2.4 Earth Motion, Regional Weather, Local Winds, and Water Phase Changes

Connect Global Weather to Local Operations

Appendix A requires the Earth's motion, regional and local weather, atmospheric wind patterns, and changes of state. These topics explain why a forecast can vary sharply between a coast, mountain valley, island, and inland plain. A dispatcher uses the physical setting to interpret observations and forecasts, rather than predicting every airport's weather from a single air-mass label.

Rotation, seasons, and unequal heating

The Earth rotates, creating the day/night heating cycle and the Coriolis effect in a rotating reference frame. Earth's axial tilt and orbit create seasonal differences in sun angle and daylight duration. Seasons are not primarily caused by the Earth being closer to the Sun in summer. Unequal heating changes pressure distribution, atmospheric circulation, and the timing of local convection and fog.

The pressure-gradient force accelerates air from higher toward lower pressure. Coriolis deflects motion to the right in the Northern Hemisphere and left in the Southern Hemisphere; it changes direction rather than acting as a source of wind speed. Surface friction slows wind and permits flow across isobars toward lower pressure. Aloft, winds are more nearly parallel to pressure-height contours when forces are approximately balanced, with curvature and acceleration producing additional differences.

Global circulation and regional types

The simplified three-cell circulation model describes tropical Hadley, midlatitude Ferrel, and polar circulation. Trade winds, prevailing midlatitude westerlies, and polar easterlies are useful broad patterns. They are not fixed winds at every flight level or airport. Jet streams, fronts, terrain, and daily weather systems modify the local result.

Maritime environments supply moisture from oceans; continental environments can have larger temperature ranges and less moisture. Polar and tropical describe thermal/source-region characteristics. A maritime tropical air mass moving over a cold surface can become stable and foggy, while cold continental air crossing warmer water can become unstable and produce showers or snow. Track how the surface modifies the air, not just where the air originated.

Coastal and island circulations

In fair conditions, land often warms faster than water during daylight. A sea breeze can bring cooler air inland, changing runway winds and creating convergence that supports afternoon showers. At night, faster land cooling can support a weaker land breeze. Synoptic winds and cloud cover can suppress, displace, or alter these local patterns; a sea-breeze forecast is not a guarantee of a particular runway direction.

Islands combine marine moisture, sea/land-breeze boundaries, terrain, and prevailing trade winds. Windward slopes can be cloudier and wetter than nearby leeward terrain. A destination and an alternate on the same small island may share a tropical rainband or wind hazard, so geographical closeness does not automatically provide independent weather protection. Evaluate forecast winds and terrain/approach exposure at each airport.

Mountain and valley weather

Upslope flow cools through expansion and can form cloud, precipitation, and icing on windward terrain. Descending leeward air can warm and dry, producing a rain shadow or foehn/Chinook conditions. Strong flow across a stable mountain environment can also support waves and rotors. The lack of a visible cloud does not eliminate the hazard.

Daytime heating can create valley breezes and upslope motion. Nighttime cooling can create mountain breezes or drainage winds carrying cold dense air down slopes and into valleys. Pooled cold air supports inversions and fog while nearby higher terrain remains clear. During winter operations, combine these local effects with cold-temperature altitude correction, runway condition, and available approach/weather reporting information.

Plains and dry environments

Broad plains expose fronts and thunderstorm outflows with relatively little terrain obstruction. Moisture convergence, drylines, and low-level jets can support strong convection. Dry inland surfaces can heat rapidly and produce high density altitude, thermal turbulence, blowing dust, and dry microbursts when precipitation evaporates below high-based storms.

A high surface temperature is therefore relevant to both performance and weather. It can lower allowable takeoff weight while also contributing to convective potential. Forecast the actual combination of moisture, instability, and lifting rather than assuming heat alone guarantees a thunderstorm. Strong surface wind may be an outflow or pressure-gradient event without any storm directly over the airport.

Water changes of state and latent heat

Evaporation cools air, supporting a precipitation-cooled downdraft or microburst. Condensation releases latent heat and modifies a saturated parcel's cooling rate. Supercooled liquid can remain liquid below freezing and then freeze on an aircraft. Frost can form by deposition when a surface is sufficiently cold; surface contamination must be addressed under the approved ground-icing program rather than assumed harmless because no rain occurred.

Wind-component exercise

For a hypothetical runway wind 20 knots at a 60-degree angle to the runway, the headwind component is 20 × cos(60°) = 10 knots, and crosswind is 20 × sin(60°) ≈ 17.3 knots. These are vector components, not separate winds to be added back as 27.3 knots. Apply the correct wind reference, gust treatment, and approved aircraft/operation limits before using them for performance.

In a coastal-afternoon scenario, the sea breeze shifts that crosswind while nearby convection produces gusts. Recheck runway suitability, takeoff/landing data, and alternate wind limits. The operational purpose of weather theory is a revised decision using current observations and approved performance, not a claim that the sea breeze or cloud type by itself dictates one universal outcome.

Official References

Source check: 2026-10-12. 2026 FAA Aviation Weather Handbook; Altimeter settings § 91.121. For operational use, apply current issued authorizations and the assigned aircraft’s approved data.