Permaculture Designers Manual
CHAPTER 6 –
TREES AND THEIR ENERGY TRANSACTIONS IN PERMACULTURE
Section 6.4 –
Temperature Effects in Permaculture
EVAPORATION causes heat loss locally and CONDENSATION causes heat gain locally. Both effects may be used to heal or cool air or surfaces.
The USDA’s Yearbook of Agriculture on Trees (1949) has this to say about the evaporative effects of trees:
“An ordinary elm, of medium size, will get rid of 15,000 pounds of water on a clear dry hot day” and “Evapo-transpiration (in 40 inches of rainfall) is generally not less than 15 inches per year.”
Thus, the evaporation by day of trees cools air in hot weather, while the night condensation of atmospheric water warms the surrounding air.
Moisture will not condense unless it finds a surface to condense on. Leaves provide this surface, as well as contact cooling.
Leaf surfaces are likely to be cooler than other objects at evening due to the evaporation from leaf stomata by day.
As air is also rising over trees, some vertical lift cooling occurs, the two combining to condense moisture on the forest.
We find that leaves are 86% water, thus having twice the specific heat of soil, remaining cooler than the soil by day and warmer at night.
Plants generally may be 15°C or so warmer than the surrounding air temperature.
Small open water storages or tree clumps upwind of a house have a pleasant moderating effect. Air passing over open water is cooled in summer.
It is warmed and has moisture added even in winter. Only water captured by trees, however, has a DEHUMIDIFYING effect in hot and humid tropical areas, as trees are capable of reducing humidity by direct absorption except in the most extreme conditions.
Reddish colored leaves, such as are developed in some vines and shrubs, reflect chiefly red light rays.
Sharp decreases in temperature may result by interposing reddish foliage between a thermometer and the sun, up to 20°C (36°F) lower than with green pigmented plants (Daubenmire, 1974).
Whitish plants such as wormwood and birch may reflect 85% of incoming light, whereas the dark leaves of shade plants may reflect as little as 2%.
It follows that while or red colored roof vines over tiles may effectively lower summer temperatures within buildings or in trellis systems.
Additional cooling is effected by fitting fine water sprays and damp mulch systems under trellis, thus creating a cool area of dense air by evaporation.
This effect is of great use in moderating summer heat in buildings, and for providing cool air sources to draw from by induced cross-ventilation.