Showing papers in "Quarterly Journal of the Royal Meteorological Society in 1954"

The microwave properties of precipitation particles

Abstract: The theory of scattering and attenuation by rain, snow and cloud is reviewed and theoretical results are presented in the form of equations, tables and graphs, so that the radar response to meteorological particles can be calculated at six wavelengths (10, 5.7, 3.2, 1.8, 1.24 and 0.9 cm) and various temperatures. Particular emphasis is placed on developments since Ryde's comprehensive paper in 1946. Published experimental results are compared with the theory. All results computed from the theory are contained in Tables 4 and 5. The attenuation by water vapour and oxygen is given in an Appendix.

Solar radiation and the duration of sunshine

Abstract: Records of solar radiation (Q) and of duration of sunshine (n) on a monthly basis have been collected for 32 stations and regression equations relating the data calculated in the form Q = QA (a+bn/N) where QA is the maximum possible radiation in the absence of an atmosphere, and N is the maximum possible duration of bright sunshine. For five of these stations records for long series of individual months have been examined and for all stations mean monthly values. In the overall regression equation a = 0.23 and b = 0.48.

The terminal velocity of snowflakes

Abstract: Measurements of the terminal velocity of aggregate snowflakes at the ground indicate that the velocity is approximately proportional to the one-tenth power of the mass. The constant of proportionality is dependent on the basic crystal types in the aggregates. It is moreover greatly increased by riming or by slight melting of the snowflakes. When snow grows markedly at a generating level, the rate of fall of the snow particles as determined from radar observations is found to correspond to the velocities of aggregates. It is thus inferred that aggregation can occur at temperatures far below the freezing point.

Air pollution aspects of the London fog of December 1952

Abstract: This four-day fog caused at least 4,000 deaths and much illness; it therefore ranks as the most disastrous event of its kind known to have occurred anywhere in the world. For the first time in an incident of this kind extensive measurements were made of the daily concentrations of smoke and sulphur dioxide and of the general pattern of pollution throughout the area. Mean concentrations of smoke and sulphur dioxide at 12 daily test-sites increased sharply on the first day of the incident and, after 3 to 4 days, averaged 1.6 mg/m3 and 0.7 parts per million respectively, or about 5 to 6 times normal. At individual sites in the central London area concentrations rose to 10 times normal. There was a striking parallelism between the mean daily concentration of smoke and sulphur dioxide and the total number of deaths on those days but the latter returned the more slowly to normal. A few data, relating to a severe London fog of 1948, indicated that the concentrations of pollution then reached about 50 to 70 per cent of the 1952 values. The approximate distribution of sulphur dioxide is shown on a map which indicates zones of maximum pollution mainly in the riverside areas, and particularly Westminster and adjacent boroughs.

THE AIRFLOW OVER MOUNTAINS. A review of the state of current knowledge

Abstract: A brief survey is first given of the observational evidence regarding special airflow effects in the neighbourhood of mountains, as provided by the visual evidence of clouds, the experiences of glider pilots and effects noted by the pilots of powered aircraft. This serves to indicate the phenomena requiring explanation. The review of theoretical studies is confined mainly to the work of Queney and Scorer. Both these authors apply the perturbation method to laminar, isentropic and inviscid flow over an ideal ridge. Queney deals with a uniform airstream (constant velocity and constant stability) and derives streamlines showing wave motions, both vertical and horizontal. Queney's waves are dependent partly on the stability and partly on geostrophic forces, and the order of the wavelength is several hundred km. Scorer, in his treatment, deals with smaller-scale phenomena, so enabling geostrophic forces to be neglected, and provides for variations of stability and wind shear in the vertical. He finds that realistic wave effects, of the type observed in nature are predicted by the theory provided the variation of stability and wind shear with height is in accord with certain criteria. Past experimental work with models in wind tunnels is briefly considered in the light of what can be deduced from theoretical treatments regarding the conditions for dynamical similarity. More recent work by Long, using a tank containing several layers of immiscible liquid to simulate a density gradient is referred to briefly; this technique appears to have promise. A comprehensive field study by Forchtgott, who wed teams of gliders, is summarized. From his observations, Forchtgott has built up a theory of the flow patterns which occur in the lee of mountains. The main value of Forchtgott's work lies in the practical support it provides for some of the results deduced by perturbation theory and in throwing light on the nature of non-laminar flow patterns which cannot be dealt with by existing theoretical treatments. In a concluding section, aviation applications of existing knowledge on this subject are referred to briefly. The criteria have some practical value for forecasting purposes.

The concentration, size distribution and production rate of large salt nuclei over the oceans

Abstract: The sampling and measurment of large condensation nuclei (equivalent salt contents greater than 2 × 10−14g) on board of Weather Ships in the N. Atlantic, 200 mi from land, reveal the existence of two distinct types of size spectra. It seems that the nuclei which predominate when waves are breaking (Type I distribution) are the residues of spray droplets, but that those which occur with wind speeds of less than 7 m sec−1 have a (Type II) distribution which closely resembles that of continental aerosol. In winds of up to 15 m sec−1 the measured concentrations of large salt nuclei rarely exceeded 10/cm3. From the measured concentration and size distribution of Type I nuclei, it is deduced that in winds of 15 m sec−1 the rate of production at the sea surface of salt nuclei with mass greater than 10−13g is about 40 cm−2 sec−1; very similar values have been obtained from laboratory experiments in a wind-wave tunnel. The corresponding rate for nuclei with m >2 × 10−14g would be about 100 cm−2 sec−1. Laboratory studies show that the bursting of air bubbles of diameters between 0.3 mm and 4 mm at the surface of sea water produces small numbers of rather large drops (diameters 50-500μ), the majority of which would fall back quickly into the sea. The large salt nuclei found in the atmosphere are probably the residues of smaller drops produced by disintegration of the bubble caps.

Properties of spectra of atmospheric turbulence at 100 metres

Abstract: The properties of spectra of vertical and horizontal turbulence and of their cross-spectra, at elevations near 100m, are summarized; a few spectra from lower levels are considered. The following tentative conclusions are drawn: 1At frequencies of the order of 100c/hr and above, the spectral intensity is proportional to the square of the mean wind speed and essentially independent of the radiation intensity. At lower frequencies, the ratio of spectral intensity to the square of the wind speed increases with increasing incoming radiation. 2The intensity-ratio of the co-spectrum of vertical and horizontal velocity to the spectrum of vertical velocity increases with increasing wind shear. It also decreases with increasing frequency, n, approximately as n−1. 3At low frequencies, the spectral intensity of the horizontal velocity components is generally much large than that of the vertical velocity. At high frequencies, the spectral intensities of all velocity components are of the same order of magnitude. At low frequencies, the ratio of vertical to horizontal spectral density increases with increasing radiation. 4The quadrature spectra at a given frequency tend to be negative with large radiation, positive with little radiation. This indicates that edies are low and wide with little radiation, and tall with much radiation. 5The frequency at the maximum of the vertical velocity spectrum decreases with increasing height and increasing radiation intensity. 6The ratio of tubulent to mean energy is somewhat larger for air with over-land trajectories than for air with oceanic trajectories.

Secular changes in the rainfall regime of SE. Australia

Abstract: Analysis of long-series rainfall records from Victoria and New South Wales shows a decrease of summer rainfall to a minimum about the turn of the century, and fifty years' gradual increase since then. Winter rainfall trends were opposite. Fluctuations of a much larger amplitude affected the rainfall regime of autumn and spring, suggesting possibly a discontinuous change of the climatic development about 1893. The winter rainfall of southern New South Wales is positively correlated with the mean strength of the westerlies at 300 mb. The correlation is negative in summer. It is concluded that the changes of the rainfall regime were associated with an increase of intensity of the upper westerlies to a maximum at about 1900 and a following decrease. During the period of maximum mean westerly flow the formation of east-coast cyclones in autumn and spring may have been inhibited and the monsoon rains of tropical Queensland were below normal.

Turbulence in clouds as a factor in precipitation

Abstract: Visual and radar observations of the height of formation of precipitation in shower clouds in relation to temperature, and measurements of droplet population, show that when cumulus clouds produce showers it is often by coalescence, rather than by an ice-phase process. Langmuir's work gave the conditions necessary for collisions to take place between droplets. The droplets of young cumulus clouds are less than 20 microns in radius and calculation shows that they are too small and settle too slowly by gravity alone to grow at an appreciable rate by colliding with each other. The updraught in a cumulus cloud is highly turbulent so that the air containing the droplets is being perpetually accelerated in a random way. An analysis of random acceleration in one dimension shows that it would produce collisions if violent enough, and in conjunction with gravity would start droplets into growth. Once their radii reached about 20 microns they would continue to grow with or without turbulence. Such a mechanism is consistent with the observed changes in droplet size distribution. Provided that sufficient energy were available in a suitable frequency range, turbulence would initiate a shower in a cumulus cloud. Numerical confirmation of the mechanism awaits suitable measurements of small-scale turbulence in cloud, but meanwhile an alternative approach through macroscopic energy considerations might prove fruitful.

A study of blocking in the Atlantic-European of the northern hemisphere

Abstract: Various interpretations of what is meant by ‘blocking’ in the atmosphere are discussed. The three main aspects referred to by writers on the subject are: (a) a sharp local diminution of the mean zonal (tropospheric) flow, often associated with a bifurcation of the jet stream, which diminution is (b) rather persistent and (c) displaced westwards with time. Not all of these attributes have been insisted upon as defining blocking but usually (a) and (b) are required. The view is taken herein that, for forecasting purposes, only (a) is admissible, and a selection of instantaneous patterns having this characteristic is made, two broad types being distinguished, for the period 1949-52 and in the Atlantic-European sector of the northern hemisphere. Some statistics relating to the seasonal and geographic distribution of these patterns are given, which in the main confirm previous results. Blocking patterns were found to occur on more than half the days, individual blocks lasting on an average 16.5 days. They were most frequent in May and least frequent in July, and tended to be displaced from Europe to mid-Atlantic during the first six months of the year. Within this general westward drift several successive ‘waves’ of progression and retrogression were in evidence. Blocking over N. America was extremely rare. This position and movement of individual blocks is presented in some detail, and notes on their formation, dissipation and general character are also given.

Theory of airflow over mountains: III ‐ Airstream characteristics

Abstract: The problem of what second boundary condition (the first being the ground profile) should be applied to the differential equation for the variation with height of the wave amplitude is considered and it is thought that solutions corresponding to upward and downward progressing waves are generally irrelevant. In practice we shall be concerned mainly with those waves of length such that their amplitude decreases with increasing height. Only when there is a considerable range of such wavelenghts is the motion of the kind supposed in the perturation theory. Though it is desirable to characterize an airstream by a single number this is not at present possible for airstreams in general, and it is shown by examples how airstreams may possess the same lee wave length though their l-profiles differ considerably. These examples show that generally the maximum amplitude of the waves is found at or close to levels of maximum values of l, at sharp inversions in particular. The behaviour of lee (statopmaru) waves is fairly representative of mountain waves in general.

Provisional maps of the reliability of annual rainfall in East Africa

Abstract: East Africa is a dry country. Its increasing population, which is dependent on agriculture, seeks expansion into new agricultural lands. Most of the available land lies in regions of low and variable rainfall. One of the most important requirements in the selection of sites for future development is that they should be subject to a reasonably reliable rainfall suited to the potential crop. Earlier investigations have shown that the distribution of annual and seasonal rainfall in East Africa can be assumed to be normal for all practical purposes. On this basis it is possible to calculate the probabilities of receiving certain selected minimum values suited to a crop, and these can be combined for mapping purposes. Maps of the reliability of annual rainfall at two selected levels illustrate the potentialities of the method. The meteorological background to the maps and their application to agricultural development are briefly discussed.

Wind stress on a water surface

Abstract: Values for the shear stress τ0 of the wind on a water surface, found in seven later experiments, are compared with the data of Francis (1951). Four sets of the new data show that the stress coefficient C = τ0/ϱu2 is not constant, but that it increases somewhat with windspeed u (ϱ = air density). Three sets merely show a large experimental scatter of C, and do not support any consistent law of drag. A theory is also discussed that τ0 is mainly caused by the drag of the small ripples, and not by the drag of the big waves.

Modification of the size distribution of falling raindrops by coalescence

Abstract: The manner in which the size distribution of raindrops is modified during their fall by mutual coalescence, accretion with cloud droplets, and by evaporation below cloud base is investigated It is shown that if a constant flux of raindrops having an exponetial size distribution occurs at the level of origin (just below the melting level in a layer-cloud) to give a steady-state, this initial distribution will undergo considerable modification after a fall of 1 km, the smaller drops being seriously depleted and the larger ones increased All three processes will cause a shift in the drop-size spectrum towards larger diameters and produce an increase in the intensity of the radar echo at lower levels of the kind which is sometimes observed Because of this modification of the drop-size spectrum during fall and because of the rather rapid fluctuations which are frequently observed in the distribution at the ground, it is not often possible to correlate the latter with the intensity of the radar echo located higher up

Evaporation from an oat field

Abstract: Evaporation from natural surfaces may be evaluated by a method based on aerodynamic considerations which lead to the simple equation in which E, p and k are the rate of evaporation, the air density and von Karman's constant respectively, u1, u2, q1 and q2 are the wind speeds and specific humidities at the heights z1 and z2, and d the so-called surface zero displacement. This formula has been applied to observations taken over an oat field in order to determine the hourly values of E during a number of periods ranging in length from one to twelve days. The magnitude of d is shown to increase with increasing crop height and wind speed, the crop height increasing eightfold over the total period of observation. Daily water losses are calculated and found to vary from 0.05 mm to 5.65 mm. By using two sets of apparatus, the calculated values of E are shown, in unstable conditions, to depend on the height of exposure of the apparatus, decreasing with increasing height. A modified form of the equation, based on Deacon's generalized wind-profile law, has been used in the last period of observation and found to reduce the dependence of the calculated values of E on the level at which the apparatus was placed. The general application of the modified expression was found to be impossible owing to insufficient precision in the measurements of the wind speeds and to the uncertainty in the value to be assigned to d. In general, the computed daily water losses are considered to be underestimated to an extent not exceeding 15 per cent.

Airborne measurements of the concentration of natural and artificial freezing nuclei

Abstract: An aircraft-mounted instrument for measuring the concentration of freezing nuclei in the free atmosphere is described. It has been calibrated and its limitations investigated. Preliminary observations, in flight, of natural freezing nuclei were made. The temperature at which a given concentration of ice crystals formed on natural nuclei varied markedly from day to day, and increased with altitude. Measurements in flight of the concentration of freezing nuclei in silver-iodide smoke, released from a ground generator, are described. The nucleating properties of small particles produced by a hydrogen burner deteriorated very rapidly, while larger particles produced by a kerosene burner were more durable.

A sensitive recording dew‐balance

Abstract: The instrument described gives a continuous record on photographic paper of the change in weight of a naturally exposed grass surface, 330 cm2 in area. The sensitivity is 2 mg cm−1 per cm of record and the time scale is 0.5 hr/cm. Observations suggest that on very calm nights the condensation of vapour reaching the surface by diffusion from the atmosphere is only a small fraction of the total ‘dew.’ The maximum rate of deposition that has been observed on short grass is 0.03 mm hr−1.

A method for recording the formation and persistence of water deposits on plant shoots

Abstract: Apparatus has been designed to record the amount of water deposited on plant shoots by rain, dew and guttation, and how long the surfaces remain wet. These factors greatly influence the extent to which plants are infected by fungi causing such diseases as potato blight. The water on a cut potato shoot, sealed into a water-filled chamber placed on a balance, can be weighed by recording, on a rotating drum, the changes in equilibrium of the beam. Deposits from rain appear rapidly; their persistence depends on the weather. In contrast, dew is deposited slowly over a long period and dries more rapidly. The heaviest dew deposit recorded was 6.9 × 10−3 g cm−2 compared with 9.6 × 10 g cm−2 for the amount of water retained during rain.

Short-period variations in surface pressure and wind

Abstract: Periodic variations in surface pressure and wind velocity recorded by autographic instruments on 5 July 1952 are related to wave motion along a horizontal temperature discontinuity. The wave motion is considered to have been set up when the outflow of cold air from an intense outbreak of thunderstorms over the western English Channel temporarily blocked the north-easterly airstream beneath an inversion over southern England and the Midlands. The oscillation was propagated upstream of the lower current and across the upper south-easterly current for over 250 mi, with an amplitude approaching 200 m in the earlier stage.

Natural removal of atmospheric pollution during fog

Abstract: The average lives of smoke particles and sulphur-dioxide molecules in the air are compared in average and foggy conditions. Possible mechanisms of removal during fog are considered.

The nature of convection as revealed by soaring birds and dragonflies

Abstract: Birds and dragonflies can be used as indicators of the extent and intensity of vertical currents, particularly of the nature of convection below cloud base during its period of growth. The bubble theory of Scorer and Ludlam is used to interpret the observations of Hankin in India. The possibility of deriving information from the flight of swallows in Britain is also discussed. In hot sunshine and no wind the air appears to stream steadily upwards from fixed places for the first few metres. The birds indicate that convection develops upwards, the heavier ones which soar at greater heights taking to the air last. The upcurrents are stronger and wider at greater height and drift with the wind; they react more slowly at greater height to changes of sunshine on the ground. When convection is intense the upcurrents appear more sharp-edged. Puffs of wind are associated with the descent of air from above the superadiabatic layer to replace a part of that layer that is ascending as a newly-created bubble. When the air is stirred mechanically by wind, bubble formation in the lowest layers is reduced. It is suggested that air movements on the scale of ordinary convection are an element in the climate as important as any other in the life of many birds.

Observations of rainfall from warm clouds

Abstract: Rainfall from non-freezing clouds has been widely reported but detailed observations are sparse. During June 1952, ten days of observations were made during trade-wind weather conditions on the island of Oahu. Rainfall was observed during each of the 10 days, and in no instance did cloud temperatures below 7°C occur. Aircraft were used to obtain temperature and humidity soundings supplementary to the Honolulu radiosonde and to measure the dimensions of the clouds. Raindrop sizes were recorded on a continuously moving dye-treated paper tape at two locations. Data on rainfall were obtained from 5 recording raingauges on the crest of the Koolau Mountain Range and from 32 standard raingauges in an area of 130 m2 Pilot-balloon ascents were made daily on the windward coast of Oahu to supplement the Honolulu Weather Bureau wind data. A continuous record of cloud conditions and motions was made by lapse-time motion-picture photography and supplemented with photographs from aircraft. Rainfall intensities exceeding 3 mm/hr were observed from clouds with bases at 2,000 ft (18.6°C) and tops at 7,500 ft (10.2°C). Raindrop sizes as large as 2 mm in diameter were observed from clouds 6,000 ft deep. Light-intensity rains from small cumulus clouds frequently consisted of raindrops which were relatively large (2 mm diameter) but low in number.

Rainfall in relation to the jet stream

Abstract: A study is made of the rainfall occurring at four British stations on occasions when they lie beneath the jet stream. The mean distribution of rainfall in relation to the jet stream provides evidence, in agreement with dynamical theory, for a direct circulation at the jet entrance and an indirect circulation at the jet exit. Precipitation patterns found for uniform currents provide no satisfactory evidence with regard to the preferred vertical circulations within the central region of the jet stream.

Radar echoes from lighting

Abstract: Photographic observations of radar echoes received from lightning are given and are used to suggest a possible distribution of the electric charge within a thunderstorm cloud.

Five flights through a thunderstorm belt

Abstract: Analysis of accelerometer and other aircraft-instrument records taken on five flights through thunderstorms shows that it is possible to obtain the detailed structure of the vertical air currents within the clouds. A feature of the clouds studied was the close proximity of upward and downward air currents of comparable magnitude, exceeding 20 ft/sec on occasions. Frequency and intensity of gusts increased markedly on entry into cloud and correlation with the radar echo received from the storms showed that all the biggest gusts were recorded within the echoing volume. On four out of five occasions when lightning was seen the aircraft was not in any appreciable upward or downward current, and on all five occasions the cloud had already attained its maximum development.