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Weather: Reports, Visibility Minimums, and Flight Hazards
Weather decides whether a legal flight is also a safe one, and the exam expects you to pull that decision out of coded reports rather than a phone app summary. This chapter covers decoding METAR and TAF, the Part 107 visibility and cloud clearance limits, density altitude and wind, and the specific hazards that end drone flights.
Decoding a METAR
A METAR is a routine surface observation issued about once an hour, while a SPECI is an unscheduled report triggered by a significant change. The elements always appear in the same order: station identifier, date and time in Zulu, wind, visibility, present weather, sky condition, temperature and dew point, and altimeter setting. Winds are given as a three-digit true direction and two-digit speed in knots, so 25012G18KT is from 250 degrees at 12 knots gusting 18. Sky condition uses FEW, SCT, BKN, and OVC with heights in hundreds of feet above ground level, and the ceiling is the lowest broken or overcast layer.
Time is always Zulu
A group such as 121755Z means the 12th day of the month at 1755 coordinated universal time, not local time.
Wind direction is true, speed is knots
The first three digits are the direction the wind is coming from in degrees true, the next digits are speed in knots, and G marks the gust value; VRB means variable direction.
Sky cover heights are hundreds of feet AGL
BKN025 is a broken layer at 2,500 feet above ground level, and the lowest BKN or OVC layer defines the ceiling.
Temperature and dew point are in Celsius
A group like 18/12 is 18 degrees Celsius with a dew point of 12, and an M prefix such as M04/M07 means below zero.
Altimeter setting follows the letter A
A2992 means an altimeter setting of 29.92 inches of mercury, which is also the standard sea level pressure.
Visibility is in statute miles
Surface visibility appears in statute miles followed by SM, and abbreviations such as BR for mist, FG for fog, RA for rain, and TS for thunderstorm describe present weather.
Reading a TAF and Other Forecasts
A terminal aerodrome forecast covers the weather expected within a small radius of a specific airport, typically about five statute miles from the center of the runway complex. TAFs are normally issued four times a day and are valid for 24 or 30 hours, which makes them the natural planning tool for a job scheduled later in the week. Change groups tell you how confident the forecaster is: FM marks a rapid and permanent change, BECMG a gradual transition, TEMPO a fluctuation lasting less than an hour at a time, and PROB30 a thirty percent chance. Broader products such as area forecasts, graphical forecasts for aviation, and winds and temperatures aloft fill in the terrain between airports.
A TAF is airport specific
The forecast applies to a small area around the airport, roughly a five statute mile radius, and should not be stretched to a distant job site.
FM means a rapid permanent change
The FM group gives a time after which the previously forecast conditions are replaced entirely.
TEMPO means brief fluctuations
TEMPO conditions are expected to last less than one hour at a time and in total less than half the period covered.
PROB30 is a probability, not a promise
PROB30 indicates a 30 percent chance of the described conditions, usually thunderstorms or low visibility.
Use official aviation weather sources
Base the go or no-go decision on aviation products such as METAR, TAF, and graphical forecasts rather than consumer weather apps.
14 CFR §107.49Visibility and Cloud Clearance Requirements
Part 107 sets weather minimums that are measured from the control station, not from the aircraft, because the rule exists to protect visual line of sight and see-and-avoid. The flight visibility must be at least three statute miles as seen from where the remote pilot stands. Separation from clouds is written as a vertical and a horizontal number, and both must be satisfied at the same time. These minimums are waivable, but only through a certificate of waiver, and they are one of the most heavily tested numbers on the exam.
Three statute miles of flight visibility
Minimum flight visibility from the control station must be no less than 3 statute miles.
14 CFR §107.51500 feet below clouds
Keep the aircraft at least 500 feet below the base of any cloud.
14 CFR §107.512,000 feet horizontally from clouds
Keep the aircraft at least 2,000 feet horizontally from any cloud.
14 CFR §107.51Both distances apply together
Meeting the vertical clearance does not excuse the horizontal clearance; the aircraft must satisfy both at all times.
14 CFR §107.51Weather minimums can be waived
The visibility and cloud clearance limits may be waived only by an FAA certificate of waiver showing equivalent safety.
14 CFR §107.200Density Altitude, Wind, and Stability
Density altitude is pressure altitude corrected for nonstandard temperature, and it is the single number that predicts how well a small aircraft will perform. Hot, high, and humid conditions all reduce air density, so the propellers move less mass, the aircraft climbs slower, and endurance shrinks. Wind matters twice for a drone: it costs battery to hold position against it, and the return leg into a headwind can consume far more energy than the outbound leg. Stability determines the texture of the air, with stable air producing layered clouds, poor visibility, and smooth flight, and unstable air producing towering cumulus, good visibility, and turbulence.
High density altitude degrades performance
High elevation, high temperature, and high humidity raise density altitude, reducing thrust, climb rate, and usable payload.
The standard atmosphere is 15 degrees Celsius and 29.92 inches
Standard sea level conditions are 15 degrees Celsius and 29.92 inches of mercury, with temperature falling about 2 degrees Celsius per 1,000 feet.
Plan battery reserve for the headwind leg
A tailwind outbound becomes a headwind on return, so budget energy for the slower and more demanding trip home.
Obstacles create turbulence and rotors
Wind flowing over buildings, tree lines, and terrain breaks into swirling eddies downwind that can flip or push a light aircraft.
Stable air means stratiform, unstable air means cumuliform
Stable air brings layered clouds, steady precipitation, haze, and smooth conditions, while unstable air brings showers, cumulus buildups, gusty winds, and turbulence.
Temperature and dew point spread predicts moisture
As temperature and dew point converge the relative humidity rises, and a small spread signals fog, mist, or a low cloud base.
Thunderstorms, Microbursts, Fog, and Icing
A thunderstorm needs moisture, an unstable atmosphere, and a lifting action, and it passes through cumulus, mature, and dissipating stages. The mature stage is the most dangerous because updrafts and downdrafts exist side by side, producing heavy rain, hail, lightning, and a gust front that can arrive many miles ahead of the visible cloud. A microburst is a concentrated downdraft usually less than a mile across that can produce dramatic wind shear and lasts only minutes, which makes it especially deadly for a small, light aircraft. Fog and icing are quieter hazards that end flights by stealing visibility or by adding weight and disrupting airflow over propellers.
Thunderstorms need moisture, instability, and lift
All three ingredients must be present, and removing any one prevents the storm from developing.
The mature stage is the most hazardous
Precipitation reaching the surface marks the mature stage, when strong updrafts and downdrafts coexist along with hail, lightning, and severe turbulence.
Gust fronts arrive before the storm
Outflow from a storm can produce a sudden wind shift and strong gusts well ahead of the rain, so do not judge safety by the position of the cloud alone.
Microbursts are short, small, and violent
A microburst is typically less than one mile across and lasts only about 5 to 15 minutes, yet it can produce downdrafts and a headwind to tailwind shear that overwhelm a small unmanned aircraft.
Fog forms when the air cools to its dew point
Radiation fog forms on clear calm nights over land, while advection fog forms when warm moist air moves over a cooler surface and can persist in wind.
Icing requires visible moisture and freezing temperatures
Ice accumulation adds weight and disturbs the airflow over propellers and airframe, reducing lift and control long before the aircraft looks coated.
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