either by convection or radiation into the atmosphere from the earth's surface will vary with the nature of that surface; the relative quantities are roughly expressed in a numerical table (XIII).

The relative amounts of heat that reach the earth's surface will depend upon the percentage of cloudiness and the altitude of the sun somewhat as given in a numerical table (XIV).

(III) Resulting density of the air.-By a combination of the numbers given in the preceding tables and charts there may, in a rough way, be indicated by relative numbers written upon the tri-daily maps what it is expected will be the amount of heat and moisture added to any region of the lower atmosphere during the interval between any two weather charts. The effect of this heat and moisture is to forcibly alter the density of the atmosphere, hot or moist air being lighter than cold or dry air, assuming, of course, the barometric pressure to be the same in both cases. These relative densities are shown in the Table XV, for perfectly dry air (relative humidity, 0) and for perfectly saturated air (relative humidity 100).

By means of Table XV one is able finally to construct a map showing relative densities of the lower or surface air over the United States during the next eight or twenty-four hours. After much experience one becomes able to make a rapid general mental summary of these diverse influences without the labor of recording the items upon the daily maps, and this is practically the method followed in daily routine work.

Now, the normal distribution of density is that on which the general movements of the atmosphere depend, and the deviations shown by the above map of densities will give a clue to the new disturbances that will perhaps be initiated during the day. The normal distribution of temperature, pressure, and winds is shown by the monthly and annual maps. (Omitted.) Similar charts should be constructed for both hemispheres when studying international meteorology.

The Table XV, just explained, shows that the relative density of the atmosphere is much more likely to be disturbed by changes of temperature than of moisture; thus at 70° of temperature an increase of temperature by 100 diminishes the density 0.018, while at the same temperature an increase of moisture up to full saturation of the atmosphere diminishes the density 0.008. An abnormal change of temperature is therefore to be carefully looked for as a cause of further disturbance, but after a disturbance is once set up the moisture becomes the most important consideration, since its condensation alters the vertical distribution of temperature.

(IV) Wind, Friction, and Pressure.-The general distribution of density over the earth's surface determines the flow of the denser air of the polar regions toward and under the lighter air of the equator, and Professor Ferrel has shown that from this (and the rotation of the earth on its axis) results the general distribution of pressure prevailing through

out the atmosphere. This distribution is very much affected by the presence of oceans, continents, mountain ranges, and plateaus which determine the irregular distribution of density and the irregular resistances to the winds. Were the coefficient of friction uniform throughout the whole of the earth's surface the distribution of winds and pressure would be much simpler, but as affected by friction it is complicated, as is shown by the isobars on the charts of monthly mean values.

As the elevations throughout the United States must therefore be carefully borne in mind, because of their bearing on the question of their resistances to the motions of the atmosphere, and still more for the thermo-dynamic reasons shown further on, therefore a hypsometric map is provided. On this map may be introduced relative numbers, changing with the seasons, showing the local frictional resistance at a standard altitude, or the relative drag of the air blowing over different surfaces; approximate estimates of these numbers are given in the Table XVI.

The increase of wind velocities at various moderate heights above fields of grass, grain, etc., is given by Stevenson and by Archibald, and may be assumed to be as the square root of the altitude.

Any departure from the normal densities must be followed by a disturbance in the flow of air from the denser toward the lighter.

This disturbed movement of the air produces at once a change in the distribution of barometric pressure, which change becomes greater in proportion to the movement; the observed barometric changes are thus principally dependent upon the wind, and in daily predictions it is convenient to use the barometer as an index of what movements are going on in the atmosphere in the absence of observations of temperature and winds above or beyond the limits of our stations.

The relations between pressure and wind are given in Professor Ferrel's works, as also in those of Oberbeck and Guldberg and Mohn, from which it will be seen that a very slight difference of pressure produced by a very slight difference of density is sufficient to set the atmosphere in motion in the direction of an "initial gradient," the result of which is immediately to produce a vorticose motion and a steeper "barometric gradient" nearly perpendicular to the initial gradient and to the motion of the wind; these steep gradients accompany all storms, and are exemplified in Ferrel's "Movements on the Surface of the Earth," 1858, "Meteorological Researches, part II," 1877, and "Recent Advances,"

1886.

The resistances to the motion of the atmosphere would, however, soon bring it to rest, and the abnormal isobars would soon disappear or relapse into the normal ones, were there not some force at work maintaining the disturbance of density and the abnormal motions. A successful storm prediction must depend upon the accuracy with which one can determine the amount, location, and effects of the force that maintains this disturbance.

(V) The disturbing force.-The disturbing force is recognized as the solar heat, either directly absorbed by the air or evolved by the condensation of aqueous vapor in the atmosphere; the former has already been considered and the latter must now be studied. This involves (1) the amount of aqueous vapor and (2) its condensation.

(1) Amount of vapor.-The normal distribution of vapor in the atmosphere at sea-level is suggested by the hygrometric table (XVII), which in its three sections, a, b, and c, gives the monthly normal values of the mean dew-point, the mean weight, and tension of vapor for a few stations in North America.

Knowing the temperature of the dew-point, one can determine the weight of the vapor contained in any volume of air, and also the tension of the vapor by the use of the accompanying table, XVIII:

The geographical distribution of moisture at the earth's surface is best shown by charts of lines of equal tension or dew-point. A comparison of such hygrometric charts for each of the daily reports shows the presence of regions where moisture is in excess or is deficient, and where a given cooling will produce precipitation.

The normal distribution of moisture in successive strata of the atmosphere is shown approximately in the second column of the following table (XIX), computed by the formula of Hann, but which is based on observations in continental areas, and may not so closely represent the conditions over the oceans:

TABLE XIX.-Normal distribution of aqueous vapor at various altitudes above the earth's

The total amount of moisture present at any moment in a column of saturated air extending from sea-level up to the altitude of 6,000, 12,000, etc., feet is found from the numbers given in the preceding table, and is expressed in the following table, by the depth in inches of the corresponding layer of water that would be formed if all the moisture in such column were to fall to the earth as rain:

Besides the observation of local dew-point, the amount of moisture in the whole layer of air above us can perhaps be directly determined, as per example, by means of the spectroscope or the cyanometer, at each station; but as these observations have not as yet been reduced to a system the Signal Service makes use of what is called a sunset obser

vation, that is to say, the appearance of the sky at sunset is recorded as yellow, red, or green. These sky colors vary with the changing amounts of moisture through which the sun's rays pass. From these appearances an observer can form an estimate of the general hygrometric condition of large tracts of the atmosphere.

(2) Condensation of vapor.-When a mass of air, whether dry or moist, is, on account of its relative lightness, lifted up by the pressure of the surrounding denser atmosphere, or is drawn up by any abnor mal diminution of the pressure above, or is pushed up the incline of a mountain or plateau, it is raised into a region where the barometric pressure is less than in its initial position. Consequently the rising mass must expand to an extent proportional to the diminution of pressure. In this expansion a great amount of both internal and external work is done, corresponding, respectively, with the increased separation of the gaseous molecules and the pushing aside of surrounding air; this work is done at the expense of the internal heat of the rising air, consequently the whole expanding mass of mixed air and vapor grows cooler as it rises.

The cooling process is known as a dynamic cooling, and takes place uniformly and simultaneously throughout the whole rising mass of air. It is a very different process from the much slower processes of cooling by radiation or by convection. For dry air the rate of dynamic cooling is almost the same for all pressures and temperatures, and is approximately 10 C. for every 100 meters of ascent or 1° Fahr. for 180.5 feet; for moist air the rate of diminution of temperature is a little slower, but the above value may also be used for it so long as no moisture is condensed, that is to say, so long as the temperature does not fall below the dew-point.

From the preceding it follows that the elevation above the ground at which cloud or haze begins to be formed depends primarily upon the depression of the dew point of the rising air below its temperature at the time when it starts from the ground.

The third column of Table XXII gives an idea of this relation between altitude and temperature.