Fog in Alton/St Louis, IL

daily fog outlook & 2016–2026 climatology
No fog signal in NWS guidance for the next 48 hours. August typically brings about 3 hours of dense fog here.forecast cycle 2026-08-16T00:00:00Z · page rebuilt daily · live layers below update hourly

Measured at St Louis Regional Airport (KALN). This station is in forecast verification — percentages publish when it clears the accuracy bar.

Measuring station

stationairportfog hours/yrforecast
KALNSt Louis Regional Airport31 h/yrverifying

Fog is measured where instruments live — airports. Hours/yr = time below CAT I approach minima (visibility under ~½ mile or ceiling under 200 ft), the aviation definition of seriously dense fog.

When Alton/St Louis fogs in

Fog here concentrates in February and December — about 31 hours in a typical year at KALN, usually lasting ~2 h once it forms. Hour-by-hour patterns, live conditions and the forecast strip live on the KALN station page.

Fog season

January: ~3 hJanFebruary: ~4 hFebMarch: ~1 hMarApril: ~1 hAprMay: ~2 hMayJune: ~1 hJunJuly: ~2 hJulAugust: ~3 hAugSeptember: ~2 hSepOctober: ~2 hOctNovember: ~3 hNovDecember: ~5 hDec
Fog season is January, February, May, July, August, September, October, November, December, peaking in December; fog is mostly a 2 AM–6 AM phenomenon; typically thinning out by 9 AM; a typical event lasts about 2 hours once it forms local time · 10-yr average

Machine access: data.json · llms.txt · forecast verification. Not for operational use.