In Israel's Negev, dew supplies nearly half the water of some desert shrubs
Dew never adds up to much, rarely more than half a millimetre a night. Yet in the Negev Desert, researchers found it accounts for almost half the water in three dominant shrubs. The droplets appear because grass, leaves and car roofs radiate heat into a clear night sky faster than the ground can resupply it.
Dew forms when an exposed surface cools to the dew point, the temperature at which water vapour starts condensing faster than it evaporates. Fog and clouds follow the same physics, but they form in the air itself, around tiny particles, rather than on a surface. When it is cold enough, dew arrives as frost instead.
The key is radiation. On a clear night a surface can lose more energy as infrared than it gains, and if it conducts heat poorly or sits apart from the warmer ground, nothing replaces the loss. So dew favours thin, non-metallic, insulated objects such as grass blades, railings and bridges; shiny metal radiates infrared badly. Ideal conditions are cloudless skies, dry upper air that weakens the greenhouse effect, humid air near the ground and calm weather, since wind brings down warmer air. There is a twist: when the moisture comes from the air, a little breeze helps replace used-up vapour, and arid islands have the highest ideal wind speeds. When it rises from wet soil below, called distillation, wind only hurts.
Theory caps dew at about 0.8 millimetres a night, and in most climates annual totals are negligible next to rain. Still, lichens and pine seedlings in dry seasons benefit, and dew keeps lawn fungi and the potato blight organism Phytophthora infestans moist. The great rainless deserts of the Atacama and Namib, however, owe their natural watering mainly to fog, not dew.
Measuring it is tricky. The classic drosometer hangs a small artificial surface from a balance arm that records weight changes on a drum, but it is very sensitive to wind and only shows the potential for dew. Real amounts depend on the surface, so researchers may weigh actual plants or soil columns. The ancient treatise On the Universe already described dew as fine moisture falling from a clear sky, and hoar-frost as congealed dew.
Source: Dew
