A runway is considered to be contaminated when it has more than 25% of its surface area (whether isolated areas or not) within the required length and width being used covered by the following:
1. Surface water more than 3mm deep
2. Slush or loose snow with a water equivalent depth (WED) more than 3mm
3. Snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps, if picked up (compacted snow)
4. Ice, or wet ice
JAR-Ops 1.480(2)
Hope that helps
