Local Air Mass Research Articles

The influence of trade winds, local air-masses and topographic obstacles on the aeolian movement of sand particles and the origin and distribution of dunes and ergs in the Sahara and Australia

The Saharan ergs consist of a wide range of aeolian land forms, while in Australia, sand-ridges are the main form. The ergs are situated along major sandflows of continental extent along which sand is transported. These are oriented approximately from NE to SW in the Sahara. In Australia they form an anticlockwise pattern open to the west. The spatial and temporal factors that govern the origin of sand-ridges are reviewed.

An observation of hot air downwelling over Pantelleria Island in the Strait of Sicily

During a period of strong SW winds, a local anomalous hot air mass was observed at sea level on the lee side of Pantelleria Island in the Strait of Sicily. The hot air, some 10 °C above ambient, was produced by downward transport or mixing from an intense inversion layer occurring above 500 m. This rapidly moving layer, associated with a flow from the Libyan Desert, was apparently perturbed by the 836-m high volcanic cone of the island. Laboratory models suggest that such an interaction could produce phenomena, i.e., vortex shedding and lee waves, all of which would mix or advect the upper warm air downward. This, together with the adiabatic heating effect, would produce the hot air phenomena.

Some aspects of lightning activity and related meteorological conditions

Lightning statistics are used to define a thunderstorm activity index. The index is based upon (1) the total duration and (2) the frequency of occurrence of lightning discharges. It is suggested that the horizontal extent of a storm may be inferred from the shape of the frequency-distribution curve, whereas the mean duration of discharges is related to the vertical convective activity. The bipolar thunderstorm model of Holzer and Saxon is extended to account for the occurrence of multiple discharges to ground. A relationship between the number of intracloud discharges and the total number of separate return strokes in ground discharges may be predicted from the model. Good agreement is found with the predictions in some, but not all, storms. Statistics are cited which suggest that the ratio of intracloud to cloud-to-ground discharges may reflect the type of storm (e.g., ‘frontal’ or ‘local air mass’).

The Attenuation of Brightness Contrast Caused by Atmospheric Optical Haze*

Two physical processes causing the attenuation of brightness contrast of objects by the atmosphere have been isolated. One of these is related to scattering by the particles in the atmosphere and the other is related to convection currents of small local air masses of a variety of optical densities which cross a line of sight during the observation of an object through the atmosphere. These small air masses momentarily deflect the line of sight from the object to its background, thereby changing the apparent contrast of the object viewed. This attenuation process is designated as “optical haze” to distinguish it from reduction in contrast caused by scattering processes. Some authors have referred to this as “shimmer.” In the case of a black object viewed against the horizon as a background, the intensity of optical haze has been found to vary with time throughout the day. This temporal variation of optical haze was found to have at least one minimum during the day, usually after sunrise and before sunset. During the afternoon minimum, the optical haze was found to practically vanish at which time scattering alone was responsible for the atmospheric attenuation of brightness contrast and hence the two attenuation processes could be isolated on days in which the scattering was reasonably constant. On such days, studies of the relative importance of the two physical attenuation processes mentioned have indicated that as much as fifty percent of the atmospheric attenuation of brightness contrast can be attributed to optical haze. This suggests a need for modifying the theoretical treatment of the subject of atmospheric attenuation (to make allowances for optical haze which is ordinarily not considered as a factor).