A contaminated runway is a runway where more than 25% of the surface area, whether in isolated areas or not within the required length and width being used, is covered in either:

• More than 3 mm (0.125 in) deep of water or its equivalent in wet snow or slush or loose snow
• Compacted snow which will resist further compression into a solid mass and either hold together or break into lumps when picked up or
• Ice, including wet ice.

Wet ice is ice that is melting, or ice with a layer of water (any depth) on top.

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The various braking action of each surface is quite easy to work out.

Compacted snow has a relatively rough surface, and with heavy weight will allow some movement of the snow beneath, allowing for some traction between tyres and the surface, especially if the tyres have good tread for snow operations. This will allow moderate and moderate-to-poor braking action in many cases.

Ice, on the other hand, will not move, nor allow any purchase with treaded tyres (unless they are studded), but dry ice (not frozen carbon dioxide used in smoke machines) will at least have a small amount of traction with a (usually) grippy rubber tyre surface. Still, moderate-to-poor and poor braking action can be expected.

Wet ice has the same downsides of ice, but now that top level traction is nearly completely gone, and the braking action can be expected to be poor to non-existent.