|Agave in the Snow|
While our winter weather is milder than most, our thoughts often turn to visions of ice and snow. From first frost to ice storms to snow storms, these frozen precipitation events are sporadic in Central Texas, but when they occur, they can also be spectacular!
How does frost, this sparkling layer that sometimes covers the landscape, form? When the temperature of the air reaches a point where the water vapor in it can condense out into water, it is called the dew point. The frost point is when the dew point falls below freezing, and rather than producing dew, it creates frost. Consisting of tiny, spike-like crystal structures called ‘spicules’ that grow out from a solid surface, frost generally forms on surfaces that are colder than the surrounding air. Even the size of the crystals can vary, depending upon the amount of time they took to grow, the relative changes in temperature, and the amount of water vapor available.
|Frost covers an Evergreen Sumac|
Cold air is denser than warm air, so quite often when night skies are clear and calm, lower areas become colder due to differences in elevation. Known as surface temperature inversion, this phenomenon forms ‘frost pockets’ or areas where frost forms first, due to cold air trapped against the ground. On such days, there can be a 40 to 50 degree difference in air temperature between dawn and early afternoon. Getting out early can reward you with a rare and wonderful spectacle of nature when something called ‘hoar frost’ is formed.
|Hoar Frost formed on the edges of these Oak leaves|
Referring to white ice crystals that are deposited on the ground or loosely attached to exposed objects such as leaves and branches, hoar frost forms on cold, clear nights when heat radiates out to the open sky faster than it can be replaced by nearby sources such as wind. This allows objects in the landscape to cool below the frost point of the surrounding air, and well below the freezing point of water. Hoar frost can form in low-lying cold air even when the air temperature a few feet above ground is well above freezing. The name hoar comes from an Old English adjective meaning ‘showing signs of old age’, and refers to the frost making the vegetation look like it has grown white hair. When hoar frost forms on objects above the surface, like branches and leaves, it has a feathery-like appearance and is specifically called air hoar.
Snow, on the other hand, is an entirely different matter. When a cold water droplet freezes onto a pollen or dust particle high in the sky, it creates an ice crystal. As this primary crystal falls toward the ground, more water vapor freezes on it, building new crystals that form the six characteristic arms of a snowflake. This process of crystallization builds in a symmetrical or patterned way, because it reflects the internal order of the water molecules as they arrange themselves in pre-determined spaces to form the six-sided snowflake.
|Ice crystals highlight the leaves of a Sotol|
The most significant factor that determines the basic shape of the ice crystal is the temperature at which it forms, and to a lesser degree humidity. The intricate shape of a single arm of a snowflake is determined by these atmospheric conditions as the entire crystal falls. As slight changes in temperature and humidity occur minutes or even seconds later, a crystal that begin to grow in one way might then change and branch off in a new direction. Since all six arms of a snowflake experience the same changes in atmospheric conditions, they all grow identically. And since individual snowflakes encounter slightly different atmospheric conditions as they take different paths to the ground, they all tend to look unique, resembling everything from simple prisms and needles to intricately faceted plates and stellar dendrites.
On the surface, winter may seem as if nature is shutting down all around us, but take the time for a second look. Aside from the visual beauty they provide, the frozen forms of precipitation during the season are just another part of the ongoing cycle of life and renewal for our native plants and animals.