"Frost in outlying areas --"
For many Alaskans south of the Brooks Range, it's a time for gardening decisions. In many years, the last official frost over most of the mainland is in mid-May. But there is always the possibility of a late frost, and even if the official temperature doesn't drop below 32 degrees F, the weather forecast is all too likely to be something like, "Clear, light winds, overnight lows in the high 30's with a possibility of frost in outlying areas". Why? And which outlying areas?
The temperature drops at night because the loss of thermal radiation from plant leaves and the soil continues after the sun has set and no more solar radiation is available for heating. The air, or rather the water vapor and carbon dioxide in the air, is emitting some thermal radiation itself, and if there are clouds, they emit even more. Some of this is absorbed by the ground, but if the sky is clear, not nearly as much is absorbed as is lost. Since the leaves and ground are losing energy, they cool off, and become colder than the air. The air nearest the ground is cooled by contact with the cold ground, becoming denser as it cools. This has the opposite effect from heating during the day. Vertical mixing is cut off and a relatively thin layer of cold air develops at the ground, while plant leaves, with their radiative energy loss, stay even colder than the air. The official temperature, which is measured at about head height above the ground, can be several degrees warmer than the air temperature a few inches above the ground. Plant leaves can be colder yet, and on a very open, exposed site they can freeze while the air, even near the ground, stays above the freezing point.
The cold layer of air tends to flow downhill on sloping ground, so low lying areas can develop a relatively deep layer of cold air, and are most likely to have thermometer readings below freezing. But as long as the wind is too light to mix warmer air down to plant level, plants may freeze and frost may form on the ground even in areas where the thermometer temperature stays above freezing.
There is another factor to consider, which helps explain why this kind of frost is most likely in outlying areas. Cities (and airports, where most official temperatures are recorded) have a great deal of concrete, asphalt, and buildings around which are very good at storing the sun's heat during the day and releasing it slowly overnight. The result is that cities are generally at least a degree or two warmer than their surroundings on clear, calm nights. This phenomenon is known as the urban heat island. Urban gardeners have their own problems to contend with, but they really do have a slightly longer growing season.
There are several things that can be done to minimize the probability of damage to plants from late radiation frosts. Plants hardening off in seedling flats can be moved into a building overnight. In hardening off, plants can be placed under trees or next to heated buildings in the first place, as in these locations they are less exposed to the sky and will receive some radiation from buildings and trees. Those already in the ground can be covered with something to block thermal radiation. Blankets and old newspapers are possibilities, but must be removed as soon as it warms up so the plants can get light the next day. Polyethylene is a widely used covering, and it works partly because the condensation which promptly forms releases latent heat (which slows cooling). It is also the condensation, not the poly, which actually blocks the thermal radiation.
Recently a number of other materials, most of which are more porous than polyethylene and should thus have less condensation forming, have been introduced and advertised as crop covers. Does anyone have information on how well these covers work compared with poly? If I can get enough information from people who have used different types of covers side by side on how they compare, perhaps we can summarize the results in another Science Forum this August, when the first fall frosts are expected.
