The same clear night that looks perfect for flying can turn dangerous within hours as fog forms, temperatures drop, and visual references disappear. Night weather requires more conservative planning, higher alternate minimums, and greater awareness of rapidly changing conditions.
Radiation Fog: The Night Pilot's Enemy
Radiation fog is the most common weather hazard for night VFR pilots. Understanding its formation process helps you predict when it will form and avoid being trapped.
Formation Conditions
- Clear skies: Allow maximum radiational cooling
- Light winds: Less than 5 knots prevents mixing
- High humidity: Dew point close to temperature
- Low-lying terrain: Cool air settles in valleys
- Long nights: More cooling time in fall/winter
Predicting Fog Formation
The temperature-dew point spread is your key indicator:
The 2°C Per Hour Rule
On clear, calm nights, temperature typically drops about 2°C per hour after sunset. Use this to estimate when temperature will reach the dew point. If it will happen during your flight, plan accordingly.
Temperature Inversions
Nighttime cooling often creates temperature inversions where warm air sits above cold air. These inversions have important implications for night flight:
Effects
- • Traps fog, haze, and pollution near surface
- • Reduces visibility in lower layers
- • Smooth air below, possible turbulence at boundary
- • Wind shear at inversion layer
Recognition
- • Temperature increases with altitude climb
- • Haze layer visible from above
- • Smooth air with distinct turbulent layer
- • METAR shows visibility improving with altitude
Visibility Challenges
Reduced Visual References
Night flight dramatically reduces your ability to detect weather visually:
- Clouds are invisible until you enter them
- Haze layers invisible against dark sky
- Fog formation not apparent until visibility drops significantly
- Precipitation difficult to see without landing lights
- Horizon may be indistinct or completely invisible
Critical: The Black Hole Approach
Landing at airports with dark terrain or water between you and the runway creates a dangerous illusion. Without visual ground references, pilots tend to fly lower than intended. Use VASI/PAPI religiously, and consider flying an instrument approach even in VFR conditions.
Lighting Illusions
Narrow Runway
Appears farther away - tendency to fly low approach
Wide Runway
Appears closer - tendency to fly high approach
Upsloping Runway
Appears closer - tendency to fly low approach
Rain on Windscreen
Lights appear farther - tendency to fly low approach
Night Weather Planning
Conservative Minimums
Many experienced pilots use higher personal minimums for night VFR:
| Condition | Day VFR | Night VFR (Recommended) |
|---|---|---|
| Ceiling | 1,000 ft AGL | 3,000 ft AGL |
| Visibility | 3 SM | 5-6 SM |
| T-DP Spread | N/A | > 5°C (and stable/widening) |
| Fuel Reserve | 30 min | 45-60 min |
Pre-Flight Weather Checks
- Check sunset/sunrise times and plan accordingly
- Review T-DP trends at destination and alternates
- Check for forecast fog (FG) or mist (BR) in TAFs
- Review winds aloft for wind shear potential
- Check area forecast for frontal activity
- Identify lighted alternate airports along route
- Check for any TFRs or military night operations
Turbulence Differences at Night
Night flying often offers smoother air than daytime, but hazards still exist:
Good News
- • Thermal turbulence stops after sunset
- • Convective activity decreases
- • Air generally more stable
- • Smoother ride in most conditions
Still Possible
- • Mechanical turbulence (wind over terrain)
- • Mountain wave activity
- • Nocturnal thunderstorms
- • Low-level wind shear
- • Turbulence at inversion layers
Nocturnal Low-Level Jet
A phenomenon where winds at 1,000-3,000 ft AGL increase significantly after sunset, often reaching 30-50 knots while surface winds remain light. This creates:
- Significant wind shear during climb and descent
- Different groundspeed than planned if flying in the jet
- Potential for severe turbulence at jet boundaries
Frequently Asked Questions
Why is fog more common at night?
Radiation fog forms when the ground cools rapidly after sunset, cooling the air above it to its dew point. Clear nights with light winds are ideal for radiation fog formation.
How do I predict if fog will form?
Check the temperature-dew point spread. If the spread is 4°C or less and narrowing, fog formation is likely. Clear skies, light winds, and high humidity increase the risk.
Should I add extra fuel for night flights?
Yes. Experienced pilots typically add 30-45 minutes extra fuel beyond legal requirements. Weather can deteriorate more rapidly at night, and finding suitable diversions is more challenging.
What visual illusions affect night flying?
Common illusions include black hole approach, runway width/slope illusions, featureless terrain disorientation, and autokinesis. Rain on the windscreen can also make lights appear farther away.