A whiteout is a situation in which people lose their orientation and bearings in snowy weather. There are a number of types of whiteouts, and all of them can potentially be quite dangerous. Because people have difficulty seeing and navigating during a whiteout, it is very possible to become lost, even feet away from safety, and whiteouts can be especially dangerous for drivers and pilots.
Classically, a whiteout is caused by a blizzard. A blizzard is an extremely heavy snowfall, and when the snow falls thick and fast, it can form a solid curtain which is impossible to see through. Blizzards are also often accompanied with high winds which can push people off course and confuse them even further. In mild blizzard conditions, it is generally possible to see vague outlines of things like structures, but in a full whiteout, it is impossible to tell the difference between the sky and the ground, let alone see something.
During a normal snowfall, whiteout conditions can sometimes strike, albeit usually briefly. This can still be dangerous for people caught outdoors, as people may lose their bearings in the obscured conditions and make poor choices. In regions where whiteout conditions are common, some people use navigational aids like ropes between homes and barns to reduce the risk of becoming disoriented and lost.
Another type of whiteout can occur in areas with very clear air and fresh snowfall. When the sky is overcast in these regions, it diffuses the light, and it can make the entire environment turn white, erasing all signs of shadows and definition which could be used to navigate. This type of whiteout is very common in Antarctica, where the sky is very clear, and it becomes impossible to distinguish the horizon, to judge distances, or to see signs of danger. You may also hear this type of whiteout called “flat light,” in a reference to the fact that the world seems very flattened.
Being caught out in whiteout conditions is not desirable, and most people take steps to avoid them. The risk of whiteout conditions is one reason why people are encouraged to refrain from driving when an onslaught of snow is expected, and it explains why airports are often closed in snowy regions during the winter, as the staff at the airport view delays as better than the risk of losing an aircraft.