Fog-IndexFog-Index

Updates

What's new with Fog-Index — algorithm improvements, new features, and under-the-hood changes to help you chase fog with more confidence.

Feature

April 2026 updates

A collection of algorithm and product improvements shipped in April 2026.

Fog Seasonality Arrives For Saved Locations

Saved locations can now switch from Forecast to Seasonality to see when a place usually gets foggy across the year. It is built for long-range planning: comparing locations, spotting peak months, and timing trips before the short-range forecast window even begins.

  • Saved locations now include a new `Forecast | Seasonality` mode with month-by-month fog tendency built from a rolling 10-year historical archive.
  • Seasonality uses stacked Likely + Possible views so you can compare places quickly, then drill into a single location to inspect monthly and weekly timing.
  • The historical layer is point-based and shared across matching saved spots, which keeps common viewpoints fast and reusable as the feature grows.
  • Fog Seasonality is a planning tool, not a guarantee. Use it to choose when to go, then rely on the live forecast to decide whether the upcoming window is worth the alarm.

Spring Wet-Ground Memory + Better Sheltered Valley Calls

Fog-Index now does a better job remembering when the ground is still wet, even after the rain stops. That helps spring valleys, soaked meadows, and post-storm mornings show up more realistically in your forecasts.

  • Saved-location forecasts now keep more wet-ground memory when recent rain or saturated soil suggests the landscape is still feeding morning fog.
  • This especially helps spring mornings in valleys and lowlands where snowmelt, flooded meadows, or lingering soil moisture can support fog without fresh overnight rain.
  • We also softened a common false negative on saved locations in sheltered valleys, so some gusty clearing-storm setups are less likely to be dismissed too aggressively.
  • Forecast setup, manual refresh, previews, the free location check, and the Fog Potential Map now share the same upgraded wet-ground logic for more consistent reads.
Improvement

March 2026 updates

A collection of algorithm and product improvements shipped in March 2026.

Alerts UX Refresh: Faster Scanning, Smarter Defaults

The Alerts dashboard now surfaces the most important information sooner, with clearer hierarchy and defaults tuned for quick morning decisions.

  • A new dynamic alerts header now highlights horizon summary and best upcoming window at the top of the page.
  • Sorting now defaults to Top score, so your strongest opportunities rise to the top automatically.
  • Alert counts are now surfaced more clearly to improve at-a-glance awareness across your forecast horizon.

Fog Density + Clearer Planning Context

Fog-Index now separates surface-level fog density from above-fog planning, making saved-location forecasts easier to read whether you want to shoot in the soup or climb above it.

  • New Fog Density labels describe what the air should feel like at your saved spot, from subtle mist to white-room risk.
  • Fog Height remains the elevation-planning cue, helping you judge whether your spot is likely above the layer or inside it.
  • Planning context now more clearly separates surface atmosphere, above-fog decisions, and light above the fog.
  • The map now supports a Fog Density view for surface-opacity scouting, while Fog Potential remains the default layer.

Transport Pathway Upgrade: Upslope Mode + Fallback Clarity

Fog-Index now treats Pathway 2 as a broader transport engine for marine, inland advection, and ridge upslope setups. We also replaced ambiguous fallback blanks with clear diagnostics so inland advection windows are easier to trust.

  • A conservative upslope branch now activates for ridge locations when fallback transport is moist and wind-supported, instead of always penalizing ridge terrain.
  • When upslope mode is active, ridge and inland fallback penalties are bypassed so valid mountain-lift events are less likely to be suppressed.
  • Advection fallback rows on cards and table now show diagnostics (Cooling Source, Climatology, Inland Penalty) instead of all `--` placeholders.
  • Fog-type labels now clearly separate `Marine Fog`, `Advection Fog`, and `Upslope Fog` based on the active reason codes.

Calmer-Night Memory + Clearer Fog-Type Signals

Fog-Index now remembers overnight wind conditions for radiation-fog setups instead of judging only the final pre-sunrise hour. We also improved how score-driver metrics are shown so each forecast window better explains whether radiation or marine transport is driving the call.

  • Radiation scoring now checks the overnight prep window, so a brief calm snap right before sunrise is less likely to overstate fog potential after a windy night.
  • Gentle, steady overnight winds are now treated as a positive setup signal for stable boundary-layer development in radiation-fog scenarios.
  • Saved-location cards and table views now show pathway-aware score drivers, helping you quickly interpret inland cooling setups versus coastal transport setups.
  • When marine context is unavailable, pathway-specific fields stay neutral and surfaces gracefully show placeholders instead of forcing a misleading value.

Saved-Location Planning Context: Cloud Layers + Fog Layer Estimates

Saved-location forecasts now include planning context for photographers once a morning looks promising: cloud-layer breakdowns for light quality and conservative fog-layer estimates to help with vantage-point decisions.

  • Your saved locations now show low, mid, and high cloud layers plus a Sky Above read, so you can judge whether the light above the fog looks open, filtered, or blocked.
  • We added a fog layer estimate with an above-fog verdict for saved locations, using the location's elevation to help answer whether your spot is likely above the deck, in it, or still uncertain.
  • These planning cues appear on saved-location forecasts and related alert surfaces. They are not a new global fog-height map layer.
  • Coverage follows the saved-location forecast pipeline, so it can work outside the US anywhere Open-Meteo supports the forecast. US-only limits still apply to NWS advisory confirmation, and the separate Fog Potential Map remains CONUS.
Algorithm

February 2026 updates

A collection of algorithm and product improvements shipped in February 2026.

Global Coastal Upgrade + Better Rainy-Front Fog Detection

Fog-Index now scores marine fog using ocean temperature physics instead of fixed regional presets, which improves consistency across coastlines worldwide. We also added a dedicated rainy-front fog check to better catch saturation events during light-rain setups.

  • Coastal advection scoring now uses air-vs-sea temperature contrast with onshore wind geometry, so behavior is more portable across different countries and coast orientations.
  • A new frontal fog gate can raise fog potential when recent rain and rapid moisture increase point to precipitation-driven fog formation.
  • The same upgraded model path now powers location setup, manual refresh, scheduled alerts, daily digest, and the Fog Potential Map for strict parity.
  • If marine context is temporarily unavailable, the model falls back gracefully instead of switching to a separate algorithm family.

Terrain Hint Recommendations + Easier Override

Adding a location now includes terrain analysis from coordinates, with safer rules that recommend a terrain only when signals are strong enough. General remains the default when terrain is mixed or unclear.

  • New location setup now analyzes terrain in the background and may recommend hints like coastal, river, valley, ridge/highland, or urban.
  • Terrain recommendations stay conservative by design: if confidence is low or mixed, FogIndex keeps General instead of forcing a terrain override.
  • Fog Seasonality now resolves better historical terrain context for weak saved hints, which improves coastal and river climatology at many locations.
  • Urban handling is now stronger in city cores to reduce false river or valley calls from mixed metro terrain.
  • Existing saved locations can be re-checked from Edit Alert with the Re-detect action, and manual selections still win.

Valley Inversion Support: Better Deep Winter Fog Calls

Fog-Index can now detect the classic winter inversion setup where warmer air sits above a cold valley floor. This helps the model hold onto persistent valley fog later into the morning instead of burning it off too aggressively.

  • The model now checks vertical temperature layers, not just surface cooling. If air above is warmer than the surface, that's a strong trapped-fog signal.
  • When conditions match a true winter stagnation setup (valley-like terrain, very light wind, strong inversion), fog persistence is scored more conservatively after sunrise.
  • Pressure-fall penalties are softened during stagnation events, because those pressure changes don't always clear deep valley inversions right away.
  • Best results come when your terrain hint is accurate — especially valley, river, or lowland locations where cold air pooling is common.

Marine Fog Support: The Coastal Blanket

Fog-Index now understands marine fog — the thick, wind-driven coastal layers that define Pacific Northwest summers and San Francisco year-round. The algorithm runs two scoring pathways in parallel and automatically picks the one that best fits conditions at your location.

  • A brand-new advection fog scoring pathway runs alongside the existing radiation fog model. The algorithm evaluates both and keeps the higher score, so you always get the most accurate prediction for your spot.
  • The old model penalized wind — which is exactly wrong for coastal fog, where moderate onshore breezes push marine air inland. The new pathway flips this: 5-12 mph winds from the ocean are now a strong positive signal.
  • Wind direction matters. The model now checks whether wind is blowing from the ocean toward land (onshore flow), a key ingredient for marine fog formation along coastlines.
  • Your location cards and email alerts now tell you which type of fog to expect — "Marine Fog" or "Valley Fog" — so you know what kind of conditions to prepare for.
  • The fog potential map also scores marine fog. Coastal grid cells can now light up during summer marine layer events that the old map would have missed entirely.
  • Summer is no longer a dead zone. Marine fog peaks June through September, exactly when radiation fog is at its weakest. This fills a major seasonal gap in the model.

Smarter Fog Predictions: Dew Detection & Seasonal Dissipation

Two physics-based improvements make the fog score more accurate on very calm nights and throughout the morning hours across all seasons.

  • The algorithm now distinguishes between dew and fog. Very calm, wet nights are more likely to produce dew on the grass than fog in the valley — scores are adjusted accordingly.
  • Fog "burn-off" predictions are now season-aware. Winter morning fog lingers longer because the sun is low and weak, while summer fog dissipates faster under strong solar heating.
  • In previous versions, a single daytime penalty was applied year-round. The new system uses a graduated multiplier that varies by both season and hours after sunrise.
  • These changes were informed by an independent review of the algorithm against established radiation fog physics.
Feature

Interactive Fog Potential Map

A brand-new map view lets you scout fog potential across the entire continental United States at a glance — no saved locations required.

  • An interactive heatmap overlay on top of Google Maps shows fog likelihood for every sunrise window across the continental US.
  • Color-coded grid cells make it easy to spot promising valleys, river corridors, and coastlines: green for likely fog, amber for possible, and faint for unlikely.
  • Toggle between three forecast days to plan your shoots ahead of time — see fog potential for today, tomorrow, and the day after.
  • The map uses the same scoring algorithm as your saved locations, updated every 6 hours across ~23,000 grid cells covering the continental US.
  • Available for Pro subscribers.
Launch

Fog-Index Launches

After months of development and dozens of algorithm iterations, Fog-Index is live — purpose-built for landscape photographers who chase fog.

  • A proprietary 0-100 fog likelihood score that goes far beyond simple humidity checks, accounting for dewpoint depression, wind regimes, cloud timing, terrain, pressure trends, and more.
  • 7-day hourly forecasts with sunrise-window focus — the score highlights the exact hours fog is most likely to form and persist.
  • Multi-source weather data combining Open-Meteo, Tomorrow.io, and official NWS/NOAA fog advisories for higher confidence predictions.
  • Email alerts and daily digest — get notified when conditions look promising at your saved locations without having to check every morning.
  • Terrain-aware scoring with support for valley, river, coastal, lowland, and ridge terrain types that influence how fog forms and pools.
  • The algorithm has been through five major versions (v1.0 through v1.5) before this public launch, each refined against real-world fog events in the Pacific Northwest.