FogCast · Fog Prediction
LightCast

What Is Dew Point and Why Does It Matter for Photography?

Dew point is the most useful single number for predicting fog. LightCast FogCast monitors dew point depression overnight and scores fog formation probability for your location.

Check Fog Probability — iOS App Learn More →

iOS app: $2.99/mo · 7-day free trial · Cancel anytime

The number that matters

Dew Point vs. Humidity: Which One to Watch

Most weather apps show relative humidity. Relative humidity is useful but incomplete: it changes as the air warms and cools even if the actual moisture content stays the same. Dew point is an absolute number that tells you exactly how far the temperature needs to drop before condensation starts. For fog prediction, dew point is the variable that matters.

🌡️
What Dew Point Actually Means
The temperature at which air becomes fully saturated. When overnight cooling brings the air temperature down to the dew point, relative humidity hits 100% and condensation forms. That condensation near the ground surface is radiation fog.
📉
Dew Point Depression: The Key Signal
Dew point depression is the gap between air temperature and dew point. A gap of 20°F means dry air and low fog risk. When the gap closes to 4°F or less overnight, the air is near saturation and radiation fog is likely. FogCast monitors this gap through the 3–6am window.
💧
High Dew Point vs. Low Dew Point
A high dew point (above 60°F) means the air already holds a lot of moisture — fog and condensation form more readily. A low dew point (below 40°F) means dry air. Neither value alone predicts fog: what matters is whether the overnight temperature drop closes the gap to the dew point.
🌫️
Why Standard Weather Apps Miss It
Most apps show current temperature, humidity, and precipitation. They don't surface overnight dew point depression trends or flag the window when temperature and dew point are converging. FogCast is built specifically around this convergence to give photographers an actionable fog probability score.
LightCast FogCast
The full picture

Dew Point Is One of Five Variables FogCast Checks

Dew point depression is the primary fog signal, but it doesn't act alone. Overnight wind speed, sky coverage, recent precipitation, and terrain all influence whether a marginal dew point setup produces dense fog or nothing at all. FogCast combines all five into a single probability score and adds estimated formation and burn-off timing so you know when to set the alarm.

📍
App Exclusive
FogCast is only available in the iOS app. It runs 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 is dew point?
The temperature at which air becomes fully saturated and condensation begins. When overnight cooling brings the air temperature down to the dew point, relative humidity hits 100% and fog, dew, or frost forms depending on conditions. FogCast tracks this in real time for any location — available in the LightCast iOS app.
What dew point causes fog?
Fog forms when the air temperature drops to within 4°F of the dew point, humidity exceeds 90%, overnight wind stays below 5 mph, and skies are clear. The dew point value itself is less important than how close the overnight temperature gets to it.
Is dew point the same as humidity?
No. Relative humidity changes as air warms and cools. Dew point is an absolute value that stays constant unless the moisture content of the air actually changes. Dew point is more useful for fog prediction because it tells you the exact target temperature for condensation.
How does FogCast use dew point?
FogCast monitors dew point depression, overnight wind, sky coverage, recent precipitation, and terrain together to score fog formation probability for the pre-dawn window. A single probability score replaces checking five separate variables manually. Available in the LightCast iOS app.
What is LightCast FogCast?
FogCast scores fog formation probability using dew point depression, humidity, overnight wind, sky coverage, and terrain for any location. Includes formation window and burn-off timing. Exclusive to the LightCast iOS app. $2.99/month after a 7-day free trial.
LightCast
Know when the temperature will hit the dew point.

Dew point depression · Fog probability score · Formation & burn-off timing
7-day outlook · Saved locations · All tools in one app

Download LightCast for iOS
or
Learn More →

$2.99/mo after 7-day free trial · Cancel anytime in App Store