3.1 Weather Theory and Atmospheric Basics

Why Weather Theory Matters

Weather drives nearly every go/no-go decision a Remote Pilot in Command (Remote PIC) makes. On the FAA Part 107 Unmanned Aircraft General (UAG) knowledge test the Weather content area is weighted 11-16% of the 60-question exam per the Airman Certification Standards (FAA-S-ACS-10B) — that is roughly 7-10 questions out of 60, and you need 70% (42 correct) to pass at a PSI testing center. Mastering theory first makes the METAR and TAF decoding later in this chapter far easier.

The Troposphere

All Part 107 flight (max 400 ft AGL, or within 400 ft of a structure) happens in the troposphere, the lowest atmospheric layer.

• Extends from the surface to roughly 36,000 ft (higher at the equator, lower at the poles)
• Holds about 75% of the atmosphere's mass and essentially all weather
• Temperature normally falls at the standard lapse rate of 2 degrees C (3.5 degrees F) per 1,000 ft

The International Standard Atmosphere (ISA)

The ISA is the agreed baseline against which performance is measured.

Worked example: At a 5,000 ft elevation airport, standard temperature is 59 − (5 × 3.5) = 41.5 degrees F. If the actual temperature is 80 degrees F, the air is far warmer (less dense) than standard — a setup for the high density altitude problems in Section 3.6.

Pressure Systems and Wind

Air always flows from high pressure to low pressure; that flow is wind.

• High pressure (H): descending air, clear skies, stable, light winds. A common trap: high pressure plus a calm night is exactly when radiation fog and inversions form, so "high pressure" does NOT guarantee good visibility.
• Low pressure (L): rising air, clouds, precipitation, instability, often gustier winds.

Coriolis effect: Earth's rotation deflects moving air. In the Northern Hemisphere, surface wind spirals clockwise and outward around a high and counterclockwise and inward around a low. Wind never flows straight down the pressure gradient.

Moisture, Humidity, and Dew Point

• Relative humidity is the percent of moisture the air holds versus its maximum capacity at that temperature. Warmer air holds more moisture; as air cools toward evening, relative humidity climbs even though the actual moisture is unchanged.
• Dew point is the temperature to which air must be cooled to reach saturation. When temperature equals dew point, fog, clouds, or dew form.
• The temperature-dew point spread is the single most useful fog predictor.

Exam cue: A METAR of 18/17 (1 degree C spread) screams imminent fog; 24/09 (15 degrees C spread) means dry, stable visibility.

Stability vs. Instability

Stability decides cloud shape, ride quality, and visibility.

Temperature Inversions

An inversion is an altitude band where temperature increases with height instead of decreasing. Inversions cap vertical mixing, so haze, smoke, and moisture pile up beneath them.

• Produce stable air, smooth conditions, and poor surface visibility
• A radiation (surface) inversion forms on clear, calm nights as the ground radiates heat
• A frontal inversion forms when warm air overrides cold air at a front

How These Pieces Connect in Practice

The exam rarely tests one fact in isolation; it bundles several into a scenario, so practice reading the chain. A clear, calm autumn evening lets the ground radiate heat rapidly. The surface cools, an inversion forms, and the air just above the surface chills toward its dew point. If the temperature-dew point spread was already small, you now have the recipe for radiation fog and trapped haze beneath a stable, smooth layer. Recognizing that single weather picture answers questions about visibility, stability, fog type, and turbulence at once.

Contrast that with a hot, humid summer afternoon. Strong surface heating makes the lower air buoyant, the lapse rate steepens, and the air becomes unstable. Now expect cumulus building into towering cumulus, convective turbulence, gusty winds, and good visibility between showers — the opposite of the inversion picture. The Remote PIC who memorizes the two contrasting profiles can reason through almost any stability question.

Common Theory Traps

• "High pressure means good flying." Usually true for clouds, but high pressure on a calm night is precisely when fog and inversions degrade surface visibility. Read the dew point spread, not just the pressure.
• Confusing lapse rate units. The standard rate is 2 degrees C OR 3.5 degrees F per 1,000 ft — those are the same rate, and wrong-unit distractors (1 degree F, 4 degrees C) are common.
• Relative humidity vs. dew point. Relative humidity changes with temperature even when moisture is constant; the dew point is the absolute moisture measure that, compared with temperature, predicts fog.
• Wind direction around pressure systems. In the Northern Hemisphere it is clockwise around a high and counterclockwise around a low — reversing these is a frequent miss.

For the exam: Tie the cluster together — clear and calm night + small dew point spread + inversion = haze/fog and stable, smooth air, NOT thunderstorms.