Showing papers in "International Journal of Remote Sensing in 1991"
TL;DR: In this article, a model of crop primary production, which was originally developed to relate the amount of absorbed photosynthetically active radiation (APAR) to net production in field studies, is discussed in the context of coarse-resolution regional remote sensing of primary production.
Abstract: A model of crop primary production, which was originally developed to relate the amount of absorbed photosynthetically active radiation (APAR) to net production in field studies, is discussed in the context of coarse resolution regional remote sensing of primary production. The model depends on an approximately linear relationship between APAR and the normalized difference vegetation index. A more comprehensive form of the conventional model is shown to be necessary when different physiological types of plants or heterogeneous vegetation types occur within the study area. The predicted variable in the new model is total assimilation (net production plus respiration) rather than net production alone or harvest yield.
523 citations
TL;DR: In this paper, field measurements of primary production in semi-arid grasslands in three Sahelian countries over a period of 8 years were analyzed in conjunction with multitemporal sums of vegetation indices derived from the NOAA spacecraft's advanced very high resolution radiometer (AVHRR).
Abstract: Field measurements of primary production in semi-arid grasslands in three Sahelian countries over a period of 8 years are analysed in conjunction with multitemporal sums of vegetation indices derived from the NOAA spacecraft's advanced very high resolution radiometer (AVHRR). The results demonstrate that there is a strong linear relationship between the satellite observations of vegetation indices and the seasonal primary production in the range 0-3000 kg ha−. The confidence intervals of estimation of production were in the range ±61-161 kg ha−.
386 citations
TL;DR: In this paper, the authors proposed a method to estimate the fire size and temperature from multi-channel IR measurements, which can be used for global biogeochemical studies of biomass burning.
Abstract: Mid-infrared (IR) orbiting platforms are attractive candidates for monitoring fire on a global basis, as needed for global biogeochemical studies. In the mid IR, fires radiate intensely against a low-energy background. Thus fires occupying a fraction of a percent of a pixel can be detected. Fire size and temperature can, in theory, be calculated from multi-channel IR measurements. The fire record in raw IR data is biased by obscuration by clouds and smoke, sensor saturation, difficulties in gaining a representative sample given the diurnal variation in fire activity and the large role played in global fire emissions by cooking fires, charcoal making and very small burns on agricultural wastes. The bias is spatially variant and its nature is poorly understood. Further research and engineering is required before IR sensing can improve upon current rough estimates of biomass burning based on scattered available data.
251 citations
TL;DR: In this paper, the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator.
Abstract: Images are presented that show the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator. The variation in the growing season NDVI is represented by the coefficient of variation image that shows the large variation in the Sahelian growing season between years. It is concluded that these images illustrate some aspects of the perspective being brought to regional and continental scale processes by coarse resolution satellite sensors and the potential of these sensors to provide consistent, long-term datasets.
224 citations
TL;DR: In this article, the effects of soil optical properties on vegetation index imagery were analyzed with ground-based spectral measurements and both simulated and actual AVHRR data from the NOAA satellites.
Abstract: The effects of soil optical properties on vegetation index imagery are analyzed with ground-based spectral measurements and both simulated and actual AVHRR data from the NOAA satellites. Soil effects on vegetation indices were divided into primary variations associated with the brightness of bare soils, secondary variations attributed to 'color' differences among bare soils, and soil-vegetation spectral mixing. Primary variations were attributed to shifts in the soil line owing to atmosphere or soil composition. Secondary soil variance was responsible for the Saharan desert 'artefact' areas of increased vegetation index response in AVHRR imagery.
193 citations
TL;DR: In this article, a linear mixture model was applied to two AVHRR test images over the test sites to capture the subpixel proportions of forest cover, which produced image outputs in which pixel intensities indicated the proporton of forest coverage per km2.
Abstract: Tropical forest assessment using data from the Advanced Very High Resolution Radiometer (AVHRR) may lead to inaccurate estimates of forest cover in regions of small subpixel forest or non-forest patches and in regions where the pattern of clearance is particularly convoluted. Test sites typifying these two patterns were chosen in Ghana and Rondonia, respectively. To capture the subpixel proportions of forest cover, a linear mixture model was applied to two AVHRR test images over the test sites. The model produced image outputs in which pixel intensities indicated the proporton of forest cover per km2. For comparison, supervised maximum likelihood classifications were also performed. The outputs were assessed against classified Landsat TM scenes, converted to proportions maps and coregistered to the AVHRR images. An empirical method was applied for determining the critical forest cover per km2 needed for an AVHRR pixel to be classified as forest. The critical values exceeded 50 per cent, indicatin...
184 citations
TL;DR: In this paper, the authors used very high-resolution radiometer (AVHRR) data to produce savanna primary productivity maps for Senegal north of the Gambia in 1987 and 1988.
Abstract: NOAA Advanced Very High Resolution Radiometer (AVHRR) data were used to produce savanna primary productivity maps for Senegal north of the Gambia in 1987 and 1988. Measurements of herbaceous production, and also the sum of herbaceous and tree-leaf production, were regressed on temporal integrals of normalized difference vegetation indices (NDVI). Ground data were collected from 17 sites in the north Ferlo region in the Sahelian bioclimatic zone in both years and from an additional 10 sites located further south in the north Sudanian zone in 1988. The addition of tree-leaf production measurements to herb production increased the proportion of the variance of the NDVI integrals, which was explained by 168 per cent in 1987 and 6.7 per cent in 1988. The parameters of the regression obtained for 1987 compared favourably with Prince's synthesis of results from the Sahelian zones of Senegal, Mali and Niger. The regression obtained for 1988, however, was markedly different. Atmospheric water vapour, haze...
162 citations
TL;DR: Leaves from ten tree species, including three conifers from a woodlot in southern Ontario were sampled at weekly intervals for a period of 150 days spanning the phenological events in deciduous trees of leaf development and expansion, leaf maturity and leaf senescence.
Abstract: Leaves from ten tree species, including three conifers from a woodlot in southern Ontario were sampled at weekly intervals for a period of 150 days spanning the phenological events in deciduous trees of leaf development and expansion (flushing), leaf maturity and leaf senescence. The highly diverse seasonal red-edge reflection patterns were studied collectively and individually both from the perspective of long-term trends and relatively short-term or oscillating trends. The phenological events characteristic of deciduous trees were most effectively described using the red-edge position (λpg) and the chlorophyll-well position (λog ) derived from the inverted Gaussian model. Moreover, these parameters appeared consistent with what is known about the seasonal turnover of leaf chlorophyll and with other parameters R550 or Rog which more specifically quantify leaf chlorophyll. By these terms of reference chlorophyll declines considerably earlier in the season than the onset of other physiological or ...
148 citations
TL;DR: In this paper, an atmospheric correction approach for Thematic Mapper data is presented based solely on the evaluation of scene information and may therefore be considered operational, which is based on the determination of aerosol optical thickness from histogram minima and clear water targets.
Abstract: Many remote sensing applications, especially multitemporal approaches, require radiometric corrections of image data in which radiometric normalization to standard conditions and modelistic atmospheric corrections are often considered as alternative solutions. Successful radiometric normalization depends on the availability of suitable reference targets within the scenes under considerations, which may be critical. It is demonstrated that even simplified atmospheric correction modelling can provide a valuable alternative solution. We present an atmospheric correction approach for Thematic Mapper data, which is based solely on the evaluation of scene information and may therefore be considered operational. The method is based on the determination of aerosol optical thickness from histogram minima and clear water targets. Atmospheric conditions are assumed constant over the scene, but their variation with the Sunto-satellite scattering angle is accounted for. The environment effect is approximately...
140 citations
TL;DR: In this paper, the split-window method is used to correct atmospheric effects, but generally is valid only for sea surface temperature, and the error on the actual surface temperature obtained is I deg K for an error of 1 per cent on the mean emissivity.
Abstract: Agricultural applications using surface temperatures obtained from NOAA AVHRR thermal infrared data involve the difficulty of obtaining the actual surface temperature of the crop, due to the atmospheric and crop's emissivity effects. The split-window method is often used to correct atmospheric effects, but generally is valid only for sea surface temperature. Two original methods to obtain the split-window equation are presented: the first one uses ground measurements, the second one uses atmospheric information contained in NOAA data itself (i.e. the difference between brightness temperatures of AVHRR's bands 4 and 5). Both lead to a mean error of 2 deg K on the equivalent black body temperature. An original method to obtain directly from NOAA data the difference between emissivities of the surface in bands 4 and 5 is presented. The error on the actual surface temperature obtained is I deg K. for an error of 1 per cent on the mean emissivity. Finally the surface temperature can be estimated from ...
133 citations
TL;DR: In this article, the authors used satellite remote sensing data to supplement ground-based observations to forecast famine in sub-Saharan Africa, and found that at the continental scale AVHRR GAC data offer many advantages over traditional, ground data sources.
Abstract: Since 1984, national and international agencies have sought to improve their ability to forecast famine in sub-Saharan Africa. A number of early warning systems have been implemented for this purpose that monitor physical and social variables that may indicate the likelihood and magnitude of famine. Several famine early warning systems use satellite remote sensing data to supplement ground-based observations. These systems have demonstrated the advantages in timeliness and consistency of remote sensing data. Although user needs have not been clearly defined, experience gained in the operation of early warning systems and the results of related research suggest that: (a) at the continental scale AVHRR GAC data offer many advantages over traditional, ground data sources; (b) quantitative crop yield estimates might be improved through consideration of both photosynthetic activity of the vegetation and length of growing season; (c)qualitative comparisons of crop years have provided useful inputs to c...
TL;DR: The near-infrared channel of the NOAA advanced very high resolution radiometer (AVHRR) contains a water vapor absorption band that affects the determination of the normalized difference vegetation index (NDVI).
Abstract: The near-infrared channel of the NOAA advanced very high resolution radiometer (AVHRR) contains a water vapor absorption band that affects the determination of the normalized difference vegetation index (NDVI). Daily and seasonal variations in atmospheric water vapor within the Sahel are shown to affect the use of the NDVI for the estimation of primary production. This water vapor effect is quantified for the Sahel by radiative transfer modeling and empirically using observations made in Mali in 1986.
TL;DR: In this article, a network of sun photometers was established in the Sahel region of Senegal, Mali, and Niger to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data.
Abstract: In order to monitor the aerosol characteristics needed for atmospheric correction of remotely sensed data, a network of sun photometers was established in the Sahel region of Senegal, Mali, and Niger. Data analysis suggests that there is a high spatial variability of the aerosol optical thickness tau(a) in the western Sahel region. At a 67 percent confidence level the instantaneous values of tau(a) can be extrapolated approximately 270-400 km with an error tolerance of 50 percent. Spatial variability in the dry season is found to be of a similar magnitude. The ranges of variations in the NDVI in the Sahel region are shown to be approximately 0.02 and 0.01, respectively, due to commonly observed fluctuations in the aerosol optical thickness and aerosol size distribution.
TL;DR: In this paper, the local observing time of the sensor on National Oceanic and Atmospheric Administration (NOAA) satellites is discussed and a strategy is recommended for maintaining relatively constant time of afternoon observations by the Advanced Very High Resolution Radiometer (AVHRR) over the satellite lifetime.
Abstract: The local observing time of the sensor on National Oceanic and Atmospheric Administration (NOAA) satellites is discussed and a strategy is recommended for maintaining relatively constant time of afternoon observations by the Advanced Very High Resolution Radiometer (AVHRR) over the satellite lifetime The NOAA current orbital strategy is shown to be very conservative and a re-examination of the problem is recommended
TL;DR: In this paper, a simple exponential model using the band 3/band 1 (0-660μm/0-485μm) ratio was used for estimating total suspended solids and water transparency with an accuracy of ± 25 per cent over the calibration data range.
Abstract: This study reports on an investigation of multi-date water-quality calibration algorithms under turbid inland water conditions using Landsat Thematic Mapper (TM) multispectral digital data. TM data and water-quality observations were obtained near-simultaneously in Green Bay, an embayment of Lake Michigan, and related using linear regression techniques. The high concentrations of terrestrially-derived suspended solids appear to be controlling the water volume reflectance. A simple exponential model using the band 3/band 1 (0-660μm/0-485μm) ratio was found to be a useful index for estimating total suspended solids and water transparency (i.e. Secchi disk depth) with an accuracy of ± 25 per cent over the calibration data range.
TL;DR: In this paper, the relationship between radiative surface temperature (TR) and visible and near-IR reflectances expressed as the normalized difference (ND) from a Landsat Thematic Mapper scene is analyzed to examine the heat balance of agriculture and native evergreen forests in southeastern Australia.
Abstract: Relations between radiative surface temperature (TR) and visible and near-IR reflectances expressed as the normalized difference (ND) from a Landsat Thematic Mapper scene are analyzed to examine the heat balance of agriculture and native evergreen forests in southeastern Australia. Factors determining the residual scatter about, and slope of, the TR/ND relationships were analyzed using a coupled two-layer soil-vegetation model of the surface heat balance. Inverse linear relationships were observed between TR and ND for agriculture, but not for forests. This was due to a wide range of ND and TR values in agricultural regions caused by wide variations in fractional vegetation cover.
TL;DR: In this paper, the authors used regression analysis to explore relationships between stand reflectance in the Thematic Mapper bands and stand volume and growth measured in permanent sample plots, and good relationships were also found between live spruce-fir volume and TM band 4 (near-infrared) reflectance.
Abstract: Effective forest management requires reliable forecasts of timber growth. In New Brunswick, the growth of spruce-fir stands is greatly affected by annual and cumulative defoliation caused by the spruce budworm. This study was designed to determine whether the Thematic Mapper (TM) could detect defoliation in a way which was correlated with stand growth well enough to be used as a predictive tool. Regression analysis was used to explore relationships between stand reflectance in the Thematic Mapper bands and stand volume and growth measured in permanent sample plots. Good (R 2>0.8) relationships were found between net annual spruce-fir volume change and vegetation condition indices incorporating TM near-infrared and shortwave infrared reflectances. Good relationships were also found between live spruce-fir volume and TM band 4 (near-infrared) reflectance. These results have encouraged us to plan a province-wide calibration and measurement programme aimed at improved stand development forecasts for ...
TL;DR: In this paper, a general analysis of frequency domain SAR processing based on the relationship between the phase of the two-dimensional Fourier transform of a point response to its range-time history is presented.
Abstract: In this paper we present a general analysis of frequency domain SAR processing based on the relationship between the phase of the two-dimensional Fourier transform of a point response to its range-time history. The paper demonstrates how this provides an appropriate basis for the design of a coherent strip-mode processor, free of geometric or phase distortion and artefacts and without excessive computational cost. In consequence the paper is highly relevant to the real-time, precision, processing of SAR data. The paper comments on the relevance of the analysis to ambiguity estimation and the processing of very long integration time SAR data.
TL;DR: In this paper, the Andaman Nicobar group of islands has been used to prepare forest type maps using visual and digital methods, and the mapping techniques have been compared with respect to classification and accuracy levels.
Abstract: The Andaman Nicobar group of islands in the Andaman Sea are blessed with luxuriant tropical vegetation. During recent years, vegetation in these islands has been under tremendous pressure due to increased settlement and commercial extraction, Landsat TM data of the Baratang forest division of the Andaman group of islands has been used to prepare forest type maps using visual and digital methods. Digital enhancement techniques have been evaluated to discriminate forest types in a typical environmental set-up. The mapping techniques have been compared with respect to classification and accuracy levels. Finally, the land transformation in the forest division has been studied using past aerial photographs. The study highlights the appropriate methodology required to map forest types.
TL;DR: In this paper, the angular variation of the thermal infrared emissivity of representative samples of naturally occurring materials was studied under laboratory conditions, and the results showed that a decrease of the emissivities with increasing viewing angle does exist.
Abstract: The angular variation of the thermal infrared emissivity of representative samples of naturally occurring materials was studied under laboratory conditions. The angular response of land surface will be of primary importance for thermal infrared remote sensing when two-angle data are available from the Along Track Scanning Radiometer (ATSR) aboard ERS-1. The results show that a decrease of the emissivity with increasing viewing angle does exist. The magnitude of this effect is not simply related to the optical properties of the material but depends also on scattering mechanism, grain size, porosity etc. SiO2 sand has a very small angular variation in the 100.12.0 μm band, and a much larger one in the strong reflectivity bands. Agricultural soils may show a large angular effect, although rather constant with wavelength. The results of this study remain very local; their generalization to more complex media and larger scales is not straightforward.
TL;DR: In this paper, two major difficulties associated with SAR image change detection are addressed: the removal of speckle noise and the registration of information between images, and the problem of image registration is severe in airborne SAR imagery due to the unpredictable nature of the aircraft track.
Abstract: The two major difficulties associated with SAR image change detection are addressed. These are the removal of speckle noise and the registration of information between images. The problem of image registration is severe in airborne SAR imagery due to the unpredictable nature of the aircraft track. Although inertial navigation systems may be employed to measure this motion, the accuracy obtained is insufficient to allow the creation of large rectilinear images. However, autofocus techniques are used here to measure residual aircraft motions thus allowing the production of large geometrically accurate images. The second problem of speckle reduction has been approached in two ways. The first technique applies an intensity segmentation algorithm to each image and the regions generated by this segmentation are then compared using the change detection algorithm. An alternative approach to the problem of speckle removal is to use neural network methods to learn the speckle removal and region generation ...
TL;DR: In this article, the authors compared rainfall estimates, based on cold cloud duration estimated from Meteosat data, with vegetation development depicted by data of the normalized difference vegetation index (NDVI) from the NOAA AVHRR for part of the Sahel.
Abstract: Rainfall estimates, based on cold cloud duration estimated from Meteosat data, are compared with vegetation development depicted by data of the normalized difference vegetation index (NDVI) from the NOAA AVHRR for part of the Sahel. Decadal data from the 1985 and 1986 growing seasons are examined to determine the synergism of the datasets for rangeland monitoring. There is a general correspondence between the two datasets with a marked lag between rainfall and NDVI of between 10 and 20 days. This time lag is particularly noticeable at the beginning of the rainy season and in the more northern areas where rainfall is the limiting factor for growth. Principal component analysis was used to examine deviations from the general relationship between rainfall and the NDVI. Areas of low NDVI values for a given input of rainfall were identified: at a regional scale, they give an indication o areas of low production potential and possible degradation of ecosystems.
TL;DR: In this article, the authors compared the relationship between the log of softwood volume and the spectral bands from both sensors (P less than 0.01) using correlation and regression analyses.
Abstract: Digital Landsat Thematic Mapper (TM) and SPOT high-resolution visible (HRV) images of coniferous forest canopies were compared in their relationship to forest wood volume using correlation and regression analyses. Significant inverse relationships were found between softwood volume and the spectral bands from both sensors (P less than 0.01). The highest correlations were between the log of softwood volume and the near-infrared bands.
TL;DR: In this paper, the authors analyzed leaf reflectance changes associated with changes in water stress in two separate experiments and found that the variation in reflectance among collections of leaves of a given species all at the same level of water stress is at least as great as the variation of reflectance associated with changing water stress for a given leaf collection of that species.
Abstract: Leaf reflectance changes associated with changes in water stress were analyzed in two separate experiments. Results indicate that the variation in reflectance among collections of leaves of a given species all at the same level of water stress is at least as great as the variation in reflectance associated with changes in water stress for a given leaf collection of that species. The implications is that results from leaf reflectance-water stress studies have only limited applicability to the remote sensing of plant canopy water stress.
TL;DR: In this paper, an interpretation technique was developed to process the spectra of upwelling radiation from water in the visible to retrieve the chlorophyll-a concentration with a standard error of less than 2.6mgm−3, mineral suspended matter concentration less than 4mgl−l, and dissolved organic matter concentration more than O.5mgCl−1.
Abstract: Interpretation techniques have been developed to process the spectra of upwelling radiation from water in the visible. Spectral indices to retrieve the concentration of optically active components have been suggested. They make it possible to retrieve the chlorophyll-a concentration with a standard error of less than 2.6mgm−3, mineral suspended matter concentration less than 4mgl−l, and dissolved organic matter concentration less than O.5mgCl−1. The instrumentation is described for measurments of the upwelling radiation as well as for operational determination of the concentrations of phytoplankton chlorophyll-a, mineral suspended matter and dissolved organic matter.
TL;DR: In this article, the relationship between time-integrated normalized difference vegetation index (NDVI) statistics derived from NOAA AVHRR local area coverage (LAC) data (dependent variable) and total herbaceous biomass (independent variable) was obtained through regression analysis.
Abstract: The standing crop of herbaceous biomass produced during the 2-4 month summer rainy season by the annual grasses in the Sahel zone provides an indication of resource availability for livestock for the following 9-month dry season. Combined use of NOAA advanced very high resolution radiometer (AVHRR) local area coverage (LAC) satellite data and biomass data, obtained through vegetation sampling of 25-100 km2 areas, allowed the development of a method for biomass assessment in Niger. Vegetation sampling involved both visual estimates and clipped plots (double sampling). The relationship between time-integrated normalized difference vegetation index (NDVI) statistics derived from NOAA AVHRR LAC data (dependent variable) and total herbaceous biomass (independent variable) was obtained through regression analysis. An inverse prediction was used to estimate biomass from the satellite data. Biomass maps and statistics of the grasslands were produced for the end of each rainy season: 1986, 1987 and 1988. ...
TL;DR: In this paper, a remote sensing model was used together with allometric regression equations from the literature to estimate foliage and above-ground woody biomass in sparse woodlands in the Sudanian and Sahelian bioclimatic zones in Mali.
Abstract: Predictions of tree size and density from a remote sensing model were used together with allometric regression equations from the literature to estimate foliage and above-ground woody biomass in sparse woodlands. Study sites with woody cover ranging from 10-50 per cent were located in the Sudanian and Sahelian bioclimatic zones in Mali, West Africa. Estimates compared favourably to independent measurements of foliage biomass made in the Sahelian sites, and to typical values of foliage and woody biomass from the literature for these regions and for similar woodland types. If combined with a vegetation stratification at the appropriate scale, this approach could provide estimates of woody biomass for fuelwood, and foliage biomass for browse over large areas (103-106km2). These estimates could be used in regional scale models of biogeochemical cycling.
TL;DR: In this article, a new agrometeorological model was derived from the EARTH model, which combines water and energy exchanges between soil and atmosphere at one hour intervals through an energy budget.
Abstract: A new agrometeorological model wasderived from the EARTH model (elaborated at the French Meteorological Office by Choisnel). It uses classical meteorological network daily data, and combines water and energy exchanges between soil and atmosphere at one hour intervals through an energy budget. The surface temperature is computed every hour as the solution of the energy budget equation. The model is first briefly presented. A comparison was made between our model and NOAA AVHRR satellite derived surface temperature; it shows good consistency. Simulated data from actual meteorological information were then used to generate simplified relationships for estimating daily evaporation: ET d−RN d=A−B(T S−T a) where RN d is the daily net radiation, T S, is the surface temperature at 14.00 UT, T a, is the maximum air temperature. The sensitivity of the coefficients A and B to the roughness length is also studied. Finally, an example of mapping of evaporation is given,using our algorithm combined with meteor...
TL;DR: A light-weight hand-held instrument system based on an 8-channel Cropscan radiometer (500-850 nm) complemented with an Everest-4002 IR-thermometer for recording canopy temperature and other sensors for recording air temperature, relative air humidity and wind speed as well as a battery operated computer is described in this article.
Abstract: Experiments using hand-held instruments for remote sensing of field plots of agricultural crop plants demonstrated good correlation between spectral reflectance data and plant growth, green biomass and yield as well as infection by various leaf and root diseases A light-weight hand-held instrument system based on an 8-channel Cropscan radiometer (500-850 nm) complemented with an Everest-4002 IR-thermometer for recording canopy temperature and other sensors for recording air temperature, relative air humidity and wind speed as well as a battery operated computer is described The experiments have demonstrated a high potential of this version of remote sensing as a complement to conventional field plot experiment methods, for studies of plant growth and development, for predictions of yield, for early detection and quantifying of stress and diseases, but also to facilitate a better and more need-adapted disease control
TL;DR: In this article, the authors evaluated the potentiality of the low spatial resolution and high temporal resolution images, such as the images scanned by the AVHRR sensor of the NOAA-9 satellite.
Abstract: The principal objective of this study is the evaluation of the potentiality of the low spatial resolution and high temporal resolution images,such as the images scanned by the AVHRR sensor of the NOAA-9 satellite. These imagesare used for detection of forest fire in areas prone to be affected and for determining physiological conditions of vegetationcover during the year, in order that we can as certain the conditions under which such cover is most susceptible to fire.