This is the month for star patterns to be seen in the ice of lakes and ponds. This is an early season phenomenon. When the ice becomes thicker, they can’t form.
I offered my first posting about these star patterns a year ago, at which time I discussed some misconceptions about their formation. Now, at the beginning of this season of lake-star formation, I discuss the sequence of events that causes them to form.
When the freezing season begins, a thin layer of clear ice can form on ponds and along lakeshores. What is needed now is for the ice to be covered by some snow. However, the snow must be of uneven thickness, probably as a result of wind drift. Where the snow is thickest, it weighs down more on the ice and causes it to sink lower in the water than where the snow cover is thin.
What happens now is a bit subtle. The water immediately under a flat surface of ice has a temperature of 0 °C. However, just below that, the temperature is +4 °C (which is actually denser water). Wherever the weight of snow has depressed the ice and pushed it down into that lower and warmer water, the ice immediately above it melts, and a hole forms in the ice.
The overlying snow has yet another role to play in the formation of the star. Snow now wicks that warmer water up through the hole and onto the ice and this causes that warm water to flow up through the hole and to then radially percolate outwards to form stellar arms.
The lake stars in ice have formed. Later in the season, the greater thickness of ice doesn’t permit it, but for now, we see the patterns.
Stars appear in a pond, but the snow has largely melted.