Weather Theory
How the atmosphere makes VFR weather, and how to read the products you will use every preflight.
The atmosphere & standard conditions
Aviation weather starts with density, pressure, temperature, and moisture. Performance and altimetry depend on the atmosphere you are actually flying in, not the textbook standard day.
International Standard Atmosphere (ISA) at sea level is approximately 15°C and 29.92 inHg (1013.2 mb). Temperature normally decreases with altitude in the troposphere (about 2°C per 1,000 feet).
Exam tip
Density altitude rises when it is hot, high, or humid. Expect a longer takeoff roll and a weaker climb.
Key definitions
- Pressure altitude: altitude when the altimeter is set to 29.92
- Density altitude: pressure altitude corrected for nonstandard temperature
- True altitude: actual height above mean sea level
- Absolute altitude: height above the terrain (AGL)
Stability, clouds, and precipitation
Stable air resists vertical motion (smoother air, layered clouds, poorer visibility possible). Unstable air encourages vertical currents (turbulence, cumulus, showery precipitation, often better visibility between showers).
Ceiling is typically the lowest broken or overcast layer. Know FEW, SCT, BKN, and OVC sky cover abbreviations used in METARs.
Fronts & pressure systems
A front is a boundary between air masses. Cold fronts can bring a narrower band of more intense weather and a wind shift into cooler air. Warm fronts often bring widespread layered clouds and steadier precipitation ahead of the front.
High pressure generally means sinking air and fairer weather; low pressure is associated with rising air and more active weather. In the Northern Hemisphere, winds flow roughly clockwise around highs and counterclockwise around lows.
METAR & TAF basics
METAR is an aviation routine weather report. TAF is a terminal aerodrome forecast. Practice decoding wind, visibility, weather phenomena, sky condition, temperature/dew point, and altimeter setting.
- BKN: broken (5/8–7/8); can define a ceiling
- OVC: overcast; defines a ceiling
- TEMPO / PROB in TAFs modify expected periods
Hazards: thunderstorms, icing, shear, fog
Thunderstorms can produce hail, severe turbulence, wind shear, lightning, and IFR conditions. Do not take off or land in the face of an approaching storm; circumnavigate well clear.
Structural icing needs visible moisture and freezing temperatures at the aircraft surface. Carburetor ice can form even in mild moist air. Wind shear is especially dangerous near the ground.
Know personal minimums for fog and low ceilings, especially at night or at unfamiliar airports.
Practice questions
1. Standard sea level temperature in the ISA is approximately?
Show Answer
15°C (59°F).
2. What sky covers define a ceiling?
Show Answer
The lowest broken (BKN) or overcast (OVC) layer.
3. Why is low-level wind shear dangerous?
Show Answer
It can cause sudden airspeed/performance loss near the ground during takeoff or landing.
Next steps
Reinforce with flashcards and a practice quiz.