Taieri Pet

A cloud formation reportedly found only in one or two places in the world. It is formed by high north westerly winds being forced upward over the Rock & Pillar range - spectacular and unnerving by its sheer size and its association with the high winds.

The following article appeared in The Illustrated London News, October 13, 1951. It is reproduced here with permission. However, the accompanying aerial photograph (which is copyright protected) has been replaced:

"New Zealand, owing to its geographical situation and its many mountainous regions with an almost sub-tropical climate, is noted for its fantastic cloud formations. The cloud shown on this page was noticed by Mr. Leo L. White, a well-known New Zealand aerial photographer, while on a flight with Mr. S. J. Blackmore, a veteran airman, when they were flying about 20 miles inland, near Middlemarch, in the South Island of New Zealand."

Leading meteorologists of the day provided the following explanation of the formation:

"The cloud observed can best be described as a lenticular billow cloud. It is, however, such an unusual formation that no similar illustration appears in any of the recognised cloud atlases [several contemporary texts feature the cloud]. The physical processes underlying its formation are nevertheless the same as are commonly described in the formation of lenticular cloud. Clouds of this type are common over Canterbury and Otago provinces, when the lower atmosphere is comparatively dry on the east coast of the South Island, and very moist on the west coast. They most commonly occur with föhn winds, which in the South Island are associated with the passage of a deep depression across the South Tasman Sea, preceded by very strong north-westerly winds.

The meteorological situation, which produced the cloud form illustrated, differed from that normally producing the föhn wind, although the results were rather similar. A deep depression lay far south of New Zealand, with a cold front extending northwards from its centre oriented north-west to south-east across the South Island, moving to the north-east towards Dunedin at mid-day. An intense anticyclone was centred over the Tasman Sea and extended on to New Zealand, the isobars indicating a strong south-west to west-south-westerly pressure gradient over the South Island ahead of the cold front. The air mass was comparatively stable. At the time the photograph was taken the aircraft would be approximately 60 miles north of the cold front and it is probable that the northward movement of this front increased the pressure gradient ahead of it and at the same time caused the westerlies aloft to back towards the north-west. This effect was enhanced by the formation of an orographical low pressure area on the leeward side of the Southern Alps. This particular cloud was caused by the high north-westerly wind impinging on the westerly side of the Rock and Pillar range (in background) being deflected upward and over the high peaks. At the same time, the wind would be increased in velocity over the top of the peaks and would descend on the leeward side, before again carrying out a reflected upward movement. It is apparent from the photograph that a wave motion was commenced in the air by the obstructions, resulting in several billows of increasing wave-length.

The cloud, in spite of the remarkable impression it gives of raging across the countryside, is stationary. This is apparent from the typical lens-shaped structures occurring throughout the cloud and the generally striated form indicates that the wind is actually blowing at high velocity through it. The front edge of the cloud marks part of the crest of an air billow, the coldest temperature in the billow occurring at the crest of the cloud. The crest of a second billow appears at top right of the photograph, where the cloud has the appearance of false cirrus and is at a very considerable height.

The cloud is most dense in the middle of successive billows because of an increase in condensation there, and it thins out to the rear because of progressive evaporation. Individual droplets are quickly evaporated and the cloud form is only preserved through continuous condensation from renewed air deflected over the mountain. The evaporation taking place in the cloud is well shown in the clear space (upper right) where the wave motion is curved downward, causing dynamical heating. The alto-cumulus and altostratus cloud in the background is probably a forerunner of the advancing cold front. The small lenticular clouds in the distance (lower left) are formed by the same process as described above."