3.3 Clouds, Visibility, and Fog

3.3 Clouds, Visibility, and Fog

Understanding cloud types, visibility conditions, and fog formation is essential for determining whether conditions are safe for drone operations and for meeting Part 107 weather minimums.

Part 107 Weather Minimums

Before discussing weather phenomena, remember the Part 107 requirements:

Cloud Types and Categories

Clouds are classified by altitude and appearance:

High Clouds (above 20,000 ft):

• Cirrus (Ci) — thin, wispy, ice crystal clouds
• Cirrocumulus (Cc) — small, white patches arranged in rows
• Cirrostratus (Cs) — thin, sheet-like layer that can cause a halo around sun/moon

Middle Clouds (6,500–20,000 ft):

• Altostratus (As) — gray/blue layered clouds, often covering the entire sky
• Altocumulus (Ac) — white/gray patches, larger than cirrocumulus

Low Clouds (surface–6,500 ft):

• Stratus (St) — uniform, gray layer — like a blanket over the sky
• Stratocumulus (Sc) — low, lumpy, grayish clouds in patches
• Nimbostratus (Ns) — dark, gray, rain-producing layer cloud

Vertical Development:

• Cumulus (Cu) — puffy, white, fair-weather clouds with flat bottoms
• Towering Cumulus (TCu) — tall cumulus showing significant vertical growth
• Cumulonimbus (Cb) — thunderstorm clouds — the most dangerous cloud type for aviation

Cloud Coverage in METARs

Cloud coverage is reported using standard abbreviations:

Ceiling is defined as the lowest broken (BKN) or overcast (OVC) cloud layer. FEW and SCT layers are not considered a ceiling.

Visibility Conditions

Visibility affects Part 107 operations directly:

Types of Fog

Radiation Fog:

• Forms on clear, calm nights when the ground cools and chills the air above it
• Most common in valleys and low-lying areas
• Usually burns off after sunrise as temperatures rise
• Requires calm or light winds (less than 5 knots)

Advection Fog:

• Warm, moist air moves over a cooler surface
• Common along coastlines and near large bodies of water
• Can occur at any time of day (not just at night)
• Requires some wind to move the warm air over the cool surface
• Thicker and more persistent than radiation fog

Upslope Fog:

• Moist air is forced up a slope (mountain, hill)
• As the air rises, it cools and moisture condenses
• Common in mountainous terrain

Steam Fog:

• Cold, dry air moves over warm water
• Moisture evaporates from the warm water into the cold air
• Common over lakes, rivers, and coastal areas in autumn/early winter
• Usually shallow and localized

Precipitation-Induced Fog (Frontal Fog):

• Warm rain falls through a layer of cooler air near the surface
• Evaporation from the rain saturates the cool air
• Associated with warm fronts

For the Exam: Know the conditions that form radiation fog (clear, calm, cool nights) and advection fog (warm moist air over cool surface). These are the most commonly tested fog types.