The Philosophy of the Weather Part 21

It disappears, whether it be high or low fog, about the time when the barometer attains its morning maximum, or about 10 A.M.

At about that period, when there has been fog, or earlier, when there has not, and sometimes as early as 8 A.M., there is a tendency to trade condensation--cirrus in mid-winter, and a c.u.mulus in mid-summer, and, during the intermediate time, a tendency to cirro-stratus, partaking more or less of the character of one or the other, according to the season.

Temperature, in summer, commences its diurnal elevation about 4 A.M., also, and rises till about 2 P.M. From that time it falls with very little variation till 4 o'clock the next morning. It has but one maximum and one minimum in the twenty-four hours.

As the morning barometric maximum approaches, and the heat increases the magnetic activity, condensation in the trade appears, or induced condensation in the upper portion of the surface atmosphere, that portion near the earth is affected and attracted--and the "wind rises," according to the locality, the season, and the activity of the condensation. The tendency to blow increases with the tendency to trade and c.u.mulus condensation, and continues till toward night, when it gradually dies away, unless there be a storm approaching. As the heat increases, and stimulates magnetism into activity, the magnetic needle commences moving to the west, its regular diurnal variation, and continues to do so until about 2 P.M., when it commences returning to the east, and so continues to return until 10 P.M., when it moves west again until 2 A.M., and from thence to the east, till 8 A.M.

Similar variations also take place in the horizontal force, as evinced by the action of the magnetometer needle, and in the vertical force, as shown by the oscillations. So that it is evident that there are two maxima, and two minima of magnetic activity every day, shown by all the methods by which we measure magnetic action and force--more than double at the acme of northern summer transit over that of winter, and proceeding _pari pa.s.su_, with the other daily phenomena--evincing the same irregular action which the other phenomena evince. Still another phenomenon, which has a daily change, is electric tension, or the increase or decrease in the tension of the positive or true atmospheric electricity.

[Ill.u.s.tration: Fig. 19.]

The following table shows the mean two hourly tensions for three years, at Kew, viz.:

Hours 12 P.M. 2 A.M. 4 A.M. 6 A.M. 8 A.M. 10 A.M.

Number of observations 655 784 804 566 1,047 1,013 Tension 22.6 20.1 20.5 34.2 68.2 88.1

Hours 12 A.M. 2 P.M. 4 P.M. 6 P.M. 8 P.M. 10 P.M.

Number of observations 848 858 878 874 878 1,007 Tension 75.4 71.5 69.1 84.8 102.4 104

From this it will be seen that the tension of electricity is at a minimum at 4 A.M., also, that it rises till 10, falls till 4 P.M., but not as rapidly, rises till 10, falls again till 4 A.M., or the close of the meteorological day--having two maxima and minima, as have most of the phenomena thus far considered.

In order to see what the connections between these ever-present, daily phenomena are, and their connection with other phenomena, and that we may understand their normal conditions, I will trace them approximately in a diagram (figure 17.)

The foregoing diagram of the daily phenomena of a summer's day, when no disturbing causes are in operation, no storm existing within influential distance, and no unusual intensity or irregular action of any of the forces present, affords a basis for considering the various phenomena of the weather in all its changes and conditions.

It is obvious that the other phenomena do not all depend upon temperature merely, if indeed any of them do.

Temperature has but one maximum and minimum, and that is exceedingly regular, and does not correspond with any other.

The barometer has two; electric tension, two; magnetic activity, two; condensation, two--one the formation of cloud, and the other the formation of fog and dew; wind, one--resembling temperature in that respect, but embracing a much less period.

Fog forms at one barometric minimum, and cloud at another.

Fog forms at one period of the magnetic variation, cloud at another.

The formation of cloud corresponds with the greatest intensity of magnetic action, and its a.s.sociate electricities. But the oscillations of the barometer do not correspond with either. And thus, then, we connect them:

CAUSE. EFFECT. EFFECT.

Increase of magnetic Decrease of pressure. Increase of primary or magneto-electric condensation.

activity, as shown Of positive electric by declination and tension. Of wind.

increase of horizontal and Of surface condensation, Of electrical disturbance vertical force. _i. e._, fog and dew. and phenomena in the trade and its vicinity.

This connection is equally obvious if the order is reversed--thus;

CAUSE. EFFECT. EFFECT.

Decrease of magnetic Increase of pressure. Disappearance of primary or magneto-electric condensation.

activity. Of tension of atmospheric electricity. Of wind, and Of surface condensation, Of electric disturbance _i. e._, fog and dew. in the trade and its vicinity.

If we examine still more particularly the different phenomena, we shall find the same relative action of the forces carried into all the atmospheric conditions, however violent.

1. The barometer falls when horizontal magnetic force, and a tendency to cloud and wind, increase; and rises when they decrease. This corresponds with the character of the irregular barometric oscillation. Barometric depressions accompany clouds and winds, and are in proportion to them, and are all greatest where magnetic force is greatest. The barometer also rises as the magnetic energy decreases. Do the magnetic currents, pa.s.sing upward with increased force, lift, elevate the atmosphere? How, then, are we to explain the increased range of the oscillations, as the center of atmospheric machinery is reached, where magnetism has least intensity, and the perpendicular currents are less, and attraction is less? Attraction is greatest where intensity is greatest, and there the barometer stands highest, and the diurnal range is least. Is it then the attraction of magnetism which produces the barometric oscillations? If so, how then can we explain the diurnal fall while magnetism is most active?

Perhaps we have not yet arrived at such a knowledge of the nature of magnetism as is necessary to a correct answer of those questions. Faraday has taught us that the lines of magnetic force are close curves, pa.s.sing into the atmosphere, and over to the opposite hemisphere, and returning through the earth, out on the opposite side in like manner, and back again, pa.s.sing twice through the earth and twice through the atmosphere.

All we know of this is what the iron filings indicate, and we do not know how much reliance to place upon the indications they give. But if Faraday is right, the sun will, twice each day, intersect and stimulate into increased activity the same closed magnetic curve--once when it is coming out of the earth, during our day, when its influence will be the most active, and once when it is returning on the opposite side of the earth; and a second, but feebler magnetic and electric maximum, may be occasioned by its action on the opposite and returning closed curve of the same current. However this may be, it is exceedingly difficult to conceive, of any adequate influence exerted by the tension of vapor.

So the mid-day barometric minimum may be caused by the attraction of the earth, in a state of increased magnetic activity and intensity, upon the counter-trade, and its consequent approach or settling toward the earth.

Observation, as I have already said, pointedly indicates such a state of things. So the increased magnetic activity, with or by its a.s.sociate electricity, acts upon the electricity of the counter-trade, condensation takes place, the electricity is disturbed in the surface-atmosphere, by induction, and its tension is changed. Opposite electrical conditions are induced in the surface strata, and attraction takes place. The air moves easily, and thus the attractions originate the winds. Secondary currents are induced, as in all other cases of electric activity, and winds, in _different strata_ and directions, occur, with or without c.u.mulus, or scud condensation, according to their activity, and the proportion of moisture of evaporation they may contain.

I am well aware that the various received theories of meteorology attribute condensation to the action of cold, mingling of colder strata, etc. But I think that view will have to be abandoned.

It a.s.sumes that moisture is evaporated and held in the atmosphere by latent heat, which is given out during condensation, and actually warms the surrounding atmosphere. Thus, the Kew Committee undertook to explain the development of greater heat, at the elevation where they, in fact, found the counter-trade. But how unphilosophical to suppose a portion of the air or vapor contained in it, can give out to another adjoining portion _more heat than is necessary to produce an equilibrium_. This can, indeed, be done by experiment--_but the experiment is made with currents of electricity_. How unphilosophical, too, to talk of latent heat in connection with evaporation, _at the lowest temperature known_.

Meteorologists must revise their opinions on the subject of condensation.

This latent heat has never been actually met with; on the contrary, the most sudden and complete condensations of the vapor of the atmosphere are attended by as sudden and extraordinary productions of cold, and consequent hail, and the connection between condensation and electricity is shown by too many facts to permit the old theory to stand.

_Fog never forms with the thermometer below 32._ It is mainly a _summer condensation_, especially high fog. It has been attributed to the cooling effect of an atmosphere colder than the earth, but it often occurs when the earth is the coldest, and when the vapor, as it rises, is colder than the air, and could not give out heat to a warmer medium. (See American Journal of Science, vol. xliv. p. 40.) Again, it is not mere condensation, but a formation of globules or vesicles, hollow, and the air expanded in them, by means of which they float like a soap bubble which contains the warm air of the breath. Is not every vesicle a model shower, positively electrified on the outside, negatively in the center, or the reverse, according to the strata, with the air expanded in the middle by the excess of heat which negative electricity detains? Look at them, as they attach themselves to the slender nap of the cloth you wear, when pa.s.sing through them, and see how many of them it would require to form a large drop of rain. The clouds are of a similar vesicular character, and rain does not fall till the vesicles unite to form drops. Sudden and extreme cold is indeed produced in the hail-storm, when, above, below, and around it, the temperature is unaffected. Testu, Wise, and other aeronauts, have so found it, and the hail tells us it is so. But it is idle to say it results from radiation. All the phenomena of the sudden, violent hail-storms are electric in an extraordinary degree. The electricity is disturbed and separated--the a.s.sociated heat continues with the negative, and leaves the positive portion of the cloud, and a corresponding reduction of temperature results. So Ma.s.son found in his eudiometrical a.n.a.lytical experiments the _negative_ wire would heat to fusion, while the positive was cold. (See London, Edinburgh, and Dublin Journal of Science for December, 1853.) This disturbed electricity is diffused over the vesicles.

Listen to the thousand _crackling_ sounds which initiate the clap of thunder, and may be heard when the lightning strikes near you; produced by the gathering of the lightning from as many points of the cloud where it was diffused, to unite in one current and produce the "clap" or "peal"--and to the "pouring" of the rain, which follows the union of the vesicles, after the excess of repelling electricity is discharged.

No _change_ of temperature is observed when fogs form, except the ordinary change between night and day; and it seems perfectly obvious, in looking at all the phenomena, that fogs form at a temperature of 70 or 75, in consequence of the electric influence of the earth upon the adjoining surface-atmosphere; and, when formed, they withstand the most intense action of a summer sun, till the time of day arrives for the barometric and electric tension to fall, condensation to take place in the counter-trade above, and wind to be induced. Who that has noticed the almost blistering force of the solar rays, as they break through a section of high fog, about 10 A.M., can forget them.

Fogs form near the earth, during the night, when the atmosphere above is loaded with moisture many degrees colder, and yet remains free from condensation. On the other hand, during the heat of the day, and of the hottest days, the heavy rains condense above--nay, they frequently fall at a temperature of 75 to 80, in the tropics, and of 50 to 55 in mid-winter here.

Thus far, an adherence to the opinion that condensation was simply a cooling process; the driving out of its latent heat, not merely to another body to make an equilibrium, but "_getting rid of it_" by positive active radiation, or in some other way, so as to cool off and condense, has involved the formation and cla.s.sification of clouds in obscurity. Hopkins (Atmospheric Changes, p. 331) laments this, but fettered by a false and imperfect theory, in relation to the tension of vapor, he falls into a similar error.

Now, there are, as we have seen, peculiar, distinctly-marked varieties of cloud, connected with peculiar and distinctly-marked conditions of the atmosphere, _irrespective of temperature_. None of the theories advanced, account, or profess to account for the differences in either. No modification of the calorific theory will account for them. They differ in shape, in color, in tendency to precipitation, in line of progress, and in electrical character. The explanation of this is found in the fact, that they form in distinct and different strata, partake of the positive electric character of the one, or the negative of the other; or are secondary, induced by the action of a primary condensation in a different stratum. There is not any mingling of the different strata, as has been supposed; and many other facts than those to which we have alluded, show that the formation of cloud is a magneto-electric process.

The observations of Reid show that every violent shower cloud has the electricities disturbed, and portions of it are positive, and others negative. Howard gives us the following _resume_ of Reid's observations:

"From an attentive examination of Reid's observations I have been able to deduce the following general results:

"1. _The positive electricity, common to fair weather, often yields to a negative state before rain._

"2. _In general, the rain that first falls, after a depression of the barometer, is_ NEGATIVE.

"3. _Above forty cases of rain, in one hundred, give negative_ electricity; although the state of the atmosphere is positive, before and afterward.

"4. _Positive rain, in a positive atmosphere, occurs more rarely_: perhaps fifteen times in one hundred.

"5. _Snow and hail, unmixed with rain, are positive, almost without exception._

"6. _Nearly forty cases of rain, in one hundred, affected the apparatus with both kinds_ of electricity; sometimes with an interval, in which no rain fell; and so, that a positive shower was succeeded by a negative; and, _vice versa_; at others, the two kinds alternately took place during the same shower; and, it should seem, _with a s.p.a.ce of non-electric rain between them_."

Howard attributes, with great apparent probability, the successive differences in the electrical character of the rain, to the pa.s.sage of different portions of the cloud, having different polarity, over the place of observation. So _positive hail_, and _negative rain_ fall in _parallel bands_ from the same cloud. Many such instances are on record. It should be remembered that he is describing the phenomena in the showery climate of England.

But the most decisive, perhaps, as well as practically important evidence of the influence of magnetism, or magneto-electricity, in meteorological phenomena, is derived from the action of storms. My observation has been limited, for my life has been, and must be, a practical one. But, subject to future, and I hope speedy corroboration, or correction, by extensive systematic observation, I think I may venture to divide all storms into four kinds:

1. Those which come to us from the tropics, and const.i.tute the cla.s.s investigated by Mr. Redfield. That these are of a magneto-electric character is evident. They originate near the line of magnetic intensity, over, or in the vicinity of, the volcanic islands of the tropics; are largely accompanied by electrical phenomena; extend laterally as they progress north; induce and create a change of temperature in advance of them, and do not abate until they pa.s.s off over the Atlantic to the E. or N. E., and perhaps not until they reach the Arctic circle. Their extensive and continued action is not owing to any mere _mechanical agency_ of the adjoining pa.s.sive air, or other supposed currents, originated, no man can tell how, but they concentrate upon themselves the local magnetic currents as they pa.s.s over and intersect them, and, by their inductive action upon the surface-atmosphere, in different directions, attract it under them, and within their more active influence. Here the action of the magnetic currents is probably the primary cause, but the power of the storm to concentrate upon itself the new magnetic currents which it intersects as it enters each new, successive field, enables them to maintain and extend their action.