Introduction to Meteorology

INTRODUCTION

TO

METEOROLOGY

INTRODUCTION

Meteorology is the science of the atmosphere. With the increasing tendency toward specialization characteristic of our time, the subject matter under the general heading of meteorology may be referred to various subdivisions, or branches, depending partly on the theoretical approach and partly on the application of meteorology to human activities.

From a theoretical point of view, meteorology may be subdivided into the following categories:

1. Dynamic meteorology, which concerns itself with the forces that cremate and maintain motion and the heat transformations associated therewith. Within the field of dynamic meteorology, distinction is often made between hydrodynamics which deals with forces and motion and thermodynamics which deals with heat. The word aerodynamics is usually reserved for the study of the interaction between air currents and objects, such as airfoils.

2. Physical meteorology, which deals with processes of a purely physical nature, such as radiation, heat, evaporation, condensation, precipitation, ice accretion, and optical, acoustical and electrical phenomena.

3. Climatology, or statistical meteorology, which determines the statistical relations, mean values, normals, frequencies, variations, distribution, etc., of the meteorological elements.

From the point of view of practical application, meteorology is commonly subdivided into a number of classes, of which the following are the most important:

4. Synoptic meteorology has as its aim a coordinated study of the processes in the atmosphere on the basis of simultaneous observations over large areas. Thus, synoptic meteorology applies dynamic as well as physical meteorology and, to a lesser extent, climatology in order to obtain a synopsis of the state of the atmosphere; its main purpose is the analysis and forecasting of the weather phenomena.

5. Aeronautical meteorology deals with the application of meteorology to the problems of aviation. As far as actual weather conditions are concerned, it is related to synoptic meteorology; and as far as the normal state of the atmosphere is concerned, it is related to climatology.

6. Maritime meteorology is related to marine navigation in the same manner as aeronautical meteorology is related to air navigation.

7. Agricultural meteorology deals with the applications of meteorology to agriculture, soil conservation, etc.

8. Hydrometeorology is concerned with meteorological problems relating to water supply, flood control, irrigation, etc.

9. Medical meteorology has to do with the influence of weather and climate on the human body.

10. Aerology is the branch of meteorology that is concerned with the conditions of the free atmosphere on the basis of direct observations. In the United States the word aerology is frequently used synonymously with meteorology, meaning the science of the atmosphere.

The scope of the present book is mainly synoptic, inasmuch as it is chiefly concerned with the weather processes; in addition, a brief chapter on climates and an outline of the history of meteorology are included.

CHAPTER I

THE ATMOSPHERE

The word atmosphere derives from the Greek words atmos which means vapor and sphaira which means sphere. It is now used to denote the gaseous sphere surrounding the earth.

Composition.—The air, or the material of which the atmosphere is composed, is a mechanical mixture of a number of different gases. A sample of dry and pure air contains about 78 per cent (by volume) nitrogen, 21 per cent oxygen, and almost 1 per cent argon. In addition, it contains about 0.03 per cent carbon dioxide.

Nitrogen, oxygen, argon, and carbon dioxide constitute about 99.99 per cent of dry and pure air. The remaining 0.01 per cent represents traces of several other gases, such as neon, krypton, helium, ozone, xenon, and hydrogen. These are present in such minute amounts that they are of no practical importance for the study of the weather phenomena.

The amount of carbon dioxide is not quite constant. The vegetable world continuously consumes carbon dioxide which, again, is produced by the animal world, through burning of fuels, volcanic action, and various processes of decay in the soil. Although these processes are not always balanced, the oceans, by dissolving the excess of carbon dioxide, so effectively regulate it that the amount of carbon dioxide in the atmosphere remains almost constant.

Ozone, which is present in the lower atmosphere in minute amounts, has a maximum in the upper atmosphere between 10 and 25 km. (30,000 and 80,000 ft.) where its amount varies considerably.

Apart from the variations in carbon dioxide and ozone, the composition of the atmosphere is remarkably constant all over the earth’s surface. It is also constant with elevation as far as instruments have reached (35 km.).

The air also contains a variable amount of water vapor. In many respects the water vapor is the most important constituent of the atmosphere. The maximum amount of water vapor that the air can absorb depends entirely on the temperature of the air; the higher the temperature of the air, the more water vapor can it hold. The air is saturated with moisture when this maximum amount is reached. When air is cooled below its saturation temperature, condensation takes place, the water vapor being condensed to water droplets or, at low temperatures, to ice crystals. Small water droplets and ice crystals are kept afloat in the air by the ascending air currents. Under special conditions, which we shall describe later, these minute drops or ice crystals coalesce and form large drops or snowflakes which are precipitated from the clouds when they become too large to be kept afloat by the ascending currents.

Impurities.—Apart from the above-mentioned constituents, the air contains a variable amount of impurities, such as dust, soot, and salts.

The main source of dust is the arid regions, such as deserts and steppes. The coarser material, whirled up by the winds, is never carried far from its source, but minute dust particles are readily distributed throughout the lower atmosphere and carried far from the source. Air masses that have swept over subtropical continents normally contain considerable amounts of dust, but polar air masses are relatively pure.

The industrial regions, forest fires, and volcanoes constitute the main source of soot. When fuels burn at high temperatures, hydrogen and oxygen combine to produce water vapor, and carbon and oxygen combine to produce carbon dioxide, both of which belong to the normal constituents of the atmosphere. However, if fuels burn at low temperatures, the latter process is hindered, and carbon is carried up with the rising air and coagulates into soot.

Observations show that the air normally contains a considerable amount of salts. Through the action of the winds, spray is whirled up from the oceans, and when it evaporates the salt remains in the air in the form of minute particles.

The particles which constitute the impurities of the air are so small that they cannot be seen individually by the naked eye, but their effect on visibility and on coloring of distant objects is easily observed. Through haze, distant objects (e.g., mountains) are seen as if through a thin veil of pale blue if the object is dark or a yellowish veil if the object is white (e.g., snow-covered mountains, clouds at the horizon). At a certain distance depending on the density of the haze, all details disappear, and the objects stand out like a silhouette against the sky. The denser the haze, the shorter the distance at which the details disappear.

The presence of dust in the atmosphere is important not only because of its influence on visibility; if the air were perfectly pure, there would be no appreciable condensation of water vapor. When the air is cooled to its saturation temperature, condensation takes place on certain active (hygroscopic) nuclei. Salt particles from the oceans and various products of combustion are most active as condensation nuclei, and observations show that such particles are present in the atmosphere in abundant amounts.

Structure.—The air is highly elastic and compressible. Although extremely light, it has a perfectly definite weight. At ordinary pressure and temperature, the weight of a sample of air near the earth’s surface is about 1/800 of the weight of an equal volume of water. Thus, 1 cu. ft. of air weighs about 1.2 oz. or, in metric units, 1 eu. m. of air weighs about 1.3 kg.

In consequence of this weight, the atmosphere exerts a certain pressure upon the earth’s surface, amounting to about 15 lb./sq. in. This pressure is sometimes used as a unit and is called 1 atmosphere.

A column of air from the earth’s surface to the top of the atmosphere exerts a pressure on the earth’s surface that is equivalent to a column of water 34 ft. (or 10 m.) high; this is equivalent to the weight of a column of mercury 30 in. (or 70 cm.) high. For this reason, mercurial barometers are used to measure the pressure of the atmosphere.

The atmospheric pressure decreases with increasing altitude. The difference in pressure between two points, one above the other, is simply equal to the weight of the air column between the two points. By measuring the temperature and the pressure at two or more points in the same vertical line, it is possible to compute the difference in altitude between the points. Assuming a normal distribution of temperature, pressure becomes a simple function of altitude, and special barometers (altimeters) have been constructed to record altitude instead of pressure. Such instruments are now widely used in aviation.

Since the atmospheric pressure is equal to the weight of the air column, it follows that the pressure must decrease gradually and approach zero with increasing altitude. The same also applies to the density of the air. There is, therefore, no distinct upper limit to the atmosphere; it merges gradually into empty space. Figure 1 shows how pressure, density, and temperature normally vary with altitude within the lower 15 km.

Even though the atmosphere reaches to great altitudes, it is only the lower part of it that is of importance for the weather. The highest clouds (cirri) are seldom more than 10 km. (33,000 ft.) above the earth’s surface, and 50 per cent of the total weight and about 90 per cent of the total moisture content are within about 5 km. (16,000 ft.) of the earth’s surface.

FIG. 1.—Showing the normal variation in temperature, pressure, and density with height in middle latitudes.

Stratification.—It will be seen from Fig. 1 that the air temperature normally decreases with elevation up to about 11 km. (36,000 ft.) and then remains constant. The rate of decrease in temperature along the vertical is called the lapse rate. The lower part of the atmosphere, which normally is characterized by a relatively steep lapse rate, is called the troposphere. The upper part of the atmosphere, which is characterized by almost constant temperature along the vertical, is called the stratosphere. The layer of transition that separates the stratosphere from the troposphere is called the tropopause.

The height of the tropopause above the earth’s surface varies considerably with latitude and season. It also varies with the weather situation; it is normally lower over areas of low pressure (cyclones) than over areas of high pressure (anticyclones).

Figure 2 shows the normal height of the tropopause and the mean distribution of temperature in the lower atmosphere. It is of interest to note that the temperature of the stratosphere, on the whole, decreases from the poles toward the equator.

FIG. 2.—Mean annual temperature in the troposphere and the lower stratosphere. Note that the stratosphere is warmer at the pole than at the equator.

The greatest altitude to which meteorological instruments have reached is about 36 km. From the direct observations made below 36 km. and from recent studies of radiation, meteors, aurora borealis, the propagation of sound and radio waves, etc., it is possible to draw conclusions as to the structure of the upper atmosphere. Our present knowledge of the stratification of the atmosphere may be summarized briefly as follows:

In the troposphere the temperature normally decreases with altitude at a rate of approximately 0.6°C. per 100 m., or about 1°F. per 300 ft. The troposphere is relatively unstable; vertical currents occur frequently, leading to condensation, the formation of clouds and precipitation. All ordinary weather phenomena develop within the troposphere, particularly in its lower half.

As we ascend through the tropopause (see Fig. 3), the temperature is constant or increases along the vertical as far as meteorological instruments have reached. Above the tropopause, we meet with a layer particularly rich in ozone. Recent investigations by Dobson and others have shown that there is a noticeable correlation between the amount of ozone and the weather conditions at the ground. Although the stratosphere usually is cloudless, a special type of cloud (mother-of-pearl clouds) is occasionally observed to form in connection with the ozone layer. The space between the tropopause and the ozone layer (see Fig. 3) is always cloudless; and since the air within this layer is extremely stable, the lower stratosphere offers the nearest approach to ideal flying conditions.

FIG. 3.—Showing the structure of the troposphere, stratosphere, and ionosphere and the phenomena typical of each.

Statistical investigations show that the meteors disappear most frequently either at about 80 km. (260,000 ft.) or at about 40 km. (130,000 ft.) above the earth’s surface. This fact and the results of the study of the propagation of sound waves seem to indicate that there is a layer of air between 40 and 80 km. which is extremely warm, perhaps 120 to 130°F. (i.e., 60 to 70°C.).

At about 60 km. (200,000 ft.), there is a layer that tends to absorb radio waves. This layer is created through the action of the sun’s rays; and, as a result, the range of the radio stations, particularly that of short-wave stations, is greater by night than by day.

Above the level of about 80 km. (260,000 ft.) is the so-called ionosphere. In the lower portion of it, we find the noctilucent clouds, which are extremely rare. The ionosphere is characterized by several electrically conducting layers, of which the Kennelly-Heaviside layer (or the E-layer) is the most important. This layer reflects the radio waves back to the earth’s surface and thus accounts for the long range of radio stations. The Kennelly-Heaviside layer, which is quite distinct, is normally found between 90 and 130 km. (300,000 to 400,000 ft.). Above this layer is the so-called Appleton layer (or the F-layer), which is more diffuse and variable in altitude; it sometimes breaks up into several diffuse layers.

The aurora borealis and kindred phenomena are most frequently observed within the lower part of the ionosphere. Recent measurements of Störmer have shown that auroras may occur even as high as 1200 km. (4 million feet) above the earth’s surface. This shows that atmospheric matter is present in measurable amounts even at such great altitudes.

CHAPTER II

OBSERVATIONS AND INSTRUMENTS

The meteorological services of the various countries maintain a large number of observing stations whose function it is to make observations according to international definitions and rules and to report at frequent intervals to the central offices. Within each country the reports are collected and disseminated by means of radio, teletype, or telegraph. By international agreements, all countries are obliged to arrange; for radio transmission of meteorological reports for international use. All matters of international importance are handled by the International Meteorological Organization, which maintains a permanent secretariat in Lausanne, Switzerland. Meteorological observations are also made by a large number of selected ships whose reports are exchanged among the various countries.

The meteorological observing stations may be divided into three groups, viz.:

1.  Ordinary land and ship stations which report the conditions near the earth’s surface and the state of the sky.

2.  Pilot-balloon stations which measure the winds in the free atmosphere.

3.  Aerological stations which send up balloons or airplanes furnished with instruments to measure pressure, temperature, and humidity in the free atmosphere.

In this chapter, we shall consider the most important meteorological elements and the instruments commonly used. The reader who desires more detailed information is referred to the publications issued by the national weather services.

The Mercurial Barometer.—The atmospheric pressure is usually observed by means of a mercurial barometer. The principle of the mercurial barometer may be described briefly as follows: A glass tube, about 1 yd. long, with one end sealed and the other end open, is filled completely with mercury. The open end is now closed (e.g., by a finger), and the tube is placed in a vertical position with the open end submerged in a small vessel partly filled with mercury (Fig. 4). When the finger is removed, the mercury in the tube will sink somewhat and come to rest at a level of about 30 in. above the level of the mercury in the vessel. There is then a vacuum above the mercury in the tube and, therefore, no atmospheric pressure above the mercury within the tube.

Since the atmospheric pressure acts on the free surface of the mercury in the vessel, it is clear that the weight of the mercury column above the free surface of the mercury in the vessel must be equal to the weight of the air column above the same surface. The length of the mercury column will at any moment indicate the atmospheric pressure; it is measured by means of a scale placed along the glass tube.

FIG. 4.—Showing the principle of the mercurial barometer. The air pressure on the mercury in the vessel is balanced by the weight of the mercury in the tube.

Pressure.—The direct reading of the barometer gives the length of the mercury column whose weight balances the weight of the air column above the barometer. The length of the column of mercury depends on the temperature of the barometer. In order to render the observations of the various stations comparable, the readings are corrected for the thermal expansion of the tube, the mercury and the scale, using 0°C. (32°F.) as standard temperature.

The weight of the air column depends also on the local gravity. A second correction is applied to obtain the pressure that would be observed at the station if the local gravity were equal to the normal gravity at 45°N.

Even the most carefully made barometers have a certain error, and the reading must be corrected for the individual error of the instrument. When the reading has been corrected for instrument error, thermal expansion, and local gravity, we obtain the correct pressure at the level of the mercury cistern of the barometer.

The atmospheric pressure decreases with elevation at a rapid rate. In order to render the observations of the various stations comparable, it is necessary to add a correction so as to obtain the pressure that would be recorded if the barometer were placed at sea level. Hence, the pressure reported from all land stations and ships indicates the pressure that would be recorded by a correct barometer if it were placed at sea level, if its temperature were 0°C. (32°F.), and if the local gravity were equal to the normal gravity at 45°N.

Pressure Units.—Up to about 1914, pressure was reported in units of length, either in inches of mercury or millimeters of mercury. In later years a new unit, called the millibar (mb.) has come into general use. Normal pressure at sea level is roughly 30 in., or 700 mm., to which corresponds 1013 mb.

The conversion from units of length to units of pressure proceeds as follows: Suppose that the mercury column is 76 cm. high and of unit cross section. Since the density of