Permaculture Designers Manual
CHAPTER 5 – CLIMATIC FACTORS IN PERMACULTURE
Section 5.5 –
Radiation in Permaculture
Incoming global radiation has two components:
DIRECT SOLAR RADIATION penetrating the atmosphere from the sun, and DIFFUSE SKY RADIATION.
The latter is a significant component at high latitudes (38° or more) where it may be up to 30% of the total incoming energy.
Near the poles, such diffuse radiation approaches 100% of energy. We have reliable measures only of direct solar radiation, as few stations measure the diffuse radiation which occurs whenever we have cloud, fog or overcast skies.
Light and heat are measured in WAVELENGTHS, each set of which have specific properties.
We need to understand the basics of such radiation to design homes, space heaters, and plant systems; to choose sites for settlement; and to select plant species for sites.
Table 5.2 helps to explain the effects of differing wavelengths.
A minor component of terrestrial radiation at the earth’s surface is emitted as heat from the cooling of the earth itself.
The greater part of the energy that affects us in everyday life is that of radiation incoming from the sun.
Of the incoming or short-wave radiation (taken to be 100% at the outer boundary of the atmosphere):
50% never reaches the earth directly but is scattered in the gases, dust, and clouds of the atmosphere itself.
Of this 50%:
– Half is reflected back into spare from the upper layers of cloud and dust.
– Half converts (by absorption) into long-wave or heat wavelengths, within the dust and clouds that act as a sort of insulation blanket for earth.
– 50 % reaches the earth as direct radiation, mostly falling on the oceans.
Of this 50%:
– 6% is again lost as reflection to spare.
– 94% is absorbed by the sea, earth, and lower atmosphere and milled as heat or converted to growth.
Of the outgoing, or terrestrial, radiation (absorbed solar radiation and earth heat, including the added heat released by biological and industrial processes and condensation), the heat that drives atmospheric circulation:
67% is re-radiated to spare, and lost as heat. In the atmosphere, therefore, most heat is from this re-radiated heat derived from the surface of the earth.
29% is released from condensing water as sensible heat.
Ozone in the upper atmosphere absorbs much of the ultraviolet light, which is damaging to life forms.
Carbon dioxide, now 3-4% of the atmosphere, is expected to rise to 6%, and cause a 3°C (5.5°f) heating of the earth by the year 2060.
This process appears to be already taking effect on world climate as a warming trend, and will cause sea level changes.
The effect of radiation on plants is different for various wavelengths, as in Table 5.3.
Other sources of light for the earth are the moon (by reflection of sunlight) and star light.
Although weak, these sources do affect plant growth, and even fairly low levels of artificial light affects animal and plant breeding.
The major effects: of radiation overall are
PHOTOSYNTHESIS in plants, the basis of all life on earth.
TEMPERATURE effects on living and inorganic substances much used in-house design.
FLOWERING or GERMINATION effects in plants, of basic Importance to the spread of specific plant groups; this includes the day-length effect.
Plants actively adjust to light levels by a variety of strategies to achieve some moderate photosynthetic efficiency.
They may keep the balance between heat and light energies by adopting solar ranges to suit the specific environment (silvery or shiny leaves where heat radiation is high; red leaves where more of the green spectrum is absorbed and less heat needed).
Leaves may turn edge-on when light and heat levels get too high, or greatly enlarge their surface area under a shady canopy.
Trees have larger leaves at the lower layers.
When we look at any object, we see it by receiving the wavelengths of the light it REFLECTS or screens out.
Thus, many plants reflect green/blue wavelengths, while flowers reflect a wider spectrum of light, becoming conspicuous in the landscape.
About 10% of light penetrates or is transmitted by foliage, although the canopy of rainforests in very humid areas (tropical or temperate) may permit only 0.01% of light to pass through to the forest floor.
Absorbed light, as heat, is re-radiated or used in growth.
In addition to leaf color, plants have bark surfaces ranging from almost white to almost black, the latter good absorbers and heat radiators, the former good reflectors.
Leaf surfaces may vary from hard and shiny to soft, rough and hairy.
Typically, waxy leaf surfaces are found in coastal or cold areas, and in some understory plants, while woolly leaf surfaces are found in deserts and at high altitudes.
The waxiness often gives a greaterr reflection of light regardless or color, while dark or rough surfaces absorb light, so that dark evergreen trees become good radiators of heat.
All of these factors (color, reflection, heat radiation) are of as much use in conscious design as they are in nature and can be built into gardens or fields as aids to microclimatic enhancement.
ALBEDO AND ABSORPTION
The albedo (the reflected light value) of plants and natural surfaces determines how they behave with respect to incoming radiation.
The light reflected goes back into the atmosphere or is absorbed by nearby surfaces and by structures such as greenhouses.
The light absorbed is converted into long-wave radiation, and is re-emitted as heat (Figure 5.10).
Soils and similar dense materials normally absorb heat from daytime radiation to a depth of 50 cm (120 inches) or so.
As this takes time, the build-up of soil heat lags a few hours behind the hourly temperatures. Re-radiation also takes time, so that such absorbing surfaces lose heat slowly, lagging behind air temperatures.
Thus we have our lowest soil temperatures just after dawn. The radiation loss at night produces frost in conditions of still air (in hollows, on flats, and in large clearings of 9-30 m (100 feet) across or more in forests.
Some frost (ADVECTION frost) flows as cold air downhill slopes and valleys to pool in flat areas. Frost forms rapidly on high plateaus.
Dense autumn fogs often indicate the extent of winter frosts, and are clearly seen from high vantage points.
As designers, we use water surfaces, reflectors and specific vegetation assemblies for forest edges.
Table 5.4 gives an indication of the value of diffuse reflectors, as albedo.
A perfect reflector refuses 100% of light (mirrors); a perfect absorber is a BLACK BODY that absorbs all light and converts it to heat.
The fate of incoming waves encountering an object or substance is either:
REFLECTED: turned away almost unchanged, as light off a flat mirror or off a white wall.
REFRACTED: sharply bent or curved, as is light in water, images in curved glass, or sea waves around a headland.
ABSORBED: soaked in, as when a black object soaks up light. This changes the wavelength (light to heat or short to long wavelength). All absorbed light is emitted as heat.
TRANSMIITED: passed through the object.
Different substances pass on or are “transparent” to, different wavelengths due to their molecular structure.
Thus it is by our choice of the materials, colors, or shapes of fabricated or natural components that we manipulate the energy on a site.
We can redirect, convert or pass on incoming energy. The subject of radiation ties in with areas of technology as much as with natural systems, and this section will therefore serve for both areas of effect.
The earth itself acts like a “black body“, accepting the short wavelengths from the sun, and emitting after absorption the long wavelengths from the surface and atmosphere. Table 5.2 deals mainly with the short wavelengths, as they are those coming in as light and heat from the sun.
The long wavelengths we experience are those re-radiated to earth from the atmosphere, or emitted by the hot core of earth.
Curiously, snow is also a black body in terms of heat radiation. Black objects such as crows or charcoal can become effective reflectors if their shiny surfaces are adjusted to reflect radiation (a crow is black only at certain angles to incoming light).
HEAT (Longwave radiation)
It is difficult to store heat for long periods in field conditions, although it can be done in insulated water masses or solids such as stone and earth.
There is some heat input every day that the sun shines or diffuse skylight reaches the earth.
The mean temperature of the earth is 5°c (41°F), of the air at or near ground level 14°C (57°F), and of the outer layers of the atmosphere – 50° to 80°C (90° to l44°F). Normally, we lose about 1°C for every 100 m increase in altitude (3°F per 1000 feet).
In most conditions, we experience a reduction in temperature with increasing altitude, but in many valleys, or on plains surrounded by mountains, cool air from the hills or cold air generated by rapid radiation loss from soils creates a condition where layers of dense cooler air are trapped below warmer air. and we have a TEMPERATURE INVERSION.
It is in such conditions that fog, smog, and pollution can build up over cities lying in valleys or plains, where wind effect is slight. Such sites must be carefully analyzed for potential pollutants.
As in the case of precipitation, it is advisable to research temperature extremes for site.
Poultry (and many wild birds) do not survive temperatures greatly in excess of 43°C (109°F), nor do plants survive transplant shock from nursery stock when soil temperatures exceed 36°C (97°F), whether in deserts or in compost piles.
Many plants are frost affected at or below 0°C (324°F), and below this, sustained periods of lower temperatures will eliminate hardier plant species (even if well established).
Thus, the very widespread and sometimes economically disastrous black frosts that affect whole regions should be noted by site designers as much as flood periodicity.
Livelihoods should not depend on broad-scale plantings of frost-susceptible crops in these situations.
CONVECTION LOOPS AND THERMOSIPHONS
For building and garden designs, we should be aware of just how heat is stored and transmitted.
First, we need to distinguish between low-grade heat transmitted by CONVECTION or the passage of air and water over slightly heated surfaces.
It is this effect which operates in valley climates, and which creates valley winds.
Cool air is heavier (denser) than heated air; the same factor holds true for water or liquids, and other gases (and fluid now generally).
Thus, providing heated air or water is contained in pipes or ducts, a closed loop circulation can be set up by applying heat to the lower part of that loop, providing that a least rise or height difference of 40 cm (about 18 inches) is built in to the loop; any greater height is of course also effective in producing a thermosiphon effect (Figure 5.11).
This is the effect used in refrigerators driven by flames or heat sources.
In the atmosphere, columns of heated air over land ascend as an “Overbeck jet” (Figure 4.13), and at the top of this column, condensation and rain may occur as the air is cooled in t he upper atmosphere.
Such convectional rains are responsible for the mosaic of rainfall that patterns the deserts.
Convection loops will not occur in closed rooms where hot air (at 8-l0°C (15 -18°F) higher temperature sits in a quiet or stratified layer below ceilings.
As air is difficult to heat and stores little heat, air convection is not an efficient way to heat building interiors, although it is the main “engine” of atmospheric circulation in the global sense.
Thermosiphons are useful in transferring heat from solar ponds or Oat plate collectors to home radiators or hot water tanks; we should, wherever possible, site these heat collectors 0.5 m 0.6 feet) below the storage or use points so that they are self-regulated thermosiphons.
Heat flows from warmer to colder bodies, and just as warm air transfers heat to cool solid bodies by day, so warm bodies can heat large volumes of air at night.
Bodies that are heated expand, decrease in density, and (where there is freedom to move) heated air or water rises as CONVECTION CURRENTS.
The common heat unit is that needed to raise one gram of water from 14.5°C to 15°C. In terms of incoming radiation, gram calories per square centimeter! (g/cal/m2) are termed LANGLEYS; the sun provides about 2 Langley’s/minute to the outer atmosphere.
The quantity of heat received on earth is greatly affected by:
Latitude and season (the depth of atmosphere);
The angle of slopes (which in turn affects reflection and absorption); and
The amount of ice, water vapor, dust or cloud in the air above.
This means that the Langley’s received at ground level vary widely due to combinations of these factors.
Nevertheless, most homes receive enough sunlight on their sun-facing areas to heat the water and space of the house, if we arrange to capture this heat and store it.
However, even when the sun is directly overhead on a clear day, only 22% of the radiant energy penetrates the atmosphere (1 atmosphere depth).
In polar areas, where the slanting sun at 5° elevation passes obliquely through at a distance of 11 atmospheres, as little as 1% of the incoming energy is received!
Slope has similar profound effects, so that slopes facing towards the poles receive even less energy from radiation.
It follows that sighting houses on sun-facing slopes in the THERMAL BELT is a critical energy-conservation strategy in all but tropical climates, when sighting in shade or in cooling coastal wind streams is preferred.
Sun-facing slopes not only absorb more heat, but drain off cold air at night; they lie low the chilly hilltops, and above the cold night air of valleys and plains (Figure 5.12).
In hill country and mountains, these thermal belts may lie at 1000 – 5000 m. (3,280 – 16,400 feet), and on lower hill slopes at 100 – 200 m (330-650 feet), whereas in hot deserts the frost levels may only reach to 10 – 15 m (33-49 feet) up the slopes of mesas.
Each situation needs specific information, which we can gain from local anecdotes, the observation of existing plants, or trial plantings of frost susceptible species.
Winds travelling from warmer to cooler regions, or the opposite, bring ADVECTED (exotic or out of area) warmth and cold to local regions.
Thus we speak of advection fogs when these come inshore from coasts and advection frosts when cold air flows down mountain slopes to pool in hollows.
The invasion of cool areas by warm advected air causes moisture condensation, which is critical to precipitation in forests, but a nuisance in enclosed buildings.
Thus, we should attempt to bring only dry warm air into wooden houses, or provide ways to direct condensation moisture to the house exterior.
Intermediate grades of heat can be transmitted by CONDUCTION, as when solids are in contact. It is in this way that we heat an entire floor or wall by heating it in one place and this is the basis of the efficiency of the slab-floored house, where the floor is previously insulated from surrounding earth.
In open (un-insulated) systems, conduction effects are local, as heat is fairly rapidly radiated from solids or soil surfaces.
Pipes buried in hot solid masses have heat conducted to their contents, or hot water pipes conduct heat to slab floors in which they are buried; such heating is most efficient in homes.
Intense heat trapped in solids and liquids is RADIATED, which is the effect transmitted across space by the sun.
Radiant heaters affect air temperature very little, but radiation heats other solids and liquids (like our bodies) or dust in the air.
Thus, we can keep very warm even in a draughty or cool room by the use of radiant electric, gas-fire, or wood -heated massive stoves; these are very efficient space heaters.
As radiation crosses space, and is non directional, focused radiation can produce very intense heat locally.
PLANTS AS HEATERS
Most or all Arum lilies, and species such as Philodendron Selluum store fats which are “burnt” to create heat, so that the flowers heat up.
Philodendrons may register 46°C (115°F) when the air is 4°C (39°F), and crocuses heat up to 15°C (27°F) above the ambient air temperature.
The warmth generated is probably used to attract flies and heat-seeking insects to the pollen.
Some plants (skunk cabbage, Symplocarpus Foetidus), however, may use their heat to melt a hole in the spring snow, and so protect the blooms from cold [at 20° – 25°C (36 – 45°F) extra heat] as well as to provide a cozy incubator for the rest of the plant’s growth (New Scientist, 9 May ’85) and to scatter odorous scents that attract pollinating flies.
More amazingly, the shape of the first leaf of this species creates a vortex (from wind) that is contained within the hot leaf and carries pollen down to the un-pollinated lower flowers, thus achieving fertilization, in cold winds, without the presence of insects!
As all these “heaters” may have unpleasant smells, we should use them with caution. Under storey clumps of such species may assist frost-tender, fly-pollinated, or heat-starved plants, just as tall interplant systems may assist general heat requirements for some ground crops.
RADIATION AND GERMINATION OF SEED
The effect of soil temperatures alone on germination of a wide range of vegetable seeds can be profound.
Between 0° – 38°C (32° – 100°F) the time to germinate (in days) can be reduced to one-tenth or one-fourth of that in cold soils by increasing soil temperatures.
At the extremes of this temperature range, however, we find many plants have limiting factors which result in no germination. While almost all vegetable seed will germinate in soils at 15° – 20°C (59° – 68°F), such oddities as celery refuse to germinate above 24°C (74°F), and many cucurbits, beans, and sub tropical’s do not germinate below 10°C (50°F).
Thus, we are really talking about waiting until 10°C is reached, or warming up the soil in greenhouses or with clear or black plastic mulch in the field before planting.
Sometimes just the exposure of bare earth to the sun helps. A simple thermometer inserted 2.5 cm (1 inch) in the soil suffices to measure the soil temperature or a special soil thermometer can be purchased.
For specific crops, we can consult such tabulations as are found in Maynard and Lorenz (Knotts Handbook for Vegetable Growers, 1980, Wiley, N.Y.)
A second effect on germination is light itself, e.g. carrots need a definite quantity of light, and are usually surface-planted to effect this.
We can surface-scatter such seeds, or first soak them overnight and then subject them to a day under a low-wattage light bulb or in the open before planting and covering them lightly (they react to this light only if wetted first).
Larger seeds usually accept burial and germination in the dark, while some weed seed and desert seed will germinate deep-buried. For a few weed species such as wild tobacco, a mere flash of light (as when we turn over a clod of soil) suffices to start germination.
Next we come to cold, and we speak of the STRATIFICATION or VERNALISATION of seeds.
Cold-area seed, and specifically tree and berry seeds from boreal or cold areas, should spend the period from autumn to spring in a refrigerator when taken to warmer climates.
Apple seeds stored in sand or chestnuts in peat sprout in this way, and can be potted out as they shoot. This in fact reproduces the exposure to cold [at about 0° – 5°C (32° – 40°F)] that they normally experience at the litter level in cold forests or marshes.
Wild rice and other “soft” aquatic seeds are stored in open ponds, or under water in an ordinary refrigerator.
Stratification can often be accomplished by keeping such seeds in sand or peat (or water for aquatics) in cold shaded valleys, or under open cool trellis in warm climates.
They can be checked on late in winter and spring for signs of germination.
The opposite of this is heat treatment such as we can give to many tree legume seeds, by heating in an oven at 95°C (200°F) for a 10 – 20 minute period or by pouring very hot (near-boiling) water over them, or by burning them in a light straw fire.
Many older gardeners will also feed seeds to themselves (in sandwiches), or their animals (chicken or cattle), collect the manure, make a slurry of it, and sow such seeds as tomatoes, berries, and tree legumes.
The voyage through the digestive system is a compounded process of acid/alk1a1i, hot/cold, mechanical cracking in teeth or in bird crops and packaging in manure to which a lot of seeds are adapted.
Day length (in fact, night length, but we will take the day side) varies over latitudes, and flowering plants are adapted to bloom and set seed in response to specific day lengths and the change of seasons.
While many plants are DAY NEUTRAL and will flower if other factors are satisfactory, some will not flower at all in shorter or longer day-lengths than those to which they are adapted.
This can be put to use, as when we transfer a tropical (short-day) corn to a temperate (long-day) hot -summer climate, and get a good green-leaf crop as fodder, or take tobacco from temperate to cool areas and get leaf rather than seed production.
The same goes for some decorative foliage plants. But this effect is in fact the reason for choosing varieties from local growers or selecting for flowering in new introductions so that a local seed source is available for all those crops we want in seed.
In New Guinea highlands (short days), cabbages from long-day climates may never flower, and some Brassicas reach 1 – 3 m in height, the leaves being plucked off at regular intervals for vegetable fodder and the plant cut down only when too tall to reach!
Latitudes have specific day lengths as follows:
LOW LATITUDES (0° – 30°): Usually tropical climates, with colder mountain climates; equal or near equal days and nights.
MID LATITUDES (30 – 50°): Cool to temperate climates with boreal mountain regions; long summer days and short winter days.
HIGH LATITUDES (>50°): Very long summer days and probably good radiation from diffuse light all the growing season. No plants grow in winter.
Frost is caused by radiation loss (rapid cooling) of the earth on clear nights, in still air.
To reduce frost on any site (or in a small pit), it is necessary to have a steep-sided clearing or pit so that radiation is restricted to a small area of the sky.
In such clearings, we have two effects:
Radiant heat from the vertical edges plus the obscuring of the horizon (hence less radiant heat loss at night).
The proportion of heat loss on a cold night is proportional to the area of the night sky that is visible to the object losing heat.
For example, a mouse in a cardboard tube in the ground loses very little heat, but a mouse on a mound on a flat sight is exposed to the whole sky and loses a great deal of heat.
The second factor is that the pit or clearing should be small; large clearings will create or contain more frost.
The rule is to make the clearing (or pit) about one-half as wide as high and to keep the sides trimmed to vertical.
In forests, such clearings should not exceed 30m across (Figure 5.13).
It is necessary, therefore, to try to build up a complete crown cover to prevent frost on a site, and this is best done in stages.
For example, we could plant the whole area to frost-tolerant legume like silver wattle (Acacia dealbata), and then plant semi-hardy fruits in the shelter of these, eventually culling back the Acacia as the frost-sensitive, protected trees gain height.
It is obviously necessary to assist this process by supplying water to the selected trees, and this may also help ameliorate the frost effect on nights of high risk.
The effect of trees on soil moisture and frost may be profound at edges and in small clearings, as the tree crowns obviously create their own water distribution on the ground.
Crown drip can direct in excess of 100% of rain to a “gutter” on the ground, and for some tree species with down-sweeping limbs and leaves, this is a profound effect.
At the rain-shadow edges of forests, dry areas are to be expected. What makes this effect more pronounced is that the “wet” edges are, more often than not, also away from the sun (most rain comes from the polar side of sites). See Figure 5.14.
The sunny edges of the forests help protect seedlings from frost, and these and small clearings are used to rear small trees or to plant them out in frosty areas.
Some implications for designers are as follows:
IN COMMUNITY AND PLANT HEALTH: Areas of severe direct or diffuse radiation and especially where the atmosphere is thin (on mountains), where albedo is high (in snow, granite, or white sand areas, or over hayfields in summer) can produce severe radiation burns, skin cancers (very common in Australia ), and temporary or long-term blindness.
Plants, too, must be screened against sunburn by partial shade and by white paint on their stems in conditions of severe radiation (especially young plants). Older plants may suffer bark damage, but will survive.
FOR AUTOMATIC TRANSFER thermo siphon effects are best achieved by:
– Placing heat sources below storage and use points;
– Inducing cross-ventilation by building solar chimneys to draw in cool air.
– Actively fanning heated air to under floor gravel storages where solar attics or trapped ceiling heat is the heat source;
– Eliminating heat-induced condensation through the use of heat exchangers.
IN HEAT STOVES: Massive earth, brick, stone, or concrete heat storage masses must be insulated to retain heat that is otherwise lost by conduction to the ground, or by radiation to the exterior of houses.
Conduction is prevented by solid foam or air-trap insulation (straw).
Radiation loss is prevented by reflection from double-glazed windows or reflective Insulation hanging in air spaces.
Reflective insulation doesn’t work if it is dusty, dirty, or pressed against a conducting surface, hence it is of most use as free-hanging sheets, or ceiling sheets looped loosely across rafters. It can be kept clean (and effective) only in such situations as solar attics.
Plain white paint is an excellent reflector for everyday use on walls or in concentrators.
PLANT CHOICE: All plants with high biomass (e.g. trees) store heat in their mass (which is mainly water).
Thus, fairly small clearings may be frost-free in cold climates.
Dark evergreen trees absorb (and radiate) heat effectively; white-barked, shiny, or light-colored trees reflect heat in cool districts, on forest edges, and where light itself is a limiting factor.
WATER AND STONE are good heat storages, having a high specific heat.
Thus, bodies of water are good heat storages.
Air has a low specific heat and is a very poor conductor of heat, hence a good insulator. Many insulation systems work simply by trapping air or by being poor conductors (cork, sawdust, wood).
These short examples and some of the tabulated material, give the essential features of radiation that are applicable to everyday design.
A preliminary design choice is to choose house sites for the maximization of solar radiation in subtropical to cool climates and to shelter from radiation where excessive heat is a problem.
Excess heat in one area of a house can be used in arid and tropical areas to “fuel” a cross-ventilation system, also essential in the humid tropics for cool dry air intake to the home.
Designers should always be aware of opportunities to convert light to heat, to reflect more heat on to cool areas, to light dark areas by reflection or by skylight placement, and to store heat below insulated slab floors.