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doi: 10.1002/qj.49707934204pmid: N/A
An instrument which gives a continuous, automatic record of the size distribution of raindrops at ground level is described. The light scattered by the drops in falling through an optically defined area of a light beam is focused on to a photomultiplier cell and the resulting voltage pulses are sorted electronically into eight different size groups. The spectrometer is used to study the variation of the drop‐size spectrum with time and with the rate of rainfall. Calibration curves and some specimen results are presented.
doi: 10.1002/qj.49707934205pmid: N/A
New observational data are discussed which indicate that:
doi: 10.1002/qj.49707934206pmid: N/A
It is argued that the generation and separation of electric charge above the 0°C level in thunderclouds are consequences of the growth and fall‐motion of graupel pellets. The results of laboratory experiments which show the acquisition of a negative charge by a growing rime deposit are used to calculate the charge liberated during the growth, in a thunderstorm cell, of ice particles by the accretion of supercooled cloud droplets. The calculations show that, in a vigorous warm cumulonimbus, it is possible for this process to generate about 1,000 C of charge within about 11 min of the activation of ice nuclei at the −10°C level, and that this volume of charge can be subsequently separated by the falling precipitation elements at a rate which can account for the production of several successive lightning flashes over a period of a few minutes.
doi: 10.1002/qj.49707934207pmid: N/A
Using the results of laboratory experiments on the supercooling of purified water, the freezing of cloud and raindrops is examined. It is shown that at temperatures lower than about −30°C in cold‐box or expansion‐chamber experiments, the drops freeze in approximately the numbers that would be expected if they were pure water. Those that freeze at temperatures higher than about −20°C seem to be more numerous than would be the case if the drops were pure. An interpretation of the main features of the Findeisen and Schulz expansion‐chamber experiments is found to be possible without appealing to the action of foreign ice‐forming nuclei, although there is a discrepancy between calculation and experiment at temperatures higher than about −20°C. In the atmosphere, formation of cirrus clouds is shown to become possible at temperatures below about −35°C, glaciation in stratiform clouds to become appreciable at temperatures below about −20°C and freezing of raindrops in strongly convective clouds to become important below about −13°C, without the presence of ice nuclei. It is concluded that freezing nuclei may be important at temperatures above about −20°C, while their presence at lower temperatures will be masked by the freezing of uncontaminated drops.
doi: 10.1002/qj.49707934208pmid: N/A
A study was made of the length of dry and wet spells at five Canadian cities. From the results of the study it was learned that after a wet day the probability of the following day being wet is constant no matter how long the wet period has persisted; the same is true for the weather following a dry day except that there is a slight increase in the probability of dry weather with increasing length of the dry period.
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