The Apple Club Newsletter, Spring 2006, Page 3

This year the apples are flowering late, and so the likelihood of any damage being caused to flowers by a late frost is small. It's not the same every year however. Very often apples come into bloom in early April, and at that time a frost where air temperatures fall below 0慢 can easily occur. Apple blossoms can tolerate a little frost, but not much, so if the temperature falls below minus 3慢, even for a short while, an entire years crop can be lost.
There are two types of frost that can occur in Ireland in spring. One is where cold winds blow in from the North or East, and there is not much that can be done to alleviate such a frost. The other type of frost occurs on still cloudless nights. In such conditions, the warm air which is normally blanketed over the earths surface by the presence of clouds, can rise up higher into the atmosphere (that hot air rises and cold air descends is a fundamental fact of nature). As a consequence, cold air from higher up streams down to ground level, causing a frost. However, the warm air from below may not continue to rise up and up, and it can get caught below another layer of colder air a few hundred feet up.
When this happens, it is called an inversion. It can sometimes be seen in a valley at night, where cold air lies at the lowest point, then halfway up the valley the air is warmer, and then even further up, there is more cold air.
One apple grower in Northern Ireland is taking advantage of air inversions to protect his apple blossoms from frost. He has purchased a "wind machine" from the United States. This machine looks like a windmill, except that the blades are angled, and driven by a diesel engine. When it operates, it sucks down warm air from the inversion, mixing it with cold air at ground level, making the cold air warmer. So if it is minus 3 degrees at ground level, and 5 degrees in the inversion layer, when the wind machine operates, the ground level temperatures could rise to 1 degree celcius, which obviously protects the apple blossoms from any damage. Graham Hewitt's wind machine has already proved itself in action last year, when he harvested a record crop of Bramley's Seedling from even the most frost-prone parts of his orchard. It will be interesting to see whether others follow his example.

Newsletter Spring 2006		page 3 of 5

Apple Frost Protection