6.2 Stability, Clouds, and Moisture

ACS PA.I.C.K3a (stability), PA.I.C.K3d (moisture and precipitation), and PA.I.C.K3f (clouds) are one teaching unit. The same rising parcel that decides whether the air is stable is the parcel that reaches its dew point, forms a cloud, and either spreads into a stratiform sheet or towers into a convective build-up.

Adiabatic cooling is not the standard lapse rate

The standard tropospheric lapse of about 2 °C (3.5 °F) per 1,000 feet is an environmental average — what a thermometer would record if you climbed through a typical ISA column. A parcel of air that is lifted expands into lower pressure and cools even if it does not mix with the surroundings. That parcel process is adiabatic.

PHAK Chapter 12 and Aviation Weather Handbook Chapter 12 give the parcel numbers you must keep separate from ISA:

Cite the range, not a single invented “wet rate.” The moist rate is always less than or equal to the dry rate because condensation releases latent heat. That is why moist air, once lifted to saturation, stays warmer relative to its surroundings longer than dry air does — and why moist air is generally less stable.

Stable, unstable, and conditionally unstable

Stability is the atmosphere’s resistance to vertical motion. Compare the environmental lapse rate (ELR) of the surrounding column with the two parcel rates (Aviation Weather Handbook Chapter 13).

• Absolutely stable: ELR is less than the moist adiabatic rate (including an isothermal layer or an inversion, where temperature increases with height). A lifted parcel is colder and denser than its surroundings and sinks back. Expect smooth air, stratiform clouds or fog, poor visibility, and steady precipitation if the air is moist.
• Absolutely unstable: ELR is greater than the dry adiabatic rate (a superadiabatic layer). Any vertical nudge accelerates. Expect cumuliform clouds, turbulence, good visibility except in showers, and showery precipitation if the air is moist.
• Conditionally unstable: ELR sits between the moist and dry rates. An unsaturated parcel is still stable. If it is lifted to its lifted condensation level (LCL) and then to its level of free convection (LFC), it becomes warmer than the environment and rises on its own buoyancy. That “condition” is the lift plus saturation.

Cool, dry air is typically very stable. Warm, moist air is typically unstable. PHAK’s tropical-summer thunderstorms are the textbook unstable column: surface heating, abundant moisture, and a steep lapse.

A temperature inversion is a stable lid. Surface-based inversions form on clear, cool, relatively still nights when the ground loses heat by terrestrial radiation and chills the air in contact with it. Frontal inversions form when warm air overruns cold air, or cold air undercuts warm air. Beneath a low-level inversion with high humidity, expect smooth air and poor visibility from fog, haze, or low clouds — and wind shear if strong winds ride the top of the inversion.

Dewpoint, spread, humidity, and phase changes

Dewpoint is the temperature to which air must be cooled, at constant pressure, to reach saturation. When temperature equals dewpoint, relative humidity is 100 percent and condensation (or deposition) can begin.

Relative humidity is the actual water vapor compared with the maximum the air can hold at that temperature and pressure. A 50 percent reading means the air holds half of its capacity — not that the sky is “half wet.” PHAK notes that every 20 °F increase in temperature roughly doubles the moisture the air can hold, which is why a warm afternoon can have a large dewpoint spread even with plenty of vapor, and why the same moisture becomes fog after sunset.

Temperature–dewpoint spread is the difference in degrees. A small, decreasing spread (often taught as about 5 °F or less) is a fog and low-cloud watch, not a published VFR-minimum number. Condensation is vapor to liquid (cloud droplets, dew, fog). Evaporation is the reverse. Freezing and melting swap liquid and ice. Sublimation is ice directly to vapor; deposition is vapor directly to ice (frost on a skin that is below freezing). Use the correct name. A frost question is a deposition question even if an older paragraph says “sublimation.”

Air reaches saturation four PHAK ways: warm air moving over a cold surface; mixing of cold and warm air; nocturnal cooling by contact with the ground; and lifting.

Worked cloud-base examples

PHAK Chapter 12: unsaturated air cools at 5.4 °F per 1,000 feet while dewpoint falls about 1 °F per 1,000 feet, so temperature and dewpoint converge at 4.4 °F per 1,000 feet.

Surface temperature 85 °F, dewpoint 71 °F. Spread = 14 °F. 14 ÷ 4.4 = 3.18. Cloud base ≈ 3,180 feet AGL.

In Celsius the Aviation Weather Handbook’s dry-adiabatic 3 °C and dewpoint 0.5 °C rates converge at 2.5 °C per 1,000 feet. Surface 30 °C, dewpoint 20 °C. Spread = 10 °C. 10 ÷ 2.5 = 4. Base ≈ 4,000 feet AGL.

That estimate is the lifting condensation level for a surface-based convective cloud. It is not a METAR ceiling, and it is not valid if a layer aloft is already saturated or if an inversion caps the lift.

Cloud families, convective versus stratiform, ceiling, virga

Name clouds by shape and by height of the base (PHAK Chapter 12).

• Cumulus — heaped or piled; convective.
• Stratus — layered; little vertical motion.
• Cirrus — fibrous, ringlet, ice-crystal; high.
• Nimbus — rain-bearing (nimbostratus, cumulonimbus).
• Prefixes: alto- middle, fracto- broken/ragged, castellanus castle-like towers from a common base, lenticularus lens-shaped mountain-wave clouds.

Height families (bases):

• Low — surface to about 6,500 feet AGL (stratus, stratocumulus, nimbostratus; fog is a surface cloud).
• Middle — about 6,500 to 20,000 feet AGL (altostratus, altocumulus); water, ice, and supercooled droplets — icing is a real threat.
• High — above about 20,000 feet AGL (cirrus, cirrostratus, cirrocumulus); ice crystals, usually little icing or turbulence.
• Vertical development — towering cumulus and cumulonimbus, bases in the low or middle family, tops that can punch the tropopause. CB is the thunderstorm cloud.

Convective (cumuliform) clouds mean unstable lift, showers, and turbulence. Stratiform clouds mean stable lift or widespread cooling, layered skies, and steady precipitation or drizzle. The same mountain slope produces stratus if the upslope air is stable and towering cumulus if it is unstable.

A ceiling, for aviation, is the lowest layer reported as broken or overcast, or the vertical visibility into an obscuration such as fog or haze. PHAK: broken is five-eighths to seven-eighths coverage; overcast is the entire sky covered. Scattered and few are not ceilings. A 3,000-foot scattered layer with 8,000 broken has an 8,000-foot ceiling, not 3,000.

Virga is precipitation that falls from a cloud and evaporates before it reaches the ground. It is a dry-air and microburst clue when it hangs under convective bases, not “free rain you can ignore.” Ice pellets at the surface are a different moisture clue: rain aloft freezing as it falls through a cold layer — a temperature inversion and often freezing rain above.

Scenario: Maya’s 14-degree spread

Maya’s departure ATIS is 82 °F, dewpoint 68 °F, winds light, few clouds. Spread = 14 °F. 14 ÷ 4.4 ≈ 3.2, so she expects a convective base near 3,200 feet AGL if surface heating starts towers. That is not a ceiling until a layer is broken or overcast. If the afternoon sounding is steep (ELR faster than dry adiabatic), those towers can become TCU or CB. If an inversion caps the lift, she may see only flat cumulus or haze trapped underneath. After sunset the same moisture with a shrinking spread is a radiation-fog watch, which belongs with the hazard chapter — but the dewpoint arithmetic started here.