Convective and radiative components of wind chill in sheep: Estimation from meteorological records

Wind chill is defined as the excess of sensible heat loss over what would occur at zero wind speed with other conditions unchanged. Wind chill can be broken down into a part that is determined by air temperature and a radiative part that comprises wind-dependent effects on additional long-wave radiative exchange and on solar radiation (by reducing solar warming). Radiative exchange and gain from solar radiation are affected by changes that are produced by wind in both surface and fleece insulations. Coefficients are derived for (a) converting the components of sensible heat exchange (air-temperature-dependent including both convective and associated long-wave radiative, additional long-wave radiative and solar) into the components of the total heat loss that are associated with wind and (b) for calculating equivalent air temperature changes. The coefficients contain terms only in wind speed, wetting of the fleece and fleece depth; these determine the external insulation.Calculation from standard meteorological records, using Plymouth and Aberdeen in 1973 as examples, indicate that in April–September 1973 at Plymouth reduction in effective solar warming constituted 28% of the 24-h total wind chill, and 7% in the other months of the year combined; at Aberdeen the corresponding percentages were 25% and 6%. Mean hour-of-day estimates for the months of April and October showed that at midday reduction in solar warming due to wind rose to the order of half the air-temperature-dependent component of wind chill, with a much smaller effect in January. For about six hours at midday in July reduction in solar warming due to wind was similar in magnitude to the air-temperature-dependent component.It is concluded that realistic estimates of wind chill cannot be obtained unless the effect of solar radiation is taken into account. Failure to include solar radiation results not only in omitting solar warming but also in omitting the effects of wind in reducing that warming.The exchange of sensible (non-evaporative) heat loss between a homeothermic animal and its environment can be divided into two parts: one part is due to the temperature difference between the animal and the surrounding air, and the other part is due to additional long-wave radiative exchange between animal and environment and to solar radiation. Both parts of the heat exchange are determined in magnitude by the animal's thermal insulation, which is itself affected by windspeed and wetting. Wind diminishes as animal's external insulation, so increasing heat loss under all conditions when the air temperature is lower than the animal's surface temperature: this effect is termed wind chill. Wind chill has previously been investigated more commonly in relation to man (Burton an Edholm, 1955; Smithson and Baldwin, 1978; Mumford, 1979; Baldwin and Smithson, 1979). This paper is concerned with the separate contributions to wind chill calculated for sheep that can be associated with convective and radiative heat exchanges.     

Environmental impacts on offspring survival during the lambing period in central Patagonia

Weather conditions are the main factors affecting the survival rate of newborn lambs. In the windy climate of Patagonia, the influence of weather conditions is exacerbated as the wind speed increases noticeably in spring. If the wind increases faster than the temperature rises, wind chill conditions worsen temporarily. This departure from the predictable improvement in bioclimatic conditions as the season moves towards summer may coincide with lambing, if this is not adequately regulated. This paper explores bioclimatic conditions during the 2-month period in which lambing may take place (i.e. mid-September to mid-November), and identifies the unfavourable period that occurs in the second fortnight of October. Starting from the Mount and Brown (Agric Meteorol 27:241–255, 1982) model to assess thermal stress in sheep by means of common meteorological records, a simpler model was developed employing in long-term records. For 2-week averages of temperature (T _f), wind speed (V _f) and total precipitation (P _f), the average heat loss in sheep (HL_f) was 40.40–2.07T _f+5.64V _f+0.04P _f. (r2=0.95). A 24-year series of T _f, V _f and P _f data was employed in the calculation of HL_f, and results showed this parameter was significantly higher in the second fortnight in October. HL_f values were classified into four groups (from ”very good” to ”very bad” conditions); once again, significant differences appeared in the second fortnight in October. Finally, HL_f during the lambing period was correlated to the percentage of lambs surviving 2 months later. Significant differences between the survival of lambs born under ”very bad” and ”very good” conditions were found, but intermediate HL_f conditions did not show a clear pattern. The survival rate was best correlated to HL_f in the fortnight preceding lambing (P<0.05). This suggests that bioclimatic conditions during late pregnancy are at least as important as conditions during lambing in determining the survival lambs. Received: 14 December 1998 / Revised: 26 July 1999 / Accepted: 26 July 1999     

Modified wind chill temperatures determined by a whole body thermoregulation model and human-based facial convective coefficients

Wind chill equivalent temperatures (WCETs) were estimated by a modified Fiala’s whole body thermoregulation model of a clothed person. Facial convective heat exchange coefficients applied in the computations concurrently with environmental radiation effects were taken from a recently derived human-based correlation. Apart from these, the analysis followed the methodology used in the derivation of the currently used wind chill charts. WCET values are summarized by the following equation: $$ \mathrm{WCET}=12.87+0.5334\ast {T}_o-\left(12.66-0.4414\ast {T}_o\right)\ast {U}_{reported}{}^{0.1228} $$ Results indicate consistently lower estimated facial skin temperatures and consequently higher WCETs than those listed in the literature and used by the North American weather services. Calculated dynamic facial skin temperatures were additionally applied in the estimation of probabilities for the occurrence of risks of frostbite. Predicted weather combinations for probabilities of “Practically no risk of frostbite for most people,” for less than 5 % risk at wind speeds above 40 km h^?1, were shown to occur at air temperatures above ?10 °C compared to the currently published air temperature of ?15 °C. At air temperatures below ?35 °C, the presently calculated weather combination of 40 km h^?1/?35 °C, at which the transition for risks to incur a frostbite in less than 2 min, is less conservative than that published: 60 km h^?1/?40 °C. The present results introduce a fundamentally improved scientific basis for estimating facial skin temperatures, wind chill temperatures and risk probabilities for frostbites over those currently practiced.     

Predicting urban outdoor thermal comfort by the Universal Thermal Climate Index UTCI—a case study in Southern Brazil

Recognising that modifications to the physical attributes of urban space are able to promote improved thermal outdoor conditions and thus positively influence the use of open spaces, a survey to define optimal thermal comfort ranges for passers-by in pedestrian streets was conducted in Curitiba, Brazil. We applied general additive models to study the impact of temperature, humidity, and wind, as well as long-wave and short-wave radiant heat fluxes as summarised by the recently developed Universal Thermal Climate Index (UTCI) on the choice of clothing insulation by fitting LOESS smoothers to observations from 944 males and 710 females aged from 13 to 91 years. We further analysed votes of thermal sensation compared to predictions of UTCI. The results showed that females chose less insulating clothing in warm conditions compared to males and that observed values of clothing insulation depended on temperature, but also on season and potentially on solar radiation. The overall pattern of clothing choice was well reflected by UTCI, which also provided for good predictions of thermal sensation votes depending on the meteorological conditions. Analysing subgroups indicated that the goodness-of-fit of the UTCI was independent of gender and age, and with only limited influence of season and body composition as assessed by body mass index. This suggests that UTCI can serve as a suitable planning tool for urban thermal comfort in sub-tropical regions.     

Outdoor clothing: its relationship to geography, climate, behaviour and cold-related mortality in Europe

It has been suggested, that the inhabitants of northern European regions, who experience little cold-related mortality, protect themselves outdoors by wearing more clothing, at the same temperature, than people living in southern regions where such mortality is high. Outdoor clothing data were collected in eight regions from 6583 people divided by sex and age group (50–59 and 65–74 years). Across Europe, the total clothing worn (as assessed by dry thermal insulation and numbers of items or layers) increased significantly with cold, wind, less physical activity and longer periods outdoors. Men wore 0.14 clo (1 clo=0.115 m² K W^–1) more than women and the older people wore 0.05 clo more than the younger group (both P<0.001). After allowance for these factors, regional differences in insulation and item number were correlated (r=–0.74, P=0.037; r=–0.74, P=0.036 respectively), but not those in clothing layers (r=–0.21; P=0.61), with indices of cold-related mortality. Cold weather most increased the wearing of gloves, scarves and hats. The geographical variation in the wearing of these three together items more closely matched that in cold-related mortality (r=–0.89, P=0.003). A possible explanation for this may be that they protect the head and hands, where stimulation by cold greatly increases peripheral vasoconstriction causing a rise in blood pressure that procedure haemoconcentration and raised cardiovascular risk. Received: 30 September 1999 / Revised: 3 April 2000 / Accepted: 5 June 2000     

A note on cardiovascular diseases and physical aspects of the environment

This paper presents a biometeorological study of ischaemic heart diseases and a thermal load index combining meteorological factors, clothing insulation and metabolic rate. The study is based on records of weather and of hospital admissions. The data were grouped into four seasonal periods approximating the four major climate seasons experienced at Toronto, Canada. Statistical analysis of the seasonal data for five years (1976–1980) shows a good association between the deviations from group averages of the thermal load index and corresponding deviations of the number of patients admitted to hospitals at Toronto due to cardiovascular diseases if two out of nineteen data points are excluded from the analysis. In the authors' view, recognizing the nature of climate and other records, there is good reason to believe that this association is meaningful. The results indicate that at a given activity level and clothing insulation there is a higher incidence of coronorary diseases when the thermal sensation is to the cold side of the neutral comfort condition. Probable causes are advanced for the fact that 12% of the data appear anomalous.     

Prediction of air temperature for thermal comfort of people in outdoor environments

Current thermal comfort indices do not take into account the effects of wind and body movement on the thermal resistance and vapor resistance of clothing. This may cause public health problem, e.g. cold-related mortality. Based on the energy balance equation and heat exchanges between a clothed body and the outdoor environment, a mathematical model was developed to determine the air temperature at which an average adult, wearing a specific outdoor clothing and engaging in a given activity, attains thermal comfort under outdoor environment condition. The results indicated low clothing insulation, less physical activity and high wind speed lead to high air temperature prediction for thermal comfort. More accurate air temperature prediction is able to prevent wearers from hypothermia under cold conditions.     

Paradox: increased blood perfusion to the face <Emphasis Type="BoldItalic">enhances</Emphasis> protection against frostbite while it <Emphasis Type="BoldItalic">lowers</Emphasis> wind chill equivalent temperatures

A model of facial heat exchange in cold and windy environments is presented. The tissue is depicted as a hollow cylinder and the model includes heat conduction and heat transport by blood circulation from the warmer core. A steady-state solution facilitating the estimation of wind chill equivalent temperature (WCET) as a function of the effective wind velocity, air temperature and blood perfusion rate was obtained. The results quantify and demonstrate the elevation of skin temperatures caused by increased flow of warmer blood from the inner core to the face. Elevated facial temperatures, while enhancing protection against frostbite and other cold-related injuries, also increase heat loss to the colder environment. Paradoxically, such elevated facial temperatures cause WCETs, as estimated by the prevailing definition, to attain lower rather than higher values, indicating, in fact, increased risk of frostbite. The results of this study should be useful in understanding and quantifying the effects of blood perfusion in protection against cold-related injuries. They should also be considered in the re-evaluation and re-formulation of the concept of wind chill, which has been a useful cold weather indicator for decades.     

Determining optimal clothing ensembles based on weather forecasts,with particular reference to outdoor winter military activities

Military and civil defense personnel are often involved in complex activities in a variety of outdoor environments. The choice of appropriate clothing ensembles represents an important strategy to establish the success of a military mission. The main aim of this study was to compare the known clothing insulation of the garment ensembles worn by soldiers during two winter outdoor field trials (hike and guard duty) with the estimated optimal clothing thermal insulations recommended to maintain thermoneutrality, assessed by using two different biometeorological procedures. The overall aim was to assess the applicability of such biometeorological procedures to weather forecast systems, thereby developing a comprehensive biometeorological tool for military operational forecast purposes. Military trials were carried out during winter 2006 in Pokljuka (Slovenia) by Slovene Armed Forces personnel. Gastrointestinal temperature, heart rate and environmental parameters were measured with portable data acquisition systems. The thermal characteristics of the clothing ensembles worn by the soldiers, namely thermal resistance, were determined with a sweating thermal manikin. Results showed that the clothing ensemble worn by the military was appropriate during guard duty but generally inappropriate during the hike. A general under-estimation of the biometeorological forecast model in predicting the optimal clothing insulation value was observed and an additional post-processing calibration might further improve forecast accuracy. This study represents the first step in the development of a comprehensive personalized biometeorological forecast system aimed at improving recommendations regarding the optimal thermal insulation of military garment ensembles for winter activities.     

The physiological equivalent temperature – a universal index for the biometeorological assessment of the thermal environment

With considerably increased coverage of weather information in the news media in recent years in many countries, there is also more demand for data that are applicable and useful for everyday life. Both the perception of the thermal component of weather as well as the appropriate clothing for thermal comfort result from the integral effects of all meteorological parameters relevant for heat exchange between the body and its environment. Regulatory physiological processes can affect the relative importance of meteorological parameters, e.g. wind velocity becomes more important when the body is sweating. In order to take into account all these factors, it is necessary to use a heat-balance model of the human body. The physiological equivalent temperature (PET) is based on the Munich Energy-balance Model for Individuals (MEMI), which models the thermal conditions of the human body in a physiologically relevant way. PET is defined as the air temperature at which, in a typical indoor setting (without wind and solar radiation), the heat budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed. This way PET enables a layperson to compare the integral effects of complex thermal conditions outside with his or her own experience indoors. On hot summer days, for example, with direct solar irradiation the PET value may be more than 20 K higher than the air temperature, on a windy day in winter up to 15 K lower. Received: 14 December 1998 / Accepted: 26 May 1999     

The UTCI-clothing model

The Universal Thermal Climate Index (UTCI) was conceived as a thermal index covering the whole climate range from heat to cold. This would be impossible without considering clothing as the interface between the person (here, the physiological model of thermoregulation) and the environment. It was decided to develop a clothing model for this application in which the following three factors were considered: (1) typical dressing behaviour in different temperatures, as observed in the field, resulting in a model of the distribution of clothing over the different body segments in relation to the ambient temperature, (2) the changes in clothing insulation and vapour resistance caused by wind and body movement, and (3) the change in wind speed in relation to the height above ground. The outcome was a clothing model that defines in detail the effective clothing insulation and vapour resistance for each of the thermo-physiological model’s body segments over a wide range of climatic conditions. This paper details this model’s conception and documents its definitions.     

Improved predictive ability of climate–human–behaviour interactions with modifications to the COMFA outdoor energy budget model

The purpose of this paper is to implement current and novel research techniques in human energy budget estimations to give more accurate and efficient application of models by a variety of users. Using the COMFA model, the conditioning level of an individual is incorporated into overall energy budget predictions, giving more realistic estimations of the metabolism experienced at various fitness levels. Through the use of VO₂ reserve estimates, errors are found when an elite athlete is modelled as an unconditioned or a conditioned individual, giving budgets underpredicted significantly by −173 and −123 W m⁻², respectively. Such underprediction can result in critical errors regarding heat stress, particularly in highly motivated individuals; thus this revision is critical for athletic individuals. A further improvement in the COMFA model involves improved adaptation of clothing insulation (I _cl), as well clothing non-uniformity, with changing air temperature (T _a) and metabolic activity (M _act). Equivalent T _a values (for I _cl estimation) are calculated in order to lower the I _cl value with increasing M _act at equal T _a. Furthermore, threshold T _a values are calculated to predict the point at which an individual will change from a uniform I _cl to a segmented I _cl (full ensemble to shorts and a T-shirt). Lastly, improved relative velocity (v _r) estimates were found with a refined equation accounting for the degree angle of wind to body movement. Differences between the original and improved v _r equations increased with higher wind and activity speeds, and as the wind to body angle moved away from 90°. Under moderate microclimate conditions, and wind from behind a person, the convective heat loss and skin temperature estimates were 47 W m⁻² and 1.7°C higher when using the improved v _r equation. These model revisions improve the applicability and usability of the COMFA energy budget model for subjects performing physical activity in outdoor environments. Application is possible for other similar energy budget models, and within various urban and rural environments.     

Comparison of UTCI to selected thermal indices

Over the past century more than 100 indices have been developed and used to assess bioclimatic conditions for human beings. The majority of these indices are used sporadically or for specific purposes. Some are based on generalized results of measurements (wind chill, cooling power, wet bulb temperature) and some on the empirically observed reactions of the human body to thermal stress (physiological strain, effective temperature). Those indices that are based on human heat balance considerations are referred to as "rational indices". Several simple human heat balance models are known and are used in research and practice. This paper presents a comparative analysis of the newly developed Universal Thermal Climate Index (UTCI), and some of the more prevalent thermal indices. The analysis is based on three groups of data: global data-set, synoptic datasets from Europe, and local scale data from special measurement campaigns of COST Action 730. We found the present indices to express bioclimatic conditions reasonably only under specific meteorological situations, while the UTCI represents specific climates, weather, and locations much better. Furthermore, similar to the human body, the UTCI is very sensitive to changes in ambient stimuli: temperature, solar radiation, wind and humidity. UTCI depicts temporal variability of thermal conditions better than other indices. The UTCI scale is able to express even slight differences in the intensity of meteorological stimuli.     

An evaluation of the wind chill factor: its development and applicability.

The wind chill factor has become a standard meteorologic term in cold climates. Meteorologic charts provide wind chill temperatures meant to represent the hypothetical air temperature that would, under conditions of no wind, effect the same heat loss from unclothed human skin as does the actual combination of air temperature and wind velocity. As this wind chill factor has social and economic significance, an investigation was conducted on the development of this factor and its applicability based on modern heat transfer principles. The currently used wind chill factor was found to be based on a primitive study conducted by the U.S. Antarctic Service over 50 years ago. The resultant equation for the wind chill temperature assumes an unrealistic constant skin temperature and utilizes heat transfer coefficients that differ markedly from those obtained from equations of modern convective heat transfer methods. The combined effect of these two factors is to overestimate the effect of a given wind velocity and to predict a wind chill temperature that is too low.     

美国黑核桃实生苗生态生理过程对环境因素响应的数值模拟(Ⅱ):气象要素日变化进程的数理模型

在建立植物生态生理模型（如光合作用、蒸腾作用数理模型）的过程中,气温、太阳辐射、相对湿度等气象要素的日变化及瞬时资料是必不可少的．本文根据常规台站的日常观测资料,建立了气温、太阳辐射、风速、相对湿度等气象要素日变化的数理模型．并应用实测资料对所建模型进行了验证．实践表明：（1）描述气温日变化的正弦一指数模型通常其模拟结果不甚理想,但通过引用最高温度的时间延迟参数及惯性系数,可增加模拟结果的准确性．（2）根据测量的太阳辐射日总量值以及太阳高度的日变化值等,可准确地模拟太阳辐射的日变化;（3）根据实际水汽压的日均值和气温常规资料可准确模拟相对湿度的日变化．如果区域参数已知或被正确估计。上述方法可得到较为理想的气象要素日变化的模拟结果．     

Weather impacts on respiratory infections in Athens, Greece

In this study the contribution of meteorological parameters to the total variability of respiratory infections (RI) is analysed. For this purpose, data on the daily numbers of general practitioner (GP) consultations for RI during the year 2002 were used. This dataset has been compiled by the Local Health Service in the surroundings of Athens, Greece (Acharnes city). The meteorological data obtained by the Meteorological Station of the National Observatory of Athens comprise daily values of mean, maximum, and minimum air temperature, air temperature range, relative humidity, absolute humidity, sunshine, surface atmospheric pressure, wind speed, as well as day-to-day changes of these parameters. Furthermore, the following biometeorological parameters and thermal indices were also evaluated: mean radiant temperature (T _mrt), predicted mean vote (PMV), physiologically equivalent temperature (PET) and standard effective temperature (SET*) as well as their day-to-day changes. First, the relationship between every meteorological-biometeorological parameter and consultations for RI was examined by applying the Pearson Chi-Square Test (χ ²) to the data of the 25 compiled contingency tables. In the second stage, the application of generalised linear models (GLM) with Poisson distribution to the data revealed how much the weather variability leads to statistically important changes in consultations for RI. The results of this study contribute to the evidence that there is an association between weather conditions and the number of GP consultations for RI. More specifically, the influence of air temperature and absolute humidity on consultations on the same day is weaker than the lag effect (∼2 weeks) related to cold existence and absolute humidity, while a strong wind during the preceding 3 days drives a peak in GP consultations.     

On how Cattle Egret (Bubulcus ibis) spread to the Americas: meteorological tools to assess probable colonization trajectories

The Cattle Egret (Bubulcus ibis) is native to the old world. Before 1877 no Cattle Egrets had been sighted in the Americas. There are no written records of this species being transported to or escaping from captivity in South America and there is enough evidence to suggest that individuals are capable of making the crossing from Africa to the Americas unaided. Since long-distance movements of species are partly dependent on meteorological events we analyze the possibility of B. ibis crossing the Atlantic Ocean aided only by wind conditions evaluating existing theories and shedding light on their feasibility through the analysis of weather patterns and atmospheric circulation. Zonal and meridional wind components taken from the Twentieth Century Reanalysis Project were used to calculate trajectories between different points along the West African coast and South America between 1871 and 1920 in two seasons (March–April and September–October). From a total of 192,864 trajectories analyzed, 1,695 with origin in the west coast of Africa reached the NE coast of South America or the Caribbean islands in less than a week (successful trajectories). The probability of these successful trajectories originating in Central Africa was above 0.65 for the majority of the destinations analyzed. Particularly, in Guyana and Suriname where B. ibis was first sighted, the probability of the origin being Central Africa was 0.84, most of them occurring during March. Several weather events favored not only the colonization of the Cattle Egret but also its establishment and spread all throughout the Americas.     

Facial convective heat exchange coefficients in cold and windy environments estimated from human experiments

Facial heat exchange convection coefficients were estimated from experimental data in cold and windy ambient conditions applicable to wind chill calculations. Measured facial temperature datasets, that were made available to this study, originated from 3 separate studies involving 18 male and 6 female subjects. Most of these data were for a −10°C ambient environment and wind speeds in the range of 0.2 to 6 m s⁻¹. Additional single experiments were for −5°C, 0°C and 10°C environments and wind speeds in the same range. Convection coefficients were estimated for all these conditions by means of a numerical facial heat exchange model, applying properties of biological tissues and a typical facial diameter of 0.18 m. Estimation was performed by adjusting the guessed convection coefficients in the computed facial temperatures, while comparing them to measured data, to obtain a satisfactory fit (r ² > 0.98, in most cases). In one of the studies, heat flux meters were additionally used. Convection coefficients derived from these meters closely approached the estimated values for only the male subjects. They differed significantly, by about 50%, when compared to the estimated female subjects' data. Regression analysis was performed for just the −10°C ambient temperature, and the range of experimental wind speeds, due to the limited availability of data for other ambient temperatures. The regressed equation was assumed in the form of the equation underlying the “new” wind chill chart. Regressed convection coefficients, which closely duplicated the measured data, were consistently higher than those calculated by this equation, except for one single case. The estimated and currently used convection coefficients are shown to diverge exponentially from each other, as wind speed increases. This finding casts considerable doubts on the validity of the convection coefficients that are used in the computation of the "new" wind chill chart and their applicability to humans in cold and windy environments.     

UTCI—Why another thermal index?

Existing procedures for the assessment of the thermal environment in the fields of public weather services, public health systems, precautionary planning, urban design, tourism and recreation and climate impact research exhibit significant shortcomings. This is most evident for simple (mostly two-parameter) indices, when comparing them to complete heat budget models developed since the 1960s. ISB Commission 6 took up the idea of developing a Universal Thermal Climate Index (UTCI) based on the most advanced multi-node model of thermoregulation representing progress in science within the last three to four decades, both in thermo-physiological and heat exchange theory. Creating the essential research synergies for the development of UTCI required pooling the resources of multidisciplinary experts in the fields of thermal physiology, mathematical modelling, occupational medicine, meteorological data handling (in particular radiation modelling) and application development in a network. It was possible to extend the expertise of ISB Commission 6 substantially by COST (a European programme promoting Cooperation in Science and Technology) Action 730 so that finally over 45 scientists from 23 countries (Australia, Canada, Israel, several Europe countries, New Zealand, and the United States) worked together. The work was performed under the umbrella of the WMO Commission on Climatology (CCl). After extensive evaluations, Fiala’s multi-node human physiology and thermal comfort model (FPC) was adopted for this study. The model was validated extensively, applying as yet unused data from other research groups, and extended for the purposes of the project. This model was coupled with a state-of-the-art clothing model taking into consideration behavioural adaptation of clothing insulation by the general urban population in response to actual environmental temperature. UTCI was then derived conceptually as an equivalent temperature (ET). Thus, for any combination of air temperature, wind, radiation, and humidity (stress), UTCI is defined as the isothermal air temperature of the reference condition that would elicit the same dynamic response (strain) of the physiological model. As UTCI is based on contemporary science its use will standardise applications in the major fields of human biometeorology, thus making research results comparable and physiologically relevant.     

The use of eddy covariance to infer the net carbon dioxide uptake of Brazilian rain forest

• 1 Eddy covariance measurements of CO₂ flux, based on four and six week campaigns in Rondôdnia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions.
• 2 One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be ‘lost’ because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO₂, F_eco is a function of wind speed. If flux is ‘lost’ then F_eco is expected to decline with wind speed. In the present data set, this did not occur, and F_eco was similar to the nocturnal flux estimated independently from chamber measurements.
• 3 The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m^?2 y^?1 and ecosystem respiration was 194.8 mol C m^?2 y^?1, giving an ecosystem carbon balance of 8.5 mol C m^?2 y^?1, equivalent to a sink of 1.0 ton C ha^?1 y^?1. However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration.
• 4 The model also suggests that the effect of temperature on the net carbon balance is strongly dependent on the partial pressure of CO₂.