of water collected. At some automatic weather stations an ultrasonic snow depth sensor may be used to augment the precipitation gauge. Event Snow flurry, snow shower, snow storm and blizzard describe snow events of progressively greater duration and intensity. A blizzard is a weather condition involving snow and has varying definitions in different parts of the world. In the United States, a blizzard occurs when two conditions are met for a period of three hours or more: a sustained wind or frequent gusts to , and sufficient snow in the air to reduce visibility to less than . In Canada and the United Kingdom, the criteria are similar. While heavy snowfall often occurs during blizzard conditions, falling snow is not a requirement, as blowing snow can create a ground blizzard. Snowstorm intensity may be categorized by visibility and depth of accumulation. Snowfall's intensity is determined by visibility, as follows: Light: visibility greater than Moderate: visibility restrictions between Heavy: visibility is less than The International Classification for Seasonal Snow on the Ground defines "height of new snow" as the depth of freshly fallen snow, in centimeters as measured with a ruler, that accumulated on a snowboard during an observation period of 24 hours, or other observation interval. After the measurement, the snow is cleared from the board and the board is placed flush with the snow surface to provide an accurate measurement at the end of the next interval. Melting, compacting, blowing and drifting contribute to the difficulty of measuring snowfall. Distribution Glaciers with their permanent snowpacks cover about 10% of the earth's surface, while seasonal snow covers about nine percent, mostly in the Northern Hemisphere, where seasonal snow covers about , according to a 1987 estimate. A 2007 estimate of snow cover over the Northern Hemisphere suggested that, on average, snow cover ranges from a minimum extent of each August to a maximum extent of each January or nearly half of the land surface in that hemisphere. A study of Northern Hemisphere snow cover extent for the period 1972–2006 suggests a reduction of over the 35-year period. Records The following are world records regarding snowfall and snowflakes: Highest seasonal total snowfall – The world record for the highest seasonal total snowfall was measured in the United States at Mt. Baker Ski Area, outside of the city of Bellingham, Washington during the 1998–1999 season. Mount Baker received of snow, thus surpassing