Printer-friendly version

19 December 2003

Frost Flowers

by Forrest M. Mims III

Subfreezing nights have left bouquets of delicately sculpted frost flowers along Geronimo Creek here in South Texas.

Frost flowers, which are also called frost or ice castles, are extraordinarily delicate ribbons and whorls of ice that surround the stems of certain plants when the temperature falls below freezing.

Several species of plants produce frost flowers. The plants that produce them on our place are white crownbeards (Verbesina virginica). These plants have large leaves and can be taller than 6 feet. They are capped by a rounded cluster of small, white flowers. They are also known as tickweed, frostweed and ice-plant.

Frost flowers are formed when liquid water inside a frostweed stem freezes and splits the stem open. Water and water vapor inside the stem is then emitted directly into the air, where it is transformed into a delicate ribbon of ice. A frost flower may actually be an ice flower. That's because the ice appears to be extruded from the stem like toothpaste from a tube. This suggests that there is a liquid phase to the formation of the frost flower. If this is correct, then a frost flower is more correctly called an ice flower.

The difference between being an ice flower and a frost flower is interesting. That's because ice is frozen water. Frost occurs without the water stage normally found between a gas and a solid. Dew sometimes freezes. This happens when the temperature at which dew forms, the dew point, is above freezing. If the temperature later falls below freezing, then the dew already formed may freeze.

Frozen dew is very different in appearance from frost. A frozen droplet of dew looks like a frozen dew drop. Frost is much more complex. Just think about those intricately delicate patterns of frost that form on windows and windshields. Those patterns formed when water vapor in the air suddenly froze directly into frost. It did not change to liquid water first.

Frost also forms on vegetation, mail boxes, rooftops and many other exposed surfaces. But nowhere is it as obvious and beautiful as when it forms on glass. Frost can even form in the air itself. This is responsible for the phenomenon of ice fog, which few South Texans have ever seen up close.

High altitude cirrus clouds are formed from tiny ice crystals. An ice cloud at or near the ground is more like frozen fog.

An ice fog is most beautiful when the Sun is shining through it. Sunlight reflects from the crystals, which appear like thousands of bright speckles in the air.

Ice fogs are often accompanied by hoar frost, which is tiny crystals of ice sprinkled on exposed surfaces. The same sunlight that allows us to see those beautiful patterns of frost and frost flowers quickly brings an end to these phenomenon. When the temperature rises above freezing, the frosty crystals return directly to the water vapor they were before they froze. Scientists call this process sublimation, which is the same name they give to the formation of frost directly from water vapor. In short, sublimation simply refers to the change of state between a vapor and a solid without going through the liquid phase that is ordinarily in between.

David Cook of the Argonne National Laboratory wrote an intriguing article called "Predicting Frost At Your House" for the Journal of Meteorology (volume 15, November 1990). Cook pointed out that more frost forms when the soil is wet. That certainly explains the bumper crop of frost flowers along the soggy soil lining parts of Geronimo Creek.

Frost flowers and ice castles can be seen in the Southeastern U.S. after the first freezes. Look for them on a morning when the temperature is below freezing, the sky is clear or mostly so, and air is still. These are ideal conditions for frost and frost flowers. Drive slowly over rural bridges and along lanes that parallel rivers and creeks. If you see bright white objects scattered about brushy areas, look again. Those objects just might be frost flowers.

Forrest M. Mims III and his science are featured online at www.forrestmims.org.

Photo Caption: This is one of dozens of frost flowers on the Mims place at Geronimo Creek on a recent subfreezing morning. Photograph by Forrest M. Mims III.