In December 1983, White Rock Lake froze from shore to shore with ice thick enough that a person could walk across the 1,100 acre body without getting their feet wet. But 1983 was an exception. This is Dallas—Texas, by golly!—and though we have our freezes and our ice storms and our snow storms, usually our winters are an eclectic mix of arctic incursions coupled with subtropical comfort. In essence, today’s parka and gloves are quickly replaced by tomorrow’s t-shirt and shorts.
The last several weeks have offered a wee bit of a twist on that normalcy. Progressively cooler temperatures beginning in December ushered in a start to the New Year defined by real, honest-to-goodness cold weather, something that rarely happens here. And several days below freezing after a prolonged “cool” spell meant seeing a rarity in these parts.
Down here in the South we think of ice sheets as being measured in puddles and ponds, in small pools of water that cool off quickly and freeze in the short time that temperatures allow. Yet standing on the shore of the lake yesterday, I looked out over a true ice floe, a sheet of ice at least 100 acres/40 hectares in size. The whole of Sunset Bay, including the creeks that come together in the confluence, had frozen over, and in some places they had frozen solid.
The interesting thing about fresh water is that it reaches maximum density at 39° F/4° C. If the water temperature is above or below that, the liquid’s density lessens and it becomes lighter. That means none of the water in the lake can fall below 39° F/4° C until all the water hits that temperature. That’s because water cooled to maximum density becomes heavier than the warmer water beneath it, so it sinks and forces warmer water to the surface. This trading places continues until the entire body of water has the same density. Then and only then can the water temperature fall below that threshold.
Lake water mixes in this fashion until all the water is at or near freezing. It’s at that point when ice can form on the surface. That requires an extended period of time with freezing air temperatures, and it requires sufficient time with cold air for the water to reach maximum density in the first place. The whole process can speed up or slow down depending on the surface area (the amount of water being cooled by the freezing air) and the depth (the amount of water that has to be mixed before the entire lake reaches maximum density and can continue cooling).
The first part to freeze generally is along the shore where water is shallow and held relatively still through contact with the land. This kind of ice is called shorefast ice (or border ice). It’s the most common ice seen on any body of water. Once shorefast ice forms, it begins expanding away from land along the surface of the water. This floating ice sheet is called a floe.
Wind also impacts freezing since water in motion is much more difficult to freeze than is standing water. Also, water in motion causes friction, and friction causes heat. Even before shorefast ice forms, strong winds can create waves. Waves create spray. If the air is below freezing, that spray becomes supercooled like freezing rain (liquid water with a temperature below freezing). And what happens when supercooled water hits something? It immediately turns to ice. As in the above image, wave spray that freezes on the shore can create dazzling, otherworldly designs. This kind of formation is called an ice foot (or ice rampart) when it remains attached to both the ground and the floating ice.
As our temperatures increased, the fast ice (ice that stays where it formed) began to crack, and one massive floe broke apart into a few separate floes. The lane of open water in the above photo is called a lead, a place where floes move apart. And that separated floe the gulls are standing on? That’s called an ice cake. It’s still attached to the shore where the water froze all the way to the bottom at least ten paces out (where the water freezes all the way to the bottom, it’s called anchor ice).
That photo shows an interesting detail. There’s a gap of about six inches/15 centimeters between the frozen lake surface and the ice formations on all southern shores. The lake didn’t freeze until it became still (hence the floe is perfectly smooth). The gap between the two stems from the aggressive onshore waves. Everything above the wave height froze while everything in the waves remained liquid, at least until the winds calmed.
The effect became most evident on aquatic plants. Significant ice formed atop the stems, but then the lake froze well below that ice layer. It created a bizarre scene where plants were frozen at both ends but left bare in the middle. It’s as if the wave spray ice floats in the air above the lake ice.
In places the ice looked more like hot wax poured over the earth and allowed to cool into bizarre shapes and patterns. Where plants or other obstructions captured the spray, quickly formed ice created a barrier upon which other ice formed but beyond which the water didn’t travel. Whole walls of ice stood between the lake and the ground beyond, in places the demarcation drawing sharp dichotomies between the cold and the barren.
And finally one of the more fascinating structures. Everything facing north froze from the wave spray. Anything that could catch water became a magnet for ice, hence plants and rocks and the ground itself developed thick coatings. But this photo shows a perfectly formed echo of the waves themselves. As water splashed against the barrier, it did what you’d expect it to do: it exploded into the air, curved over itself and fell back into the lake. Only in this case it began freezing on contact. Eventually the ice developed spires that showed how the water splashed rhythmically in the same place, each time sending a spray into the air, each time adding a bit more ice to the frozen reflection that now stands. There’s no support for this structure save the ice itself and where it’s grounded on the rocks below.
— — — — — — — — — —
It’s important to note that I’ve oversimplified what’s required to freeze the surface of a lake. Many different conditions play a part. For example, the process of crystallization (turning liquid water into ice) actually generates heat. In order for a lake surface to freeze, both air and water must be cold enough to overcome that seemingly counterintuitive effect. Ground temperature, subsurface water movement, wind, pressure, humidity and many other items play a part.
As for the wildlife, all the inlets, bays and creeks are frozen. This is especially problematic for wading birds like herons and egrets, not to mention dabbling ducks like mallards. (Diving ducks are having fewer problems since plenty of open water exists in the middle of the lake, though the depths there prohibit dabblers from feeding normally.) Cormorants and pelicans have curtailed their near-shore fishing and have been forced to eat and sleep in deeper water. Smaller birds like warblers and sparrows struggle to find open water from which to drink. Mammals that don’t hibernate likewise are having difficulty finding open water. However, temperatures are moderating quickly and the ice is already breaking apart. Nevertheless, as yesterday’s duck image showed, a good deal of damage has already been done.