Seoul went deep on Stay Cool the week of this filing — 15,768 buildings in the central wards modelled block by block, the first APAC city to clear the queue. Bukchon’s hanok eaves, filed in N° 57, were the obvious editorial entry: a thousand-year-old vocabulary of giwa tile and timber, doing its original job. This dispatch is the other half of the answer. South of the river, the shadow that matters isn’t carved by an eave. It’s thrown by a 35-story apartment tower drawn in 1976.
The Gangnam grid is the largest piece of planned 20th-century urbanism in East Asia. From the early 1970s the South Korean government cleared the rice paddies south of the Han, drew a rectilinear avenue grid (Teheran-ro, Gangnam-daero, Olympic-daero), and rezoned the whole peninsula for high-rise apartment blocks — danji (단지) in the Korean term, "complexes" of 8 to 30 nearly-identical towers arranged in long parallel rows. Hyundai, Samsung, and LG built the first generations; the apartments inside them became the unit of South Korean middle-class wealth. Forty years on, the danji block is the texture of the south bank: 25 to 35 stories of cream-and-glass repetition, set on enormous slab podiums, separated by parking-lot moats. At solar noon in July the towers throw shadow stripes 80 to 120 meters long across the avenues between them. The lee side of any north–south block is the cool side; the sun side is genuinely worse than the temperature reading.
Koreans have been doing this math out loud since the 1980s. Apartment listings still publish the orientation — hyangsu (향수), literally "the way it faces" — and a south-facing unit (남향, namhyang) is premium because of winter sun. But every south-facing unit creates a north-facing one on the other side of the corridor, and that north-facing unit, in summer, gets the shadow advantage. Realtors talk around it; tenants on the north side mention it quietly. Our router doesn’t care which side of the building you live on. It does know which side of the street is in the tower’s lee at 14:00 on a 32°C August afternoon, and routes accordingly. In Gangnam more than anywhere else we’ve modelled, the gap between the sun side and the lee side is the whole walk.
A practical note on the picks below. They assume late-July to mid-August, the bad weeks — Seoul summers are now reliably 31 to 34°C with dew points in the low 70s°F, and the monsoon front (장마, jangma) is a separate question that needs its own dispatch. The Hangang Park access at the end is the only pick where the shadow isn’t architectural — the river itself does the cooling, and the parks under the bridges (Banpo, Hannam, Cheongdam) are the city’s honest summer answer. We route the lee side until the building geometry gives up, then we send you to the water.