Search

Much micrometeorological work to date has assumed, or tried to ensure, that conditions of horizontal homogeneity exist at and near the earth's surface. The present paper reports an observational study of the variation in the meteorological elements with distance and height downwind of a pronounced discontinuity from dry to wet surface conditions. In particular the temperature and humidity fields were observed together with the changes in the radiative-balance components and in the wind structure. It was found that temperature and humidity changes at a height of 5 cm within 16 m of the discontinuity were commonly as much as 5$C and 5 g m−3 respectively. Theoretical treatments of the problem are outlined, and a slight modification of Philip's (1959) analysis is used to compare observed and predicted changes in temperature and humidity with distance downwind up to a height of 150 cm. It is found that the observed humidity changes are in good agreement with expectations, but that the measured temperature changes are about half those predicted. Changes in the evaporation rate in various distance intervals downwind from the leading edge have been computed. These are compared with an estimate of the rate which would have occurred under the same conditions from a wet area of infinite extent. The discrepancies between theory and observation are attributed to the fact that the available theory fails to take into account the (demonstrable) differences in the aerodynamic properties of the adjacent surfaces. There is scope for further theoretical and experimental work on this matter.

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.

An account is given of the particular qualities which are necessary in service meteorological instruments and reference is made to the historical background which has considerable implication for current practice. Methods at present in use for the measurement of a few parameters are outlined. Finally a description of a recently introduced automatic observing system is provided to illustrate the latest techniques.

Experiments are described in which the horizontal wind speed at three heights up to 2 m is measured at various distances up and down wind from a dense hedge 1·68 m high on occasions with wind directions approximately normal to the hedge. The results show that the hedge begins to exert an influence on the wind speed at 2 m at a distance equivalent to approximately 15 hedge heights upwind, and that downwind its influence extends to a distance approximately equivalent to 60 hedge heights. At the two lower heights of wind-speed measurement the range over which the hedge has an effect is smaller. Diagrams are constructed to show the decrease in wind speed at the various heights within the distance range mentioned above.

An account is given of an experimental study of the factors which control the vertical turbulent transport of momentum, water vapour, and heat in the first 2 m of air above a short grass surface. The results of fifty-one observations, each of which extended over 30 min, are presented. Measurements were made of the vertical profiles of wind speed, temperature and humidity, and of the rate of evaporation and the aerodynamic drag of the surface. Measurement of the heat-balance components was also attempted. On any one occasion it was not generally possible to observe all the desired quantities. It is found that the wind speed and temperature profiles always have the same form, and that this form is shared on the majority of occasions by the humidity profile. The profiles depart from the logarithmic form found in adiabatic conditions in the manner suggested by Deacon. In near adiabatic conditions as defined by small numerical values of the Richardson number the established laboratory law relating the surface drag to the fluid velocity over a rough surface in turbulent flow is found to hold over the site and a value is deduced for von Kármán's constant. In conditions other than adiabatic it is shown that the explicit wind profile law suggested by Deacon holds in unstable but not in stable conditions. In the latter conditions equations proposed by Rossby & Montgomery and by Holtzman are found to represent the observations with reasonable precision. The friction coefficient of the surface is computed and found to be independent of wind speed but to increase in value in unstable conditions. Values of the eddy diffusivity for momentum, water vapour and heat are obtained on a direct observational basis from the expressions from which the diffusivities are normally defined, and it is found that the diffusivities for momentum and vapour are identical over the range of stability experienced. The magnitude of the diffusivity for heat often appears to be approximately the same as that for momentum or vapour, but exceptions occur when it is much larger than the other two. The exceptions do not appear to be related to stability.

The profiles of temperature, humidity and wind have been determined by conventional methods in the lowest 2 m of the atmosphere above a close-cut grass surface, with simultaneous observations of the heat absorbed in the soil and of the short-wave and long-wave radiative flux of heat. Twentynine sets of observations were made; the details are given in Appendix I. The ratio of the gradients of temperature and humidity is found to be independent of height, and changes with height of the vertical convective flux of heat and of water vapour are relatively small; the ratio of the coefficients of eddy diffusion of heat and water vapour is therefore independent of height. These coefficients are then assumed equal and the convective fluxes of heat and of water vapour determined (Bowen's method). The fluxes are also determined by an aerodynamic method which assumes equality of the eddy viscosity and the appropriate diffusion coefficient. The results of the two methods agree in most cases, in both stable and unstable stratifications, but there are some exceptions which cannot be explained. Absorption of radiation by the air is investigated. It is shown that this is comparable in effect with convection in the control of temperature and relative humidity. Particular attention is paid to four occasions with very light wind (including two with fog formation) when horizontal mixing appeared to be of major importance. Some examples are presented of wind profiles which do not conform to the law proposed by Deacon. A value between 0.5 and 1.5 is suggested for the critical Richardson number.

A surface energy-balance computer is described. This is fed with such information as dry- and wet-bulb temperatures and temperature differences in the vertical (in the form of fine wire thermocouple outputs) and wind speed differences. The psychrometric equation is solved automatically to yield a vapour-pressure difference and the latent-heat term is then computed on the basis of an aerodynamic equation. The sensible-heat term is similarly made available. The net flux of radiation and the soil-heat flux are also recorded. A determination of each of these four fluxes is made once per minute and the values are integrated and presented on counters which are photographed at desired intervals, the smallest interval so far used being 20 min. Results indicate that for periods of an hour or more the ‘positive’ and ‘negative’ terms in the surface-heat balance equation so obtained, agree to about 10 per cent except when the heat storage in the first two cm of soil, of which no account is taken, is obviously changing. The only complete 24 hr observational period so far covered indicates a daily balance to a fraction of a mw hr cm−2. Over approximately 80 hr observations, which were not continuous but contained about an equal number of daylight and dark hours, a balance to better than 1 per cent was obtained. A further result, which requires additional confirmation, is that the daily water loss from the centre of an extensive short-grass area which has an ample water supply may be obtained for all practical purposes by measuring the total net radiation and assuming that all is used in the evaporative process. It becomes obvious that if hourly mean values of the profiles of temperature, humidity and wind are used to compute the evaporation then, in general, the results will be in error by up to about 20 per cent, although daily totals may be more accurate than this. The machine as it stands is not reliable for continuous operation owing to the use of a number of electro-mechanical devices which are required to work under conditions for which they were not designed. Some improvements will be required before reliable continuous operation becomes possible.

Moderator Kraght summarized the progress of commercial aviation in the past 33 years. Weather problems continue to plague the airlines, although different weather parameters now cause the greatest difficulties. Aircraft icing is not the serious problem to jet transports that it is to propeller craft. Fog and ceilings below landing minima are among the more serious problems. To a degree, we have become inde­pendent of the low clouds and fog down to a certain limit by using the navigational facilities that have been provided. Thunderstorms and associated turbulence in the terminal area as well as enroute continue to be hazardous to safe flight, although encounters with severe turbulence occur on only a few per cent of all flights.

A method of digitizing the deflections of a reflecting galvanometer and presenting the results directly on electromagnetic counters is given. The use of expensive dc amplifiers and/or multipoint recorders can often be avoided and automatic integration for time mean values of physical quantities is conveniently achieved.

An account is given of measurements of temperature and humidity at a number of positions and heights within a tobacco crop during a 4 day period. Measurements of the intensity of sporulation of Peronospora tabacina Adam were made at the same time. It is shown that the capacity of P. tabacina to sporulate is controlled not only by nocturnal humidity but also by day temperature. The observations confirm that Cruickshank's laboratory findings hold in the field. The implications of the observations reported here, together with those of other workers on such factors as the spore dissemination and germination characteristics of P. tabacina, are discussed. It is concluded that the basic information necessary to initiate, on a trial basis, a blue mould forecasting and spray warning service is now available.

An investigation of the influence of Peronospora tabacina Adam on the transpiration and growth rates of the leaves of Nicotiana tabacum L. in three environments is described.