Practical Exercises in Elementary Meteorology

CLOUDS AND UPPER AIR CURRENTS

Attentive observation of clouds will soon lead to a familiarity with their common type forms. A series of cloud views,[7 - See Hydrographic Office Cloud Types, Appendix B.] with accompanying descriptive accounts, will teach the names of the clouds and give definiteness to the record. The directions of movement of clouds are determined by means of the nephoscope. Cloud observations should be made at different hours, in different weather conditions, and in different seasons. The following problems are concerned with clouds and upper air currents:—

A.The Typical Cloud Forms and their Changes.—Note carefully the characteristic forms assumed by clouds; their mode of occurrence, whether in single clots, or in groups, in lines, or all over the sky; their changes in form and in mode of occurrence. Classify and summarize your results. Compare the clouds of the warm months with those of the cold months.

Observations have shown that clouds have certain definite characteristic forms which are substantially the same in all parts of the world. This fact makes it possible to give names to the different typical forms, and these names are used by observers the world over. Hence cloud observations, wherever made, are comparable. The first classification of clouds was proposed by Luke Howard, in 1803. The classification at present in use is known as the International Classification, and was adopted by the International Meteorological Congress in 1896.

B.The Prevailing Direction of Cloud Movements.—The use of the nephoscope is necessary in the accurate determination of cloud movements. Study the prevailing directions of movement of the clouds, by means of frequent observations with the nephoscope, in different weather conditions. Separate the upper and lower clouds in this study. Summarize your results according to the weather conditions and the kinds of clouds.

C.Correlation of Cloud Form and Movement with Surface Winds, with Cyclones and Anticyclones, and with Weather Changes.—The results obtained in the working out of the two preceding problems may be used in the present problem. Tabulate your observations of cloud forms with reference to the wind directions which prevailed at the time of making the observations. Do the same with the directions of cloud movement. Determine the relation between surface winds and cloud types, and between surface winds and the direction of the upper air currents, as shown by the movements of the upper clouds. Study the control exercised by cyclones and anticyclones over cloud forms and over the direction of the upper air currents.

D.The Use of Clouds as Weather Prognostics.—Attentive observation of the forms and changes of clouds, and of the accompanying and following weather changes, will lead to the association of certain clouds with certain coming weather conditions. Make your cloud observations carefully, taking full notes at the time of observation. Give special attention to the weather conditions that follow. Continue this investigation through as long a period as possible, until you have gathered a considerable body of fact to serve as a basis, and then frame a set of simple rules for forecasting fair or stormy weather on the basis of the forms and changes of the clouds. Such local observations as these may be employed as a help in making forecasts from the daily weather maps.

Clouds were used as weather prognostics long before meteorological observations and weather maps were thought of. To-day sailors and farmers still look to the clouds to give them warning of approaching storms. Many of our common weather proverbs are based on the use of clouds as weather prognostics.

CHAPTER XXIV.

PRECIPITATION

The special study of various problems connected with precipitation involves detailed observations of the amount and rate of precipitation of various kinds, measured by the rain gauge during storms in different seasons. These observations of precipitation should, of course, be supplemented by the usual record of the other weather elements. The following problems are suggested:—

A. The relation of precipitation in general to the other weather elements, and to cyclones and anticyclones.

B. The conditions under which special forms of precipitation (rain, snow, sleet, hail, frozen rain) occur.

C. The conditions associated with light and heavy, brief and prolonged, local and general rainfall.

These problems are studied by means of a careful comparison of full weather records with the daily weather maps during a considerable period of time.

Rain is the most common form of precipitation the world over, although snow falls over large portions of both hemispheres. In the Arctic and Antarctic zones almost all the precipitation, which is small in amount, comes in the form of snow. In southern Europe snow falls at sea level during the winter as far south as 36° north latitude on the average. In eastern Asia snow occasionally falls as far south as 23° north latitude. The mean annual rainfall varies greatly in different parts of the world. In desert regions it is practically nothing. At Cherrapunjee, in India, it reaches 493 inches, or over 40 feet. A fall of 40.8 inches in a single day occurred at this station on June 14, 1876. In the United States, Upper Mattole, Cal., had an extraordinary monthly rainfall of 41.63 inches in January, 1888. An excessive daily rainfall of 8 inches occurred at Syracuse, N. Y., on June 8, 1876. At Washington, D. C., 2.34 inches fell in 37 minutes on June 27, 1881. A sudden and very heavy fall of rain occurred at Palmetto, Nevada, in August, 1890. A rain gauge which was not exposed to the full intensity of the storm caught 8.80 inches of water in one hour. In August, 1891, an observer at Campo, Cal., measured 11.5 inches as the rainfall in one hour from one very heavy downpour, and from a portion of a second storm.

CHAPTER XXV.

PRESSURE

The variations of atmospheric pressure, although insensible to non-instrumental observation, are so intimately connected with atmospheric processes that they deserve careful attention. Their observation leads to several problems.

A.The Decrease of Pressure with Height, as between Valley and Hill, or between the Base and Top of a Building.—Make these observations with the mercurial barometer, if possible. Note the air temperatures at the two levels at which the barometer readings are made. Determine the heights of hill or building by means of the following rule: Multiply by 9 the difference in barometrical readings at the two stations, given in hundredths of an inch, and the result will be approximately the difference in height between the stations in feet. A more accurate result may be reached by means of the following rule: The difference of level in feet is equal to the difference of the pressures in inches divided by their sum, and multiplied by the number 55,761, when the mean of the air temperatures of the two places is 60°. If the mean temperature is above 60°, the multiplier must be increased by 117 for every degree by which the mean exceeds 60°; if less than 60°, the multiplier must be decreased in the same way. For example, if the lower station has a pressure of 30.00 inches and a temperature of 62°, and the upper station has 29.00 inches and 58° respectively, the difference of level between the two will be

(30.00 – 29.00) / (30.00 + 29.00) × 55,761 = 945 feet

If the lower values are 30.15 inches and 65°, while the upper values are 28.67 inches and 59°, then the formula becomes

(30.15 – 28.67) / (30.15 + 28.67) × [55,761 + (2 × 117)] = 1409 feet

The determination of heights by means of the barometer depends upon the fact that the rate of decrease of pressure upwards is known. As the weight of a column of air of a given height varies with the temperature of the air, it is necessary, in accurate work of this sort, to know the air temperatures at both the lower and upper stations at the time of observation. From these temperatures the mean temperature of the air column between the two stations may be determined. Tables have been published which facilitate the reductions in this work. The heights of mountains are usually determined, in the first instance, by means of barometric observations, carried out by scientific expeditions or by travelers that have been able to reach their summits. More accurate measurements are later made, when possible, by means of trigonometrical methods.

B.The Diurnal and Cyclonic Variation of Pressure in Different Seasons.—This problem is satisfactorily solved only by a study of the curves traced by the barograph, or by plotting, as a curve, hourly or half-hourly readings of the mercurial barometer. The diurnal variation of the barometer is the name given to a slight double oscillation of pressure, with two maxima and two minima occurring during the 24 hours. This oscillation is in some way, not yet understood, connected with the diurnal variation in temperature. It is most marked in the tropics and diminishes towards the poles. Fig. 15 illustrates, in the May curve, the diurnal variation of the barometer at Cambridge, Mass., during a spell of fair spring weather, May 18-22, 1887. The maxima are marked by + and the minima by 0. The cyclonic variation of pressure is the name given to those irregular changes in pressure which are caused by the passage of cyclones and anticyclones. The second curve in Fig. 15 shows the cyclonic variations in pressure recorded by the barograph at Cambridge, Mass., during a spell of stormy weather, Feb. 23-28, 1887. These curves serve as good illustrations of these two kinds of pressure variations.

Study your barograph tracings, or your barometer readings, as illustrating diurnal or cyclonic variations of pressure. Note the character and the amount of the diurnal and cyclonic variations, and their dependence on seasons.

Over the greater part of the Torrid Zone the diurnal variation of the barometer is remarkably distinct and regular. Humboldt first called attention to the fact that in those latitudes the time of day may be told within about 15 minutes if the height of the barometer is known.

C.The Relation of Local Pressure Changes to Cyclones and Anticyclones, and thus to Weather Changes.—Make a detailed study of the relation of the local pressure changes at your station, as shown by the barograph curves, or by frequent readings of the mercurial or aneroid barometers, to the passage of cyclones and anticyclones, and to their accompanying weather changes. Classify the simple types of pressure change, so far as possible, together with the general weather conditions that usually accompany these types. Apply the knowledge of local weather changes thus gained when you make your forecast on the basis of the daily weather maps.

CHAPTER XXVI.

METEOROLOGICAL TABLES

The tables which follow are those which are now in use by the United States Weather Bureau. They were first published in the Instructions for Voluntary Observers issued in 1892, and were reprinted in 1897. The following instructions will be found of service in the use of the tables:—

Table I.—Dew-Point

The figures in heavy type, arranged in vertical columns at each side of the page, are the air temperatures in degrees Fahrenheit, as recorded by the dry-bulb thermometer. The figures in heavy type, running across the page, denote the differences, in degrees and tenths of degrees, between the dry-bulb and wet-bulb readings, or, technically, the depression of the wet-bulb thermometer. The figures in the vertical columns denote the dew-points. Make your observation of the wet and dry-bulb thermometers and note the difference between the two readings. Find, in the vertical columns of heavy type, the temperature corresponding to your dry-bulb reading, or the nearest temperature to that. Then look along the horizontal lines of figures in heavy type for the figure which corresponds exactly, or most nearly, with the difference between your wet and dry-bulb readings. Look down the vertical column under this latter figure until you reach the horizontal line corresponding to your dry-bulb reading. At this point the figures in the vertical column give the dew-point of the air at the time of your observation.

Example: Air Temperature (dry bulb), 47°; Wet Bulb, 44°; Difference, 3°. On page 148 (#x9_x_9_i3) will be found the table containing both 47° (dry bulb) and 3° (depression of the dew-point). In the twenty-eighth line of this table and in the seventh column will be found the dew-point, viz., 41°.

Example: Air Temperature, 61.5°; Wet Bulb, 55.5°; Difference, 6°.

In this case 61.5° is not found in the vertical columns of dry-bulb readings, but 61° and 62° are found. The dew-point, with a difference between wet and dry-bulb readings of 6°, for an air temperature of 61°, is 50°; for an air temperature of 62°, it is 52°. Evidently, then, for an air temperature of 61.5° the dew-point will be 51°, i.e., halfway between 50° and 52°. This method of determining dew-points at air temperatures or with depressions of the wet-bulb thermometer which are not given exactly in the tables, is known as interpolation.

Example: Air Temperature, 93°; Wet Bulb, 90.5°; Difference, 2.5°. Our table gives no dew-points for wet-bulb depressions of 2.5°, with air temperature 93°, but we find (on page 152 (#x10_x_10_i6)) that for air temperature 93° and depression of wet bulb of 2°, the dew-point is 91°, while for a wet-bulb depression of 3°, the dew-point is 89°. By the method of interpolation we can readily determine the dew-point in the special case under consideration as 90°, i.e., halfway between 89° and 91°.

Table II.—Relative Humidity

The general plan of this table is the same as that of Table I. The figures in the vertical columns are the relative humidities (in percentages) corresponding to the different readings of the wet and dry-bulb thermometers.

Table III.—Reduction of Barometer to 32°

The figures in heavy type, arranged in vertical columns at the left of the page, refer to the temperature in degrees Fahrenheit, as indicated by the attached thermometer. The figures in heavy type, running across the top of the page, are the barometer readings in inches and tenths. Make a reading of the attached thermometer and of the barometer. Find in the vertical column the temperature corresponding to the reading of the attached thermometer, and in the horizontal line of heavy figures the reading corresponding to the height of the barometer. The decimal in the vertical column, under the appropriate barometer reading, and in the same horizontal line with the appropriate thermometer reading, is to be subtracted from the height of the barometer as observed, thus correcting the reading to freezing. When the attached thermometer reads below 28°, the correction is additive.

Example: Attached Thermometer, 69°; Barometer, 30.00 inches; Correction, -.110; Corrected reading, 29.890 inches.

Example: Attached Thermometer, 73°; Barometer, 29.75 inches; Correction = ?

We do not find any column corresponding to a barometer reading of 29.75 inches. We do find, however, that with a barometer reading of 29.50, and an attached thermometer reading of 73, the correction is -.118 inch, and with a barometer reading of 30.00, the correction is -.120. By interpolating, as in the case of the humidity table above, we find the correction for a barometer reading of 29.75 inches, and an attached thermometer reading of 73°. The correction is -.119, and the corrected reading is 29.75 – .119 = 29.63 inches.

Table IV.—Reduction of Barometer to Sea Level

The figures in heavy type, in the left-hand vertical columns, are the heights, in feet, of the barometer above sea level. The figures in heavy type at the top of the columns, running across the page, are the readings of the ordinary thermometer. The numbers of inches and hundredths of inches to be subtracted from the barometer reading (corrected for temperature by Table III), for the different heights above sea level, are given in the vertical columns.

The altitude above sea level of the city or town at which the observation is made should be ascertained as accurately as possible from some recognized authority, as, e.g., from a railroad survey; from Government measurements, or from some engineer’s office. The correction to be made is determined by a simple inspection of the table or by the method of interpolation.

Example: Altitude of Barometer above sea level, 840 feet; Temperature of the air, 40°; Correction, +.931 inch.

Example: Altitude of Barometer above sea level, 205 feet; Temperature of the air, 45°; Correction = ?

Here 205 feet and 45° are neither of them found in the table. Hence a double interpolation is necessary. For 200 feet and 40° the correction is +.224 inch. For 200 feet and 50° the correction is +.220 inch. Hence for 200 feet and 45° the correction is +.222 inch. For 210 feet and 40° the correction is +.235 inch. For 210 feet and 50° the correction is +.231 inch. Hence for 210 feet and 45° the correction is +.233 inch. Now for 205 feet we should have a correction midway between +.235 inch and +.233 inch or +.234 inch.