Winter is on its Way

Photo Credit: Kate Keefe 9/07 (Mount Monroe)

With cold temperatures settling in, a magical force has taken the water around us and transformed it into complex formations. Moisture in the ground and air; freeze and form intricate patterns. In parts of the northern states snow has already fallen and covered the ground in a white blanket while further south frost coats the ground on cold nights.

The first sign of the upcoming winter is frost. Towards the end of September gardeners start watching the nightly lows and putting sheets over their garden to protect their vegetables from ice formation. Frost forms when the temperature near the ground reaches freezing. If the dew point temperature, the temperature in which water vapor condenses to form dew, is also near freezing, any moisture in the air near the ground changes directly from a gas to a solid, and deposits itself on anything that is also at freezing temperatures. These structures are usually plants, grass, park benches and even the roofs of well insulated homes.

Frost will form in valleys before it forms in higher elevations because heat rises and cool air sinks. Temperature readings are usually taken about six feet above the ground surface. Temperatures on the ground can be five to seven degrees cooler than the reported temperatures. On nights when it is either cloudy or windy mixing occurs and the temperature gradient disappears and frost doesn’t form. Ideal frost conditions include clear nights with little to no wind and air temperatures around freezing.

Photo Credit: Kate Keefe 10/07 (Needle Ice on the way up Mt. Willard)

Needle ice also forms in fine textured soils after the first hard freeze of the fall. The needles are fed by capillary action and are pushed out of the ground by the ground water. The surrounding soil temperatures must be above freezing to allow the water to be pushed out into the freezing air. These ice crystals usually grow in columns three to four inches high but can grow larger and end up looking like miniature chandeliers. These needles lift the cap of the soil and rocks and cause them to settle down slope as they melt.

Snow, as we generally think of it, is ice crystals that grow to be large and heavy enough to fall to the ground. Snowflakes are composed of one or more ice crystals. These crystals form when water vapor freezes around a particle which is known as the nucleating agent. The resulting ice crystal or snow flake grows when water molecules are transferred from water droplets in the air.

Photo Credit: Kate Keefe 11/07 (Footprints in fresh snow around Saco Lake)

Snowflakes are often portrayed in their beautifully symmetrical six pointed structure. These tiny crystals form six points because water molecules form a hexagonal structure when forming crystals. The six fold symmetry of the crystal comes from the symmetry of the water molecules in the crystal lattice. The crystal type is determined by the temperature and available vapor supply during formation.

Check back for more information on winter progresses. In the meantime check out where frost is forming or snow is falling in your area.

Fall Turnover

Ok so the fabulous colors of autumn have flared and faded, but that is not the end of extraordinary changes going on around us. Besides the scores of birds embarking on long distance journeys of 1,000’s of miles or the mammals preparing to hunker down to sleep off the more severe aspects of winter, we have a relatively quiet but essential shift going on in our lakes and ponds.

Let me start by saying that water is amazing. Without its unusual characteristics, life would not exist as we know it. Certainly not here in the north country. Let’s talk temperature…. Water is its most dense at 39° F. So what, you might think. But wait a minute, that means that water when it is frozen at 32° F is less dense. So ice floats. To understand why that is so important let me explain the dynamics of what is happening in the depths of the lake.

Think back to those steamy days of summer by the lakeside. As one would expect the surface temperatures of the water are warmer than the cooler temperatures found a few feet lower. The sun’s rays warm up the surface temperatures of the water. This warmer water at the surface being less dense than the cooler water below, sets up a temperature gradient. As the season moves along, there is relatively little mixing between the warm water at the surface and cool water below. As a result there is no mixing of oxygen, nutrients or aquatic waste products during this time so the surface waters can be depleted.

But just wait, a big shift is on the way. As summer fades into autumn, the cool nighttime temperatures allow the surface water to also cool. And since cooler water now on the surface is more dense than the warmer water just below, it begins to sink. It only sinks to where it’s temperature is the same as the surrounding water. Gradual at first, but the surface water temperature continues to cool and mix with the deeper water. Eventually the entire lake or pond is completely mixed with resources of nutrients, dissolved gasses distributed evenly throughout the body of water. This mixing process continues until the water reaches its maximum density at the magic temperature of 39°F.

What is so miraculous about that you might ask? Well if water was more dense at 32°F and if ice did not float, then the lake would freeze from the bottom up, preventing many aquatic species from surviving winter. With water at 39°F throughout the lake, as nighttime temperatures dip even colder, water at the top will freeze essentially placing a lid over the lake insulating it from freezing all the way through.

Check back for more of what is going on in our lakes and ponds as winter approaches. In the meantime checkout any local pond or lake for signs of fall turnover.