JAA says that a runway is contaminated "when more than 25 percent of the runway surface are (whether in isolated areas or not) within the required length and width being used is covered by the following: "Surface water more than 3.0 mm [millimeters] (0125 in [inch]) deep, or by slush or loose snow, equivalent to more than 3.0 mm (0.125 in) of ... More @Wikipedia
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