The synoptic situations associated with significant temperature decreases in Buenos Aires are analysed. Knowledge of these situations for the winter season is very useful for the community, especially when they are related to the consumption of natural gas, which is one of the most important sources of energy in Argentina. The rotated principal component method is used to classify the synoptic situations. The significant temperature decreases in winter during the period 1980–1988 occur in five modes at 1000 hPa and three modes at 500 hPa. The relationship between the two levels identifies two dominant patterns. The first has a strong high-pressure system with a northwest-southeast axis and a maximum strength near 80° W, 40° S; this pattern is related to south-southeast winds over Buenos Aires. The second type shows a high-pressure system at 105° W, 48° S and a minimum near 110° W, 30°–35° S; this kind of pattern is associated with persistent blocking situations in the South Pacific which result in persistent winds from the south over Buenos Aires. In addition, a relationship between such situations and the resulting increases in the demand for natural gas in residential areas is shown. This will help in the forecasting of such events in the future.

Design floods for major dams throughout the world are based on the probable maximum flood (PMF) which results from the probable maximum precipitation (PMP). Estimates of PMP can be made using a number of methods, the most favoured being the hydrometeorological method involving maximisation and transposition of historic storms. In this paper the in situ maximisation and storm transposition approach has been used for estimating PMP for a one-day duration for different locations in India. Based on these PMP values a generalised map of one-day PMP was produced. It was found that one-day PMP over the Indian region varied from 70 cm to 170 cm, with high values in the mountainous areas of the Western Ghats and in some locations in the north-eastern parts. These results are considerably higher than those obtained using statistical analysis of daily rainfall records using Hershfield's method and they have serious implications for dam safety over much of India.

This paper undertakes a preliminary study into the temporal recording of road accidents in adverse weather. Monthly police accident data, by county, have been utilised for England and Wales, and weather-related road accident findings compared with the general weather patterns experienced over the British Isles. The results show that seasonal patterns to weather-related road accidents match the occurrence of hazardous conditions, with regional variations to weather patterns identifiable within the accident data.

The significance of the atmospheric stability approach in models for estimating evapotranspiration is studied by considering the errors encountered when simply adopting neutral instead of actual stability conditions. Maize, cotton and wheat crops grown in 10 hectare fields (central Greece) were used to estimate actual and maximum evapotranspiration for both stability considerations, employing a model based on the equations of Shuttleworth & Wallace (1985). The required meteorological data were recorded in each field continuously, whereas the leaf area index and the plant height were measured weekly. Results show that when estimation of cumulative evapotranspiration losses is attempted, the choice of the atmospheric stability approach is of no importance (error ≤ 2%), as over- and under-estimation of evapotranspiration are counterbalanced when stable and unstable situations are (under the climatic conditions of experimentation) successively interchanged during the cultivating period. When, however, daily estimations are required, the actual stability conditions have to be taken into account, as the daily evapotranspiration rate estimated by regarding the atmospheric stability as neutral may deviate from the corresponding ‘actual’ rate by as much as 50 – 80%.

A regional numerical weather prediction system, consisting of an analysis scheme, a forecast model and a variety of interpretation, visualization and military application tools, named ‘BLM’ has been routinely operating at the German Military Geophysical Office (GMGO) since 1984. At that time, the grid domain was confined to Central Europe and parts of the North Atlantic, where a polar stereographic map projection was appropriate. Owing to increasing user demands, the system had to be made relocatable to provide numerical forecasts at any location of the globe. The relocatable version, called ‘RBL’, is still formulated on a stereographic plane, which is now defined using a rotated geographical system with an artificial north pole right in the middle of the domain. This approach is well known. However, most of the current numerical weather prediction systems are defined on a horizontal latitude-longitude grid, so that the equator in rotated coordinates runs through the centre of the forecast area. The main advantages of the horizontal coordinate transformation used by the ‘RBL’ are numerical stability for a large and economical time step, which enables the model to be run on a computer with limited power, and an invariant map factor all over the globe, which eases the interpretation of the model results for different locations.

An automated contrail detection algorithm has been applied to AVHRR data to obtain the position of contrails. Nearly simultaneous data from the Europe Model of the Deutscher Wetterdienst have been used to find typical weather conditions associated with 742 persistent contrails. Further, a flow pattern analysis identifies typical regions where the occurrence of contrails was above average. These regions are in the upper atmosphere: (a) ahead of a surface warm front either in moist warm layers before the cirrus clouds arrive or more likely with the cirrus in a warm conveyor belt and (b) ahead of a surface cold front in rapidly moving cold air in the turbulent regions near a band of strong wind (though the speed is not necessarily as high as in a jet). Usually, the atmosphere is baroclinic in the contrail region. Most of the detected contrails occur in divergent flow patterns in the upper troposphere in slowly rising warm or locally turbulent cold air masses.

‘Koshava’ is a gusty wind of moderate to strong intensity, blowing from a south-easterly direction, over Serbia, Romania and Bulgaria. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. This paper analyses wind data from an area where the influence the ‘Koshava’ wind is high during periods of maximum duration of ‘Koshava’. The focus of the paper is the examination of urban and suburban effects on ‘Koshava’ and the correlation between the instantaneous maximum wind speed and the hourly mean values. Various empirical distributions are fitted to the daily maximum wind speed and the best fit for each season and site is proposed. The vertical structure of the lower troposphere above a suburban site during the longest period of consecutive days of ‘Koshava’, which occurred in January and February 1972, is considered.

The difficulties of forecasting for flights of hot-air balloons and small airships are discussed, as well as methods which may be used to improve forecast accuracy. The low-wind-speed requirements and details of weather forecasts required for safe flight are near the limits of accuracy of the forecaster, even when aided by computer models. Preparations for the briefing of balloon pilots need to be carefully undertaken.

The existing models for predicting road surface temperature are often based on surface energy balance which includes contributions from sensible and latent heat exchanges between the atmosphere and road surface. The similarity theory is usually applied to compute the atmospheric turbulent heat fluxes. However, the theory only applies in horizontally homogeneous situations, which is certainly not the case for road conditions. An investigation is carried out to see how much measured profiles of mean wind and temperature from a road meteorological station deviate from the profiles given by similarity theory. The difference between the data and the theory is revealed by comparing the observed gradients with the theoretical ones. The sign of the vertical gradients of all data and the relationships between two vertical gradients at different levels under near-neutral conditions are examined. It is found that the measured profiles are all systematically different from the theoretical ones. The deviation is discussed within the context of the possible existence of an internal boundary layer. Moreover, detailed temperature profiles were measured to study the development of temperatures in the first 2.5 m. The results show that the sign of the temperature gradient reverses in a layer between 0.8 and 1.4 m and that a large change in temperature occurs in this layer; these effects were probably caused by local advection.

A small wave on the polar front crossed the Atlantic and experienced strong explosive cyclogenesis on 28 March 1997 when passing over the North Sea. It then progressed eastwards over northern Poland causing severe damage and the death of at least 11 people. A phenomenological study of the event is presented based on a potential vorticity (PV) approach supplemented by a synoptic analysis. Results confirm previous findings about the impact of a dry stratospheric intrusion on the process of explosive cyclogenesis and fit well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. Also apparent is the role of potential vorticity advection, which is associated with the rapid change of flow pattern in the upper atmosphere. The careful analysis of potential vorticity fields every 100 m suggests that further research is required into fine mesh analysis and wind field retrieval.

The performance of three automatic depression tracking schemes developed by Terry & Atlas (1996), Murray & Simmonds (1991a) and Konig et al. (1993) when applied over one month are assessed. The schemes respectively identify depressions by (a) locating the innermost closed contour in a PMSL field, (b) finding maxima in the curvature in a bi-cubic spline fitted to the PMSL data and (c) performing a grid point search to identify a minimum in a PMSL field. The largest number of depressions was found by the Murray and Simmonds scheme, with the Atlas and Terry scheme finding the least. The Murray and Simmonds scheme also found the largest number of tracks, with the other two having comparable numbers of tracks. Two possible explanations for the differences in the number of tracks are considered. Firstly, the case where one or more centres identified by one scheme as corresponding to a single track are not found by the other schemes. Secondly, the case where a single track found by one scheme is split into two or more tracks by the other schemes. All three techniques had a similar latitude of cyclogenesis, although the Atlas and Terry and Konig et al. schemes found more lows at high latitudes as a result of using data on a latitude/longitude grid. The longest mean track length was found with the Konig et al. scheme. A comparison of the PMSL fields with satellite imagery shows that the major NWP centres have trouble producing reliable analyses around the Antarctic because of the lack of data.

This paper describes the meteorological variables directly affecting precipitation in the southern region of South America. Results show the relationships between daily precipitation at Ezeiza, Buenos Aires, and 500 mb dynamic variables (e. g. geopotential heights, relative geostrophic vorticity and its advection, vorticity tendency and zonal and meridional geostrophic wind components) using regional synoptic analyses from the Servicio Meteorológico Nacional of Argentina for 1986. The distribution of significant biserial correlation coefficients reveals the preferred positions for the anomalous wind components which favour the occurrence of rainfall at Ezeiza. In the same way, spatial distributions and temporal lag relations of 500 mb relative vorticity anomalies in relation to precipitation show the positions and relative movement of centres of anomalously cyclonic and anticyclonic vorticity which increase the probability of precipitation at Buenos Aires. Vorticity advection does not yield higher correlations, at least in a daily temporal scale. The geographical distribution of temporal means and standard deviations of geopotential heights and zonal and meridional wind components at the 500 mb level are also shown.

A sub-synoptic vortex with characteristics of a tropical storm developed over the area between Italy and Greece, during the period 14–18 January 1995. Satellite imagery revealed that this vortex was associated with spirally distributed bands and a clearly defined ‘eye’. Ship reports and SSM/I retrieved winds show the existence of strong surface winds around the vortex. Simulation of the genesis and life-cycle of the vortex using the Regional Atmospheric Modelling System and the Eta model successfully reproduced the vortex formation, its strength and its path towards the northern African coast. The model results showed the importance of the surface fluxes (sensible and latent heat) in the development of the vortex on the rear side of a parent low-pressure system which was advected towards the east. During the mature stage of the vortex, latent-heat release within the convective motions was the dominant mechanism which sustained the vortex until its landfall.

The present study deals with the thermal mapping technique in a road climatological perspective. The temperature mapping technique is analysed in relation to instrument set-ups, data collecting and possible sources of errors in converting measured radiation to surface temperature values. Furthermore, analyses of thermal mapping recordings are discussed. Factors that give rise to a varying temperature pattern are presented (i.e. road-bed material, radiation and advection), and also how the thermal mapping technique can be used to determine the relative importance of these factors. The conclusion of the study is that the technique is very useful but it is very important to be aware of the limits and different source of errors involved.

Trends and periodicities of annual rainfall for 29 sub-divisions of India and all India (area-weighted average of the sub-divisions) are studied by using the rainfall series for a period of 124 years (1871–1994). The trends are evaluated by using a linear regression technique. To identify the climatic changes, the rainfall series are subjected to 11-year moving averages. These averages show some epochs. From this, it is found that in some sub-divisions the trend in one direction reverses its direction after some years. The years pertaining to these changes are identified and the trends are calculated for the segments of the series. The significance of the trend values are tested. Power spectra are obtained using the maximum-entropy method. The periods in the range 2.0–2.5 years are present in the rainfall series of the sub-divisions to the north of 20 °N, on the west coast, and in Tamilnadu and for all India. This periodicity may be attributed to the quasi-biennial oscillation. Rainfall series of the most of the sub-divisions and all India indicate a triennial cycle. Significant periods in the range from 3.0 to 8.0 years and 8.0 to 12.0 years are also identified.