WEATHER 02 / LEARN · EXPLORE · CHECK

Stability, clouds & fog

Read the shape of the sky and ask what is happening vertically.

Checking your account…

Your learning goals

Distinguish stability from surface wind or visibility.
Connect lifting and cooling to clouds.
Decode cloud names and recognize the ten main cloud types plus useful warning signs.
Recognize fog mechanisms and their operational implications.

Will a lifted parcel keep rising?

Picture a small parcel of air nudged upward. It expands and cools. If it becomes colder and denser than its surroundings, it tends to sink back: stable air. If it stays warmer and less dense, it can continue rising: unstable air. Compare the parcel with the environment, not just one surface temperature.

Unsaturated rising air cools about 3°C per 1,000 feet. Saturated air usually cools more slowly because condensation releases heat; that rate varies. The standard atmosphere’s roughly 2°C per 1,000 feet is a reference, not today’s measured profile.

An inversion means temperature increases with height. It resists convection but can coexist with wind shear or mechanical turbulence. Stable does not mean harmless.

Read the name before memorizing the cloud

Lift air by heating, terrain, convergence, or a front. As it cools toward saturation, droplets can form on condensation nuclei. Dew point is the temperature at which air becomes saturated when cooled at constant pressure without changing its water-vapor content.

Cloud names are a short description of shape, height, and precipitation. Learn the pieces and a long word becomes a useful picture.

Cirrus / cirro-: Latin for a curl or lock of hair. Think wispy strands; cirro- identifies the high-cloud family.
Cumulus / cumulo-: Heap or pile. Think cotton-like lumps with vertical growth.
Stratus / strato-: Spread out or layered. Think a blanket across the sky.
Alto-: From the Latin for high, but in cloud classification it marks the middle-level family.
Nimbus / nimbo-: Rain-bearing cloud. Think precipitation, which can also be snow.

Put it together: cirro + stratus = a high layer. Alto + cumulus = middle-level heaps. Cumulo + nimbus = a towering precipitation cloud: a thunderstorm.

High clouds: wisps, veils, and ripples

Original recognition sketches based on FAA Aviation Weather Handbook, Appendix A: Cloud Types; compare them with National Weather Service cloud photographs. Compare shape, texture and growth; the sketches do not establish a cloud’s actual altitude or clearance.

These height families are a guide, not fixed flight levels. Actual heights vary with latitude and season. Use reported bases and terrain elevation when planning cloud clearance.

Cirrus (Ci)

**Look for:** Thin, feathery streaks with curved wisps.

Curl of hair

Thin, feathery ice-cloud streaks. Watch the trend: spreading and thickening high cloud can signal a changing weather pattern, but cirrus alone does not promise a front.

Cirrostratus (Cs)

**Look for:** A translucent high veil around a visible sun and halo.

High layer

A thin veil that can produce a halo around the sun or moon. Look for lowering or thickening layers as the system evolves.

Cirrocumulus (Cc)

**Look for:** Rows of small white grains with little shading.

High heaps

Small white grains or ripples, sometimes called a mackerel sky. They describe high-level structure, not proof of an approaching thunderstorm.

Middle clouds: sheets and patches

Altostratus (As)

**Look for:** A broad gray sheet with the sun showing dimly through it.

Middle layer

A gray or blue-gray sheet; the sun may look dim, as through frosted glass. Watch for a thickening layer and deteriorating conditions along the route.

Altocumulus (Ac)

**Look for:** Larger rounded patches with shaded undersides and gaps.

Middle heaps

Rounded patches or rolls, often with shading. Turreted altocumulus castellanus—castle-like tops—suggests instability aloft; check the convective forecast rather than assuming storms are certain.

Low clouds and deep rain layers

Stratus (St)

**Look for:** A low uniform gray blanket with a fairly even base.

Layer

A low, fairly uniform gray blanket, sometimes with drizzle. For a private pilot, a low base can squeeze the space available between terrain and clouds.

Stratocumulus (Sc)

**Look for:** A broad layer of rounded rolls with shaded undersides.

Layered heaps

Low rolls or rounded clumps joined into a broad patch or layer. Breaks between clumps do not guarantee a usable route or adequate cloud clearance.

Nimbostratus (Ns)

**Look for:** A thick dark sheet with widespread persistent precipitation.

Precipitation layer

A thick, widespread sheet producing persistent rain or snow. It can extend through several levels with a low base and ragged cloud below. Plan for reduced visibility and obscured terrain; do not force it into one altitude band.

Vertical development: watch the growth

Cumulus (Cu)

**Look for:** Separate shallow puffs with flat bases and rounded tops.

Heap

Separate puffs with flat bases and rounded tops. Cumulus humilis means low or humble: shallow fair-weather-looking puffs. Keep watching whether they stay small or grow.

Towering cumulus (TCU)

**Look for:** A tall cauliflower tower with crisp rounded tops, without an anvil.

A growing heap

Cumulus congestus means piled up: tall cauliflower towers. This is strong convection, not a separate one of the ten main types. Give developing cells room and consider changing the route or departure time.

Cumulonimbus (Cb)

**Look for:** A deep tower with a spreading fibrous anvil, dark base and precipitation.

Precipitation heap

A deep thunderstorm cloud, often with a fibrous top or anvil. Avoid the storm and its outflow; hazards extend beyond the visible cloud and rain.

Extra clues: lenses, pouches, and ragged edges

**Lenticular clouds:** Smooth lens shapes stacked above mountainous terrain.

**Mammatus:** Rounded pouches hanging below a cloud layer.

**Fractus:** Ragged separate cloud fragments below a larger layer.

Lenticular means lens-shaped. Smooth, stacked lenses near mountains can mark standing waves. The cloud may appear stationary while strong wind flows through it; wave and rotor turbulence can extend into clear air.

Mammatus describes hanging pouches, from a word meaning breast or udder. Often seen below an anvil, these are a cloud feature, not a separate main type or proof of a tornado. Assess the parent storm and keep your distance.

Fractus means broken: ragged cloud fragments, often beneath a larger layer or in precipitation. They are a warning that cloud bases and visibility may vary quickly along the route.

For every cloud, ask three cockpit questions: Can I stay clear with terrain margin? Is visibility deteriorating? Is the cloud growing? A cloud name alone cannot establish a safe route or a smooth ride.

A cloud where you need to see the runway

Radiation fog: overnight surface cooling chills moist air, commonly with clear skies and light wind. Sunshine may help disperse it, but not on a guaranteed timetable. Advection fog: moist air moves over a colder surface; it can persist with wind.

Upslope fog: moist air cools as terrain forces it upward. Steam fog: cold air mixes with moisture from warmer water. Precipitation fog: evaporating rain moistens cooler air. Ice fog consists of suspended ice crystals in very cold air.

A small temperature/dew-point spread increases concern when cooling or added moisture is expected. It does not prove fog will form. Check observed trends and the forecast through arrival, not just departure.

FLIGHT SCENARIO

What would change your plan?

A coastal airport has steady onshore wind and low visibility. Inland temperatures are warmer. A passenger says, “The breeze will clear the fog.” What is missing?

NoticeWhat does this situation require?
VerifyWhat evidence is still missing?
DecideWhat keeps an option open?

Compare your reasoning

Wind can feed advection fog by carrying moist air over a cold surface. Check the coastal temperature pattern, observations, and forecast. Do not make departure or diversion plans depend on sunshine or wind clearing the field.

SUMMARY

Stable air resists vertical displacement.

Before moving on, explain the decision in your own words: what would you verify, and what would make you change the plan?

PRACTICE

Flashcards and knowledge check

Six flashcards, then five questions with explanations.

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Sources & lesson notes

FAA Aviation Weather HandbookAtmospheric processes, clouds, fronts, hazards, and aviation weather products.
NOAA Aviation Weather Center: product guideReport formats, forecast coverage, and advisory definitions.
FAA AIM: MeteorologyBriefing, datalink limitations, and weather avoidance guidance.
NOAA / NWS: Cloud classificationThe main cloud families, shapes, and development.
NOAA / NWS: Cloud developmentCloud-name roots and identifying features.

Curriculum framework: the supplied Weather Theory and weather products documents. All scenarios are educational examples, not current weather. Content reviewed September 2026.