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Arctic lowlands are characterized by large numbers of small waterbodies, which are known to affect surface energy budgets and the global carbon cycle. Statistical analysis of their size distributions has been hindered by the shortage of observations at sufficiently high spatial resolutions. This situation has now changed with the high-resolution (<5 m) circum-Arctic Permafrost Region Pond and Lake (PeRL) database recently becoming available. We have used this database to make the first consistent, high-resolution estimation of Arctic waterbody size distributions, with surface areas ranging from 0.0001 km2 (100 m2) to 1 km2. We found that the size distributions varied greatly across the thirty study regions investigated and that there was no single universal size distribution function (including power-law distribution functions) appropriate across all of the study regions. We did, however, find close relationships between the statistical moments (mean, variance, and skewness) of the waterbody size distributions from different study regions. Specifically, we found that the spatial variance increased linearly with mean waterbody size (R2 = 0.97, p < 2.2e-16) and that the skewness decreased approximately hyperbolically. We have demonstrated that these relationships (1) hold across the 30 Arctic study regions covering a variety of (bio)climatic and permafrost zones, (2) hold over time in two of these study regions for which multi-decadal satellite imagery is available, and (3) can be reproduced by simulating rising water levels in a high-resolution digital elevation model. The consistent spatial and temporal relationships between the statistical moments of the waterbody size distributions underscore the dominance of topographic controls in lowland permafrost areas. These results provide motivation for further analyses of the factors involved in waterbody development and spatial distribution and for investigations into the possibility of using statistical moments to

Arctic lowlands are characterized by large numbers of small waterbodies, which are known to affect surface energy budgets and the global carbon cycle. Statistical analysis of their size distributions has been hindered by the shortage of observations at sufficiently high spatial resolutions. This situation has now changed with the high-resolution (<5 m) circum-Arctic Permafrost Region Pond and Lake (PeRL) database recently becoming available. We have used this database to make the first consistent, high-resolution estimation of Arctic waterbody size distributions, with surface areas ranging from 0.0001 km2 (100 m2) to 1 km2. We found that the size distributions varied greatly across the thirty study regions investigated and that there was no single universal size distribution function (including power-law distribution functions) appropriate across all of the study regions. We did, however, find close relationships between the statistical moments (mean, variance, and skewness) of the waterbody size distributions from different study regions. Specifically, we found that the spatial variance increased linearly with mean waterbody size (R2 = 0.97, p < 2.2e-16) and that the skewness decreased approximately hyperbolically. We have demonstrated that these relationships (1) hold across the 30 Arctic study regions covering a variety of (bio)climatic and permafrost zones, (2) hold over time in two of these study regions for which multi-decadal satellite imagery is available, and (3) can be reproduced by simulating rising water levels in a high-resolution digital elevation model. The consistent spatial and temporal relationships between the statistical moments of the waterbody size distributions underscore the dominance of topographic controls in lowland permafrost areas. These results provide motivation for further analyses of the factors involved in waterbody development and spatial distribution and for investigations into the possibility of using statistical moments to

The provision of good IAQ in schools is important both for the health of students and in maximising educational achievement. It is, however, common for school classrooms to be significantly under-ventilated and this can lead to high levels of CO 2 and other pollutants. Natural ventilation offers the potential to improve IAQ within schools whilst, at the same time reducing running and maintenance costs. Accordingly, this article examines a natural ventilation strategy based on the use of a roof mounted split-duct Windcatcher ventilator. Here, 16 U.K. classrooms are studied and CO 2, temperature, relative humidity and ventilation rates are measured for the summer and winter seasons. Results show that, during the summer months, the ventilator is capable of significantly improving ventilation rates as well as reducing CO 2 levels, especially when used in combination with open windows. However, in the winter months, the ventilator is seen not to open for a sufficient length of time and so CO 2 levels rise above those required in the standards. Thus, the ventilator is shown to have the potential to improve IAQ within school classrooms, but the operation of the ventilator should be carefully controlled in order to realise these benefits. It is common for ventilation rates in school classrooms to fall below the levels required by relevant standards. The data presented here demonstrates that by using a top-down natural ventilation Windcatcher as part of a well designed natural ventilation strategy, ventilation rates in school classrooms can be significantly improved.

The provision of good IAQ in schools is important both for the health of students and in maximising educational achievement. It is, however, common for school classrooms to be significantly under-ventilated and this can lead to high levels of CO 2 and other pollutants. Natural ventilation offers the potential to improve IAQ within schools whilst, at the same time reducing running and maintenance costs. Accordingly, this article examines a natural ventilation strategy based on the use of a roof mounted split-duct Windcatcher ventilator. Here, 16 U.K. classrooms are studied and CO 2, temperature, relative humidity and ventilation rates are measured for the summer and winter seasons. Results show that, during the summer months, the ventilator is capable of significantly improving ventilation rates as well as reducing CO 2 levels, especially when used in combination with open windows. However, in the winter months, the ventilator is seen not to open for a sufficient length of time and so CO 2 levels rise above those required in the standards. Thus, the ventilator is shown to have the potential to improve IAQ within school classrooms, but the operation of the ventilator should be carefully controlled in order to realise these benefits. It is common for ventilation rates in school classrooms to fall below the levels required by relevant standards. The data presented here demonstrates that by using a top-down natural ventilation Windcatcher as part of a well designed natural ventilation strategy, ventilation rates in school classrooms can be significantly improved.

Windcatchers are roof mounted devices that use the action of the wind to provide top down natural ventilation to a room. Here, fresh air is channelled into a room while, at the same time, stale air is drawn out. This provides a simple but attractive natural ventilation methodology that is increasing in popularity in U.K. schools. However, an analysis of system performance has largely been limited to laboratory based measurements and the use of CFD to generate predictions. Moreover, analysis is normally restricted to the operation of an autonomous Windcatcher whereas, in reality, it is likely to operate in a building in which other sources of ventilation are present (an open window for example) which can significantly alter the performance. The aim of this paper is to provide a tool for estimating the performance of a Windcatcher from basic data that is typically available to the engineer in the building design phase. Accordingly, the methodology uses data that one could reasonably be expected to have for a building's ventilation performance. This paper also reviews in situ performance based on measurements in U.K. schools both with and without open windows. Predictions generated by a semi-empirical model are then compared against measurement data and this is shown to deliver generally good agreement between the two, both with and without open windows, provided the theoretical predictions are presented in terms of an upper and lower performance limit. Furthermore, both experiment and theory clearly demonstrate that a large increase in the ventilation rate is possible if one combines the operation of a Windcatcher with, say, an open window, and that this ventilation rate is greater than that which would be achievable from a window operating on its own.

Windcatchers are roof mounted devices that use the action of the wind to provide top down natural ventilation to a room. Here, fresh air is channelled into a room while, at the same time, stale air is drawn out. This provides a simple but attractive natural ventilation methodology that is increasing in popularity in U.K. schools. However, an analysis of system performance has largely been limited to laboratory based measurements and the use of CFD to generate predictions. Moreover, analysis is normally restricted to the operation of an autonomous Windcatcher whereas, in reality, it is likely to operate in a building in which other sources of ventilation are present (an open window for example) which can significantly alter the performance. The aim of this paper is to provide a tool for estimating the performance of a Windcatcher from basic data that is typically available to the engineer in the building design phase. Accordingly, the methodology uses data that one could reasonably be expected to have for a building's ventilation performance. This paper also reviews in situ performance based on measurements in U.K. schools both with and without open windows. Predictions generated by a semi-empirical model are then compared against measurement data and this is shown to deliver generally good agreement between the two, both with and without open windows, provided the theoretical predictions are presented in terms of an upper and lower performance limit. Furthermore, both experiment and theory clearly demonstrate that a large increase in the ventilation rate is possible if one combines the operation of a Windcatcher with, say, an open window, and that this ventilation rate is greater than that which would be achievable from a window operating on its own.

Estimating the performance of a natural ventilation system is very important if one is to correctly size the system for a particular application. Estimating the performance of a Windcatcher™ is complicated by the complex flow patterns that occur during the top-down ventilation process. Methods for predicting Windcatcher™ performance can currently be separated into simplistic analytic methods such as the envelope flow model and the use of complex and time consuming numerical methods such as CFD. This article presents an alternative semi-empirical approach in which a detailed analytic model makes use of experimental data published in the literature for 500 mm square Windcatchers™, in order to provide a fast but accurate estimate of Windcatcher™ performance. Included in the model are buoyancy effects, the effect of changes in wind speed and direction, as well as the treatment of sealed and unsealed rooms. The semi-empirical predictions obtained are shown to compare well with measured data and CFD predictions, and air buoyancy is shown only to be significant at relatively low flow velocities. In addition, a very simple algorithm is proposed for quantifying the air flow rates from a room induced by a Windcatcher™ in the absence of buoyancy effects. © 2009 Elsevier Ltd. All rights reserved.

Estimating the performance of a natural ventilation system is very important if one is to correctly size the system for a particular application. Estimating the performance of a Windcatcher™ is complicated by the complex flow patterns that occur during the top-down ventilation process. Methods for predicting Windcatcher™ performance can currently be separated into simplistic analytic methods such as the envelope flow model and the use of complex and time consuming numerical methods such as CFD. This article presents an alternative semi-empirical approach in which a detailed analytic model makes use of experimental data published in the literature for 500 mm square Windcatchers™, in order to provide a fast but accurate estimate of Windcatcher™ performance. Included in the model are buoyancy effects, the effect of changes in wind speed and direction, as well as the treatment of sealed and unsealed rooms. The semi-empirical predictions obtained are shown to compare well with measured data and CFD predictions, and air buoyancy is shown only to be significant at relatively low flow velocities. In addition, a very simple algorithm is proposed for quantifying the air flow rates from a room induced by a Windcatcher™ in the absence of buoyancy effects. © 2009 Elsevier Ltd. All rights reserved.

QC 20120228

The problems faced by the users in the language-checking software are discussed. There are false negatives, where the language-checking software fails to detect true errors, and false positives, where the software detects problems that are not errors. False negatives are troublesome because they might allow users to overlook problems that could be obvious to the human reader. False positives are also troublesome, although this issue has not been studied extensively in a usage context. The level of trust that users attribute to language-checking software may not always commensurate with the software's ability to do the job without errors.

The problems faced by the users in the language-checking software are discussed. There are false negatives, where the language-checking software fails to detect true errors, and false positives, where the software detects problems that are not errors. False negatives are troublesome because they might allow users to overlook problems that could be obvious to the human reader. False positives are also troublesome, although this issue has not been studied extensively in a usage context. The level of trust that users attribute to language-checking software may not always commensurate with the software's ability to do the job without errors.