Why is there water on top of frozen lake?

Of course getting stuck is a nuisance under any circumstances, but it can be especially troublesome in the Alaska wilderness in the dead of winter. The layer of slush is called overflow, and it occurs on frozen lakes when water from below the ice seeps up through cracks and rises above the surface of the ice layer.



That’s an excellent and very common observation! The water on top of a frozen lake is almost always meltwater, and it forms due to a combination of factors. Here’s a breakdown of why it happens:

1. Solar Radiation (The Primary Cause)

Even on cold, sunny winter days, the sun’s rays carry significant energy. Sunlight Absorption: The ice, especially if it’s clear or has a dusting of dark material (like dirt or pine needles), absorbs solar radiation and heats up. Internal Melting: This heat can melt the ice from within, not just from the surface. The ice acts like a greenhouse, trapping heat and creating a slushy layer inside or on top of the solid ice sheet below.

2. Insulating Snow

A layer of snow on top of the ice is a fantastic insulator. It traps the geothermal heat from the warmer water below (usually around 4°C or 39°F, where water is densest) and prevents it from escaping. This heat from below can slowly melt the bottom of the ice sheet. If the ice is thin enough, this meltwater can seep up through cracks or porous ice, appearing on the surface.

3. External Temperature Fluctuations

  • Daytime Thaw: Air temperatures can rise above freezing during the day, even if it’s still below freezing at night. This causes the very top layer of ice or snow to melt.
  • Nighttime Re-freeze: When temperatures drop again at night, this meltwater can re-freeze, creating a new, often smoother, layer of ice on top. If

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