Practical Exercises in Elementary Meteorology Part 11

Study the vertical distribution of temperature in the lower air under various conditions of weather and season; at various hours of the day, and with varying conditions of surface cover. Make your observations systematically, at regular hours, so that the results may be comparable.

Group together observations made under similar conditions of weather, season, time, and surface cover. Determine the average vertical distribution of temperature in the different cases. Note especially any seeming peculiarities or irregularities in this distribution at certain times. Study carefully, as in the previous problems, the relation of the different types of temperature distribution in the atmosphere to the weather conditions as shown on the daily weather maps.

Observations made in different parts of the world, on mountains and in balloons, have shown that on the average the temperature decreases from the earth's surface upwards at the rate of about 1 in 300 feet of ascent. The rate of vertical decrease of temperature is known in meteorology as the _vertical temperature gradient_. When it happens that there is for a time an _increase in temperature upwards_ from the earth's surface, the condition is known as an _inversion of temperature_.

As a result of the decrease of temperature with increasing alt.i.tude above sea level, the tops of many high mountains even in the Torrid Zone are always covered with snow, while no snow can ever fall at their bases, owing to the high temperatures which prevail there. Balloons sent up without aeronauts, but with self-recording instruments, have given us temperatures of -90 at a height of 10 miles above the earth's surface. On Dec. 4, 1896, Berson reached a height of 30,000 feet and noted a temperature of -52. Inversions of temperature are quite common, especially during the clear cold spells of winter. Under such conditions the tops and sides of hills and mountains are often much warmer than the valley bottoms at their bases. A good example of an inversion of temperature occurred in New Hampshire on Dec. 27, 1884. The pressure was above the normal, the sky clear and the wind light. The observer on the summit of Mt. Washington reported a temperature of +16 on the morning of that day, while the thermometers on the neighboring lowlands gave readings of from -10 to -24. In Switzerland, the villages and cottages are generally built on the mountain sides and not down in the valley bottoms, experience having taught the natives that the greatest cold is found at the lower levels.

CHAPTER XXI.

WINDS.

The determination of the direction of the wind (by means of the wind vane) and of its velocity (by means of the anemometer, or by estimating its strength) at different hours, under different conditions of weather and in different seasons, leads to a number of problems. The following simple investigations may readily be undertaken in schools:--

_A._ =The Diurnal Variation in Wind Velocity in Fair Weather.=--Observe and record the velocity of the wind (either estimated or registered by the anemometer) every hour, or as often as possible, on clear or fair days in different months. Can you discover any regular change in the velocities during the day? If so, what is the change? Does the season seem to have any control over the results obtained? Examine the daily weather maps in connection with your observations and determine the effect that different weather conditions have upon the diurnal variation in wind velocity.

The diurnal variation in wind velocity over the open ocean is so slight as hardly to be noticeable. Over the land, the daytime winds are commonly strongest in arid regions. Traveling across the desert often becomes extremely disagreeable, owing to the clouds of dust which these winds sweep up from the surface.

_B._ =The Variations in Direction and Velocity due to Cyclones and Anticyclones.=--Record the direction and velocity of the wind at your station at frequent intervals during the pa.s.sage of a considerable number of cyclones and anticyclones. Enter your observations in some form of table so that they may be readily examined. (See p. 113.) Note the character of the changes that occur, cla.s.sifying them into types, so far as possible. Study the control of wind directions and velocities by the special features of the individual cyclones and anticyclones as shown on the daily weather maps. How are the different types of change in direction and velocity affected by the tracks of cyclones and anticyclones? By their velocity of progression? By the arrangement of isobars around them? By the height of the barometer at the center? By the season in which the cyclones and anticyclones occur?

Frequent changes in the direction and velocity of our winds are one great characteristic of the Temperate Zones, especially in winter. The continuous procession of cyclones and anticyclones across the United States involves continuous shifts of wind. Over much of the earth's surface, however, the regularity and constancy of the winds are the distinguishing feature of the climate. Over a considerable part of the belts blown over by the northeast and southeast trades, roughly between lat.i.tude 30 N. and S. and the equator, the winds keep very nearly the same direction and the same velocity day after day and month after month.

Thus the trades are of great benefit to commerce. Sailing ships may travel for days in the trade wind belts without having their sails shifted at all, with a fair wind all the time carrying them rapidly on to their destination.

_C._ =The Occurrence and Characteristics of Local Winds, such as Mountain and Valley and Land and Sea Breezes.=--If the observer happens to be living in or near the mouth of a valley or on a mountain side, opportunity may be given for the observation of the local winds down the mountain sides and down the valley at night, and up the valley and the mountain sides by day, known as mountain and valley breezes. Keep a record of wind direction and velocity during the day, and especially during the morning and evening hours. Notice any marked changes in direction, and the relation of these changes to the time of day. Does the velocity of the daytime up-cast breeze show any systematic variation during the day? Study the relation of mountain and valley breezes to the general weather conditions shown on the weather maps. How are these breezes affected by season? By the presence of a cyclone over the region? Of an anticyclone?

By the state of the sky?

If near the seacoast (_i.e._, within 10 or 15 miles), an interesting study may be made of local land and sea breezes. The sea breeze is a wind from the ocean onsh.o.r.e, while the land breeze blows offsh.o.r.e. These breezes occur only in the warmer months. Take frequent observations during the day, as in the case of mountain and valley winds, noting especially any changes in direction and velocity, and the relation of these changes to the time of day. Study also the control exercised by the prevailing weather conditions over the occurrence and the strength of development of the land and sea breezes.

This problem may be considerably extended by adding temperature observations to the simpler record of wind direction and velocity.

In some of the Swiss valleys the mountain and valley breezes are such regular daily weather phenomena that it has become a weather proverb that a failure of the daily change in wind direction indicates a change of weather. Special names are often given to these breezes where they are well marked. In a part of the Tyrol sailing boats go up the lakes by day with the valley breeze, and sail back at night with the mountain breeze.

It is therefore unnecessary for the boats to be rowed either way. Land and sea breezes, although an unimportant climatic feature in these northern lat.i.tudes, are often of the highest importance in the Torrid Zone. The fresh pure sea breeze from over the ocean makes it possible for Europeans to live in many tropical climates where otherwise they would not keep their health. The land breeze, on the other hand, is apt to be an unhealthy wind in the tropics, especially when it blows off of swampy land.

CHAPTER XXII.

HUMIDITY, DEW, AND FROST.

The humidity of the air, as determined by the wet and dry-bulb thermometers or the sling psychrometer, and the occurrence or absence of dew or frost, should be studied together. Observations should be made at different hours, in different kinds of weather, and in different seasons.

From such observations the following problems may be solved:--

_A._ =Diurnal Variation of Relative Humidity under Different Conditions.=--Readings of the wet and dry-bulb thermometers in the instrument shelter, or of the sling psychrometer, several times during the day, will furnish data for determining the diurnal variation of relative humidity. Cla.s.sify your observations according to the weather conditions under which they were made, and by months or seasons. Summarize the results of your investigation, paying special attention to the relation between the diurnal variation of relative humidity and the temperature.

The variations of relative humidity are generally the reverse of those of absolute humidity. In the case of the latter the average diurnal variations are small. The fluctuations in the relative humidity during the day on the northwestern coast of Europe amount to about 7% in December and 17% in August, while in central Asia they average about 25% in winter and 50% in summer.

_B._ =Relation of Relative Humidity to the Direction of the Wind.=--Observations by means of the wet and dry-bulb thermometers in the shelter, or by means of the sling psychrometer, supplemented by records of wind direction, will furnish data for the solution of this problem.

Tabulate your observations according to wind directions and seasons.

Determine the characteristics of the different winds as to their relative humidities. Consider the control of these winds and humidity conditions by cyclones and anticyclones.

The warm wave, or sirocco, in front of our winter cyclones in the eastern United States is a damp, disagreeable, irritating wind. In summer, the sirocco is usually dry, and during the prevalence of such winds we have our hottest spells, when sunstrokes are not uncommon. In southern Italy the sirocco has the same position with reference to the controlling cyclone. There the wind is often so dry as seriously to injure vegetation. The cold wave, on the rear of our winter cyclones, with its low temperature and dry air, often comes as a refreshing change after the enervating warmth of the preceding sirocco. Our feelings of bodily comfort or discomfort are thus in a large measure dependent upon the humidity and the movement of the air.

_C._ =The Formation of Dew.=--The formation of dew is to be studied from the following points of view, viz., as dependent upon: _a_, the temperature and the humidity of the air; _b_, the exposure and condition of the ground; _c_, the state of the sky; and _d_, the movement of the air. The occurrence of dew on any night, as well as the amount, whether large or small, can readily be ascertained by inspection. Observe the conditions of temperature, humidity, cloudiness, and wind direction and velocity, as in previous exercises. Pay special attention to the state of the sky, the wind movement, and the vertical distribution of temperature near the ground. Under heading _b_ (exposure and condition of the ground) make observations of the amounts of dew formed on hilltops, hillsides, and in valleys; on different kinds of surface covering, as gra.s.s, leaves, pavements, etc., and over different kinds of soil. Cla.s.sify the results in accordance with the conditions under which the observations were made.

Compare the results and draw your conclusions from this study. Practise making predictions of the formation of dew in different places and under different weather conditions.

Over the greater portion of the earth's surface the amount of dew which is deposited is very small. It has been estimated that in Great Britain the total annual amount would measure only an inch and a half in depth; and in central Europe the depth is given as hardly one inch. In some parts of the Torrid Zone, on the other hand, dew is deposited in much larger quant.i.ties. According to Humboldt, the traveler through some of the South American forests often finds what seems to be a heavy shower falling under the trees, while the sky is perfectly clear overhead. In this case dew is formed on the tops of the tree in sufficiently large quant.i.ties to give a shower underneath. It is reported that on the Guinea coast of Africa the dew sometimes runs off the roofs of the huts like rain. In many dry regions the dew is an important agency in keeping the plants alive.

_D._ =The Formation of Frost.=--The formation of frost is to be studied in the same way as that suggested in the case of dew, _i.e._, as dependent upon: _a_, the temperature and the humidity of the air; _b_, the exposure and condition of the ground; _c_, the state of the sky; and _d_, the movement of the air. Frosts are usually cla.s.sified as _light_ or _heavy_.

The words _killing frost_ are also used. Study the weather and surface conditions which are most favorable to the formation of frost. Pay special attention to the relation of frost and inversions of temperature; to the frequency of frost on open or sheltered surfaces; on hills or in valleys, and on the lower and upper branches of trees and shrubs. Determine, as well as you can, the weather conditions which precede light or heavy frosts, and make predictions of coming frosts, when the conditions warrant them.

Our Weather Bureau gives much attention to the prediction of frosts and to the prompt and widespread distribution of frost warnings. Growing crops and fruits are often seriously injured by frosts, and farmers are naturally anxious to have as early warning as possible of their occurrence. Various methods of protecting crops and trees against frost are used. The method most commonly employed consists in the building of fires of brush or other inflammable material on the windward side of the field or the orchard when a frost is expected. The smoke from the fire is blown to leeward across the field, and acts as an artificial cloud, affording protection to the vegetation underneath. Such fires are known as _smudges_.

CHAPTER XXIII.

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] 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:--

[Footnote 7: See _Hydrographic Office Cloud Types_, Appendix B.]

_A._ =The Typical Cloud Forms and their Changes.=--Note carefully the characteristic forms a.s.sumed 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. Cla.s.sify 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 cla.s.sification of clouds was proposed by Luke Howard, in 1803. The cla.s.sification at present in use is known as the _International Cla.s.sification_, 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 a.s.sociation 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.