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We describe a new high-resolution sampling technique which can be used to measure concentration fluctuations simultaneously at several points in space. The technique has been used to measure the probability distribution function as a function of the detector location relative to a continuous and steady source. Results are compared to previous experiments and theoretical predictions. The spectra of the concentration fluctuations are analyzed and their behaviour as a function of downwind distance from the source is described. IntroducrionThe frequency distribution of the concentration of a passive scalar in a turbulent field is an important subject from theoretical and experimental points of view. The information obtained in these studies is relevant to air quality models, screen obscuration studies, combustion problems and other applied areas. It also has a theoretical aspect on information regarding the probability of characterization of the mechanism of the process which is responsible for these fluctuations. However, very few experimental studies have been carried out which may supply data for theoretical investigations. Experiments in a wind tunnel were carried out by Fackrell and Robins (1982). In these experiments, the concentration fluctuations were measured with a resolution of 300 Hz; and the probability distribution function for a ground-level source and an elevated source was measured, as well as the spectra. Field experiments which were carried out by the U.S. Army for observation purposes, were analysed by Hanna (1984). In these experiments, the detection system's response time was only 1 s and therefore results were limited to cases where time average of the order of 1 s are interesting. Similar information was collected and analysed by Sawford et al. (1985). In his experiments, he uses, in addition to the smoke-plume, an SF6 tracer. Sawford's experiments were carried out at distances of up to 100 m downwind from the source. Measurements in the cross-wind direction were taken as well, but not simultaneously at distinct points. However, it was argued (Csanady, 1973;Townsend, 1976), that the concentration of a plume measured with a rapid response detector appears as short bursts of high concentration with longer intervals of zero concentration. This phenomenon is more prominent near the source. Consequently, high-resolution sampling techniques are required. Such a Boundary-Layer Meteorology 45 (1988) 157-175.

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The effect of global change in the past century, which led to increased levels of pollution and augmented values of cloud coverage, on the time of the apparent Sunrise and Sunset, is suggested to have shortened the day by 1 -I.5 minutes in the past 4 decades in northern mid-latitudes. This is supported by photographs of the setting Sun taken in Jerusalem during the months ofJuly and August 2001, which reveal that in over 95%ofthe cases the Sun completely disappear to the naked eye below marked atmospheric layers at an average elevation angle of 0.5-2.5° above the solid earth horizon. Based on trends in past Sunshine Duration measurements, the day shortening effect is expected to increase in the future. SUNRISE AND SUNSETThe length of the day is determined as the time span between Sunrise and Sunset. The actual instant of Sunset (and likewise -Sunrise) is dependent on the atmospheric refraction calculated for the Standard Atmosphere, and on the altitude and its horizon for a given location.Atmospheric refraction is taken into account in the determination of Sunrise and Sunset since these concepts are related to the average disappearance of the upper limb of the Sun to the observer. Such a disappearance has been previously merely related to the actual line ofthe apparent local horizon which gave rise to a standard sunset solar zenith angle of 900 and 50' (1) an angle which was calculated based on the astronomical refraction at sea level of the apparent Sun's upper limb viewed at an apparent angle of 900 (undisturbed horizon), and for the average angular size ofthe Sun.The boundaries between civil and nautical twilights, or between nautical and astronomical twilights, are determined as the astronomical solar zenith angles of 96 and 1 02 degrees without any dependence on the atmosphere nor on the altitude of the observer. However, in the case of the determination of Sunset at sea level altitudes, the accepted value of the solar zenith angle of 90.833 degrees is a result of the dependence on the present atmospheric effect on the sun rays' path. It is widely accepted that any future calculations that would be based on past or future significant changes in the main constituents of the atmospheric gaseous content and their profiles would lead to a different astronomical refraction resulting in a modified Sunrise and Sunset solar zenith angle.It can, therefore, be stated that Sunset and Sunrise are dependent on the nature of the atmosphere the properties of which may alter the disappearance of the Sun's upper limb to the observer's. Yet, since the atmosphere does not only refract the solar light, but also attenuates its intensity, the disappearance of an astronomical object to the observer's eye is not occurring merely due to the solid and liquid earth surface curvature. The extinction of light along the solar rays' path due to scattering and absorption by the atmospheric aerosol and cloud particles plays also a crucial part. When the total extinction exceeds certain threshold values depending on the brightness ofthe sour...