I’ve been reading lots of information about site selection for avocados, as I am planning my (likely-to-fail in the best of microclimates) cold-hardy avocado breeding project. A few weeks ago, I encountered the concept of “cold air drainage” for the first time in a document intended to help Californian avocado growers avoid pockets of frost when choosing a location for their orchards.
I’m sure many of the experienced orchardists on this website are familiar with this concept, but for anyone who isn’t, the gist is that on clear nights, radiative cooling creates cold air near ground level. That cold air sinks, and if the ground is uneven, then it tends to flow downhill until reaching a valley or depression. Areas with good cold air drainage (sides or tops of hills) are often warmer than nearby areas with poor drainage, where frost pockets can form.
As soon as I learned of this concept, I began to worry about my zone-pushing aspirations. Looking at a topographic map of my neighborhood, my house sits in the middle of an elongated bowl-shaped depression, with a steep ridge to the east, a slight ridge to the west, a hill to the south, and a gentle upward slope to the north.
My nearest public weather stations are located in areas with much better cold air drainage (near the tops of ridges or hills), so I decided to get a weather station and see if my fears could be assuaged. It was installed 5 nights ago, selecting a site ~5 feet off the ground, out of direct sun and away from any buildings.
So far, it doesn’t look good. Last night, my station measured a low of 25.5°F when the nearest public stations registered lows between 31 and 33, and every night so far it has been at least 3° colder than any other station in my area, even on cloudy nights when radiative cooling is minimal.
Long story short: if you’re planning to zone push, make sure you think about cold air drainage. I’m still moving forward with my plans, but with even more pessimism than before.