Monday, May 19, 2014

Sun and shade, temperate vs tropical

May has arrived in Hawai'i, and for the next two months, the sun will shine on us from the north. Huh? From the north? Doesn't that only happen in locales south of the equator, like in Australia?

(The rest of this post assumes a northern hemisphere location, expecting the sun to shine from the south.)

I knew the sun travels at a higher angle in the tropics, and I knew that in temperate climate summers the sun rises and sets in the north. But until shortly before moving here, I had no idea that for part of the summer, our sun never even crosses over to the south. This is the case everywhere within the tropics; on the margins, the Tropic of Cancer and the Tropic of Capricorn, the sun is directly overhead at noon on June 21st. Closer and closer to the equator, the sun spends more and more time in the north, with part or much of the summer never even seeing the sun shine from the south. This makes for a bizarre solar chart!

Combined with greater tropical sun intensity and temperatures, this necessitates different strategies for shading structures. Standard temperate tricks for winter sun/summer shade don't directly apply; properly sized and angled eaves, deciduous vines on a south arbor, and a deciduous tree to the southwest don't help when the midday sun is directly overhead or shining from the north. I'm no expert at this for Hawai'i, let alone qualified to generalize to the rest of the tropics, but here are my observations and thoughts so far. Some of this also applies to plant interactions and light into understories.

  • It's not nearly as important to maximize sunlight in the winter. Though it helps combat the mildew and mustiness we encounter in this super-rainy part of Hawaii, we don't need the sun for warmth. It's OK to compromise a bit on winter sun access.
  • We don't need to design for morning sun. Unlike temperate climates, where it can still be chilly on spring, fall, and even summer mornings, we don't need a quick boost of morning sun to warm us up.
  • We don't have a large choice of trees that drop their leaves in the winter, and I have even less knowledge of exactly when they drop and regrow leaves, or whether they stick to a consistent schedule driven by the sun (rather than, for example, rainfall.)
  • We don't have many deciduous vines. The best I've found so far are the yams (Dioscorea spp.), of which the winged yam (D. alata) and air potato (D. bulbifera) seem most useful. They die down to the ground in November or December, and resprout in March or April, growing quickly to potentially cover a large area by early to mid summer. These may work well growing directly on top of the roof, or on a framework trellised over the roof.
  • Rather than a tree to the southwest, we want trees to both the east and west, or the northeast, north, and northwest. Ideally we'd have full tree canopy directly over the structure and maybe a bit to the south. This would provide full winter sun with maximum summer shade. An ideal tree, in order of priority:
    • Has a broad, spreading canopy of known width so we can plant it reasonably far from the structure and not have to prune too much
    • Produces some useful yield of fruit or leaves suitable for zone 1, perhaps harvestable from the roof
    • Doesn't drop limbs or large fruit on the roof (or loud hard nuts on a metal roof)
    • Grows to a useful size quickly, then slows down
    • Doesn't drop much leaf litter, especially if the roof is used for rainwater catchment
    • Lives a long time
    • Does not fix nitrogen, since we'll already provide ample urine to the area

I need to do more research on possible tree candidates, and probably some experimentation to figure out which will grow well on our particular site once we settle somewhere. I'll share those thoughts once I've developed them further!

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