From my understanding it depends on the tree.
By the variety:
Some varieties are very sensitive to the cold and light frost (Limes). Other trees can handle the cold and some light frost (mandarins)
Or by the environment:
Trees over time can be hardened to handle lower temps over time. By being exposed to short periods of cold.

Heres a source for oranges and lemons: http://homeguides.sfgate.com/minimum-temperatures-citrus-tree-59616.html

At 39 degrees there really isn't a chance of frost. Frost is really my only concern.

Two things to add: One is, is it really 39 degrees in your yard where your trees reside? If the weatherman says it's 39 degrees in your city, it may not be 39 degrees at your place, or even where your trees are, so I would take care with any temps that drop below 40 degrees in your area. I would buy a weather station and keep the transponder close to where your trees are, outside. Secondly, your trees are in containers, so the above information needs to be adjusted for container plants, which do not have the benefit of the protective nature of soil on roots. Container plants' roots are much more exposed and cannot withstand the same low temps as in-ground citrus can tolerate.

Patty S.

Yep, the weather forecast also can change. Night before last it was going to be 32*, then couple hours later they said 34*, and as I went to bed at 11:30 pm it was updated to 38*.

So a weather station at your house would be way more accurate. It's a great idea Patty had, and I am now thinking about getting one too.

I took some old regular mini Christmas lights, not LED, and strung them through the branches of the potted citrus on my deck. That little bit of heat has kept them warm enough, even with the true 32* we had last night.

I actually have two, Barb. A basic one that is up against my house facing east, that transmits to a receiver (two, actually) in the house. And, a more sophisticated one (Davis VantageVue) that transmits to my wireless router and to the internet (WeatherLInk). That one is down in the colder area of my yard. So, I compare the two temps to get a better idea of my temps on my property (an acre). I will be adding a second WeatherVue out on the front slope, as it has yet a different microclimate than the two other areas, and has the bulk of my citrus.

Patty S.

If you're in Alameda, it's a lot less likely to have frost because you're surrounded by water and are well inside the Bay. I live in Oakland, straight down High St. halfway up the Oakland hills, and our Meyers and Bearss are in the ground.

Occasionally the fruit gets nipped by frost (interior dries out) but the issue is in-ground plants don't suffer because the ground NEVER freezes here. You may want to be more careful with container plants because most people use the smallest containers possible since large ones aren't easily moved....and smaller pots may be more susceptible to frost nipping.

Keep them a foot or so off the ground; that will avoid the coldest air currents. Setting them against a south- or west-facing wall will help, too.