Treebeard's Stumper Answer 25 January 2002

Last Friday was Pajama Day at Dunn Middle School where I teach science and shop. Silly, but fun! We brushed the frost from the outdoor picnic tables, ate waffles, and played a chess tournament in our PJs in the crisp sunny morning. Then we all watched the remarkable Japanese animated film Grave of the Fireflies. It's animation, but "cartoon" is the wrong word. Excellent morning!

Frosty Leaves

Last week we had to scrape the morning frost from the outdoor picnic tables to play a chess tournament in our PJs! It was a good reminder that it's still winter despite the sunny and dry weather lately. Look close at the frosty grass in the morning, and notice how the heaviest frost appears on the edges of the leaves. That's stumper enough, but I also noticed that the edges of the picnic tables are the very first part to thaw! Dry sunny days seem to go with cold frosty nights, and sometimes those stormy nights that feel coldest have no frost at all. How does this work?

Close shot of the frost on clover leaves at school. Note how the frost forms mostly on the edges of the leaves.	Frost on a picnic table at school. Note how the edges are the first to thaw!

Tables thaw from the edges just like ice cream melts on the outside. It takes time for heat to penetrate. If leaves start freezing on their thin edges, it would take longer for that heavier frost to thaw. Leaves often have minute spines and hairs on their edges that act as nucleation sites for ice crystals to form. Many plants undergo a process of guttation at night to force out drops of excess water through special pores on their leaf edges and tips. If this water freezes as it is extruded, it would explain the long ice crystals I see on frosty morning leaves.

Notes:

Here are few more factors (and complications) that effect frosty leaves:

• I took those pictures at school in the early morning before my DMS classes, so the angle of the sun might be a factor. The low winter sun hits the picnic tables before the low grass blades. I think the order of thawing is still a stumper.

• Some plants (and animals) produce natural antifreeze compounds called antifreeze glycoproteins or AFP/AFGPs. This protection may not be available along the thin edges of the leaves.

• Many of our local chaparral plants have waxy coatings on their leaves to conserve water during the dry summers in our California Mediterranean climate. That waxy coating repels water droplets and probably prevents frost formation on leaf surfaces. I don't notice much frost on our native shrubs and trees.

• Bacteria on leaf surfaces can act as nuclei for frost formation, so antibiotic sprays with streptomycin can provide cold protection to some plants. The genetically modified bacteria known as FrostBan takes this a step further by introducing a Pseudomonas bacterium that reduces frost damage because it does not contain the gene coding for the formation of ice crystals. The release of this bacterium into the wild has stirred up some controvery, as it should. See the writings by Jeremy Rifkin.

• Water expands into crystals when it freezes, which damages plant tissues from the inside. But forming frost on the leaf surface can actually keep a leaf from freezing! Solid ice is a molecular form of water that contains less heat energy than the liquid form. The extra latent heat is released to the immediate environment when ice forms, which can prevent damage to underlying tissue. This is the reason why farmers use sprinklers when frost is in the forecast. Sweat cools us when it evaporates, but this is the opposite warming effect. Water from a pipe gives off 11 calories per gram as it cools to freezing, and then gives another 80 calories per gram when it turns to ice. Freezing water can keep plants warm!

I felt better about my answer when I read Graybear's reply. He mostly agrees:

As to the frost stumper, I believe it's because the edges have more surface area per volume so they adjust to the temperature faster. Since frost is a crystalline form, it may need the crisp edge of the leaf to begin forming. (Remember the crystal-growing experiment where you need a seed-crystal to begin the process.)

"Guttation" (as in "gutta-percha") is an unfamiliar word that's not in the M$ Word spell-check dictionary, but it's a real word and a natural process. Here's a description from the New Scientist Last Word

On the surface of leaves there are stomata or pores through which water is lost by transpiration. At night, the stomata close, causing a reduction in transpiration. Drops of water are then forced out of the leaf through special stomata or hydathodes. These special stomata are found along the edges of the leaves or at the tips. It is believed that guttation is caused by high root pressure. Grasses often force water out of the tips of their blades...

I haven't found a published connection between guttation and frosty leaves on the Web, but New Scientist also discusses another example of ice formation where water freezes as it is exuded, with the result that interesting ice crystals are extruded from the tiny pores of a substrate. You can almost see it happening in my photo (left) of ice and water drops on the leaves of Foothill Lupine (Lupinus longifolius) in our garden last week. It's interesting how the downward-pointing appressed leaf hairs direct the water droplets to the center of the palmate leaf cluster and hold them there as a single drop. Is this a deliberate adaptation to protect the center of the leaf from frost damage?

Every stumper is something new to watch for, and I've learned that frost is more complicated than I first thought. An Arctic storm with little moisture settled over California this week. We only had an inch of rain and a few snow flurries, but it brought very cold air. We've had temperatures in the teens, and ice patches by our house haven't thawed all week. That's unusual for central California! (See my Big Chill (1 Feb 02) stumper.) We've had a different kind of frost in our graden this week, with ice crystals on the leaf surfaces and little on the edges (right photo). We've lost some garden plants.

Our cold front this week brought advective frost as frigid Arctic air moved into California. That's different from the usual radiative frost we have in the open fields at school in the valley. Plants radiate infrared energy at night, but they don't get much back from the clear night sky, so there can be frost even when the air temperature is slightly above freezing. That's also why your upward-facing car windshield usually gets thicker frost than the side windows. This is an important difference that explains a number of frosty stumpers. See my Frosty Windows (19 Jan 01) stumper for more details.

There are many links embedded above, and here are some more links for further research on frost and frosty leaves:

• The New Scientist Last Word has articles on Hard Frost, Knowing Your Dews, and Thin ice that relate to this stumper. (There's lots more good stuff here!)

• There's info on growing your own crystals here, here, and here, and many more if you search! These sites explain the importance of crystal nucleation sites and seeds. There are striking Scanning Electron Microscope (SEM) images of ice crystals here. There are classic snow crystal photographs from Wilson A. Bentley, and even better new images at SnowCrystals.net from Caltech.

• The difference between advective frost and radiative frost is very important for farmers as well as naturalists. I found good practical discussion at UC Davis and the Government of Ontario, Canada.

• It's not just Eskimos that have many words for snow! We have frost, hoar frost, hail, rime, glaze, snow, corn snow, powder, slush, red klister snow, and many more forms of ice and snow in nature, many of which we've never seen here in Santa Barbara County, California. There's a glossary with photos at the Atmospheric Meteors site.

• The genetically engineered Frostban bacteria were created by Advanced Genetic Sciences (AGS), but they have no Web presence that I can find. I think they changed their name to DNA Plant Technology Corporation, also with no Web presence. I'm not surprised there's some paranoia about the biotech industry! There's some discussion here and here. Much of the controvesy about FrostBan comes from author Jeremy Rifkin, eg his book The Biotech Century: Harnessing the Gene and Remaking the World (1998). I haven't read it.

• I have more frosty stumpers, including: The Big Chill (1 Feb 2002), Frosty Windows (19 Jan 2001), Mediterranean Climates (19 Oct 2001), Keeping Warm (2 Mar 2001), Snow on the Hood (6 March 2000), Storm Damage (26 March 1999), Cold Winter Sun (6 Dec 1996), and The Big Freeze (18 Oct 1996).

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