Weather Sources, METAR and TAF

Weather Sources, METAR and TAF

Chapter Overview

This chapter covers the primary aviation weather products used by remote pilots for preflight planning and in-flight decision-making under 14 CFR Part 107. The focus is on reading and interpreting routine and special surface observations, primarily METARs and SPECIs, and terminal forecasts, primarily TAFs. Remote pilots must understand how to decode wind, visibility, weather, sky condition, and forecast change groups, and must know how to apply that information to small unmanned aircraft system (sUAS) operating limits such as visual line of sight, cloud clearance, and safe wind conditions.

Weather is a leading cause of sUAS accidents and unplanned landings. Part 107 requires the remote pilot in command (PIC) to become familiar with all available weather information before flight. This chapter explains that obligation and provides the technical knowledge needed to interpret official weather products.

The Preflight Weather Requirement

Before every Part 107 operation, the remote PIC must complete a preflight familiarization. The applicable regulation is 14 CFR § 107.49, which requires the remote PIC to become familiar with relevant weather reports, forecasts, and other weather information before flight.

A complete weather check should include:

Remote pilots should not rely on a single weather source. Current observations and forecasts should be cross-checked with actual conditions at the operating site before and during flight.

METAR and SPECI

Purpose of METARs

A METAR is an aviation routine weather report. METARs are issued hourly, normally within a few minutes before the observation time. They provide a snapshot of current surface weather conditions at a specific airport or observing station.

METARs are reported in Zulu time, also called Coordinated Universal Time or UTC. Remote pilots must be able to convert Zulu time to local time when planning operations.

A METAR may be several hours old by the time it is used. Local weather can change quickly, especially near terrain, fronts, or convective activity. Best practice is to obtain the most current official weather product possible and validate that report with actual conditions at the flight site before and during the operation.

SPECI

A SPECI is a special METAR issued when a significant weather change occurs between routine hourly observations. Significant changes can include:

A SPECI is a signal that conditions are changing. Remote pilots should treat a recent SPECI as a warning to re-evaluate the go/no-go decision.

Decoding a METAR

A typical METAR includes the station identifier, observation time, wind, visibility, present weather, sky condition, temperature/dewpoint, altimeter setting, and remarks.

Example METAR:

KXYZ 101752Z AUTO 24012KT 10SM CLR 27/17 A3006 RMK AO2

Decoded:

Wind Group

The wind group is expressed as a three-digit direction followed by a two- or three-digit speed, with the suffix KT for knots.

Remote pilots must plan for wind speed and direction because wind affects sUAS endurance, battery life, stability, launch, and recovery. A forecast or reported wind shift can significantly change aircraft performance.

Visibility and Present Weather

In United States METARs, visibility is reported in statute miles. A value such as 10SM means 10 statute miles visibility. A value such as 1/2SM means one-half statute mile visibility.

Common weather codes in METARs and TAFs include:

VCTS means a thunderstorm is located within 5 to 10 statute miles of the airport or station. Even when the thunderstorm is not directly over the field, convective activity can produce gusty, turbulent winds and lightning hazards that exceed safe sUAS operating capabilities.

VCSH means showers are in the vicinity, typically within 10 miles but not over the station. Showers in the area can still affect a planned route by reducing visibility or producing shifting winds.

NSW means no significant weather. It is commonly used in a TAF after a previous weather group ends. It indicates that precipitation or other significant weather is not expected during that later period.

Sky Condition

The sky condition group describes cloud cover using abbreviations and height in hundreds of feet above ground level.

For example, OVC005 means an overcast ceiling at 500 feet AGL. BKN003 OVC008 means a broken layer at 300 feet and an overcast layer at 800 feet.

A low ceiling is critical for sUAS operations because Part 107 requires the aircraft to remain clear of clouds and within visual line of sight. A ceiling at 500 feet AGL may leave only limited usable airspace below the clouds and increases the risk of losing sight of the aircraft.

Temperature, Dewpoint, and Altimeter

Temperature and dewpoint are reported in degrees Celsius. A prefix M indicates a negative value. For example, 12/11 means temperature 12°C and dewpoint 11°C.

The altimeter setting is reported in inches of mercury, such as A2992 or A3006. The altimeter setting is used to calibrate pressure altimeters; for sUAS operations, it is still useful as an indicator of changing pressure systems.

AUTO Identifier

The AUTO identifier means the METAR was produced by an automated observing system. Automated systems provide valid weather reports but may not detect or report all significant weather elements. Some automated stations cannot observe deep convective clouds, fog, or thunderstorms in all directions.

Remote pilots may use an automated METAR for preflight planning, but they should be aware of its limitations and cross-check with other sources such as TAFs, SIGMETs, and actual site conditions.

TAF

Purpose and Format

A TAF is a Terminal Aerodrome Forecast. TAFs are issued for specific airports by forecast offices, typically every 6 hours, and cover a 24-hour or 30-hour period. They provide expected wind, visibility, significant weather, and sky condition for the airport area.

A remote pilot should use a TAF to determine whether conditions are forecast to remain within Part 107 operating limits for the planned flight time.

Wind in TAFs

TAF wind groups use the same format as METARs. For example:

Winds are a primary planning factor for UAS operations. High winds or gusts can exceed aircraft limitations, reduce endurance, and make launch or recovery hazardous.

Visibility in TAFs

TAF visibility may be reported in statute miles or meters, depending on the station and forecast format.

When visibility is reported in meters, it often appears as a four-digit group with no decimal. For example, 3000 means 3,000 meters, approximately 1.8 statute miles.

Forecast Change Groups

TAFs use change groups to describe when and how conditions will change during the forecast period.

FM — From

FM indicates a rapid or immediate change to the listed conditions at the specified time. The new conditions fully replace previous conditions.

Example:

FM010000 32015G25KT 4SM -RA BR OVC008

This means that at 0100 UTC, conditions become wind from 320° at 15 knots gusting to 25 knots, 4 statute miles visibility in light rain and mist, and an overcast ceiling at 800 feet. If a flight occurs after that time, the pilot must plan for these new conditions.

BECMG — Becoming

BECMG indicates a gradual change to a new set of conditions over a specified time period. The change is expected to occur at a fairly constant rate and reach the listed values by the end of the period.

Example:

BECMG 1814/1816 02008KT

This means the wind is expected to become from 020° at 8 knots between 1400 UTC and 1600 UTC. A flight during the BECMG period may encounter changing conditions, not just the old or the new values.

Another example:

BECMG 1500/1700 32010KT P6SM

This means winds are expected to become from 320° at 10 knots and visibility to improve to greater than 6 statute miles between 1500 UTC and 1700 UTC. A flight at 1530 UTC should anticipate the beginning of this trend.

TEMPO — Temporary

TEMPO indicates a temporary fluctuation from the prevailing forecast conditions. TEMPO conditions are expected to last less than one hour at a time and may occur at any time during the stated period.

Example:

TEMPO 1718/1722 1/2SM FG

This means that between 1800 UTC and 2200 UTC, temporary periods of one-half statute mile visibility in fog are expected. The fog will not be continuous, but it is a real possibility during the entire window.

Remote pilots should treat a TEMPO group as a meaningful risk. Even if the prevailing forecast is good, temporary low visibility or low ceilings can make a sUAS operation unsafe.

PROB30 and PROB40

PROB30 or PROB40 indicates the probability of a forecast condition occurring during a specified period.

Example:

PROB30 2212/2214 1/2SM TSRA

This means there is a 30 percent probability of one-half statute mile visibility in thunderstorms with rain between 2200 and 2214 UTC. It is not a certainty, but it is a significant risk that must be included in the go/no-go decision.

Even a relatively low probability of thunderstorms should trigger contingency planning if the operation falls within the forecast window.

NSW in TAFs

NSW stands for no significant weather. It is used when precipitation or other significant weather previously forecast is no longer expected. For example, after a rain period ends, the TAF may show NSW in the following time period. NSW does not always mean favorable flying conditions, because low ceilings or high winds may still exist.

Other Weather Sources

PIREPs

A PIREP is a pilot report of actual weather encountered in flight. PIREPs provide real-time descriptions of:

PIREPs are especially valuable when no automated weather station is near the planned operating site. Remote pilots may use PIREPs for preflight planning, although Part 107 remote pilots do not file PIREPs.

SIGMETs and AIRMETs

SIGMETs are advisories warning of significant or severe weather that affects all aircraft. SIGMETs may include:

A SIGMET may be in effect even if the nearest METAR shows no precipitation at a given moment. SIGMETs describe broader hazardous weather patterns that may affect an area. Remote pilots must factor SIGMETs into the go/no-go decision.

AIRMETs are advisories for less severe conditions that may still be hazardous to smaller aircraft, such as moderate turbulence, moderate icing, or widespread visibility restrictions. Remote pilots should consider AIRMETs as part of their preflight weather assessment.

AWOS and ASOS

AWOS and ASOS are automated surface observation systems that provide weather information by broadcast or telephone.

These broadcasts may include wind, temperature, dewpoint, and altimeter setting. Depending on the system type, they may not include sky condition or visibility. AWOS and ASOS data should supplement, not replace, a complete METAR or other official weather product.

Flight Service Briefings

The FAA Flight Service provides official weather briefings by telephone at 1-800-WX-BRIEF.

Types of briefings include:

A standard briefing gives the remote pilot a structured weather and NOTAM review. Under 14 CFR § 107.49, remote pilots should obtain weather information from official sources before flight.

Part 107 Weather and Operating Limits

Part 107 establishes specific operating limits that interact with weather conditions.

Visual Line of Sight

The remote PIC must maintain visual line of sight with the sUAS at all times during flight. The aircraft must be visible without vision aids, except corrective lenses.

Low visibility, fog, mist, haze, heavy rain, or low clouds can make it impossible to maintain visual line of sight. A METAR reporting 1/2SM FG with BKN003 OVC008 means the remote pilot likely cannot see the aircraft at a safe distance and should postpone the flight.

Cloud Clearance

Part 107 requires sUAS operations to remain:

An overcast ceiling at OVC005, or 500 feet AGL, may leave no usable airspace between the ground and the required 500-foot buffer. A remote pilot cannot operate within clouds, and a low ceiling can force the aircraft too close to clouds or obstruct visual line of sight.

Wind and Aircraft Performance

Manufacturer limitations and safe operating procedures often constrain sUAS wind operations. The remote PIC must assess reported or forecast wind speed and gusts. Wind directly affects:

A forecast wind shift, such as BECMG 2100/2300 30012KT, must be considered in the flight plan and battery estimate.

Preflight Go/No-Go Decision

A remote pilot should:

Common Relationships Between Concepts

METAR vs. TAF

A METAR is a current observation; a TAF is a forecast. METARs answer, “What is the weather doing now?” TAFs answer, “What is the weather expected to do at my planned flight time?” Remote pilots must use both products together.

Current Observations and Forecast Timing

Even if a METAR shows calm winds and clear skies, a TAF may indicate a significant change before the planned flight. Forecast change groups such as FM, BECMG, TEMPO, and PROB describe when and how conditions will change.

A flight scheduled during a BECMG period may encounter changing winds or visibility. A flight scheduled during a TEMPO period must plan for temporary deteriorations. A flight scheduled after an FM time must use the new forecast conditions as the primary planning data.

Vicinity Weather and Local Risk

VCTS and VCSH mean the phenomenon is not directly over the reporting station but is nearby. Thunderstorms in the vicinity can still produce gusty winds and lightning that affect the flight site. Showers in the vicinity can still cause changing visibility and wind conditions along the route.

SIGMETs and Local METARs

A SIGMET can warn of severe weather over a broad area even when the nearest METAR currently shows no precipitation. Remote pilots should not assume that a clear METAR means safe conditions if a SIGMET or convective outlook indicates developing or embedded thunderstorms.

Automated vs. Human Observations

An AUTO METAR is valid but may not report all significant weather. A remote pilot should cross-check automated observations with TAFs, radar, PIREPs, SIGMETs, and an on-site weather assessment before flight.

Low Ceilings and Visibility vs. Part 107 Limits

Low cloud ceilings and poor visibility are directly related to the Part 107 visual line-of-sight and cloud-clearance requirements. Even if the wind is calm, a flight may be unsafe or noncompliant if ceilings or visibility are below Part 107 minimums or if the remote pilot cannot see the aircraft.

Conclusion

Weather interpretation is a core skill for Part 107 remote pilots. The ability to decode METAR wind groups, visibility, ceiling layers, and present weather, and to understand TAF change groups, is required for safe sUAS operations. Remote pilots must combine current observations, terminal forecasts, advisories, PIREPs, and real-world site validation to make an informed go/no-go decision before every flight.