Meteorology for VFR flight

This is the meteorology element — the longest in Task C. The DPE expectation isn't recitation; it's that you can explain why the atmosphere behaves the way it does, recognize what the clouds and pressure trend are signaling, and decide what it means for this flight.

Weather is the variable you don't control. The skill is reading what the atmosphere is telling you — clouds, pressure, temperature, wind — and deciding what it means for this flight.

Atmospheric stability and clouds

Stability is the atmosphere's resistance to vertical motion AC 00-6. Lift a parcel of air; it cools by adiabatic expansion. Compare its temperature to the surrounding air:

• Lifted parcel cooler than surroundings → sinks back → stable
• Lifted parcel warmer than surroundings → keeps rising → unstable

Lapse rates

• Dry adiabatic — 3°C per 1,000 ft (unsaturated parcel)
• Saturated adiabatic — roughly 1.5–2°C per 1,000 ft (latent heat of condensation slows cooling)
• Standard atmosphere — 2°C per 1,000 ft up to the tropopause

Stable vs. unstable signs

• Stable — stratus, fog, drizzle, smooth air, poor visibility, surface inversions
• Unstable — cumulus, showers, bumpy air, good visibility from vertical mixing

Inversions

A temperature inversion (warmer aloft) traps pollutants and moisture below, often produces poor visibility, and caps convection. Smooth above the inversion; reduced visibility and possible IFR below.

Cloud formation

Clouds form when air cools to its dewpoint and water vapor condenses on condensation nuclei. Two basic families:

• Cumuliform — vertical development from unstable air; cumulus → towering Cu → cumulonimbus
• Stratiform — horizontal layers from stable air; stratus, altostratus, cirrostratus, nimbostratus

Cloud-reading signals

• Stratus thickening → stable, smooth, possible IFR
• Cumulus building → unstable, bumps, showers possible
• Towering Cu — strong instability, growing toward thunderstorm
• Cumulonimbus — thunderstorm; avoid by 20 NM
• Cirrus thickening from the west — warm front approaching
• Lenticular — mountain wave; expect turbulence and rotor downwind of the ridge

Fronts and air masses

An air mass takes on the temperature and moisture of its source region AC 00-6:

• cP — Continental Polar — cold, dry; winter Canada
• mP — Maritime Polar — cool, moist; North Pacific and North Atlantic
• cT — Continental Tropical — hot, dry; SW deserts and Mexico
• mT — Maritime Tropical — warm, moist; Gulf of Mexico and tropical Pacific

A front is the boundary between two air masses with different properties.

Cold front

• Cold air pushes warm air aloft sharply
• Narrow band of weather, often thunderstorms
• Fast-moving; clears quickly behind
• Post-frontal: cooler, drier, gusty surface winds

Warm front

• Warm air rides up over retreating cold air
• Broad area of layered clouds
• Steady precipitation, low ceilings
• Freezing rain possible in winter (warm layer aloft, cold surface)
• Slow-moving; deteriorating conditions ahead

Stationary front

• Neither air mass advances
• Prolonged unsettled weather along the boundary

Occluded front

• Cold front overtakes a warm front
• Embedded thunderstorms in stratiform precipitation
• Hardest to fly through visually — the TS hide in the layered cloud

The planning question on any flight crossing a front: when does the front cross my route, and is my flight before, during, or after passage?

Turbulence and wind

Air moves from high pressure to low pressure, deflected by Coriolis (right in the Northern Hemisphere) and modified by friction near the surface.

• Above ~2,000 ft AGL — wind flows roughly parallel to isobars (geostrophic)
• In the friction layer — wind backs counterclockwise and slows; surface wind is typically 30° offset and about half the speed of winds aloft

Local wind effects

• Sea / lake breeze — daytime onshore flow as land heats faster than water
• Land breeze — nighttime offshore flow as land cools faster than water
• Mountain / valley winds — daytime upslope, nighttime drainage
• Mountain wave — standing waves downwind of ridges; rotor below ridgetop, lenticular cloud above

Density altitude

Pressure altitude corrected for non-standard temperature. High density altitude means thin air and degraded performance — longer takeoff roll, reduced climb rate, lower engine output. Critical at high-elevation airports in summer, and also relevant at lowland fields on hot days. Always cross-check performance charts at the actual DA, not just at field elevation.

Turbulence sources

• Mechanical — wind over terrain or structures; worst near the surface and downwind of mountains
• Convective — thermals from surface heating; afternoon over hot terrain
• Frontal — shear at the air-mass boundary; strongest with fast-moving cold fronts
• Clear-air turbulence (CAT) — sharp wind changes between layers; often near the jet stream

Severity (PIREP scale)

• Light — slight, erratic changes
• Moderate — definite changes; positive control; strain against seat belts
• Severe — large, abrupt changes; aircraft momentarily out of control
• Extreme — violent; structural damage possible

When penetrating known turbulence, slow to maneuvering speed (V_A) to limit load factor.

Wind shear

A sudden change in wind direction or speed across a short distance — most dangerous in the critical phases of takeoff and landing. Encounter zones: thunderstorm gust fronts and microbursts (worst), frontal passages, inversions, jet stream / CAT, and terrain-induced shear. Cues include TS or virga in the area, gust fronts on radar, LLWS reports in METARs (the WS group with altitude AGL), and PIREPs. LLWAS and TDWR alerts at major airports.

Defense: anticipation, energy reserve on approach (add airspeed for known shear), and willingness to go around.

Thunderstorms

Three ingredients are required for thunderstorm formation AC 00-6:

1. Moisture
2. Instability
3. A lifting force — front, terrain, convergence, or surface heating

All three required.

Lifecycle stages

• Cumulus — strong updrafts dominate, cell growing vertically, no surface precipitation yet
• Mature — updrafts and downdrafts coexist; heaviest precipitation, lightning, hail, gust front, microburst risk; the most dangerous stage
• Dissipating — downdrafts dominate, weakening cell, lightning continues

Lifecycle is 30–60 minutes per individual cell. Multicell and supercell systems persist much longer.

Types

• Air mass / single-cell — afternoon convection, isolated, summer
• Multicell — clusters or lines, longer-lived than single-cell
• Squall line — line of TS ahead of or along a cold front
• Supercell — single rotating cell with mesocyclone; tornadoes
• Embedded — TS hidden within stratiform precipitation; not visible from outside the cloud

Microbursts

A microburst is a small, intense downdraft — typically less than 2.5 NM in diameter — descending from a thunderstorm or even a virga shower. Hazard sequence on approach:

1. Headwind — lift increases, pilot reduces power
2. Downdraft — sinking rapidly
3. Tailwind on exit — lift decreases; aircraft now low, slow, with reduced power

Total airspeed change is 30–90 knots in seconds. Duration is 5–15 minutes. Visual cues: virga, a dust ring at the surface, a localized rain shaft. Recovery is max power, pitch for best climb, accept airspeed loss. Avoidance is the only real defense.

Avoidance rules

• 20 NM from severe thunderstorms; never under an overhang; never through a line
• Never try to thread visually through a line — embedded TS look identical to harmless rain shafts
• Convective SIGMETs along route trigger deviate / delay / scrub

Icing, fog, and visibility hazards

The atmospheric conditions that take VFR off the table.

Structural icing

Forms when supercooled liquid water droplets strike a surface below 0°C and freeze on contact AC 00-6. Three types:

• Rime — small droplets, freeze on contact, opaque/milky, follows the airfoil shape; common in stratus
• Clear — large droplets spread before freezing, smooth/transparent, much heavier and more aerodynamically destructive; common in cumulus and freezing rain
• Mixed — combination, worst characteristics of both

Effects: weight, drag, lift loss, control degradation, prop ice, blocked pitot/static, AOA increase.

VFR rule: avoid visible moisture when temperatures aloft can be at or below freezing. Exit options if ice forms: descend to warmer air, climb above (rarely possible in a 172), or divert.

Freezing-level products: the aviationweather.gov freezing-level chart, AIRMET Zulu, FB winds and temps aloft, and PIREPs (urgent UUA for severe icing).

Fog

Fog reduces visibility below 5/8 SM; mist (BR) covers 5/8 to 6 SM. Five common types:

• Radiation — clear, calm nights; surface cools by radiation, chilling moist air to its dewpoint; common in valleys; dissipates after sunrise
• Advection — warm, moist air over a cold surface; coasts; persistent; needs wind
• Upslope — moist air lifted up sloping terrain; lee of mountains, Plains east of the Rockies
• Steam — cold air over warm water; lakes in fall and winter
• Precipitation-induced — warm rain falls through cooler air below, evaporates, and saturates it; frontal warm sectors

The single most important METAR indicator: temperature/dewpoint spread within 5°F means fog is imminent.

Frost

Forms when the surface temperature drops below the dewpoint AND below freezing — moisture deposits directly from vapor to ice. A thin layer disrupts the boundary layer over a wing, reducing lift by up to 30% and increasing drag enough to prevent takeoff. Never take off with frost on lifting surfaces. Polish, brush, melt by sun, or de-ice — full removal required.

Precipitation types

Determined by the temperature profile from cloud base to surface:

• Rain (RA) — entire profile above freezing
• Snow (SN) — entire profile below freezing
• Freezing rain (ZR) — warm layer aloft melts snow; freezes on contact at the surface (severe icing hazard)
• Ice pellets (PL) — rain refreezes before reaching the surface; warning sign that ZR is likely above
• Drizzle (DZ) — small droplets from low stratus

Other obstructions to visibility

Beyond fog and precipitation:

• Haze (HZ) — fine particles in stable air; summer; industrial
• Smoke (FU) — wildfires, agricultural, industrial
• Dust (DU) and sand (SA) — strong winds over dry terrain
• Volcanic ash (VA) — rare, extreme; SIGMET coverage

Stable air traps these obstructions; unstable air mixes them out.

Worked examples:

• Approaching cold front. West-southwest surface winds, towering Cu and lowering ceilings to the west, scattered to broken at 4,000 ft with isolated TS embedded in the cumulus line. The Convective SIGMET issues 90 minutes before predicted passage. Options: depart early enough to clear the route before frontal arrival, delay 4–6 hours for the front to cross and post-frontal clearing to set in, or scrub. Post-frontal flight is often the smoothest — cooler, drier, gusty surface winds, generally good visibility behind the front.
• Summer convective afternoon. Warm, humid morning, light surface winds, scattered Cu by 11Z growing taller through midday. By 14–15Z, towering Cu and isolated air-mass TS over hot inland terrain. Coastal sea-breeze convergence triggers additional cells along the shoreline. Decision: route early-morning departure (before 13Z) or late-evening (after the cells dissipate around 21–22Z) — the midday window is the avoidance period, not the launch period.