In this past spring we cleared out approximately 0.5 acre to open up the area around the vineyard. There were several reasons for this. First the larger opening in the forest should foster a higher degree day accumulation in the vineyard. Secondly it would allow the cool air that gets trapped behind the trees to drain away from the vineyard. Lastly it would provide more morning sun, as the tree to be cleared were those to the east of the vineyard.
We cleared the trees at the end of April and we were finally able to see the lay of the land around the test vineyard. To our surprise we found out that the area of the test vineyard in in a small low spot and is prone to cold air accumulations in spring and fall - naturally this is my luck. This also means that the larger opening we created now provides a greater area to drain cold air into this low spot that the vineyard is in and heightens the frost potential.
So now we were faced with trying to eliminate the amount of cold air that flows off the mountain side and settles in the large opening surrounding the vineyard. There are a few measures that can be taken to mitigate this. First, we have cleared the rest of the trees out to the edge of the valley drop off, about another 0.5 acre. The bench that the vineyard sits on is approximately 100-120 feet above the lower valley below. Clearing these trees out has changed the movement of air dramatically. If you stand at the low end of the clearing at the edge of the valley drop off after the sun goes down you can feel the rush of wind moving down the valley and through the vineyard and the clearing and over the edge to the valley below. Clearing the cool air away is a good start.
The picture below is of the valley below the Lower Bench in winter. You can see the warm air fog that has formed in the middle because the cold air is filling in the low area. The Lower Bench that the test vineyard is on is to the left of and 100-120 feet above the road. There is another bench, the Upper Bench, which is separated by a gravel road and it is another 100-120 feet above the Lower Bench (see image below).
Next we want to limit how much cold air enters the vineyard and the clearing. So we are planting a large wind break hedge at the upper end of the vineyard clearing to deflect the cool air that otherwise flows down the mountain and enters the vineyard. This will take a few years to grow and to have some effect but in the interim we are discussing erecting a mesh or wood wind break to do the same thing. The one we build will not be as high as the tree hedge but should help deflect the cool air to some degree.
We placed temperature data loggers down slope from the vineyard towards the valley drop off and have found that the area just down slope from the vineyard (but out of the low spot that the vineyard is in) registers between 0.0 - 0.5 degree warmer each night. Also, a second data logger placed near near the bottom of the slope registers between 0.5 - 1.0 degrees warmer than the vineyard for overnight lows. Another placed in a clearing on the lower valley bench registers night temperatures 0.5 degrees colder than the vineyard. It appears that allowing the cold air to drain away provides warmer night temperatures in the vineyard but in addition there may be some warm air funneling into the clearing from the bottom end of the slope as it rises up from the valley below.
We have recorded similar but more dramatic readings when we placed a data logger on another part of our property. This area is a smaller bench that is approximately 100 feet above the vineyard. The data logger at this level has consistently recorded night time lows that are 2.5 degrees warmer than at the vineyard location in June, July, and August - no winter records have been taken.
While it is nice to see these warmer night time temperatures at different locations on the property, where we'd really like to see them is in the test vineyard. With out getting into wind machines or artificial heating of the air the best we can do to reduce cold air in the low spot of the test vineyard is to provide a place for the cold air to go. Also we can to try to limit how much cold air enters that area. Clearing the land for air drainage and creating a wind break hedge to deflect cold air should help with these.
At the same time we know that if the vines grow well in the test vineyard, then climatically speaking they should grow great in the other warmer locations.