Monday, February 20, 2012

Gully winds and range clouds - local weather conditons and ripening in McLaren Vale

View from Colville Rd, near the airport, looking South East.
20/12/2012 – At this time of the year grapegrowers are constantly looking at the weather. The pictures on this page show a common localised weather pattern in the McLaren Vale Wine Region.

Locals call it a gully breeze.


In this picture a strong breeze is blowing from the ‘range’ (the Southern Mt Lofty Ranges) towards the camera, while a ‘range cloud’ sits on top of the hills. These 'range cloud' conditions slow grape and horticultural ripening rates, in February, March and April, on the Fluerieu and at Kuitpo. The corresponding gully breeze is famous for its strength and duration in McLaren Vale.

Cloud over the Range at Willunga.
The technical name for this cloud formation and accompanying wind is 'Orographic.' Orographic clouds form when humid air blows over the top of the hill range. The air first rises to go over the hill range then, on the downwind side of the range, the air sinks back into the valley and warms. During warming the water droplets (i.e., clouds) evaporate into invisible water vapor.

Orographic cloud.
Humid air moves from the left, eg. Lake Alexandrina and Encounter Bay, is pushed up across the Fluerieu, until it rapidly falls down the leedward side spilling into the McLaren Vale region - (diagram above left).

It is fascinating to watch orographic clouds and understand that a single cloud is not hanging onto the mountain range. Rather the cloud is rapidly forming and dissipating at the speed of the wind as air rises over the mountain range then sinks on the other side. The parcel of air suddenly becomes visible as it passes over the top of the mountains and clouds temporarily form.

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