Out walking his dog one winter evening, a YouTube user noticed a roughly round patch of black, open water in the middle of a frozen, snow-dusted pond. Its rim was crusted with feathery ice crystals, and he described something like vapour rising from it. His phone video, “Strange Phenomenon – WHAT THE HELL IS THAT?”, asks a question that puzzles plenty of winter walkers: why does one spot stay open when everything around it is frozen?
How a pond freezes from the top down
Water has an unusual property: it is densest at about 4°C. As autumn air chills a pond, the surface water cools, sinks and is replaced by warmer water from below until the whole pond sits near 4°C. After that, colder surface water stays on top because it is lighter. The surface reaches 0°C, ice forms, and because ice floats, it seals the pond with a lid while the bottom stays near 4°C all winter.
That means every frozen pond has relatively “warm” water just under the ice. Anything that brings that water up to the surface, or keeps it moving, can stop ice from forming in one spot.
Video: Strange Phenomenon - WHAT THE HELL IS THAT? (published by Lukas).
Why holes stay open in pond and lake ice
Ice scientists and safety agencies point to a handful of usual suspects:
- Springs and inflows. Water flowing in from springs or streams keeps stirring the surface and stops ice from thickening above it.
- Moving water. The Minnesota Department of Natural Resources warns that ice over currents, near inlets, bridges and culverts is often dangerously thin.
- Fish. According to the Minnesota DNR, the movement of fish can bring warmer water up from the bottom and open holes in the ice, holes that have let snowmobiles and even cars break through.
- Rising gas. On some northern lakes, methane from decomposing organic matter bubbles up so steadily that strong seeps keep the surface from sealing at all. Researchers led by Katey Walter Anthony, working with satellite radar data from NASA's Alaska Satellite Facility, map these “hotspot” seeps; on one lake she saw a huge open area that “looked like it was boiling.”
The vapour above the opening
The mist the filmmaker described has a well-known name: steam fog, also called sea smoke or water smoke. The American Meteorological Society defines it as fog that forms when water vapour enters much colder air, most often when very cold air moves over relatively warm water. Water vapour from the open patch mixes into the much colder air and condenses at once. On a cold evening, water near 0–4°C can steam visibly against air far below freezing.
Ice thickness around open water
The filmmaker estimated the ice at about 20 cm where he stood. That can be true a few metres from water that has never frozen at all. The Minnesota DNR notes that ice can be a foot thick in one spot and only an inch or two just a few feet away. Its guidelines call for at least 4 inches (10 cm) of new, clear ice for walking, and double that for white, snowy ice. Admire an opening like this one from the shore or from well-tested ice, and give it a wide berth.
For more freezing puzzles, see why hot water can freeze faster than cold and how melting river ice moved an entire bridge.
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