Stratus Layers
Part of Weather Forecasting
Reading flat, layered clouds to predict overcast conditions, drizzle, steady rain, and fog.
What Stratus Clouds Are
Stratus clouds form when air cools uniformly across a wide area rather than rising in individual columns. Instead of puffy cumulus towers, you get flat sheets and layers. They are the most common clouds on Earth, covering vast areas and persisting for days.
Stratus clouds are not dramatic, but they dominate more of your weather experience than thunderstorms do. Learning to read them means knowing whether today will be grey and damp, when the rain will stop, and whether fog will lift or thicken.
Stratus — The Basic Layer
Appearance: A uniform, grey, featureless sheet covering the sky. It looks like a ceiling. No texture, no breaks, no individual cloud elements. The base is typically below 6,500 feet and often much lower — sometimes just a few hundred feet off the ground.
Precipitation: Light drizzle at most. True stratus is too thin to produce significant rain. The drops are tiny and fall slowly. You may feel moisture on your face without seeing visible rain.
Duration: Hours to days. Stratus often forms overnight when the ground cools and persists until solar heating warms the surface enough to break it up. In maritime climates or during winter, it can persist for weeks.
What it tells you: The air is stable and moist at low levels but lacks energy for convective development. No storms will form. Conditions are safe but dreary. Stratus often lifts into stratocumulus as the day progresses, with breaks appearing.
Stratocumulus — The Lumpy Layer
Appearance: A low layer of grey-white cloud with visible texture — rounded lumps, rolls, or patches with darker crevices between them. Gaps of blue sky may be visible. It looks like stratus that has been given some shape.
Precipitation: Occasionally light rain or drizzle, but usually dry. The individual cloud elements are not thick enough for significant precipitation.
How it differs from stratus: Stratocumulus has visible structure — you can see individual cloud elements. Stratus is featureless. If you can see any pattern, rolls, or lumps, it is stratocumulus.
How it differs from altocumulus: Size of individual elements. Hold your hand at arm’s length. If individual cloud lumps are larger than your fist, it is stratocumulus (low). If they are about thumb-sized, it is altocumulus (mid-level).
What it tells you: Low-level moisture with mild instability — enough to create some vertical motion (the lumps) but not enough for storms. Stratocumulus typically breaks up during the afternoon. If it thickens instead, a front may be approaching.
Nimbostratus — The Rain Machine
Appearance: Thick, dark, featureless layer covering the entire sky. The base is ragged and indistinct, often obscured by falling rain. You cannot see the sun through it. Low, torn cloud fragments (scud) race beneath the main layer.
Precipitation: Continuous moderate rain or snow, lasting hours. This is not showery weather — it is steady, relentless precipitation from a deep cloud layer that may be 10,000-15,000 feet thick.
How it differs from stratus: Darkness and precipitation. Stratus is grey. Nimbostratus is dark grey to nearly black. Stratus produces drizzle at most. Nimbostratus produces real rain.
What it tells you: A major weather system is overhead — typically a warm front or occluded front. The rain will continue for 6-24 hours. There is no quick fix. Settle in, protect your gear, and wait it out.
Transition sequence: You can see nimbostratus coming. Cirrus appears first (24-36 hours out). Cirrostratus follows (12-24 hours). Altostratus thickens (6-12 hours). Altostratus lowers and darkens into nimbostratus (rain begins). This entire sequence gives you up to 36 hours of warning.
Fog — Stratus on the Ground
Fog is simply stratus cloud at ground level. Visibility drops below 1,000 meters. The physics are identical — air has cooled to its dew point and water vapor is condensing — but the practical implications are different because you are inside the cloud.
Radiation Fog
How it forms: Clear nights with light winds allow the ground to radiate heat away, cooling the air in contact with it. When this air reaches the dew point, fog forms. Most common in valleys and low-lying areas where cold air pools.
When: Late night and early morning, peaking near dawn. Requires clear skies (to allow radiative cooling) and light winds (strong wind mixes the air and prevents fog).
Burn-off prediction: Radiation fog lifts when the sun heats the ground enough to warm the air above the dew point. Thin fog (you can see blue sky above) burns off within 1-2 hours of sunrise. Thick fog (no sky visible) may persist until late morning or even noon.
Rule of thumb: If you can see the sun as a bright disc through the fog, it will burn off within 2 hours. If you cannot see the sun at all, expect fog through mid-morning.
Advection Fog
How it forms: Warm, moist air moves over a cold surface (cold ocean current, snow-covered ground, cold lake). The air cools from below, reaching dew point. Common along coastlines.
When: Any time of day or night. Not dependent on solar heating cycle.
Duration: Can persist for days because the cold surface remains, and the warm air keeps flowing over it. Advection fog does not “burn off” like radiation fog — it lifts only when the wind direction changes or the air mass moves away.
Upslope Fog
How it forms: Air forced up a slope (by wind or terrain) cools as it rises. If it reaches the dew point, fog forms on the slope. Common on mountain flanks facing the prevailing wind.
Duration: As long as the wind continues pushing air upslope. Can last days.
Predicting Fog
Conditions that favor fog:
- High humidity (small gap between temperature and dew point)
- Light winds (less than 8 mph for radiation fog)
- Clear skies overnight (for radiation fog)
- Proximity to water bodies
- Valley or basin terrain (cold air pooling)
The dew point spread rule: If the difference between air temperature and dew point is less than 5 degrees F (3 degrees C) and dropping, fog is likely overnight. If the spread is less than 2 degrees F (1 degree C), fog is almost certain.
Fog thickness estimation: Step outside at dawn. If you can see objects 200+ yards away, the fog is thin and will lift quickly. If visibility is under 100 yards, the fog is thick and will persist. If you cannot see 50 yards, expect fog through mid-morning at minimum.
Layer Thickness and Precipitation Type
The thickness of a stratus layer determines what falls from it:
| Layer Thickness | Precipitation | Duration |
|---|---|---|
| Under 3,000 ft | None or light mist | Hours to days |
| 3,000-6,000 ft | Drizzle | Hours |
| 6,000-10,000 ft | Light rain | Hours to half a day |
| Over 10,000 ft | Moderate steady rain | 6-24 hours |
How to estimate thickness without instruments: If you can see any lightening of the cloud where the sun is, the layer is thin (under 3,000 ft). If the sky is uniformly grey with no hint of brighter areas, the layer is thick. If the sky is dark grey with scud clouds racing beneath, you are looking at nimbostratus — deep, thick, and rain-bearing.
When Stratus Becomes Dangerous
Stratus itself rarely kills, but the conditions it creates can:
Hypothermia: Days of cold drizzle under nimbostratus, with temperatures in the 35-50 degrees F range, is the prime hypothermia window. You are wet, cold, and unable to dry out. More people die of hypothermia in these conditions than in blizzards.
Flash flooding: Nimbostratus producing steady rain for 12+ hours saturates the ground. Even moderate rain rates become dangerous when they persist. Watch streams and rivers during extended nimbostratus events.
Navigation: Thick stratus and fog eliminate landmarks. If you must travel in these conditions, use compass bearings and pace counting. Do not rely on visual navigation.