The Greasy Black Cloud

By Norman B. Peake

Reproduced with kind permission of the Airship Heritage Trust.

Curious – or sinister – cloud formations play a prominent part in the study of airship losses. The loss of the Shenandoah is a classic case: with aviation meteorology in its infancy, no one was quite sure what would happen when a large rigid airship met a violent line-squall. Cloud formation featured in the loss of NS11, and here Norman Peake reconstructs the probable weather conditions that contributed to the tragedy.

The NS11 was attempting to better her own world endurance record for airships when, heading seawards from the north Norfolk coast, she was seen to pass under (or into) a long “greasy black cloud”. Eye-witnesses saw no lightning, but heard a thunderous explosion as she fell towards the sea in a ball of fire.

The “greasy black cloud” which features in several accounts given to the Inquiry following her loss is typical of an advancing cold front along which it extends. The dry, denser, air of the front pushes beneath the moisture-laden clearer air which is, paradoxically, lighter. As this is forced upwards it expands and cools, whereupon its moisture starts condensing out as discrete droplets – the resulting cloud appears to be “greasy” since it has no clearly defined edge whilst it continues to grow. But condensation of water vapour releases heat (the opposite to that which happens when we wish to turn water into vapour – steam – which we do by putting heat IN): this causes a warm updraft which becomes self-sustaining as it initiates further condensation and the cloud builds. Eventually, the updraft attains a height of 20,000 feet or more, whereupon ice-crystals form the “anvil” head of a thunder-cloud, hail or torrential rain falls from the cloud and lightning occurs.

The official Inquiry into NS11’s loss postulated a lightning strike, although none was seen and it was some twenty minutes after the explosion that a mild thunderstorm developed. However, the “greasy” cloud may have already acquired some static charge, inducing a brush-discharge (St. Elmo’s Fire) near a valve on NS11’s envelope. Frequent past experiences of this phenomenon by airshipmen had convinced many that, being cold though luminous; it would not ignite hydrogen since it was not seen to do so. But in 1915 the Zeppelin L10 had burned over Neuwark Island in identical circumstances, and in 1936 the loss of the Hindenburg was attributed to St. Elmo’s Fire, subsequently claimed to have been seen playing along the top of the ship by a retired professor of physics (who had been young at the time, and had subsequently not testified). Further, Anthony Smith’s gas balloon Jambo was burned on the ground when hydrogen being released through her valve ignited – on a clear evening. There is some evidence to suggest that a rapid flow of hydrogen may ionize and ignite under certain conditions.

Possibly NS11 had set out more fully “gassed-up” than normal to maximise lift with a heavy fuel load and, with her ballonets emptied of air at a comparatively low altitude (pressure height), would automatically have valved hydrogen upon rising; or possibly her coxwain, sensing the updraft associated with rapidly-building clouds, had deliberately valved hydrogen to save a greater loss by expansion if she ascended. Whichever, or even through a leak, St. Elmo’s Fire could have been enough – especially if her hydrogen was of low purity, and near to an explosive mixture. Postwar parsimony may well have restricted hydrogen purification, which was done, in wartime, simply by valving off impure, and replacing with fresh. That she carried a crew of nine instead of the usual ten does suggest that special efforts were being made to enhance lift, which low purity of hydrogen may have reduced.

First published in 1994 in ‘Dirigible’ the journal of the Airship Heritage Trust