Characteristics of the electrification in dust storms have been studied by making field observations at different places in the southwestern United States. Measurements consisted of the atmospheric electric potential gradient at ground level, space charge and wind speed (both at 1.25 meters above the ground), atmospheric temperature and relative humidity, and size distribution of dust particles at ground level. It has been observed that during dust storms both signs of potential gradient can occur at ground level, and any sign of potential gradient can be associated with the same or opposite sign of space charge at 1.25 meters above the ground. Potential gradients of many kilovolts per meter are not uncommon during these dust storms, and sometimes these high values can be sustained for many minutes, The space charge values at 1.25 meters above the ground associated with these high values of potential gradient can be as high as 105–106 el cm−3. In some dust storms the polarities of potential gradient and space charge have been observed to change if the blowing dust, rather than reach high altitudes, is raised because of low wind speeds only to lower levels. Size distributions of different types of dust particles collected at ground level are also given.

Abstract:Research work in the area of the Global Electric Circuit (GEC) has rapidly expanded in recent years mainly through observations of lightning from satellites and ground-based networks and observations of optical emissions between cloud and ionosphere. After reviewing this progress, we critically examine the role of various generators of the currents flowing in the lower and upper atmosphere and supplying currents to the GEC. The role of aerosols and cosmic rays in controlling the GEC and linkage between climate, solar-terrestrial relationships and the GEC has been briefly discussed. Some unsolved problems in this area are reported for future investigations.

The onset of disintegration and corona in water drops falling at their terminal velocity in a vertical wind tunnel and exposed to horizontal electric fields has been investigated. Contrary to previous observations, the drops elongate in horizontal direction and distort into the shape of a concavo‐convex lens with a convex bottom and a sharp‐edged rim facing upward. Drops of diameter < 6.6 mm which do not break up in absence of electric field in this wind tunnel, break up in presence of the horizontal electric field. The values of horizontal electric field required for instability of the drops are much lower than those either predicted by Taylor's criterion of instability or observed in previous experimental studies. The criterion for instability of water drops freely suspended in presence of horizontal electric fields can be expressed as FH (ro/σ)½ = 0.98 ± 0.03 where FH is the horizontal electric field in esu, ro is the drop radius in centimeters and σ is the surface tension in dynes per centimeter. Most of the drops produce corona just before their breakup. Among various drops that are freely suspended in the wind tunnel, one by one, the number of drops that produce corona and/or breakup increases with increase in the electric field and/or drop size. While all drops of diameter ≥ 7.1 mm produce corona in a horizontal electric field of 500 kV/m, only a small fraction of very large drops of 8.0 mm diameter produce corona when the electric field is equal to 200 kV/m. Comparatively, very low values of instability field observed in our experiment are qualitatively explained because of long exposure of the freely suspended drops to the horizontal electric fields. The drops become unstable and produce corona when the drop's oscillation amplitude overshoots its equilibrium value and the plane of the drop oscillation coincides with the direction of electric field. From the results, it seems likely that horizontal electric fields in the bases of thunderclouds may cause disintegration of large raindrops and the occurrence of corona from their surfaces may trigger a lightning discharge.

Time for which charged or uncharged water drops of different sizes can be suspended over a vertical wind tunnel before their spontaneous breakup and the size distribution of droplets resulting from their breakup have been determined. Probability of spontaneous breakup of a drop has been found to increase with the size and charge of the drop. It has been observed that water drops carrying a charge of 5×10−10 C breakup immediately after their suspension if their diameter > 8 mm. Total number of droplets produced on spontaneous breakup of a drop increases with the size of the drop, and if the drop size is > 6.6 mm, the total number of droplets is more when the drop is uncharged than that when it is charged. However, the number of droplets larger than a critical size is more if the parent drop is charged and the number of droplets smaller than that critical size is more if the parent drop is uncharged. It has been attempted to qualitatively explain the experimental results as the result of enhanced surface charge density around the waist of the drop during its oscillation. Charge on the drop has been suggested to cause an increase in width of the base of suspended drop.

No abstract