http://safety.dri.edu/FieldSafety/Gu...ons_on_Ice.pdf
Excerpt:
Ice safety
Ice strength is dependent on appearance, thickness, daily temperature, ice history, and snow cover, plus water depth under the ice, the size of the water and water chemistry, currents, and distribution of the load on the ice. Additional points to consider:
�� Don’t judge ice strictly by appearance. Ice can change with the surrounding climate conditions. Temperature, precipitation (such as snow, sleet, rain), wind speed, ice age, water depth and water quality are all factors that affect ice strength and thickness.
�� New ice is usually stronger than old ice because bonds between the crystals decay with age making the ice weaker even if melting has not occurred.
�� Ice thickness over a body of water is not constant. Water currents, particularly around narrow spots, bridges, inlets and outlets, are always suspect for thin ice. Beware of ice around partially submerged objects, such as trees, logs, brush, embankments or dam structures.
�� Snow can act like a blanket insulating the water below. Snow can insulate ice and keep it strong. It can also insulate it to keep it from freezing. Snow cover also hides the surface and can mask rotten ice and thin spots. You should always be cautious when moving across snow-covered ice.
�� Springs and currents can create areas of extreme thickness and patches that are just wafer thin.
�� Wind speeds influence ice formation. Light winds speed up the formation. Strong winds force water from beneath the ice and can decay the edges of the ice.
�� Air temperature influences the quality of ice. A rapid large drop in air temperature may actually make the ice brittle and it may not be safe for more than twenty four hours after this drastic temperature change. Above freezing temperatures for six or more hours in a twenty four hour period can weaken the ice. Staying above freezing for 24 hours or more will result in loss of ice strength to the point where the conditions may no longer be safe. Stay off the ice if this has occurred.
�� Slush is a danger sign. It indicates that ice is no longer freezing from the bottom and indicates weak or deteriorated ice.
�� Loud cracks and booms coming from the ice. On river ice this may indicate the ice is about to break up or move. One large lakes (several acres) this may only indicate harmless thermal expansion and contraction