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Three types of thermocouple sensing elements were used to measure potato leaf temperatures in the uppermost layer of the canopy. A comparison between thermocouple hypodermic needles inserted into the midrib of the potato leaf and thermocouple clips fastened on its surface gave near-identical temperature readings. Miniature screens housing thermocouple sensors exposed at the upper canopy level and measuring ambient air temperatures showed excellent agreement (r = 0.996, N = 68) with potato leaf temperatures. This was not the case for temperatures recorded at the standard meteorological screen 2 m above the ground. It is therefore suggested that for micrometeorological measurements, where accuracy of crop temperature values is biologically significant, accurate information can be obtained with such miniature screens. 相似文献
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T. Ben-Gai A. Bitan A. Manes P. Alpert A. Israeli 《Theoretical and Applied Climatology》1998,61(3-4):207-215
Summary Recent studies imply that significant climatic changes over the central and southern coastal plain in Israel may be due to
changes in land usage, which have taken place since the National Water Carrier operation in the early 1960’s. Such changes
are reflected in the spatial distribution of the surface albedo pattern, obviously resulting in changes in the surface radiation
balance and, subsequently, modifying the surface heat fluxes and the stability conditions of the Planetary Boundary Layer
(PBL).
An Eppley PSP Pyranometer facing downward was mounted on a small Cessna aircraft and flown along the coastal plain from Tel
Aviv to the northern Negev south of Beer Sheva, at an altitude of approximately 500 feet, measuring surface reflection. The
incoming solar radiation was measured simultaneously, at several surface radiation stations of the Israel Meteorological Service,
along the flight path. The results show large differences in surface reflection distributions, between the cultivated areas
in southern Israel (as low as 0.15), and the adjacent arid regions (with values of up to 0.35). Historical albedo maps were
reconstructed according to land utilization maps of the 1930’s and the 1960’s. A comparison between recent albedo map and
the reconstructed maps, indicates temporal changes in the surface albedo pattern during the last decades.
Received January 15, 1998 Revised June 23, 1998 相似文献
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Rock texture has a critical influence on the way rocks weather. The most important textural factors affecting weathering are grain size and the presence of cracks and stylolites. These discontinuities operate as planes of mechanical weakness at which chemical weathering is enhanced. However, it is unclear how different rock textures impact weathering rates and the size of weathered grains. Here, we use a numerical model to simulate weathering of rocks possessing grain boundaries, cracks, and stylolites. We ran simulations with either synthetic or natural patterns of discontinuities. We found that for all patterns, weathering rates increase with discontinuity density. When the density was <~25%, the weathering rate of synthetic patterns followed the order: grid > honeycomb > Voronoi > brick wall. For higher values, all weathering rates were similar. We also found that weathering rates decreased as the tortuosity of the pattern increased. Moreover, we show that textural patterns strongly impact the size distributions of detached grains. Rocks with an initial monomodal grain size distribution produce weathered fragments that are normally distributed. In contrast, rocks with an initial log-normal size distribution produce weathered grains that are log-normally distributed. For the natural patterns, weathering produced lower modality distributions. 相似文献
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