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A technique has been developed for deriving estimates of condensed water mixing ratio in cumulus clouds from measurements of potential temperature in the air in the compressor of a jet engine. Condensate that enters the engine at low temperatures (near −10°C) is evaporated in the compressor, causing a cooling of the air that is proportional to the amount of condensate evaporated. An important element of the technique is a correction for the time response of the temperature measurement system mounted in the compressed air. This new technique is simpler and more robust than an earlier technique based on measurement of the vapor concentration in the compressed air by an optical extinction (at Lyman-alpha wavelength) method. The values of condensed water mixing ratio derived from the new system are shown to be similar to those from the optical extinction system, previously reported in the literature.

Some possible effects of hygroscopic seeding with flares are explored by calculating how such seeding would modify the initial size distribution of cloud droplets and the subsequent evolution of that size distribution by coalescence. To be representative of recent experiments in South Africa, the calculations emphasize the effects of hygroscopic particles that can be produced by flares, instead of the larger particles used in most past hygroscopic-seeding experiments. Parcel calculations representing simultaneous condensation and coalescence suggest that the formation of rain through the warm-rain process can be accelerated significantly by the addition of such hygroscopic particles. Some observations of the effects of hygroscopic material near cloud base support at least the early stages of the calculations. The results suggest that the positive effects being obtained in the South African experiment may occur through such acceleration of the warm-rain process. Possible cloud-seeding applications and climate implications are discussed.

While conducting a randomized seeding experiment, a storm was selected whose microphysical characteristics were so far from what was expected, given the existing thermodynamic environment, that some explanation was needed to account for the apparent errant behavior of this storm. More than a decade of sampling at −10°C using the project Lear jet has led to a simple classification technique, based on cloud-base temperature and buoyancy, which predicts the absence or presence, and to some extent the degree of coalescence (coalescence-freezing) precipitation growth in local convective storms. The unusual nature of this storm was recognized against this extensive microphysical database. The probable cause is attributed to emissions from a Kraft paper mill 10 km south of the storm's position. The mill had recently undergone an expansion program that had quadrupled its output of paper products. Using the radar characteristics of this unusual storm as a guide, a search of one season of radar data reveal...

Daily precipitation records during the period of the hail suppression seeding program in the Eastern Transvaal around Nelspruit, South Africa are examined for evidence of persistence effects from silver iodide iodine similar to those reported by Bigg and Turton in Australia. Using their method of analysis, a positive anomaly is found in the comparison of target-to-control rainfall commencing 11 to 12 days after a seed day and lasting 10 to 11 days. The Australian anomaly commences 9 to 10 days after a day and lasts about 9 days. Otherwise, the “peristence signatures” in the two areas are very similar. This repetition of apparent persistence effects in another area on a different continent raises serious doubts about conventional statistical assessments of the success of randomized seeding experiments when silver iodide is used.

For the past three years, a Learjet has been making microphysical measurements in new cloud development on the flanks of multicellular storms in the eastern Transvaal area of South Africa. Data from an imaging probe and a forward scattering spectrometer have been averaged for each storm for all first cloud penetrations between −8° and −12°C. Clear images of drops of diameters greater than 300 μm are found in 40% of the 42 storms measured. Most of the observed drops are associated with the more “maritime” droplet spectra. Also, the appearance of coalescence around −10°C appears to be related to cloud base temperatures and buoyancies, rather than changes in air masses, suggesting that cloud thermodynamics may play a dominant role in determining cloud microphysics in the Nelspruit area.

Eastern Transvaal summer rainfall from 1951 to 1982 is examined for possible changes coincident with the operational hail suppression program, which ran from 1972 to 1981. Rough exploratory methods are used initially and lead to fitting various models by standard regression techniques. The importance of station location (geography) and year of observation (trends over time), as well as possible seeding effects, are assessed by successively fitting models incorporating these effects and comparing their fits. Finally, graphical displays are used for further exploration of residuals from those models. The results from all these analyses show unusually large amounts of rainfall on the target area in six out of the ten seeded summers. These augmented amounts apparently occurred in summers when natural rainfall was plentiful. Since the allocation of the seeding was not randomized, one should not apply statistical significance tests, and definitive conclusions about the effect of seeding cannot be drawn.

A commercial hail suppression program has been in operation at Nelspruit, South Africa (25°S, 31°E) since December 1971. The seeding technique employed has been the “on-top” method, which consists of dropping silver iodide pyrotechnics into the tops of cloud turrets growing on the flanks of mature storms. Daily crop insurance records of hail damage to tobacco are compared for seeded and nonseeded days. The control (unseeded) days consist of historical data and of days during the operational period when storms in the area were definitely not seeded, usually for operational reasons. A “hail severity ratio”, defined as the ratio of the crop lost to the total crop planted in the areas hit by hail, is derived from the hail-damage assessments and appears to be the parameter most sensitive to the claimed treatment effect. A comparison of seeded days during the initial phases of the program, when propeller aircraft were used, with more recent days when one or two jets were used for the seeding, indicates...

Determination of the habits (ice or water) and therefore the densities of particles whose images are acquired by 2D probes is often an ambiguous process. A Learjet's radar measurements of equivalent reflectivity factors from a range gate 1800 m ahead of the aircraft are compared to reflectivities calculated from images acquired by a 2D-C probe over a range of assumed particle densities from 0.2 to 1 g cm−3. Although the comparisons suffer from many uncertainties, such as the vast disparity between the volumes sampled by the 2D-C probe and the aircraft radar, the method does discriminate well between water drops or recently frozen riming water drops and low density graupel particles.

Many of the severe and persistent hailstorms in Alberta propagate by means of new cloud development on the southern or western flank. A concept is proposed whereby these cumulus towers are seeded with freezing nuclei early in their development. In order to accomplish this a droppable pyrotechnic flare system was developed and tested. A T-33 jet aircraft was used as the seeding platform. It was equipped with a Ku-band weather radar, a flare rack and firing control panel, a 14 channel recorder and a 3-cm transponder. Seven inch pyrotechnic flares were manufactured to the following specifications: delay burn 50 sec, silver iodide burn time 30 sec, and flare output 24 gm of silver iodide producing a total of 2.4 × 1014 freezing nuclei active at −10C. A unique feature was the incorporation of 10-cm radar chaff which was released at the flare burn-out and used as a position marker. Flare performance was evaluated using radar, visual and photographic tracking. Total fall distance as a function of releas...

At the National Aeronautical Establishment, Ottawa, Canada, a T-33 jet aircraft has been instrumented for turbulence research (Fig. 1). The system is capable of measuring the three components of atmospheric turbulence, longitudinal, lateral, and vertical (with respect to aircraft axes) in terms of true gust velocities. A doppler-radar installation on the T-33 provides groundspeed and drift measurements, and, along with altitude, airspeed, heading and temperature information, permits simultaneous fine scale measurements of the temperature and wind fields in the turbulence.