In this viral video, two friends walk on crystal clear ice on a frozen lake in the High Tatras Mountains of Slovakia. The ice is so clear it appears as though they are walking on water.
The color of ice, how clear or white it is, can indicate its thickness.
Clear ice is about twice as strong as white ice since it is much more dense, according to the Minnesota Department of Natural Resources. Ice looks cloudy and white when it has air bubbles due to thawing and refreezing.
It is always important to check the thickness of ice before venturing out for skating or ice fishing.
A general ice thickness guideline from the Minnesota Department of Natural Resources indicates that there should be at least 4 inches of clear ice for people to walk on, 5 inches of clear ice for someone on a snowmobile to ride on and 8-12 inches of clear ice for a car to drive on. White ice needs to be twice as thick to hold the same weight.
The ice on the frozen lake in the Slovakian Mountains was likely sufficiently thick for people to walk on due to cold air entrenched across the region since mid-November.
Lomnicky Stit, Slovakia, which sits at an elevation of 8,638 feet, has been gripped by subfreezing temperatures since the middle of November, according to data obtained from the location's observation site. This is cold enough to keep a thick layer of lake water frozen.
At a slightly lower elevation, 6,532 feet, an observation site at Kasprowy Wierch, Poland, reports that cold air has been in place since the middle of November, but there have been some days above freezing which could compromise the thickness of ice.