Fog vs Mist: What's the Difference?
Fog and mist are meteorologically identical phenomena — both are clouds at ground level consisting of tiny water droplets or ice crystals suspended in the air. The only official difference is visibility. Fog reduces horizontal visibility to less than 1,000 metres (1 km). Mist reduces visibility to between 1,000 and 2,000 metres. Same physics, different visibility thresholds.
The distinction between fog and mist is less about physics and more about visibility thresholds. Both are ground-level clouds — visible masses of water droplets or ice crystals suspended in the lower atmosphere, formed when air is cooled below its dew point. The World Meteorological Organization (WMO) defines the boundary at 1,000 metres of horizontal visibility. In everyday language, "mist" suggests something lighter or more romantic; "fog" suggests denser, more dangerous conditions. Meteorologically, this intuition is correct — but the dividing line is a specific number, not a general impression.
Key Differences at a Glance
| Feature | Fog | Mist |
|---|---|---|
| Official definition | Visibility < 1,000 m (1 km) | Visibility 1,000–2,000 m |
| Physical composition | Water droplets or ice crystals | Water droplets or ice crystals (identical) |
| Formation mechanism | Same as mist — radiative, advection, or evaporative cooling | Same as fog |
| Safety impact | Significant — fog warnings issued | Minor — generally considered low impact |
| WMO classification | Reduced visibility phenomenon | Reduced visibility phenomenon (different threshold) |
| Density | Higher droplet concentration (visible difference) | Lower droplet concentration |
| Common types | Radiation fog, advection fog, sea fog | Usually radiation mist, valley mist |
How Fog and Mist Form
Fog and mist form by the same mechanisms — any process that cools air to its dew point (the temperature at which air becomes saturated and water vapour condenses into droplets). Radiation fog forms overnight when the ground cools by emitting infrared radiation, chilling the air above it below the dew point — common in valleys and lowlands on clear, calm nights. Advection fog forms when warm, moist air moves over a cool surface — common on coasts when warm maritime air moves over cold ocean upwellings (San Francisco's famous fog is largely advection fog). Evaporation fog forms when water evaporates from a warm surface into cooler air above it — sea smoke and steam fog are examples. The same processes create both mist and fog — the denser the resulting droplet concentration, the lower the visibility.
The 1,000-Metre Rule
The official WMO boundary of 1,000 metres visibility is not a physical threshold — it is an operational one, chosen because it is the point at which visibility begins to significantly affect road, rail, and air transport safety. Fog warnings, METAR codes at airports (FG for fog), and road safety advisories are triggered at sub-1,000m visibility. Between 1,000m and 2,000m (mist), conditions are recorded but typically no formal warnings are issued for surface transport. Aviation uses a more complex classification with additional thresholds for instrument flight rules. Below 200m visibility is classified as "dense fog" in many national systems.
Types of Fog Worth Knowing
Several distinct fog types have different causes. Radiation fog is the classic overnight fog that "burns off" by mid-morning as the Sun warms the ground — it is shallow (often less than 100m deep) and tends to lie in valleys. Advection fog is often deeper and more persistent, as it depends on a continuous flow of warm moist air over a cool surface. Hill fog is simply cloud that intersects with high ground — it is ordinary cloud base coinciding with terrain. Freezing fog consists of supercooled water droplets that freeze on contact with surfaces, coating everything in white rime ice. Sea fog (haar in Scotland) can persist into the afternoon even after fog has cleared inland.
Frequently Asked Questions
What is the difference between fog and mist?
The only official difference is visibility. Fog reduces horizontal visibility to below 1,000 metres. Mist is the same phenomenon but reduces visibility to between 1,000 and 2,000 metres. Both consist of water droplets or ice crystals suspended in the air at ground level and form by identical physical processes.
Is fog just a cloud at ground level?
Effectively yes. Meteorologically, fog is a cloud (stratus cloud) in contact with the ground. The same water droplets that form cloud at higher altitudes form fog when the cloud base is at or below ground level. The physics is identical — the distinction is height relative to the surface.
Why does fog burn off in the morning?
Radiation fog forms when the ground cools overnight. After sunrise, solar radiation heats the ground, which heats the air above it, raising the temperature above the dew point — the air can now hold the water vapour again, so the droplets evaporate. This typically happens from the ground up, which is why fog often "lifts" rather than simply disappearing.
What is freezing fog?
Freezing fog consists of supercooled liquid water droplets — water that remains liquid below 0°C because it contains few nucleation sites for ice crystal formation. When these droplets contact cold surfaces (car windscreens, power lines, trees), they freeze instantly, depositing rime ice — a coating of white, opaque ice that grows into branching crystals away from the wind direction.
How We Write These Comparisons
SmartAss Facts comparisons are written to be the clearest, most accurate answer to "what is the difference between X and Y?" on the internet. We start from the primary definition — taxonomic, scientific, or linguistic — and work outward to the practical distinctions most people actually need.
Each comparison table row is independently sourced. If a distinction is more nuanced than a table cell allows, the detail appears in the body sections below the table. Last reviewed: 2026-05-25.
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