The 3D model on the Sun & Shadow sheet shows the shadow at one instant. It is good for dragging the clock and watching shadows sweep, and useless for "how many hours of sun does this spot get in a day?" The sun-hours map answers that: shadows are solved every ten minutes from sunrise to sunset and added up.
The same computation appears in the diagram set as the SUN HOURS panel. This page explains what is calculated, where the three numbers on the sheet come from, and why the plate is not a shadow study in the regulatory sense.
| Value | Meaning | Origin |
|---|---|---|
| Mean sun hours | Average total daily sunlight on ground inside the plate | — |
| Area with ≥ 2 continuous hours, 09–15 solar | Share of ground that is sunlit for at least two unbroken hours between 9 and 15 true solar time | Korean Building Act Enforcement Decree, art. 86 |
| Area with ≥ 4 total hours, 08–16 solar | Share of ground with at least four hours in total between 8 and 16 true solar time | Korean case law (tolerance threshold) |
Both criteria are Korean, because the tool was built there first: the decree's wording is "two or more continuous hours between 9 and 15 on the winter solstice", and the four-hour total is the courts' alternative test. They are shown on the US edition as reference criteria, not as anything a US jurisdiction requires. There is no federal sunlight standard; where shadow analysis is regulated it is local and framed differently. New York's CEQR Technical Manual, for instance, assesses shadows on sunlight-sensitive resources over a window from 1.5 hours after sunrise to 1.5 hours before sunset on four representative dates, and asks about the duration and extent of new shadow, not a continuous-hours threshold. If your jurisdiction has its own test, the plate will not compute it — but the underlying map of sunlit hours is the same picture that test starts from.
The windows are in true solar time: the sun is due south at 12:00. Sunlight is a physical quantity governed by the sun's position; clock time is administrative and offset from it by longitude and daylight saving. In New York solar noon falls around 11:57 EST, so a 09–15 solar window is about 08:57–14:57 standard time; in Seoul it is 09:30–15:30. The sheet shows the conversion so you know which axis was used.
Solar noon comes from the site longitude and the time-zone value in the EPW file header. Estimating the zone from longitude alone is a trap — it puts Korea at UTC+8.5 and rotates the sun's azimuth by 7° — so the header value is read instead.
The building height map used by the wind and thermal-comfort sheets is reused. No CFD is involved; shadow is pure geometry, so there is no reason to wait for a three-minute GPU job. A 300 m radius takes about 4 s, 500 m about 14 s, 800 m about 28 s.
For the chosen date (solstices, equinoxes or any day) the sun's position is found at ten-minute steps from sunrise to sunset; at each step every ground cell is tested for sun or shade and the results summed. Ten minutes differs from six by 0.01 h.
The first version worked differently. Bake, once per cell, a horizon map by azimuth: how high the skyline is in each direction. Then any date and any hour is free — just test whether the sun is above the horizon in its azimuth. Four to nine seconds to bake, then zero. Attractive.
Integrated over a day, though, it returned 0.15–0.29 more hours of sun than the truth. Each instant was off by only 1–5%, but the errors were all in one direction and accumulated. Doubling the azimuth resolution did not help — the cause was the distance sampling along each ray, which stepped over thin buildings.
This value sits next to a regulatory criterion. A bias in the direction of "it's fine" is not acceptable. We discarded it.
Instead the shadow test was folded into a recurrence: walking away from the sun, carry a running maximum of "highest thing seen so far minus the drop over distance". Nothing can be stepped over, and it is 30% faster. Cost was cut not by lowering precision but by restricting the computation to the plate (dropping the buffer ring outside it, a factor of 5.5).
The thermal-comfort sheet's shadow function had the same flaw and now shares this one.
| Winter solstice | Summer solstice | |
|---|---|---|
| Mean sun hours | 1.8 h | 6.9 h |
| ≥ 2 continuous hours, 09–15 | 36% | 89% |
| ≥ 4 total hours, 08–16 | 15% | 79% |
Most side streets in a dense core get under an hour of winter sun — the dark slate-blue on the plate. Midtown Manhattan reads the same way: side streets nearly sunless in December, the avenues bright.
Colour breaks at 0·1·2·3·4·5·6·8·10·12 hours. Two and four hours are the thresholds above, but they are deliberately not hard edges in the ramp — a hard edge invites reading pass/fail off the colour, which this plate cannot deliver (below).
Buildings are filled grey. In the first version buildings and the outside of the plate were both white and a downtown plate looked empty — the same fix as greying the right-of-way on the plot size panel.
A "plate only" save button drops the title, colour bar and caption. The default save keeps them; if you take the bare plate, add the caption yourself.
What the plate is for: reading, at concept stage, where winter sun reaches around the site and where it never does — where outdoor space will be usable in January, which frontage a ground-floor shop will be dark on, which part of your lot the neighbour's tower shades all day. A regulatory shadow study needs a survey and a validated tool, and should be done as such.