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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 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:

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Copyright © 2002 by Marc Kummel / mkummel@rain.org