A few days ago when I woke up on a very still winter morning, the official outside temperature on my iPhone app was 5 degrees F (-15 C). But on the thermometer here at the house it was 0 (-18 C). Zero.
On such mornings I am reminded that I live and garden in a cold sink.
My yard sits at the bottom of a ridge, and on a still morning the temperature in my garden can be four or five degrees cooler than the general conditions.
This shows you the bare contours of the rise behind our house, when we first moved in. My gardens have been built in the flat area below this little hill:
Cold air is heavier than warmer air because the molecules slow down and stay closer together, making it denser. Heating air makes the molecules expand and move apart, and the whole air mass becomes lighter, and it moves upward.
Cold air shrinks and sinks.
On cool spring mornings the new leaves on my trees in the yard at the bottom of this ridge get frost nipped, when there is no general frost around. It's that much colder. The heavier air sits hard on my garden.
Years after we moved in, the bare hillside is now covered in lupines and daisies in spring:
In fall the new trees I have planted hide the contours of the hill:
But this 15 foot drop drives the conditions in my gardens and sometimes, when conditions flirt with frost, disastrously so.
Orchardists around here used the principles of sinking air when they planted apple trees. Have you noticed that old apple orchards spill down slopes?
Partly for the drainage that apple trees want, but also because earlier blooming apple varieties were planted at the top where it would be five degrees warmer on a spring morning,
Cold air would sink down the slope on a still morning in early May, and the trees at the bottom would bear the brunt of a frost. If those trees at the bottom were later blooming types, with their buds still tight until late May, the apples were not as vulnerable. The early blooming varieties planted upslope, with their delicate flowers open, would be spared.
These heirloom apple trees at Tower Hill Botanical Garden in western Massachusetts march down a hillside that drops very steeply off to the right of this picture. Late blooming kinds like Northern Spies are growing below, earlier blooming Pearmains further up:
Almost every suburban town in New England has a road called Apple Hill, a reminder that developments were built in old orchards, and old orchards were built on slopes with the hardiest trees at the bottom.
Most of the time it makes no difference in my garden. When the breezes blow it equalizes the temperatures, and when there is no danger of frost it doesn't matter.
But on a still morning like the other day when it was 5 degrees in the neighborhood, but 0 degrees at my house, I am aware that my half acre of land sits in a cold trap.