An interactive map of where coastal redwoods could grow along the Northern California coast — based on fog and rainfall.
The Wikipedia range map is a static PNG drawn from survey data. It answers: where are redwoods growing today?
This map asks a different question: where could redwoods grow today, given enough fog and rainfall? That's very close to asking where redwoods might have existed in the 1700s, before urban development and logging reshaped the landscape.
Why fog? During the rainless summer, coastal redwoods harvest moisture from fog directly through their needles and drip the rest to the forest floor — supplying up to 40% of their annual water and effectively letting the trees water themselves when no rain is falling. A three-year study in northern California found the canopy itself roughly doubles fog-water input to the ground: about 34% of annual water under redwoods came from fog drip off the trees, versus 17% in nearby areas without them. (See also Save the Redwoods League on the science of redwoods and fog.)
We follow a simple heuristic: a place likely supported redwoods if it is north of San Simeon (~35.6°N), receives at least 20 inches of rain in the wet season, and has fog on at least 80 days of the dry season.
Green pixels on the map are places that meet all three criteria. Red dots are ground-truth redwood locations used to sanity-check the model.
v0 — Nighttime fog detection only. A future version will incorporate daytime fog measurement for a more direct match to the "fog past noon" heuristic.