Frost-covered trees and Glastonbury Tor
Frost-covered trees and Glastonbury Tor. Hoar frost covering three willow (Salix sp.) trees in a field on a farm. St Michael's Tower can be seen on the summit of Glastonbury Tor in the background. Hoar frost results when heat is lost through radiation into the open sky on cold clear nights, causing objects to become colder than the surrounding air. This temperature difference causes the water in the warmer air to condense on the colder objects and then freeze into small ice crystals. Photographed in Somerset, England, UK, in December.
© DUNCAN SHAW/SCIENCE PHOTO LIBRARY
Snowflake. This is an ice crystal that forms in air that has a temperature near the freezing point of water. If the air is calm, then a symmetrical, hexagonal snowflake can form. The two main growth patterns observed are faceting and branching. When growth is fast and unstable, branching patterns create a dendritic snowflake. Slower growth allows the straight lines and hexagonal shapes of a plate snowflake. This snowflake displays both types of growth. The type of growth also depends on temperature, which changes in a different way for each snowflake, causing the wide variation in snowflake patterns.
© Kenneth Libbrecht/Science Photo Library
Glacier crevasses, Canada
Tellot Glacier. A glacier is a huge mass of ice slowly moving over a land mass, formed from compacted snow. The deep crevasses seen here are created when planes of ice within the glacier move past each other, causing shear forces that cause trenches to develop. The mountains at upper left are a section of the Waddington Range, part of the Canadian Rockies. Photographed in British Columbia, Canada.
© David Nunuk/Science Photo Library