Water is in clouds.
Because the area where clouds are in the sky is cold, water in the clouds freezes when the temperature is right. The water molecules freeze into ice crystals, forming a hexagonal lattice. When vertical air currents toss the crystals up and down in the atmosphere, the crystals join, making them larger.
Ice to Snow
Ice hugs windowpanes.
Ice crystals are also called snow crystals, and some of them fall to the Earth. When enough crystals join by adhering to one another, they create a snowflake. If even more join together, the result may be large and heavy enough to be hail. That depends on the path traveled from the clouds to Earth.
Characteristics
Ice crystals form many shapes.
Even though each ice crystal is one-of-a-kind, ice crystals have shared characteristics. For example, they form when the temperature is below freezing. Each ice crystal is porous. All ice crystals are hexagonal, meaning they have six sides. Ice crystals are in different forms, however, including plates, stars (dendrites), hollow columns, solid prisms and needles. Scientists measure the effect of temperature and the level of water vapor on the various shapes of ice crystals.
Morphology
A crystal's shape depends on temperature and water vapor.
At just below freezing, ice crystals form dendrites and thin plates, according to research done at California Institute of Technology. As the temperature becomes colder and the level of water vapor in the cloud rises, the crystals formed tend toward the shape of long needles and hollow columns. As the temperature drops and the level of water vapor rises, a wide variety of plates -- sectored, thin and solid -- as well as dendrites form. As the temperature drops well below 0 F and water vapor also drops, the crystals form columns and simple plates.
The reasons behind the formations are a mystery. Because ice crystals generally formed in cirrus clouds have an observable effect on Earth's atmosphere, scientists using Pennsylvania State University's cloud chamber investigate the growth processes of ice crystals. All the crystals start out sharing characteristics. As a crystal combines with other crystals, becoming a snowflake, it travels through different temperatures, concentrations of water vapor and grows at varying rates. According to Hans Verlinde, associate professor of meteorology at Pennsylvania State University, "the probability of finding two identical ice crystals...is vanishingly small. The bigger the crystals get, the greater the freedom for different growth paths, and the lower the probability of finding identical crystals even at the macroscopic visual level."
Ongoing Research
Scientists throughout the world study ice crystals.
In 1951, the International Commission on Snow and Ice formed a system to categorize types of ice crystals and described seven forms: plate, stellar, column, needle, spatial dendrite, capped column and irregular. Each generation of scientists builds upon the knowledge shared by its predecessors, and the study of ice crystals similarities and differences continues.
