Cold air drainage & freeze/chill update

In a comment earlier in the week, Doug Young noted that at his location in El Cajon (roughly 25 miles southeast of here) there were no freeze/frost problems. He mentions the location is about 1100 feet above sea level. That is much higher than here, where the Ladera Frutal office is about 525 feet elevation, as are the bananas that have been so badly ravaged by our freezing conditions. (Day by day, they continue to look worse than in the photo I posted on 14 January).

Doug notes how he can “feel the cold air draining downhill on a calm night.” I know what he means. That is usually the case here, too. On many a clear, windless winter evening, as I walk down the hill from the office to the house (elevation 450 or so), I can feel the air getting significantly colder as I descend. But not so on the recent evenings when the cold air mass settling overhead.

Compare 6 January, a more typical dry clear winter night, to 14 January, the coldest night of the recent snap. On 6 January, the low temperature down at the lower level (below the house, at maybe 350 feet) was 30 degrees, while up here at the office it got to only 36. Six degrees difference over an elevation change of 175 feet. Yet on 14 January, when the temperature reached 24 at the lower level, it was 27 at the office level. The cold air just was not draining. Instead, it was parked firmly overhead and the differences from elevation to elevation just were not very significant.

The steep slope here normally makes for a near-perfect mix of micro-climates–above-freezing at the top but very chilly down below. But it looks a lot less perfect than it did as of 12 January. This week, it would have been good to have been at 1000 feet (as the highest nearby ridgelines are). As an aside, I wonder how high, under local conditions, one can be and still get the advantage of cold-air drainage. At some point, the “thin air” effect of higher elevation has to take over, and one would be too high for cold-sensitive crops. One can see this effect on some of the hills east and south of here in Valley Center and San Marcos: Avocado groves begin part way up the slope and stop before the top, taking advantage of the parts of the slope that are out of usual freeze range. Of course, there was nothing “usual” about the weather this past week–and the impact on the local ag industry will be devastating.

Every day, more damage appears as the plants’ diminished ability to take in moisture leads to more and more collapse of tender young tissues and browning/yellowing and curling of foliage, even on some trees that appeared unaffected as of two days ago (such as the sapodillas, lychee, and some of the citrus).

On the upside, the cold snap has left the chill-hour estimate down below at 370+ hours, or about 100 hours ahead of where it was at this point a year ago.

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