The routes, categories and fields are on the low visibility reference.
WxAlerts sets no thresholds
This product describes; it does not decide. It never says “dangerous”, never
issues an alert, and never hides a category as too minor to show. Every area
carries its category, confidence, cause and sources, and what is worth drawing
or alerting on is your call. A traffic app might alert only on very_low and
worse at medium confidence or higher; a marine app might care about any
marine_fog. Filter on the fields.
Where it comes from
Four independent sources, fused on a 2.5 km grid:
| source | what it adds |
|---|---|
analysis |
NOAA’s RTMA Rapid Update visibility analysis, every 15 minutes, the base layer everywhere |
station |
airport (METAR) and buoy (NDBC) visibility, which pulls the analysis toward the measurement within 15 km |
advisory |
NWS Dense Fog, Marine Dense Fog, Freezing Fog and Dense Smoke Advisories. Inside one, the category is at least very_low and the cause is the advisory’s |
reports |
clusters of anonymized driver reports from partner apps. No single driver is trusted: ten or more independent reporters within 2 km and 30 minutes create an area, marked reported: true. Individual reports are never served by this API |
sources on each area lists the ones that agree with it.