Low Sun Elevation Research Articles

Turbidity of the atmosphere determined from satellite: Calculation of optimum wavelength

With the object of determining the atmospheric turbidity (aerosol optical depth) from upward emerging spectral radiances, an evaluation was made of that wavelength at which the variability of the radiance owing to the variability of the turbidity predominates the variability caused by the perturbing optical parameters. This has been done by numerical modeling of the variability of the radiances owing to the variability of all optically acting atmospheric constituents (no clouds) and the underlying rough ocean for maritime aerosols and Saharan dust. It is found that the optimum wavelengths are 0.75 and 0.87 μm. In addition, it has been found that low sun and low observation elevation angles are preferable.

Digital pre-processing techniques of landsat data in high relief area—an Himalayan example

The analysis of the Landsat data for a Himalayan Test area recorded with a low sun elevation has shown that most of the information is due to illumination and shadow effects, which can be satisfactorily separated from the effects of the ground-cover on the radiance levels, by using the PC analysis and the Euclidean Distance and Directional Cosine techniques. At the same time effective data compression is achieved without loss of information. For a visual interpretation, both the images of “relief” and “groundcover” are necessary. Proper location, using drainage divides and channel network may be done on the “relief” image. Vegetation boundaries appear on the other image. Comparison of groundcover images of September 1972 and May 1974 indicates the utility of these these techniques for sequential studies, particularly the separation of deciduous and non-deciduous vegetation in a broad manner.

The distribution of 660 and 730 nm radiation in corn canopies

Light sensors were constructed which measured narrow wavelength bands of radiation at 660 and 730 nm under field conditions. Both the spectral quality of radiation incident to the top of a corn crop and within the crop were recorded. The sensors were traversed horizontally through five types of corn canopies and at several heights. The radiation incident to the crop was spectrally stable through themidday, regardless of sky conditions. However, at lower Sun elevations a relative increase of 730 nm radiation was observed. Within the corn canopies the red wavelength is absorbed by the leaves, but the far-red is highly scattered. Consequently, the radiation within the crop is heavily enriched with 730 nm radiation with increasing depth into the crop. The enrichment of far-red within the crop canopy was greater at low Sun elevations. Some implications of the effects of these environments on the behavior of the phytochrome system of plants are discussed.

Radiation measurement on the Antarctic snowfield, a preliminary report

Observations of solar and terrestrial radiation and associated energy exchange at the geographic South Pole were first taken during the International Geophysical Year. An attempt is made to show the problems of radiation observation when standard instrumentation is used on a polar snowfield. Low sun elevation, temperatures 150°F below calibration, and human limitations during the winter night all affect in varying degree the observations of fluxes of solar and terrestrial radiation. A preliminary study of the radiant energy exchange is presented to show the absorption of solar radiation at the snow surface and the effect of ‘warm’ clouds on surface temperatures during the winter night.