FogCast · Fog Prediction
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How to Predict Morning Fog

Five variables determine whether fog forms. LightCast FogCast checks all of them and scores formation probability for your location, with estimated burn-off timing.

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The variables that matter

What Actually Causes Radiation Fog

Morning fog forms when a specific combination of overnight conditions aligns. None of the variables alone tells the full story — it's the combination that matters. Standard weather apps show you temperature and precipitation. They don't show you dew point depression, overnight humidity trends, or the wind threshold that determines whether fog will actually form.

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Dew Point Depression
The gap between air temperature and dew point. When this gap closes to 4°F or less, the air is saturated enough for fog to form. This is the primary signal FogCast monitors overnight.
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Overnight Wind Speed
Wind under 5 mph is required for radiation fog. Wind mixes the air and prevents the near-surface cooling that allows fog to form. Even a light breeze can break up a fog setup that would otherwise produce dense conditions.
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Relative Humidity
Baseline humidity above 90% is a prerequisite. Rain in the 24–48 hours prior raises soil moisture and near-surface humidity, significantly improving the fog setup even when the forecast looks marginal.
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Clear Overnight Sky
Fog needs radiative cooling: heat escaping the surface into the atmosphere overnight. Cloud cover acts as a blanket and prevents the surface cooling required for fog formation. Clear or mostly clear overnight skies are essential.
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Terrain
Cold air drains into low-lying areas. Valleys, river corridors, and basins concentrate fog more reliably than elevated terrain. FogCast factors terrain type into the probability score for your location.
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Timing the shoot

Formation Window and Burn-Off

Fog typically forms in the hour before sunrise, between 4am and dawn, when overnight cooling reaches its minimum. The most photogenic phase is usually burn-off: the 30–60 minutes after sunrise when fog thins and patches separate but hasn't lifted entirely. FogCast shows both the estimated formation window and burn-off time so you know when to be on location.

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App Exclusive
FogCast is only available in the iOS app. It includes saved locations, a 7-day fog outlook, and runs alongside GoldCast, StarCast, DroneCast, CloudCast, and TriCast. GoldCast and StarCast also send push alerts when golden hour or night sky conditions hit your threshold — so a single app covers your full forecast workflow.
Common Questions
How do you predict morning fog?
Check dew point depression, overnight humidity, wind speed, and sky coverage for the 3–5am window. When all four align, fog probability is high. LightCast FogCast automates this check and gives a single probability score — available in the LightCast iOS app.
What time does morning fog form?
Most radiation fog forms in the hour before sunrise, when overnight cooling reaches its minimum and air temperature converges with dew point. Valley fog can form earlier, around 2–3am, and often persists longer than fog on open terrain.
What time does fog burn off?
Typically 60 to 90 minutes after sunrise, as solar heating warms the surface. Dense valley fog can hold until mid-morning. The thinning, patchy phase just before full burn-off is usually the most photogenic window.
Does recent rain help fog formation?
Yes. Rain in the prior 24–48 hours raises soil moisture and near-surface humidity, which lowers the bar for dew point convergence overnight. Marginal setups that might not produce fog on a dry week often produce dense fog after a rain event.
What is LightCast FogCast?
FogCast scores fog formation probability using dew point depression, humidity, overnight wind, sky coverage, and terrain for any location. It includes estimated formation windows and burn-off timing. Exclusive to the LightCast iOS app. $2.99/month after a 7-day free trial.
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Know when to set the alarm.

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