FogCast · Fog Formation Guide
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What Causes Morning Fog?

Morning fog isn't random. Five variables determine whether it forms, and knowing them tells you when to set the alarm. LightCast FogCast monitors all five and scores probability for your location.

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The science

Radiation Fog: What Actually Happens Overnight

Most morning fog is radiation fog, named for the radiative cooling process that causes it. On clear nights, the ground surface loses heat upward into the sky. The air immediately above the ground cools as heat leaves it. When that air cools to the dew point, water vapor condenses into tiny suspended droplets — and fog forms. Every variable that accelerates or slows this cooling process affects whether fog appears and how dense it gets.

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Clear Overnight Sky
Cloud cover acts as a blanket, trapping heat near the surface and slowing radiative cooling. Clear skies allow maximum heat loss from the ground, accelerating the temperature drop toward the dew point. A single layer of stratus overnight can prevent fog from forming entirely.
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Dew Point Convergence
Fog forms when air temperature drops to the dew point. The smaller the gap between the two at sunset, the less cooling required overnight. A dew point depression of 4°F or less by the pre-dawn hours means the air is near saturation and fog is likely. This is the signal FogCast monitors most closely.
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Calm Wind
Wind mixes air vertically, distributing heat and preventing the near-surface cooling that causes fog. Winds above 5 mph typically prevent radiation fog from forming or break it up before it develops density. Even a light breeze can shift a marginal fog setup from dense to nothing.
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Terrain: Where Cold Air Pools
Cold air is denser than warm air and flows downhill overnight, collecting in valleys, basins, and river corridors. These low-lying areas reach the dew point faster and hold fog longer than elevated terrain. Shooting from a ridge looking down into a fog-filled valley is one of the most reliable fog photography setups.
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Recent Precipitation
Rain in the prior 24 to 48 hours saturates the soil and raises near-surface humidity. Marginal setups that wouldn't produce fog on a dry week often produce dense fog the night after rainfall. FogCast factors recent precipitation into the probability model alongside the five core variables.
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Why checking the forecast isn't enough

Standard Weather Apps Don't Show Fog Variables

A standard forecast shows temperature, precipitation chance, and maybe humidity. It doesn't show overnight dew point depression trends, wind speed at 3am, or whether the terrain at your specific location drains cold air. FogCast combines all five variables into a single probability score, adds formation and burn-off timing, and updates daily so you can plan shoots around fog windows rather than guessing.

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App Exclusive
FogCast is only available in the iOS app, alongside GoldCast, StarCast, DroneCast, CloudCast, and TriCast. GoldCast and StarCast send push alerts when golden hour or night sky conditions hit your threshold — so one app covers your full photography forecast workflow.
Common Questions
What causes morning fog?
Radiative cooling of the ground surface overnight. On clear, calm nights, the ground loses heat upward, cooling the air above it. When air temperature drops to the dew point, water vapor condenses into fog. FogCast monitors this process and scores probability for any location — available in the LightCast iOS app.
Why is fog thickest just before sunrise?
Overnight cooling reaches its minimum in the pre-dawn hours, when the ground has been losing heat for the longest period without solar input. This is when air temperature converges most closely with the dew point and fog reaches its maximum density before burn-off begins.
Why do valleys get foggier than hilltops?
Cold air drains downhill and pools in low-lying areas. Valleys, basins, and river corridors concentrate cold, moist air and are the first places where temperature reaches the dew point. Elevated terrain often sits above the fog layer entirely.
Does rain help fog form?
Yes. Rain in the prior 24 to 48 hours raises soil moisture and near-surface humidity. Marginal overnight setups often produce dense fog after recent rainfall that they wouldn't produce on a dry week. FogCast factors precipitation history into its probability score.
What is LightCast FogCast?
FogCast scores fog formation probability using dew point depression, humidity, overnight wind, sky coverage, terrain, and recent precipitation for any location. Includes formation and burn-off timing. Exclusive to the LightCast iOS app. $2.99/month after a 7-day free trial.
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Know the night before whether fog will form.

Fog probability score · Formation window · Burn-off timing
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