Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1950"

The size distribution of raindrops

Abstract: Experimental data relating to drop size distribution in rain are examined and it is shown that in many cases the size distribution is in accordance with the following formulae 1 - F = exp [-(x/a)n] a = A IpW = C Ir where F = fraction of liquid water in the air comprised by drops with diameter less than x. I = rate of precipitation. W = amount of liquid water per unit volume of air. A, C, p, r and n are constants. If × is measured in mm, I in mm/hr and W in mm3/m3 the mean values of A, C, p, r and n are 1.30, 67, 0.232, 0.846 and 2.25 respectively. There may be appreciable variations from these mean values, particularly in the case of n, if the precipitation is essentially of a showery or orographic nature. Tables based on these formulae are given showing values of W and of the number and total volume of drops between certain diameter limits for various rates of rainfall.

The application of the similarity theory of turbulence to atmospheric diffusion

Abstract: The purpose of this paper is to draw the attention of meteorologists to the potential usefulness of the similarity theory of the small-scale structure of turbulent motion first put forward by Kolmogoroff. An outline of the theory is given first and the possibility of its application to atmospheric and oceanic turbulence is considered. As an example of its use, the theory is applied to the problem of the relative diffusion of neighbouring particles moving with the fluid, and predictions about the rate of increase of mean square distance between the particles are made. Some data obtained recently by Durst are compared with these predictions by Charnock, and similar observations made by Richardson and Stommel are considered herein. The empirical proportionality between the diffusion coefficient and the 4/3 power of the scale of observation, obtained many years ago by Richardson, is shown to be in general agreement with the theory.

Types and spells of weather around the year in the British Isles : Annual trends, seasonal structure of the year, singularities

Abstract: From a classification of 50 years' daily weather maps (1898—1947) the outstandingly long spells of weather, lasting over 25 days, have been picked out. Frequency curves show the occurrences of these long spells on each day of the year and also the occurrences of each of the seven defined weather-types individually. This leads to a suggested division of the year into five periods, to be considered as natural seasons, and appears to support the definition of more numerous shorter seasonal phases or episodes (singularities). Good correspondence is found with the long-year daily or weekly mean figures of pressure, temperature, rainfall, sunshine, etc. The significance of these phenomena, their dependability as regards date and their secular persistence are discussed.

Empirical formulae for the terminal velocity of water drops falling through the atmosphere

Abstract: From a consideration of experimental measurements of the terminal velocity of water drops and of the drag coefficient of rigid spheres it is suggested that a formula of the type V = A exp bz { 1 – exp [– (d/a)n]} may be used to represent the terminal velocity of water drops which are larger than those to which Stokes's law is applicable and which are falling through a standard atmosphere. If V (the terminal velocity) is measured in cm/sec, z (the height) in km and d (the drop diameter) in mm the values of the constants are as follows:— I.C.A.N. atmosphere Summer tropical atmosphere Range of d mm A b a n A b a n 0.3 to 6.0 932 0.0405 1.77 1.147 958 0.0354 1.77 1.147 0.05 to 0.3 191 0.0290 0.316 1.754 188 0.0256 0.304 1.819 If the drops have diameter less than 0.05 mm, Stokes's law is applicable and V = A d2 exp bz where A is 3040 and 2840 in the I.C.A.N. and S.T. atmospheres respectively and b has the values 0.0191 and 0.0172 in these two atmospheres.

The theory and use of upper air thickness patterns in forecasting

Abstract: The combination of the ordinary synoptic chart with the 1,000—500 mb thickness pattern, and the implied 500 mb contour chart, provides a first approximation to a three-dimensional representation of the troposphere. Although incomplete, the representation permits the introduction of dynamical and thermodynamical theory and provides a basis for scientific forecasting. The forecasting of thickness patterns, in effect of the temperature field, is a thermodynamical problem. The theory of advective, adiabatic and non-adiabatic temperature change is discussed. A useful approximation to advection is obtained by regarding the thickness lines as material lines embedded in the surface geostrophic wind field. Climatological monthly means and extremes of thickness are given; these provide a practical framework to the forecast. The dynamical development is related with the geometry of the thickness patterns as indicating the vertical variation of vorticity transport. Attention is directed to the development field associated with thermal troughs and ridges, confluence and difluence, and certain combinations of these. When the dynamical development so produced sets up a circulation which by advection maintains or increases the thermal distortion in the same sense, the situation is self-maintaining or self-developing. The warm sector depression, the baroclinic anticyclone and the jet stream complex are examples of such systems. A step-by-step procedure of forecasting prebaratic and pre-thickness charts, which has been used in experimental four-day forecasts, is outlined.

Evaporation over the British Isles

Abstract: Theoretical values of average annual evaporation from the British Isles are calculated from long-term averages of air temperature, duration of sunshine, vapour pressure and wind speed for about 100 stations. Direct estimates of the same quantity are obtained from long-term averages of annual rainfall and annual run-off from about 40 catchment areas. The two kinds of estimate agree and are plotted on a map to show the geographical variation of evaporation. This shows an increase from 14 in. per annum in Scotland to 20 in. along the south coast.

Natural removal of pollution from the atmosphere

Abstract: Observations of atmospheric pollution throughout Britain are considered in relation to the amounts of pollution emitted through the combustion of coal. The estimated weight of ash emitted into the air and the estimate, from deposit gauges, of the ash deposited, are considered likely to agree fairly well at a figure of rather over one million tons per annum (one million tonnes/yr). On the assumption that about 1.5 million tons of smoke are blown to sea each year, it is shown that about 1.1 million tons of sulphur dioxide are blown to sea each year; the remaining 0.8 million tons of smoke and 3.9 million tons of sulphur dioxide must be deposited in Britain, irrespective of the quantities measured in deposit gauges. The average life of a smoke particle before deposition on land is probably of the order of 1–2 days; that of a molecule of sulphur dioxide is estimated with rather more certainty to be less than 12 hours. Of the chlorides collected in deposit gauges more than half, as a rule, come from the sea, and less than half from the utilization of coal; but the ratio must vary with the position of the deposit gauge relative to the sea and to industries.

The composition of coagulation‐elements in cumulonimbus

Abstract: Calculations are made from which it appears that the rate of loss of heat from ice particles growing by coagulation in dense supercooled clouds may be insufficient to allow all the acquired water to freeze. The larger coagulation-elements within cumulonimbus clouds may therefore have liquid skins at levels where the temperature has fallen to −20° or −30°C. The rain-drop multiplication process envisaged by Langmuir may thus proceed also at temperatures well below 0°C, and the effect may also be important in the production of ice nuclei by‘splinter’ formation and in the generation of thunderstorm electricity.

The measurement and recording of the heat flux into the soil.

Abstract: Relatively simple instruments for measuring the heat flux into or out of the soil are described together with methods of calibration. One form consists of a suitable metal disc of sandwich construction producing a thermal e.m.f. proportional to the heat flux through the disc, which is placed in the soil just below the surface. Details are also given of another form of flux plate which is suitable for continuous recording using a thread recording galvanometer.

Some further considerations of the measurement and indirect evaluation of natural evaporation.

Abstract: A previous observational study (Pasquill 1949 a) has been supplemented by further measurements of evaporation and the associated vertical profiles of wind velocity and vapour pressure in the first two metres above a clayland pasture with grass of moderate length, mainly in effectively adiabatic conditions of flow. The evaporation measurements, using a simple form of soil evaporimeter, included two series of statistically designed measurements, in order further to examine the “reliability” of the technique. The data so obtained, though not comprehensive, indicate no substantial error due to the necessary isolation of test soil from underlying layers, but there is a suggestion that the rates of evaporation ultimately adopted might be underestimates, owing to unavoidable disturbance of root and soil continuity in the vertical at the perimeters of the soil cores contained in the evaporimeters. The observed rates of evaporation (Eo) are compared with those computed (Ec) from the following formula, which is based on the Prandtl form of the law relating drag and wind profile over a rough surface in adiabatic conditions, Ec = ρk2 (u2–u1) (q1–q2)/[log (z2–d/z1–d)]2 where u1, u2, q1, q2 are the wind velocities and specific humidities at heights z1, z1 above the soil surface, d is a‘zero point displacement’ associated with the vegetation cover, ρ is air density and k is von Karman's constant, of magnitude 0.4. For the present measurements the usual “roughness parameter,” deduced from the wind profiles, was 0.51 cm as compared with 0.25 cm for the previous series. The two series include 19 separate determinations of rate of evaporation over periods of about one hour, and in terms of different sections of the observed profiles, provide 41 values of Ec/Eo in effectively adiabatic conditions, with wind velocities at a height of 2 m ranging from 3 to 9 m/sec. The mean values of Ec/Eo is 0.99 and the individual deviations from 10 are within 10, 20 and 30 per cent in 24, 66 and 90 per cent of cases. It is demonstrated that deviations of this order are to be expected from local variations in the various factors involved. Although some a priori uncertainty remains concerning the precise validity of the evaporation measurements, the consistency so demonstrated in terms of a semi-theoretical treatment which has been established in other connections (Calder 1949) suggests that the ultimate result may be accepted with confidence. Brief consideration is then given to the validity and practicability of applying the above formula to evaporation from areas of growing crops.

The nature of ice‐forming nuclei in the atmosphere

Abstract: During the last decade many experiments have been designed to investigate the initial processes of ice formation in the atmosphere. The results of these experiments are examined for clues as to the identity of the responsible nuclei and the following tentative conclusions are reached: (a) The nuclei which cause freezing between 0°C and −32°C are mainly solid, insoluble particles which are wetted by water and produce ice particles through the freezing of water drops. (b) The nuclei which become operative in the range −32° to −11°C consist of droplets of sea salt solution, ice and salt crystallising out on contaminating foreign particles at a temperature depending on the efficiency of these particles and the rate of expansion of the ambient air. (c) The nuclei effective at just below −41°C consist of droplets of pure salt solution and possibly of “ gaseous” nuclei formed industrially or by the action of ultra-violet light on the gases of the upper atmosphere.

The microclimate of the potato crop

Abstract: Shade temperatures and humidities in a standard screen and at an arbitrary level of 15 cm in potato crops were continuously recorded during three summers, 1947–49. In dry sunny periods : (1) the maximum temperature in the crop was from 0° to 13°F higher than in the screen, and over a period of eleven weeks in 1947 averaged 6°F higher (2) the crop minimum was about 2°F lower than in the screen (3) over a period of five weeks in 1949 the average daily mean temperature in the crop was 2.2°F higher than in the screen, and (4) the average daily temperature range was 8°F greater than in the screen. Wind, wet soil and cloudy weather greatly reduced these contrasts; occasionally the crop minimum was higher than the screen minimum. By day, humidity was higher in the crop than in the screen, the average difference at the minima being 5 per cent in relative humidity, and 7°F in dew point. By night, the dew point in the crop was, on average, 2°F lower than in the screen, corresponding to the observed difference in mean minimum air temperatures. During 1948 and 1949, more precise discontinuous records were taken of temperature, humidity and wind speed in and above potato crops with different densities of foliage in a variety of weather conditions. Temperature and humidity were measured at 10, 20 or 30, and 60 cm; wind at 20, 30 and 200 cm above ground level. In the crop there was only a slight temperature gradient in the early morning and throughout cloudy days, but during sunny days gradients were produced, depending on crop density; it was hottest at 10 cm in an open crop, at 30 cm in a dense crop; in both it was usually coolest at 60 cm when the soil was dry, but over wet soil the lowest temperature was found at 10 cm. Except on cloudy days, temperature inversion took place before sunset whatever the foliage density or moistness of the soil. Humidity (dew point) was usually greatest at 10 cm, but in a dense crop over dry soil water vapour transpired from the leaves often caused the air at 30 cm to be more humid than that at 10 cm. Wind affected both temperature and humidity by increasing lapse rates and causing rapid fluctuations within the crop, particularly of humidity. Changes of wind speed within the crop took place every few seconds, the amplitude of the fluctuations depending on the speed of the wind above the crop.

The significance of vertical stability in synoptic development

Abstract: The connection between the development of depressions and anticyclones and the degree of vertical stability of the atmosphere is examined. It is concluded that in middle latitudes the damping due to stability is not important in the largest-scale circulations of diameter greater than about 2,000 km, but has controlling significance for systems of diameter less than about 1,000 km. The difference in the rate of development between an ordinarily stable and a neutral atmosphere may be ten-fold or more for the smaller systems. A very small degree of instability in depth will permit small-scale cyclonic development to proceed rapidly. Furthermore, the damping effect of atmospheric stability is inversely proportional to the square of the Coriolis parameter. At 10° latitude, for example, the effect becomes dominant for all practical diameters, and it would appear difficult for cyclones or anticyclones of any reasonable size to develop significantly in low latitudes, with the exception of cyclones in a vertically unstable atmosphere.

The bright band — a phenomenon associated with radar echoes from falling rain

Abstract: Frontal rain when illuminated by radar transmission gives rise to echoes which exhibit a characteristic layer of greater intensity near to the freezing level. Observations which establish the height of this layer or “bright band,” relative to that of the freezing level, are described. Measurements of the terminal velocities of snowflakes of known mass, a factor of importance in support of a possible explanation of the phenomenon, are also given.

On the theory of large‐scale vortex motion in the atmosphere

Abstract: A consideration of the parameters best suited to characterising a circular vortex leads to the conclusion that an intensity factor, the central pressure defect (or excess) and a scale, or size factor are fundamental. General expressions are given for the mass defect of a cyclone, its total potential energy, and the total kinetic energy of geostrophic winds. These parameters are estimated for 17 cyclones on the basis of an assumed pressure-profile shape. It is shown that the ratio of the mean geostrophic kinetic energy to the mean potential energy is of the order 0.08. A dimensionless measure of the‘flexure’ or bending of a pressure profile is shown to vary relatively slightly from cyclone to cyclone, suggesting that the profile shape is roughly the same for all symmetrical lows. The kinetic energy arising from ageostrophic components of wind are also discussed and it is shown that a quasi-geostrophic vortex in bodily movement has the same (ageostrophic) kinetic energy as a rigid body moving with the same speed and having a specified mass, the virtual mass of the vortex. It is shown that the virtual mass of a cyclone is of the same order as the total mass defect. It is shown how elleptical vortices, troughs and wedges may be treated, and a‘front’ is shown formally to be representable as a line distribution of circular vortices. On the basis of the models introduced, a dynamic theory of quasi-geostrophic vortex interaction is put forward, based on the hypothesis that in mutual interaction the pressure profiles of individual vortices are conserved. This theory leads to the conception of a potential barrier inhibiting the over-close approach of two vortices, and permits vortex interaction to be conceived as the motion of attracting and repelling'centres of action.

Variations in thunderstorm severity in Great Britain

Abstract: The paper gives an account of variations in thunderstorm severity in Great Britain based on statistics of breakdowns caused by lightning on the Grid system during the 14-year period 1934 - 1947. Data are given for the seasonal and diurnal variations and for the fluctuations from year to year. The breakdown data are compared with the corresponding statistics of thunderstorm-days, and it is shown that the thunderstorm-day observations, if used directly, do not give an adequate indication of storm severity. It is proposed to introduce a “severity index” (s) such that the severity is proportional to Ts, where T is the number of thunderstorm-days. For the data examined, s has values of 1.4 and 1.9.

A study of the deviations of wind speeds and directions from geostrophic values

Abstract: From a knowledge of the three-dimensional wind field as a function of space and time, the total wind acceleration and the trajectory curvature can be computed, permitting the evaluation of the geostrophic wind vector without recourse to analysed isobars or height contours. A study of 618 cases at 10,000 ft for a 3-day November sequence, using observed winds over the North American continent, revealed that the mean absolute angle between the reported wind and the geostrophic wind directions was approximately 15°. Large deviation angles were associated with low geostrophic wind speeds, and thus indirectly with low latitudes. The corresponding mean cross-isobar wind component was slightly greater than 5 mi hr−1. The mean absolute deviation between wind and geostrophic wind speeds was of the order of 7 mi hr−1, the corresponding mean relative deviation being just under 30 per cent. Large relative deviations were associated chiefly with low geostrophic wind speeds, hence indirectly with low latitudes. The mean total wind acceleration slightly exceeded 40 mi hr−1 day−1. The mean trajectory curvature corresponded to a radius of curvature of 175 mi. It was found that the mean components of these terms associated, formally, with horizontal and vertical advection were virtually identical in magnitude, illustrating the importance of the vertical velocity contribution to wind accelerations and curvatures. The mean local tendency component was numerically smaller than the two advective terms in the care of accelerations, but numerically larger in the case of trajectory curvatures.

The climatic fluctuation since A.D. 1850 in Europe and the Atlantic

Abstract: Trend-curves of thirty-year means are discussed (a) in general terms and then applied specifically to (b) temperature, (c) pressure, (d) rainfall, (e) wind, (f) temperature gradient and (g) temperature amplitude. Correlations between the departures of the different elements suggest that we can consider the various anomalies as depending on those of pressure and wind. This is notably true of rainfall departures which can be regarded as the sum of two terms, the “area” term depending on the pressure anomaly and the “local” term depending on the vector wind anomaly. High pressure is associated not only with consistently dry years but also with cold years. Variations of wind or pressure gradient are particularly felt as changes of frequency of the prevailing wind. The southwesterly component is of course linked with the “zonal index” and the circulation of the wind systems of the earth. The increase of this component gives more maritime conditions and, except to the lee of hills or mountains, wetter weather. The recent increase explains the main differences between nineteenth and twentieth century climates but not the phases of the transition. These phases necessitate first the concept of a ring of high pressure which, by some south-steering process, left the Arctic Circle about 1880 and reached the Tropic of Cancer about 1915. Preceding and following this ring are rings of high rainfall, low pressure and high temperature-gradient that can be loosely regarded as frontal belts. At any one place therefore, the following sequence of five phases should occur : the first front (Frontal-A stage), the easterly or cold period (T-min), the pressure maximum (P-max), the westerly period culminating in the second front (Frontal-B stage). In the British Isles the five phases explain the climatic fluctuations from 1875 to 1925 and much of the same sequence can be followed in Scandinavia from 1865 to 1915, Italy from 1885 to 1935 and in the United States from 1895 to 1945. Temperature data suggest that this equatorward movement with time may be of more general application, and that the preceding West European climatic fluctuations can also be interpreted as anomaly patterns which spread from the north to the south. Pressure anomalies are normally greater over oceans than continents. The south-steering of anomaly patterns necessitates, therefore, a backing of the prevailing wind in NW. Europe after the pressure maximum (P-max) of about 1895. Such a change is confirmed by the wind records themselves. An effect of persistent wind on ocean temperatures and therefore pressures might well account for the south-steering, but there is as yet no evidence that delayed action effects of this kind occur.

The temperatures at the tops of radar echoes associated with various cloud systems

Abstract: Evidence is produced to support the belief that temperatures at the tops of weather echoes are an indication of the relative strengths of the vertical currents within the echoes. Two types of weather echo are indicated according as the vertical currents are strong or weak, giving support to a theory for two different methods for the production of water drops of raindrop size in the atmosphere.

A note on nocturnal cooling

Abstract: The published methods of calculating the minimum ground temperature after a clear night accompanied by radiational cooling are examined and a more comprehensive calculation is given which takes into account the physical properties both of the air and of the underlying surface. Some recent experimental data are adduced to support the contention that both the ground and the air supply the heat lost by radiation.

Troughs in the Upper Westerlies and cyclonic developments in the Nile Valley

Abstract: In the Middle East Area (M.E.) troughs in the upper westerlies are found, generally, at levels over 2 km above M.S.L. intruding into low latitudes as far south as the Northern Sudan (lat. 20° N.). These troughs usually travel eastwards, but tend to trail over the NW. of the Abyssinian Plateau. They also frequently remain stationary for about three days (on the average) over the SE. Mediterranean, owing to the effect of the plateau of Asia Minor. At the southern limits of these troughs, centres of convergence are usually set up within the westerlies, which have thus extended (at upper levels) into low latitudes, between lat. 22° N. and lat. 15° N. For the purpose of studying them the stream-line map has been found the most satisfactory; the usual pressure map is of little assistance. As a rule a cyclone (say of type “A”) tends to form at a centre of convergence, and travels towards the NE., NNE., or even N.; the northward movement of the cyclones being due to the tendency for them to move towards the “centres” of the quasi-stationary troughs over the SE. Mediterranean. Cyclones may eventually appear on the surface synoptic charts as shallow areas of low pressure. Another set (say of type “B”) of upper-air cyclones, which move south-westwards, tends to form farther south by the setting up of wave-like perturbations in the usual E.–NE. upper current of the atmosphere of the Central Sudan. These perturbations appear at the time of re-establishment of the E.–NE. current over the N. Sudan with the northward retreat of the westerlies, extend into low latitudes, and generally, when an upper-air cyclone of type “A” has reached the M.E., another cyclone of type “B” develops over the East-Central Sudan. The analysis has some value in forecasting upper winds on the Cairo—Khartoum air-route and the local weather associated with these cyclones, and also in studying the general circulation in these latitudes, more especially the characteristic features of these cyclones.

Constant-Height Balloons Calculation of Geostrophic Departures

Abstract: The accelerations observed during the flight of seven constant-height balloons are examined. These observations extended over periods of 2 to 4 1/2 hr. It is found that the values of the geostrophic departures determined from the accelerations are closely comparable with the values deduced independently from contour charts and observed winds. The variations in the acceleration are greater than would be expected from errors in wind determination alone; it is deduced that this is due to the wind at the level of flight being affected by eddies giving rise to fluctuations of the order of 3 to 4 kt in the value of the mean wind determined over half-hour intervals.

The ratio between observed velocities of the wind at 50 feet and 2000 feet over the North Atlantic Ocean

Abstract: The ratio of the wind at 50 ft to the wind at 2000 ft has been calculated for a number of cases from ocean weather ship observations over the North Atlantic and mean values classified according to the lapse rate and the velocity of the wind at 2000 ft. Curves have been drawn showing the variation of the ratio v50/v2000 with lapse rate and v2000 respectively.

Recent work of the Thunderstorm Census Organisation

Abstract: The weakness of the definition of a day with a thunderstorm as any civil day on which thunder is heard at a station, and the great difficulty of interpreting data on such a basis, is illustrated. An example of a thunderstorm chart based on the daily areas of overhead thunderstorms drawn from a close network of recording stations is presented and is compared with the “thunder-heard” results. A method of producing detailed period and average maps, published as County Thunderstorm Surveys, is shown. The similar charting of daily areas of lightning damage provides a convenient and reliable method of examining the distribution of storm severity, and has many practical applications. Analyses of thunderstorms on individual days by means of hourly isochrones, published as Daily Thunderstorm Charts, present new opportunities for detailed comparison with rainfall maps and with radio thunderstorm recordings.