The Magic of Frost

Diamonds and pearls in our midst

Looking out the window this morning, I was greeted by a magical scene here in Central Iowa. The trees, bushes and ground were covered completely in white, but it wasn’t snow. A stunning layer of sparkling crystal blanketed everything in sight. 

“Did we have an ice storm?” I asked my husband, who was already up and reading the newspaper while nursing a steaming cup of coffee. No, he said, it was frost. We’ve been married long enough for him to anticipate my next question so he added: “And it isn’t hurting the trees.” 

A bit later, my dear friend Jen drove down to Huxley to go for a walk on the nature trail not far from my house. As we headed out in the freezing weather, we were again captivated by the loveliness of the landscape. I wondered out loud about the heavy frost and why it was not damaging to the trees. Jen remembered that this particular phenomenon was called Hoar Frost, which sounded right to me, and we resolved to study up on it when we got back. 

As we walked, we were mesmerized by the sparkling scene and the pale, ethereal vistas all around us. Each scene was more beautiful than the last, and only the extreme cold prevented me from exposing my fingers numerous times to take even more photos. Every branch and bush seemed to be painted with a fuzzy, spikey coating of crystal. While the visual impression was similar to an ice storm, which wreaks havoc on trees and crops, up close we could see that the frost was relatively light and seemed harmless. The overall effect on our wooded trails was of a dreamlike and enchanted winter forest. I half expected a tiny snowflake-covered fairy with an icicle wand to fly out and land on the path in front of us.

Upon returning to the warmth of my kitchen, we did some googling. Jen had been right about the Hoar Frost. According to a Wisconsin State Farmer article, Hoar Frost gets its name from the old English word hoary meaning “getting on with age, or “hair like.”

While as a farm kid and the member of a serious gardening family, I had long been cognizant of killing frosts. But I hadn’t really given much thought to exactly what frost is or how varied it can be beyond what I learned in junior high science. I always thought frost was a type of crystal formed from water vapor in the air. 

My memory was correct. Frost forms when an outside surface cools past the dew point. You’ll also recall from junior high science that the dew point happens when the air gets so cold that the water vapor in the atmosphere turns into liquid. When this liquid freezes and gets cold enough, tiny bits of ice form: frost. The ice is arranged in ice crystals. “Frost is simply water vapor, or water in gas form, that becomes solid,” says National Geographic. 

According to weather.gov’s “Forecasting of Frost,” frost falls into two categories: depositional frost and frozen dew. 

Depositional frost, also called Hoar Frost or White Frost, usually forms on objects – including trees – that are outside in air that is saturated with moisture. Caltech’s SnowCrystals.com site says that like snow crystals, frost crystals grow from water vapor in the air. “But while snow crystals form on suspended dust particles high in the clouds,” says the site, “frost crystals form near the ground -- on window panes, blades of grass, or just about any other solid surface.” If depositional frost is thick enough, it can look like a light snowfall.

Another form of frost is frozen dew. When dew forms at first, generally the dewpoint and temperature are above freezing. “Longwave radiational cooling gradually lowers the temperature to at or below freezing during the night,” says weather.gov., and when the temperature falls to freezing, the condensed dew droplets freeze. 

Frozen dew looks different from white frost because it does not have the same type of crystal patterns. White Fost tends to look… well, whiter; frozen dew has a slicker appearance and can be more difficult to see. 

So my memory of what frost is was true, but it is more complicated, and more poetic than that simple definition. 

“Frost is to dew as snowflakes are to raindrops,” says the Caltech SnowCrystals.com’s Guide to Frost. It continues, “When water vapor condenses into liquid water, you get raindrops and dew. When water vapor condenses directly into ice, then you get snowflakes and frost.”  Therefore, Caltech tells us, snowflakes are not frozen raindrops, and likewise frost is not really frozen dew. I found something especially satisfying in the fact that Caltech, one of the most foremost science institutions in the world, used such lyrical language about frost. 

There are actually numerous types of frost, all with their own story.

Hoar Frost: Hoar Frost is a particular kind of frost that resembles spiky hairs. It refers to the old age appearance of the frost, or the way the ice crystals form makes it look like white hair or a beard. According to the Farmers Almanac site, Hoar Frost happens when water vapor freezes instantly after coming into contact with a very cold surface. No wonder Hoar Frost seemed very familiar to me – it also forms in refrigerators and freezers. I’ve scraped my fair share of these crystals out of old appliances. A type of depositional frost, it is also referred to as White Frost or Radiation Frost. Note: It is not as pretty when it is in your freezer.

Surface Hoar: The most frequently appearing type of Hoar Frost is Surface Hoar, which consists of ice crystals that form on the top of snow banks. What we see as sparkling snow on a field is often simply reflections off the facets of Surface Hoar crystals. Surface Hoar tends to form when a snowback warms up, cools again, and then recrystallizes; the best time to see Surface Hoar is in the early hours of the morning. In Iowa, where the fields stretch out in the light right after dawn, the view of Surface Hoar is especially lovely. I will now know to call it by its name, versus just “sparkling fields.”

Black Frost: Black Frost is literally a dry freeze, and is always a killing frost. It gets its name from the blackened appearance of vegetation that has not been protected by moisture. According to the Britannica science site, Hoar Frost is not guaranteed by the occurrence of temperatures below freezing (32 degrees F or 0 C). For Hoar Frost to happen, there must also be enough moisture in the air for condensation to occur when it reaches saturation. If there is not enough moisture, Hoar Frost may not form, but the water in the plant tissue may freeze. This produces Black Frost, a condition with which many of us Midwesterners are very familiar. The hideous black appearance of a beloved flower or a houseplant not moved inside fast enough in the fall is emblazoned in my memory.

Fern Frost: The Farmers Almanac also describes Fern Frost, which appears on windows when the pane of glass is exposed to freezing temperatures on one side and moist air on the other, causing tiny water drops to form and freeze in patterns that look like ferns or intricate leaves. According to the Caltech SnowCrystals.com site, this type of frost can form especially elaborate patterns depending on the type of window surface, saying that scratches or soap streaks can influence the growth of the crystal’s nucleate. This is also called Window Frost, and was more common in the past when single-pane windows were the norm. Window frost is familiar to people who live in cold climates. I have fond memories of tracing the intricate designs on my grandmother’s old farmhouse porch windows with my fingers when we visited in winter.

Frost Flowers: An interesting phenomenon cited on Caltech’s SnowCrystals page is Frost Flowers, which are curved ice filaments that look like cotton balls. These apparently grow on water-logged wood. I have never seen these, and Caltech reports they are quite rare.

Rime Ice: The word rime derives from the Middle English word rim, which is akin to the Old Norse word hrīm, which means frost, according to Mirriam Webster.  This is actually a type of frost that forms rapidly and tends to happen in very cold, wet climates or in windy weather. It is more like accumulating freezing than like thick frost.

According to the Caltech site, snow crystals accumulate rime when they collide with water droplets in the clouds, saying that when the clouds are near the ground fog occurs, and sometimes the fog is made of water at a temperature below the freezing point.  When that happens, the droplets freeze on contact to anything they come in contact with. 

Rime can resemble sugar sprinkled onto the edges of plants or surfaces, but can sometimes look like solid ice depending on the surface. Wind-driven rime can yield elaborate formations.  

Ships traveling through arctic waters or other cold places often end up with rime covering parts that are exposed. “The Rime of the Ancient Mariner,” a poem published in 1798 by Englishman Samuel Taylor Coleridge, features a sailor recounting how his rime-covered ship is lost in a storm and ends up in the Antarctic (told in rhyme, in a lovely double entendre).

Frost is magical but it can be damaging to crops and plants. It was not surprising that after my initial reaction of wonder this morning, my first thought was of harm to the trees. Farmers know that severe weather of any kind can damage crops and plants, and live in fear of an early frost. And gardeners are well aware that any plant with a thin skin can be ruined by a killing frost. Woe to a tomato against the power of the frost.

While Hoar Frost is relatively benign, in extreme cases Rime Frost can accumulate. Over time, it can weigh down trees, power lines and communication towers to the point of causing significant damage.  

Frost is also dangerous to drivers and to infrastructure. Frost on car windows can hinder visibility and frosty roads can be slippery. Because its creation is facilitated by moisture, Hoarfrost can form on bridges over unfrozen bodies of water like streams, lakes or bays, which can be especially hazardous for unsuspecting motorists.

At the very least, frost or frozen dew can cause delays for commuters. According to weather.gov’s “Forecasting of Frost, some frosts and frozen dews are easier to scrape off windshields than others – not surprisingly, the closer the temperature is to the freezing point, the less resistance there is, and the more the temperature drops, the bonding of the ice crystals is stronger.  

Traditionally farmers consulted almanacs and maps to predict frost. Today, we rely on meteorologists and weather sites to guide us both in farming, gardening and managing our commutes.

Jack Frost nipping at your nose? Given both the beauty and danger of frost, it is no wonder that it has sparked our imagination throughout history. Frost is mentioned in poetry and songs throughout cold northern countries. In North America and Britain, it is personified in the character of Jack Frost. According to the Farmers Almanac, Jack Frost is a mischievous, but mostly benign, white haired, blue-skinned elven figure said “to paint designs on our windows and nip at unsuspecting noses (and fingers and toes) during the winter months.” 

Lore has it that Jack Frost stems from the Norse figures Jakul, meaning icicle, and Frosti, meaning frost. Over time, the names were allegedly combined, and Jakul Frosti was seen as a wicked frost giant who served as the personification of ice and snow. Given the powerful and often uncontrollable force of nature, it is not surprising that a frost giant would emerge in Scandinavian mythology. Similarly, in Russia, Finland, and Japan, deities related to frost emerged and were feared.

Over time, these mythological characters became less threatening, and more mysterious, like the intricate feathery designs on windows or the sparkling fields. While I might choose a nonbinary figure for modern times, Jack Frost conjures up the elven spirits that I felt this morning on the silvery path. Surely something as beautiful as the Hoar Frost reminds us that despite all the turmoil in the world, there is magic in our midst. 

As the poet Hannah Flagg Gould (1789-1865) wrote in “The Frost:” 

“Then he went to the mountain, and powdered its crest,

He climbed up the trees, and their boughs he dressed

With diamonds and pearls…”

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