What are these strange looking objects?

These bizarre looking objects look as though they're manmade, but they're actually snow crystals; not ones that have just fallen but rather ones that have resided for a while in seasonal snow packs. A scanning electron microscope was used to make these images. In recent years, intact snow crystals have been observed clearly focused and in great detail using electron microscopy. The powerful electron microscopes don't utilize light to observe different specimens, as do conventional microscopes, instead, electrons are emitted at the specimen being observed, and the resulting image is recorded on Polaroid film.

About 100 years ago, microscopic imaging techniques were first combined with photography enabling scientists to record fine variations that existed in the structure of different materials, including snow crystals. Many people have seen the fabulous snow crystal photomicrographs made by Wilson Bentley more than 100 years ago. It's likely that the decorative six-sided snowflakes you see during the holiday season in store windows and newspaper advertisements are copied from Bentley's photographs. Despite having to do his work in sub freezing temperatures, he was able to produce over 6,000 micrographs of snow crystals. Although Bentley's work was remarkable considering the working conditions and his relatively unsophisticated equipment, he didn't include information about the atmospheric conditions at the time the crystals were collected or the magnifications at which the crystals were photographed. For images of Bentley's work, see for example, http://epod.usra.edu/archive/epodviewer.php3?oid=169760.

Snow crystals can occur in a variety of hexagonal or six-sided shapes. They're structure is controlled by how hydrogen and oxygen molecules bond, so the crystal lattice is always hexagonal, however, there's a myriad of possible hexagonal shapes that can form depending primarily on the cloud temperature at the time the water vapor freezes. For instance, when the temperatures are close to freezing and the air is moist, hexagonal plates with intricate dendritic structure or elongated hexagonal needles are likely to fall.

A frequently asked question about snow crystals is - are any two identical? Well, for starters, the millions of molecules (10 15 or more) than compose an individual snow crystal are unlikely to be arranged the same way in any other crystal. Moreover, probably no two crystals look exactly the same (at least under the microscope) because their histories are different. As the snow crystals fall through the air, each one encounters minutely different temperature, humidity and pressure conditions. Impurities or defects on crystal surfaces effects their growth by helping or hindering them in attracting water vapor. Competition for water vapor also plays a big role in determining the shape of the crystal. For example, a crystal falling through a layer of supersaturated air at a temperature near or slightly below freezing (32 degrees F or 0 degrees C) will collect more vapor on its upper and lower sides or facies as it tumbles through the air, and as a result, a plate type crystal is produced.

Once a crystal falls to the ground it almost immediately begins to change or metamorphose. The constant jostling and rubbing of crystals against each other causes edges to become chipped and broken. As the snow settles under its own weight, melts and refreezes and is buffeted by the wind, individual crystals are further altered so after a few days they have little resemblance to their original shape. In general, the crystals are smaller and more rounded. This process is referred to as destructive metamorphism. See Figure 1 below.

Figure 1

Beginning metamorphism of dendrite crystal

Crystals may also grow during the time they're on the ground. This process is referred to as constructive or temperature-gradient metamorphism and usually happens in places where the snow remains on the ground continuously for several weeks or longer, such as in mountainous areas, northern lands, the upper Mid-West, and parts of New England. Over time, the crystals at the bottom of the snowpack become larger. This because the crystals in closer proximity to the ground gain heat and vapor from the soil, which permits them to grow and attain a more angular or multifaceted shape. These kinds of crystals are called "depth hoar." The greater the differences in temperature between the ground below and the air above, the greater the growth rate potential. Some of these crystals may be 1/4 inch (6 mm) in diameter, which is about ten times as large as crystals normally found near the top of the snowpack. Depth hoar crystals are rather loosely bonded since their increasing size creates larger spaces between adjacent crystals. In mountainous areas, once the depth hoar layer is well established, a heavy snowfall can compromise the stability of the depth hoar layer, triggering avalanches. See Figures 2 and 3 below.

Figure 2

Depth hoar crystals, Colorado Rocky Mountains (Feb. 2002)

Figure 3

Large, mature depth hoar crystals

In order to more accurately and reliably evaluate snow cover and depth observed from satellite sensors, a better understanding of the structure of snow is required. Scientists at Goddard Space Flight Center are using data on variations in the size and shape of snow crystals, collected during field-work and measured using scanning electron microscopes, to improve the algorithms or models used to estimate how much water is stored in snow packs, such as those in the Rocky Mountains. The water storage component of snow packs is referred to as the snow water equivalent. This kind of information is useful for better managing our precious water resources and for improving flood forecasts.

Merry Christmas, Happy Holidays, and all the best in the coming year!

This week's question is provided by Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for EIGHT YEARS! This year Dr. Foster has decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis.