Met. Office wants hailstorm reports

Press, 12 April 1984, Page 2

Met. Office wants hailstorm reports

The Meteorological Service would like more reports from Canterbury Sle who experienced the torm last Sunday, to enable it to map the path of the storm.

This exercise is part of its continuing project to determine whether some parts of Canterbury are more susceptible to hail. A meteorologist at the Christchurch Weather Centre, Mr lan Miller, said he would like anyone willing to report such future storms to write to him at the centre, P.O. Box 14-004, Christchurch Airport. A squall over Canterbury on Sunday caused gusts of about 90 to 100 kilometres an hour. Near West Melton it was accompanied by hail up to 50mm (2in) in diameter.

Mr Miller said the Canterbury Plains experienced extremes of weather during the week-end. Temperatures recorded at Christchurch Airport ranged from 25.3 deg. about 2 p.m. on Saturday to a chilly —o.6deg. on'Monday morning, after a squally southerly change with hail and sleet on Sunday evening. The thunderstorms which accompanied the squall brought hail up to the size of walnuts in Tinwald and Ashburton, and as the squall line passed Rokeby inland from Rakaia a tornado up-

rooted trees and scattered big hay bales. The marked change was mainly caused by a cold front moving over the South Island but the influence of the Southern Alps on Canterbury’s weather was also apparent, Mr Miller said. A cold front passed over Canterbury on Sunday morning, preceded by a north-westerly flow. However, most of the rain was confined to the main ranges and the West Coast, and very little reached the eastern foothills and there was none over the plains. This was because the ascent of the air over the Alps increased the rainfall there while the cloud and rain cleared as the air descended over the plains.

As the air ascended over the Alps the condensation of water vapour which formed cloud and rain created a lot of heat, and when this air descended over the plains it was much wanner (a nor’wester).

Therefore while the maximum temperature at Hokitika was 18deg., at Timaru it reached 27deg. Because a lot of moisture had left the air as rain when it crossed the Alps the humidity at Christchurch Airport dropped to 30 per cent, illustrating the major effects of the Alps on the Canterbury climate, Mr Miller said.

A brief south-westerly change moved quickly north-eastwards over Canterbury on Sunday morning behind the front but the winds soon dropped and skies cleared.

However, in the late afternoon a squall line developed over South Canterbury and raced northwards over the plains, reaching Christchurch about 6 p.m. As it passed, the wind, which had turned to a light north-easterly, swung rapidly to the. south with gusts of about 90km to 100 km an hour. The temperature plummeted more than 7deg.

The cumulo-nimbus clouds which made up the squall line were triggered by the convergence of the north-easterly ahead of it and the southerly behind the squall. They rode to heights of 9000 m or more because of the very low temperatures aloft after the passage of the cold front. Near the top of the clouds the temperature was about —4sdeg.

The cold downdraughts associated with the thunderstorms caused the strong gusts and rapid temperature drop as the squall line passed. In some parts of Christchurch there was sleet and on Monday morning there was a thin coating of snow over the Port Hills and in the ranges, Mr Miller said.