How do smudgepots in orange groves prevent fruit from freezing?

[astro]

I just always wondered. I didn’t really see how a few burning smoky torches could prevent fruit freezing over a large outdoor area.

[Ice_Wolf]

Interesting stuff. According to this site from Oregon:

Orchardists have used smudge pots to guard against freeze that can ruin a fruit crop. The smudge pots don’t put out enough heat to warm the orchard, but do create a thick smoke cloud. This acts to reflect heat radiated from the orchard, thus “trapping” heat and cooling the ground more slowly. Hopefully, it keeps the crop above the freezing point. It takes 390 smudge pots burning 4,880 gallons of diesel per season to protect 13 acres of crop.

However, pollution-wise, this method has become something of a villain, and alternatives are suggested more and more.

[mangeorge]

Not from freezing, from frost. They change the humidity, IIRC. I’ll check.
Peace,
mangeorge

[kniz]

I agree with mangeorge although everything I get from a google search agrees with Ice Wolf. Moisture needs a speck of something to form around and the soot from the pots gives the moisture in the air something to bound to. It turns to fog, rather than forming frost on the fruit. I believe I was taught this in meteorology in flight school 40 years ago.

[Ice_Wolf]

Well, this is something I’ve learned today. I’d always thought the two terms, frost and freeze were interchangeable. But, not according this report by Katherine Perry.

Frost vs. Freeze; Although the terms frost and freeze are often interchanged, they describe two distinct phenomena … An advective, or windborne, freeze occurs when a cold air mass moves into an area bringing freezing temperatures. Wind speeds are usually above 5 mph and clouds may be present. The thickness of the cold air layer ranges from 500 to more than 5,000 feet above the surface. Attempts to protect crops by modifying the environment are very limited under these conditions.

Sorry, I should have looked further into this.

[Joey_P]

Another thing they do to prevent frost (I’m not sure which crops this is for, strawberries maybe) is…When they know there’s going to be frost coming they water all the plants, this creates a layer of ice over the crop that will insulate them from the frost.

How exactly the crops can be hurt by the frost but not the ice I’m not exactly sure, maybe it keeps them from drying out as well.

[Cillasi]

It could have something to do with the size or arrangement of the crystals. Also, building a “roof” over the crop probably protects the lower, more vulnerable fruit and leaves from frost. It may be quite possible to lose the top leaves of the plants to ice damage without destroying the entire plant or the majority of the fruit. The ice then acts as an “igloo” staving off the wind and keeping out the mists that turn to frost.

[RealityChuck]

*Originally posted by Joey P *
**Another thing they do to prevent frost (I’m not sure which crops this is for, strawberries maybe) is…When they know there’s going to be frost coming they water all the plants, this creates a layer of ice over the crop that will insulate them from the frost.

How exactly the crops can be hurt by the frost but not the ice I’m not exactly sure, maybe it keeps them from drying out as well. **

Oranges freeze at something like 28 degrees F (the sugar and pulp inside brings the freezing point down). Water freezes at 32 degress F. And water has one interesting property: in a mixture of water and ice, the temperature of the ice does not go below 32 until all the water is frozen.

Thus, if you spray the oranges with water and don’t allow it to freeze solid, the coating of ice around the orange remains at 32, above the freezing point of the orange.

Of course, eventually the water will freeze solid and the orange ruined. But this allows oranges to survive a night or two of below-freezing weather.