Canterbury and other walnut growing areas can has suffer a devastating “advection” frost. Devastating because it kills off all tender shoots even several meters above the ground, including flowers.
Richard L. Snyder, Extension Biometeorologist University of California, Atmospheric Science describes advection frost in an article on the “Principles of Frost Protection”:
An advection frost occurs when cold air blows into an area to replace warmer air that was present before the weather change. It is associated with moderate to strong winds, no temperature inversion, and low humidity. Often temperatures will drop below 32°F (0°F) and stay there all day. Advection frosts are difficult to protect against, but fortunately they are rare in California fruit growing regions.
Another helpful explanation that includes reference to the affect of terrain (like snow on the Port Hills or foot hills draining down onto the plains) can be found at “The Weather Doctor”. The way they put it is:
Whereas frost masses move across continents under the push of global air currents, flood frosts fall downslope under the pull of gravity — their cold air heavier than the surface air undercutting the lighter, warmer air below. Flood frosts form at high elevation and flood down hillsides and valleys, forming frigid pools in depressions and low terrain. Unlike the frost masses, flood frosts usually whisper as they roll down the hills and valleys at night, stirring dead leaves in passing and blackening low-lying vegetation with their touch.
Flood frost is a form of atmospheric motion known as katabatic flow.
We have put up more information about preventing frost damage on the NZWIG website. There are some links to other material, and there is an edited version of a useful 1971 article from the DSIR by E.W. Hewitt.
Actually it seems that there is not much any of us could have done to prevent major damage from the advection frosts but there are some helpful ideas for less severe events.