Temperatures here in the South were really cold in the single digits a few mornings here recently.
We did a little experiment.
Here is the observation journal. We kept track of our observations every 40 minutes or so...We tracked 4 cups of water. 2 cups were placed in the sun and 2 in the shade.
1 of the 2 were filled and the other had just a small amout of water.
After 4 1/2, hours only the full cup in the sun was frozen. And the full cup in the shad was slushy.
They were only filled part of the way!