Weather

The Part 107 weather minimums

• 3 smminimum visibility from the control station
• 500 ftbelow clouds
• 2,000 fthorizontally from clouds
• 900 ft AGLthe lowest ceiling that lets you fly at 400 ft legally

That last number is a derived one: 400 ft of altitude plus 500 ft of cloud clearance. A METAR reporting BKN008 (broken at 800 ft) grounds a 400 ft flight.

Decoding a METAR

A METAR is an hourly observation. Read this one left to right:

KRVN 151953Z 27015G24KT 10SM SCT035 BKN050 24/09 A2998 RMK AO2

Sky cover: SKC or CLR (clear), FEW (1/8 to 2/8), SCT (3/8 to 4/8), BKN (5/8 to 7/8), OVC (8/8). A ceiling is the lowest broken or overcast layer, or the vertical visibility (VV) into fog or precipitation. A layer with CB (cumulonimbus) or TCU (towering cumulus) appended means thunderstorms.

Weather codes go between visibility and clouds: RA rain, SN snow, DZ drizzle, TS thunderstorm, SH showers, FG fog (visibility below 5/8 mile), BR mist, HZ haze, FZ freezing, GR hail. A minus sign is light, no sign is moderate, a plus is heavy: -RA, RA, +RA. SPECI in place of METAR means a special, off-schedule observation because conditions changed significantly.

Decoding a TAF

A Terminal Aerodrome Forecast covers the area within 5 statute miles of an airport, usually for 24 or 30 hours, in METAR-style code with time groups:

TAF KRVN 151720Z 1518/1618 27012KT P6SM SCT040
     FM160200 30006KT P6SM SKC
     TEMPO 1610/1613 4SM BR
     FM161500 22010G18KT P6SM BKN030

• 151720Z issued on the 15th at 1720Z; 1518/1618 valid from the 15th at 1800Z to the 16th at 1800Z.
• FM (from) begins a new period with a complete change: FM160200 means from the 16th at 0200Z.
• TEMPO describes temporary fluctuations lasting less than an hour at a time within the period shown: between 1000Z and 1300Z, expect brief periods of 4 miles in mist.
• BECMG (becoming) is a gradual change over the time range; PROB30 is a 30 percent probability.
• P6SM is visibility greater than 6 statute miles; SKC is sky clear.

Other weather products

• PIREPs (UA, or UUA for urgent) are reports from pilots in flight: the only source of actual turbulence, icing, and cloud-top information.
• SIGMETs warn of weather hazardous to all aircraft: severe turbulence, severe icing, dust or sand storms, volcanic ash. Convective SIGMETs cover thunderstorms. AIRMETs (now issued graphically as G-AIRMETs) cover moderate conditions: Sierra for IFR conditions and mountain obscuration, Tango for turbulence and surface winds over 30 knots, Zulu for icing.
• Winds and temperatures aloft (FB) forecast winds at 3,000 ft and above. Not directly useful at 400 ft, but they show whether winds strengthen with height.
• Graphical Forecasts for Aviation (GFA) on aviationweather.gov replaced the old text area forecasts and are the easiest way to see the big picture.
• Flight Service at 1-800-WX-BRIEF provides a standard briefing, including TFRs.

The atmosphere in six facts

• Standard sea-level conditions: 15 °C (59 °F) and 29.92 inHg (1013.2 hPa).
• Temperature falls about 2 °C per 1,000 ft (the standard lapse rate); pressure falls about 1 inHg per 1,000 ft in the lower atmosphere.
• Warm air is less dense than cold air; humid air is less dense than dry air; low pressure means less dense air. All three reduce aircraft performance.
• The dew point is the temperature at which air becomes saturated. When temperature and dew point meet, you get clouds, fog, or dew. A spread of 2 °C or less with a falling temperature is a fog warning.
• Most weather happens in the troposphere, which extends to roughly 36,000 ft at mid latitudes.
• Uneven heating of the Earth's surface drives everything: pressure differences, wind, convection, and the water cycle.

Stable versus unstable air

A temperature inversion, where temperature increases with height, is the most stable condition of all. It traps haze and smoke, gives smooth air, and can hide a sharp wind shear between calm surface air and moving air above. Inversions form on clear, calm nights as the ground cools.

Clouds and fog

Clouds are grouped by height: low (stratus, stratocumulus, nimbostratus, up to about 6,500 ft), middle (altostratus, altocumulus, 6,500 to 20,000 ft), high (cirrus, cirrostratus, cirrocumulus, above 20,000 ft, made of ice), and vertical development (towering cumulus, cumulonimbus). For a drone pilot the important ones are the low clouds that set your ceiling and the vertically developed ones that mean turbulence, downdrafts, and lightning. Lenticular clouds near mountains mark mountain wave turbulence even though they look stationary.

Fog is a cloud at the surface, and it is the weather most likely to catch you on a calm morning:

• Radiation fog: clear, calm nights; the ground radiates heat away and the air above it cools to the dew point. Common in valleys and fields, burns off after sunrise.
• Advection fog: moist air moves (is advected) over a colder surface such as cold water or snow. Needs wind; common on coasts; can persist for days.
• Upslope fog: moist air cools as it is pushed up terrain.
• Steam fog: cold air over warm water, common over lakes in fall.
• Precipitation-induced (frontal) fog: warm rain falling through cool air ahead of a warm front.

Fronts

A front is the boundary between two air masses. The test wants the contrast between the two common ones:

An occluded front forms when a fast cold front overtakes a warm front and can combine the worst of both. A stationary front stalls and can bring days of the same weather. Squall lines, which are bands of severe thunderstorms, often form well ahead of a fast-moving cold front.

Thunderstorms

Three ingredients: moisture, unstable air, and a lifting action (heating, terrain, or a front). Three stages:

1. Cumulus stage: updrafts build the cloud. Updrafts can exceed 3,000 ft per minute.
2. Mature stage: precipitation reaches the surface, downdrafts begin alongside the updrafts. The most violent stage: heavy rain, hail, lightning, wind shear, microbursts, and gust fronts that race ahead of the storm.
3. Dissipating stage: downdrafts dominate and the storm collapses.

Hazards extend well beyond the cloud: lightning can strike 10 miles from rain, and the outflow gust front can knock a small aircraft down minutes before a storm arrives. Manned pilots are told to stay 20 miles from severe storms; a drone has no business being airborne when a thunderstorm is anywhere near.

Wind and wind shear

Wind flows from high to low pressure, deflected by the Coriolis force. Near the surface, friction slows it and bends it toward low pressure. Wind usually increases with altitude, even within your 400 ft: expect stronger wind at the top of a climb than at the launch point.

• Sea breeze: during the day, land heats faster than water, air over land rises, and wind blows from the water onto the land. At night the land breeze reverses it.
• Mechanical turbulence: wind tumbling over buildings, trees, and terrain creates eddies that extend downwind. Launch upwind of obstacles, not in their wake.
• Convective turbulence: thermals rising from heated surfaces (parking lots, dark fields) on sunny afternoons.
• Wind shear: a sudden change in wind speed or direction over a short distance, horizontal or vertical. It can occur at any altitude and is most dangerous near thunderstorms, fronts, and temperature inversions, where a layer of still air sits under moving air. Low-level wind shear can overpower a small aircraft's ability to hold position.
• Microbursts: intense downdrafts from thunderstorms that spread out at the surface, producing shear in every direction.

Density altitude

Pressure altitude is the altitude in the standard atmosphere that matches the current pressure. Density altitude is pressure altitude corrected for temperature: the altitude at which the air density equals what you have now. Hot, high, humid, and low pressure all raise density altitude, and the aircraft performs as if it were at that higher altitude: less thrust, less lift, slower climb, longer takeoff roll for fixed wings, and shorter endurance for multirotors because the motors draw more current to hover.

Temperature also hits the battery directly. Cold lithium batteries deliver less capacity and sag under load; warm them before flight and plan a larger reserve. Heat accelerates battery aging and can trigger thermal protection on controllers and cameras.

Test tips for weather questions

• Practice decoding until every group in a METAR is automatic: time, wind, visibility, weather, clouds, temperature and dew point, altimeter. Expect at least one question that is pure decoding.
• Convert Zulu time with your own offset (Central: UTC minus 5 in daylight time, minus 6 in standard time).
• Learn the stable/unstable table and the cold/warm front table cold. Most theory questions come from them.
• Density altitude is always about performance getting worse when it goes up.

Quiz this module Next: Loading and performance