Standard Meteorological Stations Research Articles

Spatio-temporal variation of reference evapotranspiration and aridity index in the Loess Plateau Region of China, during 1961–2012

Analyses of the spatial and temporal variation of reference evapotranspiration (ET0) and aridity index (AI) can further understand climate change and its effect on hydrology. In this study, based on date from 58 standard meteorological stations, the spatio-temporal variations and trends of ET0 and AI were calculated for the Loess Plateau Region (LPR) from 1961 to 2012 using the Mann–Kendall test, the moving t-test, and the Morlet wavelet methods. The results demonstrated: (1) the annual ET0 displayed a statistically significant decrease of −10.3 mm/10 y (P < 0.05) during 1961–2012. At that time, AI also had a decreasing trend of −0.013/10 y over the past 52 years. Compared with ET0, the inter-decadal variations of AI over the LPR seemed to show similar declined trends. (2) Spatially, about 81% stations displayed a decreasing trend for annual ET0 during 1961–2012, and most of them were distributed in the western, northern and southeastern LPR. In addition, about 60% of stations showed statistically decreasing trends for AI, and the major decreases were mainly in the east Qinghai division and northern LPR. (3) Abrupt changes were detected in annual ET0 series in the 1980s and in annual AI series in 1987. Wavelet analysis indicated significant 2 year and non-significant 7, 10 and 14 years in annual ET0 series, and there existed periods of 4, 8 and 12 years for AI. (4) For ET0, wind speed was the most sensitive meteorological factor, followed by sunshine hours, precipitation, and relative humidity. For AI, ET0 and precipitation were the sensitive factors. This study provides an understanding of the effect of recent climate change on hydrological cycle in the LPR.

Spatial and temporal trend of potential evapotranspiration and related driving forces in Southwestern China, during 1961–2009

Potential evapotranspiration (ETo) is an integral component of the hydrological cycle and the global energy balance, and its long-term variation is of much concern in climate change studies. Trends of temporal and spatial patterns in potential evapotranspiration and related driving factors over southwestern China are evaluated for the period 1961–2009, based on data from 111 standard meteorological stations. The results showed that southwestern China is experiencing a statistical decrease of ETo at a rate of 5.0 mm/10 y during 1961–2009. This decrease mainly started in the 1980s when accelerated warming occurred, which reflected the existence of the “evaporation paradox”. However, autumn ETo showed a similar inter-annual variation with temperature rise. In spatial characteristics, the statistically significant decrease of ETo mainly occurred in the lower altitude region, whereas stations with increases or non-significant decreases are mainly in higher altitude regions. Through the analysis, the driving factors are the following: (1) the decreased wind speed is the main driving force for ETo decrease because the higher saturated vapor pressure would restrain evaporation owing to the weak water vapor exchange under lower wind speed. (2) Increased sunshine hours lead to stronger evaporation. This influence mainly occurred in the Xizang plateau–Hengduan Mountains and Yunnan–Guizhou plateau. (3) The weakly increased precipitation is also linked to the decreased ETo because the precipitation increase will cancel the partially increased ETo. (4) The decreased relative humidity would cause a corresponding decline of saturated vapor pressure, which must lead to the rise of ETo. (5) Climate warming is not the crucial driving factor for ETo inter-annual variation, but the accelerated warming had some influence on summer and autumn ETo, especially during the period of 1991–2009. In addition, the topography also had some influence. Thus, variation is dominated by the comprehensive functions of multi-factors, owing to the complex nature of ETo.

Forest fire danger rating in complex topography – results from a case study in the Bavarian Alps in autumn 2011

Abstract. Forest fire danger rating based on sparse meteorological stations is known to be potentially misleading when assigned to larger areas of complex topography. This case study examines several fire danger indices based on data from two meteorological stations at different elevations during a major drought period. This drought was caused by a persistent high pressure system, inducing a pronounced temperature inversion and its associated thermal belt with much warmer, dryer conditions in intermediate elevations. Thus, a massive drying of fuels, leading to higher fire danger levels, and multiple fire occurrences at mid-slope positions were contrasted by moderate fire danger especially in the valleys. The ability of fire danger indices to resolve this situation was studied based on a comparison with the actual fire danger as determined from expert observations, fire occurrences and fuel moisture measurements. The results revealed that, during temperature inversion, differences in daily cycles of meteorological parameters influence fire danger and that these are not resolved by standard meteorological stations and fire danger indices (calculated on a once-a-day basis). Additional stations in higher locations or high-resolution meteorological models combined with fire danger indices accepting at least hourly input data may allow reasonable fire danger calculations under these circumstances.

Comparing Measured Duff Moisture with a Water Budget Model and the Duff and Drought Codes of the Canadian Fire Weather Index

Fuel moisture plays an important role in predicting wildfire spread rates, fuel consumption, and heat output. The purpose of this study was to find how much we can simplify an F and H layer moisture model by comparing an empirical-phenomenalistic drying model with a mechanistic water budget that included all of the major water fluxes. Traditionally fuel moisture has been calculated using phenomenalistic exponential drying rate models, which use standard meteorological station variables (temperature, precipitation, relative humidity, day length, and others). First, we report on comparisons between field-measured F and H soil layer moisture and moisture estimates based on the Duff Moisture Code and Drought Code of the Canadian Forest Fire Weather Index (FWI) in Pinus contorta and Picea engelmannii forests during dry and wet years. Next, an F and H layer water budget is used to understand possible reasons for the differences between the Duff Moisture Code, the Drought Code, and the moisture measured in the field. For the pine forest, the Duff Moisture Code was a good estimate of the F layer moisture during and shortly after precipitation events in both the dry and wet years but underestimated moisture when duff was drying. The pine Drought Code underestimated the H layer moisture in the dry year and overestimated it in the wet year. For the spruce forest, in the dry year the Duff Moisture Code overestimated the moisture during and after precipitation and underestimated it in dry periods. However, in the wet year, the code overestimated moisture most of the time. The spruce Drought Code underestimated the moisture content in both the wet and dry years. Results from the water budget model suggest that the difference in the F layer moisture between the field measurements and both the Drought Code and Duff Moisture Code is due to the lack of coupling of water flow between the F and H layers in the codes. In particular, the diurnal water movement from the H to F layer during the drying part of the season is integral to the water budget. To improve predictions based on the fuel moisture codes, coupled water and heat budgets along with the hydrologic properties of the F and H layers should be incorporated into the codes to enable more accurate prediction of duff moisture and calibration for different types of duff. FOR. SCI. ❚❚(❚):000–000.

Central Svalbard 2000–2011 Meteorological Dynamics and Periglacial Landscape Response

Local mountain meteorology of the landscape around Longyearbyen in central Svalbard is analyzed through the decade from 2000 to 2011. Standard meteorological stations from close to sea level and up to 464 m a.s.l. located on different periglacial landforms, have been used. During winters with little sea ice, strong temperature inversions do not develop, and then there is a distinct cooling with height, as opposed to when sea ice is present. Airflow is accelerated due to topography and direction deflected in the confined valleys, whereas open plateaus have on average 1 m/s lower wind speeds with a regional SE direction. The permafrost thermal state is largely controlled by meteorology, with permafrost in the valley bottoms as cold as on the mountain plateaus. The periglacial landform most exposed to climatic variability is ice-wedges, which seem to crack mainly during shorter cooling periods. Such activity is also linked to temperature inversions, and thus also occur mainly when sea ice is present. Solifluction is mainly controlled by the balance between summer thawing and winter freezing in combination with snow dynamics, whereas avalanches are mainly wind controlled. Avalanches and avalanche controlled landforms are least sensitive to climatic variability.

Analysis of All-China Sunshine Conditions of Site Selection for Large Solar Telescopes

The yearly and monthly data sets of sunshine duration and percent- age obtained at the 756 principal and primary standard meteorological stations all over China from 1971 to 2000, compiled by the Meteorological Reference Room of the National Center for Meteorological Information, are used to study the countrywide sunshine conditions and to analyze the factors which have an effect on the sunshine, in combination with the data of the key elements, such as the cloud cover and water vapor, recorded in China from 1951 to 2008. The obtained result shows that the distributions of the sunshine duration and sun- shine percentage in China are consistent with each other: small in southeastern China and large in northwestern China, increasing from southeast to northwest. In eastern China, the sunshine duration gradually increases from south to north and the sunshine condition in the western area is integrally better than that in the eastern area, tallying with the general rule. The sunshine condition in the Shiquanhe area in southwestern Tibet is the optimum, the sunshine conditions in the vicinity of the latitude zone of 40∘ N and in the area of southern Tibet take the second place, those in the south of the Yangtze River, in southern China, Sichuan Basin and in the eastern area of the Yunnan-Guizhou Plateau are rela- tively poor, and those in the other areas are placed in the middle. The sunshine duration is affected by the key elements like the cloud cover and water vapor, exhibiting a negative correlation with them.

Decreasing trend of sunshine hours and related driving forces in Southwestern China

Total hours of sunshine are one of the most important factors affecting climate and environment, and its long-term variation is of much concern in climate studies. Trends of temporal and spatial patterns in sunshine hours and related climatic factors over southwestern China are evaluated for the period 1961–2009 based on data from 111 standard meteorological stations. The results showed that southwestern China is experiencing a statistical decrease of sunshine hours, at the rate of 31.9 h/10a during 1961–2009. The decline was particularly strong in summer, whereas it is nonsignificant in winter. Spatially, statistically significant decreases of sunshine hours mainly occurred in lower altitude regions, especially in the Sichuan basin and Guizhou plateau. Sunshine hours have a high correlation with wind speed, relative humidity, precipitation, cloud cover, surface downwards solar radiation flux, and cloud water content, with wind speed showing the strongest relationship to sunshine hours, implicit in the close correlation (temporally and spatially) between the two variables. Changing water vapor and cloud cover influence sunshine hours in southwestern China. In addition, the increased surface downwards solar radiation flux also made some contribution to a rise of sunshine hours during 1991–2009. The larger decreasing trends of sunshine hours at urban stations than rural stations may reflect the effect of urbanization on sunshine hours. Variations are dominated by the comprehensive functions of multiple factors owing to the complex nature of effects on sunshine hours.

Empirical models for estimating daily maximum, minimum and mean air temperatures with MODIS land surface temperatures

Daily air temperature is a measurement that is required by many biogeochemical models. This study compared daily maximum (T max), minimum (T min) and mean (T mean) air temperature observations collected at 678 standard meteorological stations of China in 2003 with estimates derived from daytime and night-time land surface temperature (LST) observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board TERRA and AQUA satellites. Correlation analysis showed that the determination coefficients (R 2 > 0.81) between models using night-time LSTs and the observed air temperatures were higher than those using daytime LSTs (R 2 > 0.57), but with significant seasonal variation. Though estimates derived from coupled daytime and night-time LSTs were more accurate than using night-time or daytime LSTs alone, the available pixels were substantially reduced. Four empirical models were established for T max, T min and T mean with MODIS night-time LSTs alone, or with coupled daytime and night-time LSTs, respectively. Solar declination was incorporated into the models to simulate seasonal variation of the correlations. Model validation showed that percentage of residuals within –3°C to 3°C ranged approximately from 60.2% to 74.3%, 64.4% to 69.9% and 76.8% to 85.7% for T max, T min and T mean, respectively. It was concluded that night-time LST was the optimum predictor for estimating daily T min, T mean and even T max when considering both the performance of the models and the availability of the LST data. Moreover, there was no significant difference between LSTs of TERRA and AQUA for estimating daily air temperatures.

Estimation of the Wind Speed in Urban Areas - Height less than 10 Metres

Wind is an important factor that should be considered in the process of urban design. It is particularly important for natural ventilation design as well as providing guidance on heat transfer, the dispersion of air pollution, wind loading and pedestrian exposure. This article introduces a set of graphs produced by the author to estimate the wind speed around low rise buildings at heights of less than 10 metres above ground level. By applying these graphs the architect is able to easily and rapidly estimate the wind speed for urban terrain for heights below 10 metres without using complex calculation procedures. This therefore allows the transfer of data from meteorological stations, taking into account terrain and the degree of urbanisation.These graphs are based on the power law formulation of wind profile and are used to convert data from the nearest standard meteorological station. This procedure is aimed at assisting the architect or designer in decision making at the initial design or planning stage of a development.

A case study of the development of nocturnal slope flows in a wide open valley and associated air quality implications

This paper documents the development of nocturnal flows in the wide open Phoenix, Arizona (U.S.A) valley (30 km x 100 km) that is bordered by a large nearly flat plain to the west and high mountains to the north and east. Local thermally driven winds concomitant with the absence of significant synoptic pressure gradients dominate typical winter conditions in the Phoenix valley. The purpose of the Phoenix Air Flow Experiment (PAFEX-1) was to study the development of thermally driven flows during the evening transition in a sloping valley and describe the general pattern of transport and dispersion of contaminants during transition periods and at night. Measurements were made using a tethered balloon, sonic anemometer, balloon-based aerosol sampler, radiation sensors, cup anemometers, thermistors and humidity sensors in conjunction with data collected from 44 standard meteorological stations located throughout the valley. Over the period of 15 days in January and February 1998 the general diurnal flow patterns were repeatable, but varied substantially around the valley. This paper focuses on a case study of the evening transition, nocturnal circulation and morning breakdown of the nocturnal circulation on the night of 31 January and morning of 1 February. Central valley measurements were consistent with the notion that the evening transition is associated with a moving front, followed by intense mixing and the movement of the front to establish down-valley winds. Flows originating from different slopes led to the arrival of fronts at the various measurement locations at different times. These flows intrude into the valley and interact with each other, often causing multi-layered vertical structure. The intrusions respond to the evolving stratification and cause striking variability of these layers, for example, periodic wind and temperature disturbances corresponding to the arrival of new intrusive fronts. The evolution of the boundary layer was found to have a direct bearing on the pollution concentrations in the Phoenix air shed.

Decreasing trend of sunshine hours and related driving forces in North China

Global dimming is currently an active area of research in climate change. Trends of temporal (on the order of decades, years, seasons or even months) and spatial patterns in sunshine hours and associated climatic factors (average air temperature, relative humidity, precipitation and wind speed) over North China are evaluated for the period 1965~1999 based on data from 81 standard meteorological stations. The results show that: (1) North China is experiencing decreasing sunshine hours (–82.855 h/decade); (2) seasonally, decline in sunshine hours is highest in summer and lowest in winter; (3) spatially, decrease in sunshine hours is highest in inland and plain regions and lowest in the northwest mountain and coastland regions; (4) sunshine hours have a high correlation with precipitation, relative humidity and wind speed, with wind speed having the strongest influence on sunshine hours implicit in the close correlation (temporally and spatially) between the two variables; (5) cloud cover could not be any significant driver of sunshine-hour decline because it is more or less stable; (6) spatially and seasonally, wind speed is an important driving factor of decreasing sunshine hours in North China. Furthermore, the interactions between wind speed and aerosol loading may be an enabling factor of wind speed in driving (strongly) the changes in sunshine hours.

Estimating evaporation from a wet grassland

Abstract. Wet grasslands are being restored across the UK and Europe to reinstate their high biodiversity following over 50 years of drainage and conversion to arable agriculture. The water balance of many wet grasslands is dominated by precipitation and evaporation and it is essential to quantify evaporation rates to understand the hydrological functioning of wetlands and the implications for water resources in catchments where wetlands are being restored. This paper considers data from direct measurements of evaporation from the Pevensey Levels wet grassland using the eddy correlation method. Equations are derived to predict actual evaporation using meteorological data on the site or from standard meteorological station observations. It was found that evaporation could be estimated reliably from meteorological variables, such as wind speed, temperature and humidity and by water availability. It was also found that when water availability is high, evaporation is high and may exceed reference evaporation values, raising questions over the deployment of the two-step Penman-Monteith model unless reliable crop coefficients and relative evaporation figures can be determined.

Meteorological observations 2005 and in a longer perspective at the Sermilik Station, Ammassalik Island, Southeast Greenland

Geografisk Tidsskrift, Danish Journal of Geography 107(2):59–68, 2007 Meteorological stations have been in operation since 1993 at Sermilik (65°40'N, 38°10'W), Ammassalik Island, southeast Greenland. This note presents meteorological observations for the year 2005 from the two meteorological stations: Station Nunatak (515 m a.s.l.) and Station Coast (25 m a.s.l.). The year 2005 was dominated by a temporal break down of Station Nunatak and defect instrument recordings at both stations. Average yearly measurements at both meteorological stations: Station Nunatak (1994–2004) and Station Coast (1998–2005) are presented to illustrate the climatic variability within the Mittivakkat Glacier catchment, and further compared with simultaneous standard synoptic meteorological DMI station climate observations (1994–2004) at the town Tasiilaq (Ammassalik) in order to establish transfer functions (linear regressions) and describe the spatial variability of climate variables.

Simulating air temperature in an urban street canyon in all weather conditions using measured data at a reference meteorological station

AbstractA model is proposed that adapts data from a standard meteorological station to provide realistic site‐specific air temperature in a city street exposed to the same meso‐scale environment. In addition to a rudimentary description of the two sites, the canyon air temperature (CAT) model requires only inputs measured at standard weather stations; yet it is capable of accurately predicting the evolution of air temperature in all weather conditions for extended periods. It simulates the effect of urban geometry on radiant exchange; the effect of moisture availability on latent heat flux; energy stored in the ground and in building surfaces; air flow in the street based on wind above roof height; and the sensible heat flux from individual surfaces and from the street canyon as a whole. The CAT model has been tested on field data measured in a monitoring program carried out in Adelaide, Australia, in 2000–2001. After calibrating the model, predicted air temperature correlated well with measured data in all weather conditions over extended periods. The experimental validation provides additional evidence in support of a number of parameterisation schemes incorporated in the model to account for sensible heat and storage flux. Copyright © 2006 Royal Meteorological Society

Determining long‐term effective groundwater recharge by analyzing vertical soil temperature profiles at meteorological stations

Vertical water seepage in soils results in convective heat transport that modifies the temperature profiles and their variations with time; consequently, there is a relationship between temperature profile variations with time and the vertical Darcy velocity associated with the seepage. Considering the annual sinusoidal time variation of the temperature at the soil surface, it can be shown that convective heat transport has a significant effect on the amplitude damping with depth and a negligible effect on the phase lag with depth of the temperature time signal. Standard meteorological stations constitute a relatively dense network, and we show that their routinely collected data can be used to determine an average value of the vertical Darcy velocity, uz, representing the effective annual recharge over long time periods (several years). A new procedure for determining uz from these temperature records is presented. First, the layering of the medium is determined by an electrical sounding. Then the thermal properties of each layer are inferred from the phase lag with depth. Finally, uz is calculated from the amplitude damping. After having tested this approach with synthetic data, we used the 1984‐2001 Abbeville (Somme, France) data to determine the average recharge over six 3‐year periods. The results are in good agreement with classical meteorological recharge estimates and show a significant increase in the recharge during the last 3‐year period, consistent with the observed phreatic 2001 flood event. Specific temperature measurements at appropriate depths and time steps could drastically improve the sensitivity of the method.

Using midday surface temperature to estimate cooling degree-days from NOAA-AVHRR thermal infrared data: An application for Athens, Greece

Cooling degree-days (CDD) are a practical method for assessing the effect ambient air temperature has on the energy performance of buildings. In this study, the relationship between midday land surface temperatures derived from NOAA-AVHRR data and mean daily air temperature observations recorded at standard meteorological stations is defined and statistically validated. The relationship is further used for the calculation of CDD. The benefit of this approach is the direct application of daily satellite data for the definition of CDD in urban areas at a spatial resolution of 1.1 km.

Performance of the Priestley–Taylor equation in the semiarid climate of North China

The reference crop evapotranspiration is the most important component in the determination of agricultural water use. Since too many different methods are available in computing this value, the evaluation of these methods is the first step before using them. The objective of this paper is to assess the applicability of the Priestley–Taylor equation, widely used in many crop simulation models, by comparing its value with that of the Penman utilizing 46–50 years data series from six weather stations in the semiarid north china. The FAO Penman 1979 version is adopted because of its wide use in China. The input data includes monthly average sunshine duration, air temperature, relative humidity, air pressure and wind speed. Daily rate of monthly average is first obtained, and then multiplied by the days of each month to get the monthly total. During the calculation, the intermediate net radiation is estimated, and the soil heat flux is omitted. The required data, subjected to strict quality control before delivering and can make sure the accuracy of this study, is provided by the Climate Data Center, National Meteorological Bureau. The six locations are all national standard meteorological stations with long data record and are almost evenly distributed in the region. The evaluation was conducted at three time scales, annually, monthly and daily. The Priestley–Taylor equation underestimated the reference crop evapotranspiration significantly at yearly time scale, ranging from 128.6 to 364.1 mm, or 14–31% in terms of percentage. Spatially, it performed decreasingly according to Taiyuan > Shijiazhuang > Zhengzhou > Tianjin > Beijing > Jinan. At monthly time scale, the performance in most locations depends largely on month. In the humid months of July and August, it is comparable to the Penman equation. But when the wind speed is above certain level, the agreement decreased. In other months, its performance is unacceptable. At daily time scale in July and August in the best-performed year and the worst performed year, the Priestley–Taylor equation also performed well. At the latter two time scales, the spatial performing order varies from that at yearly time scale, and varies for different months and years. At all time scales considered, the performance of the Priestley–Taylor equation in the semiarid region is inversely affected by the ratio of aerodynamic term to radiation. This was supported by the correlations between this ratio and the ratio of the Priestley–Taylor value to the Penman value for annual, monthly and daily series. In addition, precipitation and wind speed also play a part in the performance. Years, months or days with higher precipitation or lower wind speed usually result in better performance, or vice versa. In summary, care should be taken when applying the Priestley–Taylor equation in the semiarid climate in north China. Temporally, it can be used in July and August and at daily time scale in these two months, but unsatisfactorily in other months and at yearly time scale. Spatially, except locations with wind speed higher than 2.61 and 2.36 m s −1 as Jinan in July and August, it is applicable in other locations in these two months. With the widespread use of this equation in crop simulation modeling, the possible modification to the coefficient α in the region deserves further investigation.

Modelling leaf wetness duration during the rosette stage of oilseed rape

In this paper, we develop a variant of the energy budget model approach, as presented by Pedro and Gillespie, to model leaf wetness duration for use in forecasting the oilseed diseases, stem canker and light leaf spot. The model overcomes three shortcomings of existing models: (i) the model includes rain as a source of leaf wetness, (ii) the model is simple enough to be integrated in disease forecasting systems and (iii) the model only uses meteorological measurements available from standard meteorological stations. The model is tested using two data sets from sites on the experimental farm at Rothamsted Research, UK. Model performance is discussed on basis of (i) the performance of the model for predicting consecutive multiple day periods of leaf wetness duration above a threshold and (ii) the percentage of days where the model correctly/incorrectly predicts whether or not infections could occur. These criteria are important for forecasting both light leaf spot and stem canker in oilseed rape crops.

Comparison of energy fluxes at the land surface-atmosphere interface in an Alpine valley as simulated with different models

Abstract. Within the framework of a research project coupling meteorological and hydrological models in mountainous areas a distributed Snow-Soil-Vegetation-Atmosphere Transfer model was developed and applied to simulate the energy fluxes at the land surface – atmosphere interface in an Alpine valley (Toce Valley - North Italy) during selected flood events in the last decade. Energy fluxes simulated by the distributed energy transfer model were compared with those simulated by a limited area meteorological model for the event of June 1997 and the differences in the spatial and temporal distribution. The Snow/Soil-Vegetation-Atmosphere Transfer model was also applied to simulate the energy fluxes at the land surface-atmosphere interface for a single cell, assumed to be representative of the Siberia site (Toce Valley), where a micro-meteorological station was installed and operated for 2.5 months in autumn 1999. The Siberia site is very close to the Nosere site, where a standard meteorological station was measuring precipitation, air temperature and humidity, global and net radiation and wind speed during the same special observing period. Data recorded by the standard meteorological station were used to force the energy transfer model and simulate the point energy fluxes at the Siberia site, while turbulent fluxes observed at the Siberia site were used to derive the latent heat flux from the energy balance equation. Finally, the hourly evapotranspiration flux computed by this procedure was compared to the evapotranspiration flux simulated by the energy transfer model. Keywords: energy exchange processes, land surface-atmosphere interactions, turbulent fluxes

A dynamic simulation model for powdery mildew epidemics on winter wheat*

A system dynamic model for epidemics of Blumeria graminis (powdery mildew) on wheat was elaborated, based on the interaction between stages of the disease cycle, weather conditions and host characteristics. The model simulates the progress of disease severity, expressed as a percentage of powdered leaf area, on individual leaves, with a time step of one day, as a result of two processes: the growth of fungal colonies already present on the leaves and the appearance of new colonies. By means of mathematical equations, air temperature, vapour pressure deficit, rainfall and wind are used to calculate incubation, latency and sporulation periods, the growth of pathogen colonies, infection and spore survival. Effects of host susceptibility to infection, and of leaf position within the plant canopy, are also included. Model validation was carried out by comparing model outputs with the dynamics of epidemics observed on winter wheat grown at several locations in northern Italy (1991–98). Simulations were performed using meteorological data measured in standard meteorological stations. As there was good agreement between model outputs and actual disease severity, the model can be considered a satisfactory simulator of the effect of environmental conditions on the progress of powdery mildew epidemics.