Good ice for weight-bearing is thick, black or blue and clear, without air bubbles.
Features of weak ice:
(1) thin, relative to weight applied
(2) presence of wet cracks
(3) presence of a layer of air underneath the ice sheet (ice must be on water if it is to bear any weight)
(4) recent temperature above freezing for >= 24 hours
(5) recent rapid, steep fall in temperature (makes ice brittle)
(6) location on river, lake or stream with water flowing underneath
(7) slushy ice (layer of wet snow over ice) or snow ice (white with air bubbles)
(8) presence of pressure ridges
Weak ice needs to be either avoided (for example, if there is a layer of air beneath) or 2-3 times thicker than the thickness of clear ice required to support a load.
Activities that may make good ice weak:
(1) driving too fast over ice (>= 15 miles per hour), due to generation of waves in the water beneath the ice
(2) leaving a load on the ice too long in one place (results in ice "creep")
(3) having heavy loads too close together
(4) driving parallel to cracks
(5) leaving a load near the edge of a large ice opening
(6) not distributing a load over a large enough area
