When the Going Gets Hot
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
...the hot go to a frost pocket. No it isn’t a Hot Pocket® you put into the freezer and forget about; it is one of the interesting habitats on island that many folks don’t know much about. I was not very familiar with these until I stumbled upon one on the Land Bank’s Shawkemo Hills Trail off of Polpis Road (www.nantucketlandbank.org/Map/Trails/10_shawkemo.pdf).
This is one of my favorite walks on island due to sweeping views of Nantucket Harbor and the Middle Moors from Radar Hill and neighboring peaks. The cool paths lined with pine needles spread out on various loops that can take from 15 minutes to over an hour depending on how far you decide to roam. This trail starts at a small parking lot on the right not long after you pass North Pasture Lane on Polpis Road. If you take the first left on the trail (the Low Trail), you’ll eventually find a classic frost pocket on your right. It is a relatively obvious low point in a micro valley with scrub oak and shrubs on the upper portions of the bowl and grasses and lichens dominating the lowest point. This deep glacial valley and the surrounding higher points were formed as part of the terminal moraine which marks the maximum advance of the glacier that formed the glacial deposits of Nantucket. Our glacial origins make Nantucket an ideal spot for frost pockets. Depressions formed as large block of ice were left behind by the retreating lobes of the glacier creating natural bowl shapes.
The definition of a frost hollow or frost pocket is any low-lying area (e.g. a valley bottom or a smaller hollow) where frosts occur more frequently than in the surrounding area. This normally happens as cold air drains down neighboring slopes into a localized pocket from which it is slow (or unable) to escape, not unlike the trapped feeling you may get listening to someone long winded at a cocktail party. Both late and early frosts are more likely to occur in frost pockets, making them bane of gardeners and farmers and the boon of hikers. Frost hollows of larger scale (a valley or basin) are also known as cold pools. Cold pools are areas where cold air is trapped under an inversion under calm winter weather conditions. A frost pocket is sometimes sparse in vegetation, especially in those species that can be damaged by late frosts in spring or early frosts in autumn.
When air at higher elevations cools during nighttime radiational heat loss it becomes much heavier than the surrounding air as well as air layers well beneath it. Its negative buoyancy causes it to sink downward until it reaches a layer of air cooler than itself or bottoms out against the earth's surface. The preferred cold-air drainage route descends downhill following the same local terrain drainage patterns taken by rain or snow runoff. Once it has reached its resting level, it may continue to cool through radiational heat loss to even lower temperatures. Cold-air drainage flows can occur even with terrain differences of only a few feet. These pools of colder air are often observed in warmer weather as patches of light fog forming over low-lying wetlands, ponds and stream beds, or wet vegetation. These “foggy bottoms” are seen all over Nantucket. In colder weather, when the cold air drops below the freezing mark, frost will form in these locations. In the extreme cases — i.e., with very cold temperatures and/or very steep slopes, these drainage flows, known as katabatic winds, may attain great speeds, as high as 44 mph, but usually they slide gently downhill at a leisurely 4-8 mph.
Let’s look at one of the typical ways in which frost forms. As air cools at night, its ability to hold water vapor decreases, causing it to become saturated. If the air continues to cool, excess vapor condenses, and forms as dew on the surface of plant leaves. This dew then freezes if the temperature continues to drop to 0°C (32°F) or lower. As mentioned above; as the cooling of the air occurs, heat accumulated in the soil radiates back into the atmosphere. This loss of heat continues until just after dawn, i.e. until such times as the sun warms it up again, so this is why the sharpest frosts often occur in early morning.
Differential heating can occur when night-time terrestrial radiation, or how much heat is given off by vegetation, is greatest on valley slopes. Air above these slopes becomes colder and hence denser, therefore it flows down slope. Minimum temperatures in the pocket may be tens of degrees below the surroundings. For this reason, fruit growers try to avoid frost pockets. Frost pockets can also be bothersome if they are in your garden. Common solutions for garden cold spots include leaving an area in a fence for cold air to flow out of the garden at a low spot, watering your garden on frosty nights as wet soil holds more heat, covering tender plants with insulative material on nights when frost is predicted, and placing a barricade against the cold air flow on the highest ground above your garden made of a fence or a row of plants to divert the cold air.
With our recent hot spells, it is tempting to think of fall issues like frost. On a hot day, it is extremely refreshing to encounter a frost pocket along the trail where the temperature can be ten degrees cooler than the higher areas on either side. Some plants thrive in these microclimates and these areas may shelter plants that would normally by susceptible to our hotter summer temperatures.
In keeping with the scale of these small cold pockets, microclimatology is the science of studying and measuring climate in a small area, from cities to tiny valleys like our glacial ones. Three researchers from the Harvard Forest, Glenn Motzkin, Stephanie C. Cicarello, and David R. Foster, wrote in a 2002 Journal of the Torrey Botanical Society paper (Vol. 129:2 pages 154-163) that frost pockets can even form in some glacial plain areas when extreme radiative cooling occurs. This creates microclimates where leaf budding and vegetative growth may slow down and prevent scrub oak and other shrubs from overtaking some rarer species, in effect, helping to maintain priority habitat. They observed that burning and mowing these flatter terrains increased the chances for microclimatic effects.
I bet you feel cooler already. The flattened grassy mats in the frost pocket described above attest to the fact that the deer certainly appreciate the cool oasis. If the heat starts to get to you, go find this trail and look for the tell-tale signs of the frost pocket.
