local moisture of
evaporation.
The term _storms_, seems to be used by European meteorologists to denote
what we term thunder showers or gusts, and tornados; while what we call
storms are denominated by them regular rains. As the terms are extensively
in use in this country, we must adhere to the meaning attached to them
_here_ rather than _there_.
Storms with us, then, are regular rains of from six to forty-eight or more
hours' continuance: generally without lightning, or thunder, or gusts, and
usually with wind of more or less force, from some easterly point. They
are called north-east storms, or south-east storms, according to the
point from which the surface winds blow. Practically we shall find that
this distinction is of some importance, for the north-east storms are the
longest, lasting generally twenty-four hours, or more, while the
south-east ones seldom, if ever, continue as long.
These storms extend over a considerable surface, rarely less than one
hundred miles in one direction or another, and sometimes fifteen hundred,
or more. Distinct showers cover but a small surface, sometimes not more
than forty to one hundred rods, as in the tornado, and rarely more than
ten miles. Belts of showers, each new one forming a little more to the
south, often, in summer, pass across the country, following each other in
succession; and these belts may be of considerable width, say thirty to
one hundred and fifty miles.
The clouds which constitute the storms and showers differ in appearance
and character, as well in the active as in the forming state. Clouds are
of distinct characters, alike, substantially, every where under like
circumstances; and a distinct nomenclature has been applied to them by Dr.
Howard, of London. He notes three kinds of primary clouds: _viz._, cirrus,
stratus, and cumulus; and inasmuch as the boundary line between them is
not very distinct, certain compounds of the three, _viz._: cirro-stratus,
cirro-cumulus, and cumulo-stratus. This nomenclature is every where
received, and portions of it are of great practical importance.
The three principal descriptions of cloud, _viz._: the cirrus, the
stratus, and the cumulus, we have very much as they have in Europe, and
doubtless as they exist every where outside of the tropics. The nimbus,
another cloud described by him, is not distinct from the cumulus or
stratus. An isolated, limited thunder-shower in a clear sky, presents the
appearance of a nimbus, as shown in the cuts, but the basis of it is a
cumulus, and it differs from an ordinary fair-weather cumulus merely in
the dark and fringe-like appearance of the rain as it is falling from its
lower surface, and sometimes in the existence of a stratus above and in
connection with it. A similar form is often assumed by the peculiar clouds
of the N. W. winds in March or November, when they assume the form of
_squalls_, and drop flurries of snow. The nimbus, therefore, is not a
distinct cloud, but an appearance which the cumulus, stratus, or
cirro-stratus has in a stormy or showery state, and does not deserve a
distinct name. It is but a cumulus, or a stratus, or cirro-stratus
dissolving in snow or rain. It is important that this term should be
abandoned. It tends to confuse and prevent a clear understanding of the
difference in the character of the clouds, and in relation to which
precision is both difficult and desirable.
The figures on pages 27 and 29, show the different kinds of clouds as
designated by Howard. They are copied from the engravings in the sixth
edition of Maury's "Sailing Directions."
[Illustration: Fig. 5.]
[Illustration: Fig. 6.]
Figure 5.
The cirrus is indicated by 1 bird.
The cirro-cumulus by 2 "
The cirro-stratus by 3 "
The cumulo-stratus by 4 "
Figure 6.
The cirrus by 1 "
The cumulus by 3 "
The stratus by 2 "
The nimbus by 4 "
How far these representations correspond with the actual appearance of the
different compound forms in England, I can not say. But although they
convey a _general_ idea, _they are not sufficiently accurate for practical
illustration or observation here_. Indeed Howard himself has omitted from
his last edition his plate of the clouds, assigning as a reason, "that the
real student will acquire his knowledge in a more solid manner by the
observation of nature, without the aid of drawings, and that the _more
superficial are liable to be led into error by them_." The collection of
forms in the cuts _does not contain some very important ones_, and
contains some which are not distinct forms; but they may aid us somewhat
in this inquiry, and, therefore, I have copied them. It is well, also, for
the reader to have the generally received description before him.
But for the purpose of _practical_ illustration hereafter, and greater
precision, I shall follow a somewhat different order in describing them,
and introduce two forms of _scud_ quite as important, practically, as any
other.
First, then, commencing at the earth, we have what may be properly termed
_fog_, or low fog. This forms, in still clear weather, in the valleys, and
over the surface of the rivers and other bodies of water, during the
night, and most frequently the latter part of it, and is at its acme at
sunrise, or soon after, limiting vision horizontally and perpendicularly,
and dissolving away during the forenoon. It is rarely more than from two
to four hundred feet in height at its upper surface, and often much less,
and is composed of vesicular condensed vapor, sometimes sufficiently dense
to fall in mist, and is doubtless in composition substantially what the
clouds are in the other strata of the atmosphere, as observed by us, or
passed through by aeronauts. I have never seen it carried up to any
considerable height into the other strata by any of the supposed ascending
currents, to form permanent clouds, and shall have occasion to allude to
the fact in another connection. It disappears usually before mid-day, and
has, when thus formed, no connection with any clouds which furnish rain.
To this Dr. Howard originally gave the name of stratus, and so it is
represented upon the cut; but the latter term may be with greater
propriety applied to the smooth uniform cloud in the superior strata from
which the rain or snow is known to fall, and I shall retain and so apply
it.
The next in order, ascending, is high fog. This is usually from one to
two thousand feet in height at its lower surface. It forms, like low fog,
during the night and in still weather; and is rarely, if ever, connected
with clouds which furnish rain. It breaks away and disappears between ten
and twelve in the forenoon, usually passing off to the eastward. This fog
is most commonly seen in summer and autumn, particularly the latter, and
unless distinguished from cloud will deceive the weather-watcher. It is
readily distinguishable. Although often very dense, obscuring the light of
the sun as perfectly as the clouds of a north-east storm, it differs from
them. It forms in still clear weather, is present only in the morning, is
perfectly uniform, and, before its dissolution commences, without breaks,
or light and shade, or apparent motion, and unaccompanied by scud or
surface wind. The storm clouds are never entirely uniform, or without
spots of light and shade, by which their nature can be discerned, and
rarely, when as dense as high fog, without scud running under them and
surface winds.
There is another fog still, connected with rain storms, but it does not
often precede them; occurring at all seasons, but most commonly in
connection with the warm S. E. thaws and rains of winter and spring; and
which usually comes on _after_ the rain has commenced and continued for
awhile, and the easterly wind has abated; occupying probably the entire
space from the earth to the inferior surface of the rain clouds or
stratus. Practically this does not require any further notice. It is an
_incident_ of the storm. When formed it remains while the storm clouds
remain, and passes off with them. It is sometimes exceedingly dense in
February and March, when it accompanies a thaw, and if there is a
considerable depth of snow, it has the credit of aiding essentially in its
dissolution.
Mingled with the smoke of London, it produced there the memorable _dark
day_ of the 24th of February, 1832, and at various other times has
produced others of like character. (See Howard's Climate of London, vol.
iii. pp. 36, 207, 303.) These fogs have been so dense there that every
kind of locomotion was dangerous, even _with lanterns, at mid-day_.
The next in order, ascending, are the storm scud, which float in the
north-east or easterly, south-east or southerly wind, before and during
storms.
These, as the reader will hereafter see, are, _practically_, very
important forms of cloud condensation--although they have found no place
in any practical or scientific description given of the clouds, and are
not upon the cuts. They are patches of foggy seeming clouds of all sizes,
more or less connected together by thin portions of similar condensation,
often passing to the westward, south-westward, north-westward, or
northward with great rapidity. Their average height is about half a mile,
but they often run much lower. They are usually of an "ashy gray" color.
The annexed cut shows one phase of them, from among many taken by
daguerreotype. The arrows pointing to the west show the scud distinguished
from the smooth partially formed stratus above. This view was taken a few
hours prior to the setting in of a heavy S. E. rain storm. It is a
northerly view.
[Illustration: Fig. 7.]
At about the same height, but in a _different state of the atmosphere_,
float the peculiar fair-weather clouds of the N. W. wind. They usually
form in a clear sky, and pass with considerable rapidity to the S. E.
Sometimes they are quite large, approaching the cumulus in form, and
white, with dark under surfaces, and at others, in the month of November
particularly, are entirely dark, and assume the character of squalls and
drop flurries of snow; and then resemble the nimbus of Howard. They assume
at different times and in different seasons, different shapes like those
of the scud, the cumulus, or the stratus.
[Illustration: Fig. 8.]
They form and float in the peculiar N. W. current which is usually a
fair-weather wind, and are never connected with storms. In mild weather
they are usually white, and in cold weather sometimes very black, and at
all times differ _in color_ from the ashy gray scud of the storm. This
variety is not represented upon the general cuts. The annexed diagram
shows one phase of them, but they are readily observable at all seasons of
the year, when the N. W. wind is prevailing; differing in appearance
according to the season. Let these, as well as the storm scud, be
carefully observed and studied by the reader, and let no opportunity to
familiarize himself with their appearance be lost. A brief glance at
each recurrence of easterly or north-westerly wind will suffice.
[Illustration: SUMMER CUMULI.]
The _cumuli_ appear in isolated clouds of every size, or in vast clouds
composed of aggregated masses, as the peculiar cloud of the thunder
shower. They form as low down as the scud or fair-weather cloud of the N.
W. wind, which, for convenience, I will call N. W. _scud_; and often in
violent showers, and particularly in hail storms, extend up as far as the
density of the atmosphere will permit them to form. Professor Espy thinks
he has measured their tops at an altitude of ten miles. Others have
estimated their height, when most largely developed, at twelve miles; but
it is very doubtful whether the atmosphere can contain the moisture
necessary to form so dense a cloud at that elevation. It is their immense
height, however, whether it be six, or eight, or ten miles, together with
the sudden and violent electric action, condensing suddenly all the
moisture contained in the atmosphere within the space occupied by the
cloud, which produces such sudden and heavy falls of rain or hail. As the
rain drops or hail, when formed at such an elevation, in falling through
the partially condensed vapor of the cloud must necessarily enlarge by
accretion from the particles with which they come in contact, and probably
also by attraction, their size when they reach the earth, though
frequently very considerable, is not a matter of astonishment. The cumulus
is represented in the general plate with sufficient accuracy to show its
peculiar character.
In summer, when the air is calm, the weather warm, and no storm is
approaching, there is always, in the day time, a tendency to the formation
of cumuli. This tendency exhibits itself about ten o'clock in the
forenoon, and they gradually form and enlarge until about two in the
afternoon; and after that, if they do not continue to enlarge and form
showers, they melt away and disappear before nightfall. Sometimes in July
and August the atmosphere will be studded with them at mid-day, floating
about three-quarters of a mile from the earth (in a level country), gently
and slowly away to the eastward. At times it may seem as if they must
coalesce and form showers, yet they frequently do not, but gradually melt
away, as before stated.
The cumulus is the principal cloud of the tropics, and is not often seen
with us except in summer, or when our weather is tropical in character.
The engraving on the preceding page, shows a phase of these fair-weather
summer cumuli.
The last in order occupying (with their compounds) the higher portions of
the atmosphere, are the cirrus and stratus. The cirrus is often the
skeleton of the other, and precedes it in formation.
These are the proper clouds of the storm, in our sense of the term. While,
however, the cirrus remains a cirrus, it furnishes no rain. When it
extends and expands, and its threads widen and coalesce into cirro-stratus
and stratus, or it induces a layer of stratus below it, the rain forms.
The following is Dr. Howard's description of cirrus: "Parallel, flexuous
or diverging fibers, extensible by increase in any or in all directions.
Clouds in this modification appear to have the least density, the greatest
elevation, and the greatest variety of extent and direction. They are the
earliest appearance after serene weather. They are first indicated by a
few threads penciled, as it were, on the sky. These increase in length,
and new ones are in the mean time added to them. Often the first-formed
threads serve as stems to support numerous branches, which in their turn,
give rise to others."
The illustrations in the general cut are imperfect, and do not represent
the delicate fibers of the cloud, for it is a difficult cloud to
daguerreotype or engrave, but the representation is sufficiently accurate
to give the reader a general idea of the different varieties, and enable
him to discover them readily by observation. They are the most elevated
forms, always of a light color, and often illuminated about sunset by the
rays of the sun shining upon their inferior surface; the sun, however,
often illuminates, in like manner, the dense forms of cirro-stratus, and
the latter, from their greater density, are susceptible of a brighter and
more vivid illumination.
The stratus is a smooth, uniform cloud--the true rain cloud of the storm;
often forming without much cirrus above, or connected with it. It may be
seen in its partially formed state in the bank in the west, at nightfall,
or in the circle around the moon in the night. When it becomes
sufficiently condensed, rain always falls from it, but in moderation. If
there be large masses of scud running beneath it for its drops to fall
through (especially as is sometimes the case, in two or more currents),
the rain may be very heavy. But more of this hereafter.
[Illustration: Fig. 10.]
The annexed cut shows the forming stratus, light and thin, passing to the
east, as indicated by the short arrows just before a storm, while the scud
beneath is running to the west.
It was copied from a daguerreotype view, facing northwardly.
Intermediate between the fibrous, tufted, cirrus, and the smooth uniform
stratus, there is a variety of forms partaking more or less of the
character of one or the other, and termed _cirro-stratus_. No single
correct representation of cirro-stratus as a distinct cloud, can be
given--but several varieties will be hereafter alluded to, under the head
of prognostics. Several modifications are represented with tolerable
accuracy upon the cuts.
The cirro-cumulus is a collection in patches of very small distinct heaps
of white clouds; they are called fleecy clouds, from their resemblance to
a collection of fleeces of wool, and are imperfectly represented on the
general cut. They do not appear often, and are usually _fair-weather
clouds_.
This form has none of the characteristics of the cumulus, and does not
appear in the same stratum. It was probably called cumulus because its
small masses are distinct, as are those of the ordinary cumulus. It occurs
in the same stratum as cirro-stratus, and properly belongs to that
modification. I retain the name inasmuch as the cloud is of some practical
importance.
The cumulo-stratus is seldom seen in our climate, as it is represented in
the cut. Stratus condensation _above_, and in connection with cumulus
condensation, is not uncommon, but that precise form is rare.
This, too, is practically of no consequence, and I shall take no further
notice of it.
Recapitulating, I give (in a tabular form) the three principal strata and
their modifications, located with sufficient accuracy for illustration.
The clouds which are found in an upper or lower portion of a stratum are
so represented by the location of their names; those which appear at all
heights in the stratum, with the names across. The elevation is the
average one--although there is no limit to the cirrus above, except the
absence of sufficient moisture. It was seen by Guy Lussac, and has been by
other aeronauts, at an elevation of five miles, or more, when too delicate
to be visible below.
+------------------------------------------------+
| | 3 miles.
|Cirrus. |
| Cirro-cumulus. |
| Cirro-stratus. |
| |
Primary | { Cumulus extending up |
stratum. | { in violent showers. |
| |
| |
|Stratus. |
|------------------------------------------------|
| | 1-1/2 miles.
Scud & |N. W. scud. { Cumulus Storm scud. |
cumulus |Fair-weather { ordinarily and |
stratum. | { its base always. |
| |
|------------------------------------------------|
| | 1/2 mile.
Fog |High fog. Storm fog. |
stratum. | |
| Low fog at the surface of the earth. |
| |
+------------------------------------------------+
With the assistance of this table of elevations, and a careful
observation, the reader can soon become familiar with the forms of clouds
and their relative situations.