Puddle Holes
6 ft long puddle hole in about 6" of ice. In these conditions these holes can be hard to spot. The slight lip around the edge and the rough ice on the surface skim were the primary indicators. The thin snow drifts were a secondary indicator.
Puddle holes are a form of wind holes. They are common during windy thaws on ice that is less than a roughly 6" thick. They can also occur in ice at least 15" thick. The mechanism is a warm wind blows and warms the surface water to the downwind end of the puddle. The return flow along the bottom melts ice off the bottom of the puddle. There is usually a connection between the puddle on the surface and the water underneath the ice that replaces surface water that gets blown out of the downwind end of the puddle. Shallow puddles with no connection to the water under the ice are likely to get blown down in high winds rather than staying in the same place so they can erode a hole. The erosion mechanism appears to work best once the puddle gets to be a couple inches deep, allowing significant ripples to form on the surface and a strong return flow at the bottom of the puddle. Johan Porsbey pointed out that solar heating can be a significant part of heating the puddle, particularly later in the season. Water absorbs or transmits sunlight almost completely while an ice surface is more reflective (particularly when the sun is at a low angle).
The puddles holes form a number of ways. The following are a couple of common examples.
When the ice sheet warms it expands putting compression stress in the sheet. This often results in local shallow buckling of the sheet, often with a mostly tight crack acting as the hinge point. Puddles form where the sheet has buckled down, creating a puddle that is resistant to being blown down wind. In a thawed state the ice sheet usually has pores through the sheet at grain boundaries. Wind gusts tend to pull water up through the ice sheet and lulls let it go back down. This enlarges the pores over time. A warm wind on a few inches of ice can turn the puddle into a hole in as little as a day.
Puddle holes can be hard to see on thawed state with lots of other shallow puddles on the surface of the ice. Once it gets colder they often develop rims of white splash out ice and/or catch blowing snow around their edges. Both of these features make them easier to spot (see the picture above). It is common for there to be a long line of holes scattered along the one particular crack while many other visible cracks do not have holes associated with them.
Puddle holes along a crack in about 6" of ice. Scale: the holes about a foot wide.
A thick, water saturated, snow drift is heavy enough to depress the ice enough to let water seep to the surface at each side. These puddles often form in pairs with one hole on each side of the drift ('saddlebag holes') .
This drift shape created a single hole rather than two saddlebag holes on the sides of the drift. The test pole is in the center of the puddle and near the edge of the actual hole (there is still ice under some of the puddle).
Addition feb 17, 2020
We have had two wind hole events so far this year. All of them involved ice less that 4" thick, a day of two wind and temperatues above 32 degrees. All three of these events had winds over 20 mph (often over 30). They can be difficult to see without a cover. With a cover you still have to pay attention. These events had holes about 100 feet appart. In an event on thick ice a couple years ago they were seen every half mile or so. If you see one, expect to see lots more.
