Practical Exercises in Elementary Meteorology

Make forecasts of wind direction and velocity from the daily weather maps for your own station. Continue these for a week or two, keeping record of the verification or non-verification of each of your forecasts. Then make daily forecasts of temperature and of wind direction and velocity together. Write out your own forecast for each day before you compare it with the official forecast, and if the two differ, keep note of which one seemed to you to be the most accurate.

Weather.—What general relation between kind of weather and cyclones and anticyclones was illustrated on the six maps of our series? What is the average distribution of the different kinds of weather around cyclones and anticyclones, as shown by your composites? (Chapter XVI.) What changes in weather will ordinarily be experienced at a station as a cyclone approaches, passes over, and moves off? What conditions will prevail in an anticyclone?

Make a series of daily forecasts for your own station of probable weather changes, omitting temperature and winds at first. Include in your weather forecasts the state of the sky (clear, fair, cloudy); the changes in the state of the sky (increasing or decreasing cloudiness); the kind of precipitation (rain or snow) and the amount of precipitation (light or heavy). Write out your forecasts; compare them with the official forecasts, and notice how fully they are verified. Then add temperature and winds to your forecasts so that you will make a complete prediction of probable changes in temperature (kind, amount, and rapidity of change), wind (direction and velocity), and weather. Practice making these complete forecasts for several weeks, if time allows. Use all the knowledge that you have gained in the preceding work to aid you in this. Study each weather map very carefully. Do not write down your forecast until you are sure that you have done the best you can.

Fig. 53.

Vary this exercise by extending your forecasts so as to embrace the whole section of country in which your station is situated (as, e.g., New England, the Gulf States, the Lake region). Pay special attention to making forecasts of cold waves, of heavy rain or snowstorms, of high winds over the lakes or along the Atlantic coast, etc. When possible, obtain from the daily newspapers any particulars as to damage done by frost or gales, or concerning snow blockades, floods due to heavy rains, etc.

Fig. 53 summarizes what has thus far been learned as to the distribution of the various weather elements around a well-developed center of low pressure. The curved broken lines represent the isotherms (Chapter XIV). The solid concentric oval lines are the isobars (Chapter XI). The arrows represent the winds, the lengths of the arrows being roughly proportionate to the wind velocities (Chapter XII). The whole shaded area represents the region over which the sky is covered by heavy lower clouds. The smaller shaded area, within the larger, encloses the district over which rain or snow is falling (Chapter XVI). The lines running out in front of the cloudy area represent the light upper clouds (cirrus and cirro-stratus) which usually precede an area of low pressure.

Imagine this whole disturbance moving across the United States in a northeasterly direction, and imagine yourself at a station (1) directly in the path of the cyclone; (2) south of the track; and (3) north of the track. In the first case, as the disturbance moved on in its path, you would successively occupy the positions marked A, B, and C on the line AC, passing through the center of the cyclone. In the second case you would be first at D, then at E, and then at F. In the third case you would be at G, H, and J in succession. What changes of weather would you experience in each of these positions as the cyclone passed by you? Imagine yourself at some station halfway between the lines AC and DF. What weather changes would you have in that position with reference to the storm track? In what respects would these weather changes differ from those experienced along the line DF? Imagine your station halfway between the lines AC and GJ. What weather changes would you have there? How would these changes differ from those experienced along the line GJ?

It must be remembered that Fig. 53 is an ideal diagram. It represents conditions which are not to be expected in every cyclone which appears on our weather maps. If all cyclones were exactly alike in the weather conditions around them, weather forecasting would be a very easy task. But cyclones are not all alike—far from it. Some are well developed, with strong gradients, high winds, extended cloud areas, heavy precipitation, and decided temperature contrasts. Others are but poorly developed, with weak gradients, light winds, small temperature differences, and it may be without any precipitation whatever. Some cover immense districts of country; others are small and affect only a limited area. It therefore becomes necessary to examine the characteristics of each approaching cyclone, as shown on the daily weather map, very carefully. Notice whether it is accompanied by heavy rain or snow; whether its winds are violent; how far ahead of the center the cloudy area extends; how far behind the outer cloud limit the rain area begins; what is the position of the cloud and rain area with reference to the center, and other points of equal importance, and govern yourself, in making your forecast, according to the special features of each individual cyclone. Well-developed cyclones will be accompanied by marked weather changes. Weak cyclones will have their weather changes but faintly marked.

The distance of your station from the center of the cyclone is of great importance in determining what the weather conditions and changes shall be, as may easily be seen by examining Fig. 53. If the storm passes far to the north or far to the south of your station, you may notice none of its accompanying weather conditions, except, perhaps, a bank of clouds on your horizon. You may for a few hours be under the cloudy sky of some passing storm, and yet not be reached by its rainy area. The shifts in the wind may be marked and the wind velocities high, or the expected veering or backing may hardly be noticeable, owing to the weakness or the distance of the controlling cyclone.

Again, the rapidity with which weather conditions will change depends upon the rate of movement of the cyclone itself. The better developed the cyclone, the higher its velocity of progression, and the nearer its track lies to the station, the more emphatic and the more rapid are the weather changes it causes. On the other hand, the weaker the cyclone, the slower its rate of progression, and the further away its track, the less marked and the slower the weather changes. The probable track of a coming storm, and its probable rate of movement, therefore, need careful study if our forecasts are to be reliable.

There are many other obstacles in the way which combine to render weather forecasting extremely difficult. Some of these difficulties you will learn to overcome more or less successfully by the experience you will gain from a careful and persevering study of the daily weather maps; others, the best forecast officials of our Weather Bureau have not yet entirely overcome. The tracks followed by our cyclones vary more or less from month to month, and even if the average tracks for each month are known, individual cyclones may occur which absolutely disregard these tracks. While the average hourly velocity of cyclones is accurately known for the year and for each month, the movements of individual storms are often very capricious. They may move with a fairly uniform velocity throughout the time of their duration; they may suddenly and unexpectedly increase their rate of movement, or they may as suddenly come nearly to a standstill. The characteristics of cyclones vary in different portions of the country and at different times. Cyclones which have been accompanied by little precipitation on most of their journey are apt to give increased rain or snowfall as they near the Atlantic Ocean and Gulf of St. Lawrence. Cyclones which over one portion of the country were rainy, may give little or no precipitation in another portion. Cyclones and anticyclones are found to have considerable influence on one another, retarding or accelerating one another’s advance, or changing one another’s normal path of progression. While this mutual interaction is clearly seen, and may be successfully predicted in many cases, many other cases arise in which, under apparently similar conditions, the result is very different from the anticipation. Such are some of the difficulties with which weather forecasters have to contend, and which prevent the attainment of greater accuracy in weather prediction.

Part V.—Problems in Observational

Meteorology

CHAPTER XX.

TEMPERATURE

The chief interest and value of the instrumental work in meteorology are to be found not only in the taking of the daily observations at stated hours, but in the working out of numerous simple problems, such as may readily be undertaken with the help of the instruments already described. Thus, the temperature of the air (obtained by the sling thermometer, supplemented by maximum and minimum thermometers, and by the thermograph if available) can be determined under a variety of conditions, e.g., close to the ground, and at different heights above the ground; at different hours, by day and night; in different seasons; in sunshine and in shade; during wind and calms; in clear and cloudy weather; in woods and in the open; over bare ground, grass, snow, or ice; on hills and in valleys. Observations may also be made of the temperature of the ground and of a snow cover, at the surface and at slight depths beneath the surface, in different seasons and under different weather conditions. Among the problems which may be worked out by means of such observations as these are the following:—

A.The Diurnal Range of Temperature under Different Conditions and at Different Heights above the Ground.—Under the influence of the sun the regular normal variation of temperature during 24 hours is as follows: A gradual increase, with the increasing altitude of the sun, from sunrise until shortly after noon, and a gradual decrease, with decreasing altitude of the sun, from the maximum, shortly after noon, until the minimum, about sunrise. This variation is known as the diurnal variation of temperature. Curve a in Fig. 12 illustrates well the normal diurnal variation of temperature, as recorded by the thermograph during a period of clear, warm spring weather (April 27-30, 1889, Nashua, N. H.). The diurnal range of temperature is the difference between the maximum and minimum of the diurnal oscillation. The regular normal diurnal variation in temperature is often much interfered with by other controlling causes than the sun, e.g., cyclonic winds, clouds, etc.

I. Study and compare the diurnal ranges of temperature as indicated by the maximum and minimum thermometers, or the thermograph, in the instrument shelter, in clear, fair, cloudy, and stormy weather, during winds and in calms, in different months. Summarize your results by grouping them according to the general weather conditions, and according to the months or seasons in which the observations were made. For example, group together and average the ranges observed on clear, calm days in winter; on similar days in early summer or autumn; on clear days with brisk northwest winds in winter; on similar days in early summer or autumn; on calm days with overcast sky in the different seasons; on stormy days with strong winds, etc. Study carefully the weather maps for the days on which your observations are made. Pay special attention to the relation between the diurnal ranges and the control exercised over these ranges by cyclones and anticyclones through their winds and general weather conditions.

II. Observations of diurnal ranges of temperature at different heights above the ground may be made by means of maximum and minimum thermometers fastened (temporarily) outside of the windows of different stories of the school or of some other building. These observations should be made out of windows facing north, and care should be taken to check, so far as possible, any draft from within the building out through the window during the taking of the observation. If a fire escape is provided on the building, the instruments may often be conveniently fastened to that.

Study the ranges under different conditions of wind and weather at various heights above the ground, and compare these results with those obtained under I. Notice the relations of all your results to the cyclonic and anticyclonic areas of the weather maps.

The diurnal range of temperature in the air over the open ocean from the equator to latitude 40° has been found to average only 2° to 3°. In southeastern California and the adjacent portion of Arizona the average diurnal temperature range in summer is 40° or 45°. Over other arid regions, such as the Sahara, Arabia, and the interior of Australia, the range also often amounts to 40°. Observations of temperature above the earth’s surface, in the free air, made on mountains, in balloons, and by means of instruments elevated by kites, indicate very clearly that the diurnal range of temperature decreases with increasing elevation above sea level. The results obtained at Blue Hill Observatory, Massachusetts, by means of kites, show that the diurnal range of temperature almost disappears, on the average, at 3300 feet (1000 meters).

B.Changes of Temperature in the Lower Air, and their Control by the Condition of the Ground, the Movement of the Air, and Other Factors.—Determine the changes of the temperature in the lower air by making frequent readings of the ordinary thermometer in the instrument shelter, of the sling thermometer, or by an examination of the thermograph record. Group these changes, as in Problem A, so far as possible according to the weather conditions under which they occurred, and try to classify the kinds of change roughly into types. Study the control of these various types by the wind and other weather conditions accompanying them, as illustrated on the daily weather maps. The control exercised by different conditions of the earth’s surface may be studied by means of observations made with the sling thermometer over different surfaces, such as grass, bare ground, snow, etc.

Examples of temperature changes in the lower air, under different conditions of weather, recorded on the thermograph, are given in Fig. 12, and are briefly referred to their causes in the text accompanying that figure.

C.Vertical Distribution of Temperature in the Atmosphere.—The vertical distribution of temperature in the lower air may be studied by having ordinary thermometers or thermographs exposed at different heights above the ground, e.g., close to the surface; in an instrument shelter; out of windows on successive stories of some high building; and on the roof of the building. They may also, in cases where there is a hill in the neighborhood, be exposed in a valley at the bottom of the hill and at successive elevations up the side of the hill. It is, however, usually much simpler, as well as more practicable, to take these temperature readings by means of the sling thermometer. In the case of observations made out of the windows of a building, one observer can take the readings at different elevations in succession. When the observations are made at different altitudes on the side of a hill, it is best to have the coöperation of several observers, who shall all read their thermometers at the same moment of time. The results obtained in the previous problems (A and B) may, of course, also be utilized in studying the vertical distribution of temperature in the atmosphere.

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 altitude 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 aëronauts, 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 passage 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 (#x7_x_7_i24).) Note the character of the changes that occur, classifying 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 latitude 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 onshore, while the land breeze blows offshore. 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 latitudes, 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. Classify 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 grass, leaves, pavements, etc., and over different kinds of soil. Classify 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 quantities. 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 quantities 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 classified 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.