Field trials were conducted to determine the effectiveness of shields in reducing off-target droplet drift from ground-rig sprayers. Sprayer booms ranging in width from l0 to 13.5 m and equipped with com--e.ciully avaltitti shields *ere operated along a 150-m swath in a field of approximately 2O-cm-tall spring wheat in wind speeds ranging from lb to 35 km h-r. Airborne drift was measured using aipiriteO air samplers. ihe ure of an 80" flat fan tip (8001) at a pressure of 275 kPa and a ground speed of 8 km h -I resulted in 7 .5% of the 50 L ha -r spray solution drifting off the target area. The ule of protective cones with 8fi)1 tips without lowering the boom reduced airborne drift by 337o at_ a 20 krir h-r wind speed, while a Si-gSn drift reduction was accomplished with the combination of solid or perforated s-hielding and lowering the sprayer boom. Increasing the application rate to 100 L ha -r by using 8002 tips reduced drift of the unshielded sprayer by 65% . Decreasing application rate to 15 i ha-riy using tOOOtZ tips increased drift by 29% despite the use of a shield. Off-target drift increased withincreaiing wind speeds for all sprayers, but the increase was less for shielded sprayers and coarser sprays. The decreased droplet size of spray from 1 10" tips increased drift when the boom height was the same as for 80o tips. High wind speeds, lower carrier volumes and finer sprays, 110" tipJ, and solid shields tended to dicrease on-swath deposit uniformity, whereas a perforated shield or cones did not affect deposit uniformity.
The freezing of water drops is often accompanied by shattering of the ice shell and the production of small ice splinters. These phenomena have been studied in relation to the drop diameter in the range 30 p to 1 mm, the nucleation temperature of the drop, the air content and purity of the water. The number of splinters produced is largely governed by the nucleation temperature (the degree of supercooling) which controls the quantity of air released during freezing and is not very dependent on drop size. Slightly supercooled drops of +c-l mm diameter produced, on average, 20 to SO splinters. The mechanism of drop fragmentation is discussed and the potential importance of splintering in the ice-nucleus economy of clouds is assessed.Fragmentation of freezing drops is accompanied hy electrical charging, usually with the splinters positively charged and the drop residue carrying a negative charge. The magnitude of the residual charge which, on average, is observed to be about lo-$ e.s.u. for a millimetre drop, is related to the same factors which control splinter production. Charging by this mechanism would not appear to be of major importance in the electrification of thunderstorms.
SUMMARYA fairly wide range of 28 naturally-occurring mineral dusts has been tested for ability to act as ice nuclei. 19 substances, mainly silicate minerals of the clay and mica groups, are found to be effective at -18°C; seven of these are active above -10°C. The most abundant of these is kaolinite with a threshold temperature of -9°C. 10 of the 28 substances, again mainly silicates, are found to become more efficient ice nuclei having once been involved in ice-crystal formation, i.e. they can be pre-activated. Thus, ice crystals grown on kaolinite nuclei, which are initially active at -9°C when evaporated and warmed to near 0°C in a dry atmosphere, leave behind nuclei which are thereafter effective at -
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