I thought this may be an interesting topic for some folks here on the forum of observations I have made in the past 6 years of waiting impatiently for the ice on our pond to become usable for ice skating. It is often a very frustrating wait compounded by an obviously changing climate.

1. On the occasion we have a severe and quick cold snap early in the winter and the water was previously open, this ice will thicken very slowly, especially with snow on it. The theory here is there is a lot of latent trapped heat in the water column that is insulated by the layer of ice and snow. This remaining heat is forming circulations that are melting ice from below while trying to freeze from above. This is what we are experiencing now. Two weeks of sub 20's and holes keep appearing in the ice, and a large variety of thicknesses. Dangerous stuff!
This is where even a light weight of snow is enough to crack the ice to where it leaks water and adds a layer of slush to the top. You get those cool "tree" patterns where the heat is welling from below, melting the ice in interesting patterns.

2. If the chill is gradual and the ice can close off slowly, this is the best stuff. Ice grows thick faster, is harder, and doesn't leak under the weight of snow. The theory here is the water column gets chilled and stratified so there is not much mixing going on melting the ice from below so it just remains solid.

3. Sandwich ice is terrible. Had this stuff last year where we had an early freeze, thaw, rain and snow, and another freeze. The snow mixed with water to a 3" layer of slush on top of old ice, and then froze again on the top. The slush refused to freeze. It seemed thick, tested thick, and your skate would blow right through.

4. Ground water ponds are dangerous. The water entering the pond from ground water is significantly warmer than water already chilled in the pond, and will act like #1, melting ice from below. Snow on top as an insulator? The ice can completely melt from below due to this heat rising to the top.

5. Skipping rocks across <1" thick black ice is where Star Wars got its blaster sounds. Try it some time!

I am interested to hear some other observations.