3.1 Weather Theory and Atmospheric Basics

3.1 Weather Theory and Atmospheric Basics

Weather is one of the most significant factors affecting drone operations. Understanding basic atmospheric principles helps you interpret weather reports, predict hazardous conditions, and make informed go/no-go decisions. Weather questions account for approximately 11-16% of the Part 107 exam.

The Atmosphere

The atmosphere is divided into layers, but drone operations occur in the lowest layer:

Troposphere:

• Extends from the surface to approximately 36,000 feet (varies by latitude and season)
• Contains 75% of the atmosphere's mass and virtually all weather
• Temperature decreases with altitude at the standard lapse rate of 3.5°F (2°C) per 1,000 feet
• This is where all Part 107 operations occur

Standard Atmosphere

The International Standard Atmosphere (ISA) provides baseline reference values:

When actual conditions differ from standard, aircraft performance changes:

• Higher than standard temperature → lower air density → reduced performance
• Lower than standard pressure → lower air density → reduced performance

Air Pressure and Wind

Air flows from high pressure to low pressure — this movement creates wind:

• High pressure (H) — associated with clear skies, stable conditions, descending air
• Low pressure (L) — associated with clouds, precipitation, unstable conditions, ascending air

Coriolis Effect: The Earth's rotation causes wind to curve:

• In the Northern Hemisphere, wind flows clockwise around high pressure and counterclockwise around low pressure
• Wind does not flow straight from H to L — it spirals

Temperature and Moisture

Relative Humidity: The percentage of moisture in the air compared to the maximum it can hold at that temperature:

• 100% relative humidity = air is saturated = potential for fog, clouds, or precipitation
• As temperature drops, relative humidity increases (same amount of moisture, less capacity)

Dew Point: The temperature at which air becomes saturated:

• When the temperature equals the dew point, fog or clouds form
• A small temperature-dew point spread indicates high humidity and potential visibility issues
• Spread of 5°F or less = high moisture content, potential fog

Dew point spread decreases as conditions become more humid. When the spread approaches zero, expect fog, low clouds, or precipitation.

Stability and Instability

Stable Air:

• Resists vertical motion
• Produces stratiform (layered) clouds — stratus, nimbostratus
• Associated with smooth air, poor visibility (haze, fog), steady precipitation
• Temperature decreases slowly with altitude (or a temperature inversion exists)

Unstable Air:

• Encourages vertical motion (convection)
• Produces cumuliform (puffy, towering) clouds — cumulus, cumulonimbus
• Associated with turbulence, good visibility (except in precipitation), gusty winds, thunderstorms
• Temperature decreases rapidly with altitude

Temperature Inversions

A temperature inversion occurs when temperature increases with altitude instead of decreasing:

• Traps pollutants, haze, fog, and smoke below the inversion layer
• Creates very poor visibility near the surface
• Restricts vertical air movement, creating stable conditions
• Common during clear, calm nights when the ground cools rapidly (radiation inversion)
• Can also form when warm air overrides cold air (frontal inversion)

For the Exam: Temperature inversions are associated with poor visibility (haze, fog, smog), stable conditions, and smooth air. They often form on clear, calm nights.