I finally decided to cover my plants in place last night. Good thing, too, because it got down to 7 deg F (-13 deg C). It got all the way up to 37 deg F (2 deg C) this afternoon, with a light breeze. I'm not going to transcribe any more numbers, but if you're interested go to Weather Underground and enter WJF for the airport.
I'm going to go fetch my sansevierias in for tonight. I don't think a cardboard box is enough protection, somehow. I think I got lucky last night.
A friend and former cow orker has twenty acres of lemons down in the Coachella Valley. He gave us a call as he drove down there and told me he was about to lose a lot of money from the freeze. His workers were there picking as many lemons as they possibly could, because it was going to be so cold tonight that he was going to lose the remaining lemons and some of the trees. It takes seven years to replace a frost-killed lemon tree. He isn't alone, of course. He suggested that I buy some lemons now, as the price was going to soar, perhaps to the point of dollar lemons being a bargain.
We have a lemon tree in the back yard of our Low Desert house, across the Coachella Valley from his grove. I probably won't lose a single lemon because it's not going to get as cold there. The NWS has a freeze warning for a hard freeze over all of SoCal tonight but historically our area is warmer than the other side. Of course, all the agriculture is on the other side.
What no one much mentions is that this is going to be very expensive for a lot of ordinary people. A lot of plants that commonly grow in SoCal can't stand a hard freeze, so a lot of landscaping is going to be damaged or even killed. That's expensive and it's not covered by insurance.
Incidentally, given the right conditions (mostly a clear sky and no wind) the air doesn't have to be freezing for fruit and other things to freeze. This is because of black body radiation, with the heat of the object being radiated, some of it to space. Years ago I took Heat Transfer and this was one of the examples the professor worked out. As I recall, citrus can freeze when the air temperature is as high as 37 or 38 deg F (3 deg C). That's why citrus groves in some areas have those great big fans, because moving air disrupts the radiation. Ditto smudge pots back before air quality was an issue. Another method for saving citrus is overhead watering to make "freezing rain". Covering the trees with ice keeps them from getting any colder than 32 deg F (0 deg C).