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 classification 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 résumé 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.