3: Weather

Introduction

Wind

Air Masses and Fronts

Atmospheric Stability

Visibility and Clouds

Thunderstorms

Icing

Fog

Density Altitude

Weather Briefing

Weather Reports, Forecasts and Charts

Introduction

As with any flight, the remote PIC should check and consider the weather conditions prior to and during every sUAS flight. Even though sUAS operations are often conducted at very low altitudes, weather factors can greatly influence performance and safety of flight. Specifically, factors that affect sUAS performance and risk management include:

The major source of all weather is the sun. Every physical process of weather, change, or variation of weather patterns is accompanied by or is a result of unequal heating of the Earth’s surface. The heating of the Earth (and therefore the heating of the air surrounding the Earth) is imbalanced around the entire planet. Both north and south of the equator, due to the different angle sunlight hits the Earth, one square foot of sunrays is not concentrated over one square foot of the surface, but over a larger area. This lower concentration of sunrays produces less radiation of heat over a given surface area; therefore, less atmospheric heating takes place in that area. The unequal heating of the Earth’s atmosphere creates a large air-cell circulation pattern (wind) because the warmer air has a tendency to rise (associated with low pressure systems) and the colder air has a tendency to settle or descend (associated with high pressure systems) and replace the rising warmer air. This unequal heating, which causes pressure variations, will also cause variations in barometric altimeter settings between weather reporting points.

Different surfaces radiate heat in varying amounts. The resulting uneven heating of the air creates small areas of local circulation called convective currents. Convective currents can cause turbulent air that has the potential to dramatically affect the remote PIC’s ability to control unmanned aircraft at lower altitudes. For example:

Wind

Wind and currents can affect sUAS performance and maneuverability during all phases of flight. Be vigilant when operating sUAS at low altitudes, in confined areas, near buildings or other manmade structures, and near natural obstructions (such as mountains, bluffs, or canyons). Consider the following effects of wind on performance:

Local conditions, geological features, and other anomalies can change the wind direction and speed close to the Earth’s surface. For example, when operating close to a building, winds blowing against the building could cause strong updrafts that can result in ballooning or a loss of positive control. On the other hand, winds blowing over the building from the opposite side can cause significant downdrafts that can have a dramatic sinking effect on the unmanned aircraft that may exceed its climb performance.

The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary velocity of the wind. This same condition is even more noticeable when flying in mountainous regions. While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar manner. As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become. Due to the effect terrain has on the wind in valleys or canyons, downdrafts can be severe. Even small hills or odd shaped terrain can have similar effects on local wind conditions. Remote pilots should be aware that terrain/object wind effects may exist for some distance downwind of the actual terrain or object.

Air Masses and Fronts

When a body of air comes to rest on, or moves slowly over, an extensive area having fairly uniform properties of temperature and moisture, the air takes on these properties. The area over which the air mass acquires its identifying distribution of temperature and moisture is its source region. As this air mass moves from its source region, it tends to take on the properties of the new underlying surface. The trend toward change is called air mass modification. When an air mass that is different in such properties advances upon a dissimilar air mass, the division line is referred to as a front.

A ridge is an elongated area of high pressure. A trough is an elongated area of low pressure. All fronts lie in troughs. A cold front is the leading edge of an advancing cold air mass. Cold fronts are often accompanied by poor weather ahead of the front, which passes relatively quickly. Once the front has passed, there is a wind shift and, due to the increased wind speeds, turbulence is common for a period of time. More severe cold fronts can also produce thunderstorms, hail, and tornadoes.

A warm front is the leading edge of an advancing warm air mass. Warm fronts move about half as fast as cold fronts and have more widespread impact on weather. They are often preceded by lowered ceilings, increased precipitation, and reduced visibilities. Remote PICs should be aware of ambient and approaching weather systems as they can significantly impact their operations and safety of flight. Frontal waves and cyclones (areas of low pressure) usually form on slower-moving cold fronts or stationary fronts. These types of systems are often accompanied by conditions that may be unfavorable to sUAS operations. Figure 3-1 shows the symbols that would appear on a weather map.

Figure 3-1. Weather map symbols

The physical manifestations of a warm or cold front can be different with each front. They vary with the speed of the air mass on the move and the degree of stability of the air mass being overtaken. A stable air mass forced aloft will continue to exhibit stable characteristics, such as stratus clouds, calm air, steady precipitation, and poor visibility, while an unstable air mass forced to ascend will continue to be characterized by cumulus clouds, turbulence, showery precipitation, and good visibility.

Frontal passage will be indicated by the following discontinuities:

  1. A temperature change (the most easily recognizable discontinuity);
  2. A continuous decrease in pressure followed by an increase as the front passes; and
  3. A shift in the wind direction, speed, or both.

Atmospheric Stability

Atmospheric stability is defined as the resistance of the atmosphere to vertical motion. A stable atmosphere resists any upward or downward movement. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical (convective) current.

Determining the stability of the atmosphere requires measuring the difference between the actual existing (ambient) temperature lapse rate of a given parcel of air and the dry adiabatic (3°C per 1,000 feet) lapse rate. Because sUAS operate at low altitudes, it may seem as though lapse rate may not be a factor, but the stability of the local air mass can have significant impact on ambient conditions. Unstable air can often result in weather conditions unfavorable to sUAS operations.

A stable layer of air would be associated with a temperature inversion (a condition in which warm air is situated above cool or cold air). Warming from below, on the other hand, would decrease the stability of an air mass. The conditions shown in Figure 3-2 can be characteristic of stable or unstable air masses.

Figure 3-2. Characteristics of stable and unstable air masses

Visibility and Clouds

As in manned aircraft operations, good visibility and safe distance from clouds enhances the remote PIC’s ability to see and avoid other aircraft. Similarly, good visibility and cloud clearance may be the only means for other aircraft to see and avoid the unmanned aircraft. Prior to flight, the remote PIC must determine that visibility from the control station is at least 3 SM and that the sUAS is kept at least 500 feet below a cloud and at least 2,000 feet horizontally from a cloud. These standards must be maintained throughout flight operations.

One of the ways to ensure adherence to the minimum visibility and cloud clearance requirements is to obtain local aviation weather reports that include current and forecast weather conditions. If there is more than one local aviation reporting station near the operating area, the remote PIC should choose the closest one that is also the most representative of the terrain surrounding the operating area. If local aviation weather reports are not available, then the remote PIC may not operate the sUA if he or she is unable to determine the required visibility and cloud clearances by other reliable means.

The remote pilot can determine local visibility by verifying that a known point at least 3 SM away is visible. Similarly, an object with a known height can be observed to have clouds above that object, providing information about how much operational altitude is available for use. For example, you know a nearby tower extends to 1,000 feet AGL (referencing a Sectional Chart). The clouds are above the top. Thus you know that even if you climb to the maximum altitude of 400 feet AGL, you have more than the required 500 feet of separation from the clouds. When in doubt, always choose the most conservative and safest option, which sometimes may be to delay the operation. It is imperative that the sUA not be operated above any cloud, and that there are no obstructions to visibility, such as smoke or a cloud, between the sUA and the remote PIC.

Thunderstorms

Thunderstorms present many hazards to flying. Three conditions necessary for the formation of a thunderstorm are:

  1. Sufficient water vapor;
  2. An unstable lapse rate; and
  3. An initial lifting force (upward boost).

The lifting action can be caused by heating from below, frontal lifting, or by mechanical lifting (wind blowing air upslope on a mountain). There are three stages of a thunderstorm: the cumulus, mature, and dissipating stages. See Figure 3-3.

Figure 3-3. Stages of thunderstorms

The cumulus stage is characterized by continuous updrafts, and these updrafts create low-pressure areas. Thunderstorms reach their greatest intensity during the mature stage, which is characterized by updrafts and downdrafts inside the cloud. Precipitation inside the cloud aids in the development of these downdrafts, and the start of rain from the base of the cloud signals the beginning of the mature stage. The precipitation that evaporates before it reaches the ground is called virga. Virga is typically associated with very turbulent conditions and should be avoided. The dissipating stage of a thunderstorm is characterized predominantly by downdrafts.

Lightning is always associated with a thunderstorm. The frequency of lightning is a good indicator of the severity of the storm. Upon observing frequent nearby lightning, remote PICs should recover their crew and sUAS in order to seek cover. One way to determine the distance a storm is from your location is to count the number of seconds it takes between when you see lightning and then hear thunder. Take the result and divide by 5 to give you the distance in statute miles. If there is frequent lightning or multiple storms, this method may not be feasible. Ideally, remote pilots can monitor the weather by electronic means and observe incoming systems with precise detail.

Hail is formed inside thunderstorms by the constant freezing, melting, and refreezing of water as it is carried about by the up- and downdrafts.

Thunderstorms that generally produce the most intense hazard to aircraft are called squall-line thunderstorms. These non-frontal, narrow bands of thunderstorms often develop ahead of a cold front. Embedded thunderstorms are those that are obscured by massive cloud layers and cannot be seen. A pilot should always expect the hazardous and invisible atmospheric phenomena called wind shear turbulence when operating anywhere near a thunderstorm (within 20 NM).

Microbursts are small-scale intense downdrafts which, as they get near the ground, spread outward from the center in all directions. Maximum downdrafts at the center of a microburst may be as strong as 6,000 feet per minute far exceeding the capabilities of sUAS. Also, wind speeds in excess of 45 knots and shears of 90 knots or more may exist which may cause an upset or loss of control of an sUAS. An individual microburst will seldom last longer than 15 minutes from the time it strikes the ground until dissipation. The horizontal winds continue to increase during the first 5 minutes with the maximum intensity winds and last approximately 2–4 minutes.

The most violent thunderstorms draw air into their cloud bases with great vigor. If the incoming air has any initial rotating motion, it often forms an extremely concentrated vortex from the surface well into the cloud. Meteorologists have estimated that wind in such a vortex can exceed 200 knots with pressure inside the vortex quite low. The strong winds gather dust and debris and the low pressure generates a funnel-shaped cloud extending downward from the cumulonimbus base. If the cloud does not reach the surface, it is a funnel cloud; if it touches a land surface, it is a tornado. Tornadoes occur with both isolated and squall line thunderstorms. Remote pilots should avoid operations during or in close proximity to thunderstorm activity, especially those that appear to be severe.

Remote pilots should be aware that some phenomenon (e.g. hail, lightning, windshear, and microbursts) can occur well away from the center of the storm. Extreme caution is advised when operating in conditions that may generate thunderstorms.

Icing

Structural icing occurs on an aircraft whenever supercooled condensed droplets of water make contact with any part of the aircraft that is also at a temperature below freezing. An inflight condition necessary for structural icing to form is visible moisture (clouds or raindrops). Icing in precipitation (rain) is of concern to the remote pilot because it can occur outside of clouds. Aircraft structural ice will most likely have the highest accumulation in freezing rain which indicates warmer temperature at a higher altitude. The effects of structural icing on an sUAS are as follows:

The presence of ice pellets at the surface is evidence that there is freezing rain at a higher altitude, while wet snow indicates that the temperature at your altitude is above freezing. A situation conducive to any icing would be flying in the vicinity of a front.

Remote pilots should avoid flight in conditions that may produce icing. If it appears that ice is accumulating on the sUAS, it should be recovered immediately to avoid loss of control.

Fog

Fog is a surface-based cloud (restricting visibility) composed of either water droplets or ice crystals. Fog may form by cooling the air to its dew point or by adding moisture to the air near the ground. A small temperature/dew point spread is essential to the formation of fog. An abundance of condensation nuclei from combustion products makes fog prevalent in industrial areas.

Fog is classified by the way it is formed:

Density Altitude

Manufacturer performance information conveys to the remote pilot what can be expected of an sUAS (rate of climb, takeoff roll, etc.) under ideal conditions. Prediction of performance is based upon the conditions a manufacturer chooses, but is often similar to manned aircraft calculations which use a sea level temperature of +15°C (+59°F) and atmospheric pressure of 29.92 "Hg (1013.2 mb). This combination of temperature and pressure is called a standard day. When the air is at a standard density, temperature and/or pressure deviations from standard will change the air density, or the density altitude, and that change affects sUAS performance. Remote pilots should be aware that conditions which deviate away from the manufacturer-calculated or standard conditions will impact performance.

Relative humidity also affects density altitude, but should have minimal impact on sUAS performance. Overall, sUAS flight performance will decrease as atmospheric pressure decreases, altitude increases (in flight or at higher launch sites), temperature increases, and marginally as humidity increases. A combination of high temperature, high humidity, and high altitude result in a density altitude higher than the pressure altitude which, in turn, results in reduced aircraft performance.

Weather Briefing

Remote PICs are encouraged to obtain weather information from Flight Service prior to flight by visiting 1800wxbrief.com. Remote PICs can create a free account in order to use the briefing service. While Flight Service does offer a telephone-based service, it is intended for manned aircraft pilots only.

Remote PICs are also encouraged to visit the NWS Aviation Weather Center (AWC) at aviationweather.gov. This free, web-based service does not require registration and offers all of the weather products important to a remote PIC, such as aviation routine weather reports (METAR) and terminal aerodrome forecasts (TAF). While reviewing the weather for your intended operation, it is also critical that the remote PIC review any temporary flight restrictions (TFR) at tfr.faa.gov.

Request a standard briefing to get a complete weather briefing. Request an abbreviated briefing to supplement mass disseminated data or when only one or two items are needed. Request an outlook briefing whenever the proposed departure time is six or more hours from the time of briefing.

Remote pilots need to learn the various abbreviations and symbols of textual weather reports in order to interpret them properly, and should familiarize themselves with available graphical products to be able to understand available weather maps and images.

Weather Reports, Forecasts and Charts

Remote pilots can access weather reports, forecasts and charts by visiting aviationweather.gov or 1800wxbrief.com.

The Automated Surface Observing System (ASOS) is the primary surface weather observing system of the U.S. Automated weather reporting systems are increasingly being installed at airports. These systems consist of various sensors, a processor, a computer-generated voice subsystem, and a transmitter to broadcast local, minute-by-minute weather data directly to the pilot. The Automated Weather Observing System (AWOS) observations will include the prefix “AUTO” to indicate that the data is derived from an automated system.

An international weather reporting code is used for weather reports (METAR) and forecasts (TAFs) worldwide. The reports follow the format shown in Figure 3-4. For aviation purposes, the ceiling is the lowest broken or overcast layer, or vertical visibility into an obscuration.

Figure 3-4. TAF/METAR weather card

Surface Aviation Weather Observations

Surface aviation weather observations are a compilation of elements of the current weather at individual ground stations across the United States. The network is made up of government and privately contracted facilities that provide continuous up-to-date weather information. Automated weather sources, such as AWOS, ASOS, as well as other automated facilities, also play a major role in the gathering of surface observations.

Surface observations provide local weather conditions and other relevant information for a specific airport. This information includes the type of report, station identifier, date and time, modifier (as required), wind, visibility, runway visual range (RVR), weather phenomena, sky condition, temperature/dew point, altimeter reading, and applicable remarks. The information gathered for the surface observation may be from a person, an automated station, or an automated station that is updated or enhanced by a weather observer. In any form, the surface observation provides valuable information about individual airports around the country. These reports cover a small area and will be beneficial to the remote pilot.

Aviation Weather Reports

Aviation weather reports are designed to give accurate depictions of current weather conditions. Each report provides current information that is updated at different times. Some typical reports are METARs and PIREPs. To view a weather report, go to aviationweather.gov.

Aviation Routine Weather Report (METAR)

A METAR is an observation of current surface weather reported in a standard international format. METARs are issued on a regularly scheduled basis unless significant weather changes have occurred. A special METAR (SPECI) can be issued at any time between routine METAR reports.

Example:

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

A typical METAR report contains the following information in sequential order:

  1. Type of report—there are two types of METAR reports. The first is the routine METAR report that is transmitted on a regular time interval. The second is the aviation selected SPECI. This is a special report that can be given at any time to update the METAR for rapidly changing weather conditions, aircraft mishaps, or other critical information.
  2. Station identifier—a four-letter code as established by the International Civil Aviation Organization (ICAO). In the 48 contiguous states, a unique three-letter identifier is preceded by the letter “K.” For example, Gregg County Airport in Longview, Texas, is identified by the letters “KGGG,” K being the country designation and GGG being the airport identifier. In other regions of the world, including Alaska and Hawaii, the first two letters of the four-letter ICAO identifier indicate the region, country, or state. Alaska identifiers always begin with the letters “PA” and Hawaii identifiers always begin with the letters “PH.” Station identifiers can be found by searching various websites such as DUATS and NOAA’s Aviation Weather Aviation Digital Data Services (ADDS).
  3. Date and time of report—depicted in a six-digit group (161753Z). The first two digits are the date. The last four digits are the time of the METAR/SPECI, which is always given in coordinated universal time (UTC). A “Z” is appended to the end of the time to denote the time is given in Zulu time (UTC) as opposed to local time.
  4. Modifier—denotes that the METAR/SPECI came from an automated source or that the report was corrected. If the notation “AUTO” is listed in the METAR/SPECI, the report came from an automated source. It also lists “AO1” (for no precipitation discriminator) or “AO2” (with precipitation discriminator) in the “Remarks” section to indicate the type of precipitation sensors employed at the automated station. When the modifier “COR” is used, it identifies a corrected report sent out to replace an earlier report that contained an error (for example: METAR KGGG 161753Z COR).
  5. Wind—reported with five digits (14021KT) unless the speed is greater than 99 knots, in which case the wind is reported with six digits. The first three digits indicate the direction the true wind is blowing from in tens of degrees. If the wind is variable, it is reported as “VRB.” The last two digits indicate the speed of the wind in knots unless the wind is greater than 99 knots, in which case it is indicated by three digits. If the winds are gusting, the letter “G” follows the wind speed (G26KT). After the letter “G,” the peak gust recorded is provided. If the wind direction varies more than 60° and the wind speed is greater than six knots, a separate group of numbers, separated by a “V,” will indicate the extremes of the wind directions.
  6. Visibility—the prevailing visibility (¾ SM) is reported in statute miles as denoted by the letters “SM.” It is reported in both miles and fractions of miles. At times, runway visual range (RVR) is reported following the prevailing visibility. RVR is the distance a pilot can see down the runway in a moving aircraft. When RVR is reported, it is shown with an R, then the runway number followed by a slant, then the visual range in feet. For example, when the RVR is reported as R17L/1400FT, it translates to a visual range of 1,400 feet on runway 17 left.
  7. Weather—can be broken down into two different categories: qualifiers and weather phenomenon (+TSRA BR). First, the qualifiers of intensity, proximity, and the descriptor of the weather are given. The intensity may be light (–), moderate ( ), or heavy (+). Proximity only depicts weather phenomena that are in the airport vicinity. The notation “VC” indicates a specific weather phenomenon is in the vicinity of five to ten miles from the airport. Descriptors are used to describe certain types of precipitation and obscurations. Weather phenomena may be reported as being precipitation, obscurations, and other phenomena, such as squalls or funnel clouds. Descriptions of weather phenomena as they begin or end and hailstone size are also listed in the “Remarks” sections of the report. See Figure 3-5.
  8. Figure 3-5. Descriptors and weather phenomena used in a typical METAR

  9. Sky condition—always reported in the sequence of amount, height, and type or indefinite ceiling/height (vertical visibility) (BKN008 OVC012CB, VV003). The heights of the cloud bases are reported with a three-digit number in hundreds of feet AGL. Clouds above 12,000 feet are not detected or reported by an automated station. The types of clouds, specifically towering cumulus (TCU) or cumulonimbus (CB) clouds, are reported with their height. Contractions are used to describe the amount of cloud coverage and obscuring phenomena. The amount of sky coverage is reported in eighths of the sky from horizon to horizon. See Figure 3-6.
  10. Figure 3-6. Reportable contractions for sky condition

  11. Temperature and dew point—the air temperature and dew point are always given in degrees Celsius (C) or (18/17). Temperatures below 0°C are preceded by the letter “M” to indicate minus.
  12. Altimeter setting—reported as inches of mercury ("Hg) in a four-digit number group (A2970). It is always preceded by the letter “A.” Rising or falling pressure may also be denoted in the “Remarks” sections as “PRESRR” or “PRESFR,” respectively.
  13. Zulu time—a term used in aviation for UTC, which places the entire world on one time standard.
  14. Remarks—the remarks section always begins with the letters “RMK.” Comments may or may not appear in this section of the METAR. The information contained in this section may include wind data, variable visibility, beginning and ending times of particular phenomenon, pressure information, and various other information deemed necessary. An example of a remark regarding weather phenomenon that does not fit in any other category would be: OCNL LTGICCG. This translates as occasional lightning in the clouds and from cloud to ground. Automated stations also use the remarks section to indicate the equipment needs maintenance.

Example:

METAR KGGG 161753Z AUTO 14021G26KT 3/4SM +TSRA BR BKN008 OVC012CB 18/17 A2970 RMK PRESFR

Explanation:

Routine METAR for Gregg County Airport for the 16th day of the month at 1753Z automated source. Winds are 140 at 21 knots gusting to 26. Visibility is ¾ statute mile. Thunderstorms with heavy rain and mist. Ceiling is broken at 800 feet, overcast at 1,200 feet with cumulonimbus clouds. Temperature 18 °C and dew point 17 °C. Barometric pressure is 29.70 "Hg and falling rapidly.

Aviation Forecasts

Observed weather condition reports are often used in the creation of forecasts for the same area. A variety of different forecast products are produced and designed to be used in the preflight planning stage. The printed forecasts that pilots need to be familiar with are the TAF, inflight weather advisories such as Significant Meteorological Information (SIGMET) and Airman’s Meteorological Information (AIRMET), and the winds and temperatures aloft forecast (FB).

Terminal Aerodrome Forecasts (TAF)

A TAF is a report established for the 5 SM radius around an airport. TAF reports are usually given for larger airports. Each TAF is valid for a 24- or 30-hour time period and is updated four times a day at 0000Z, 0600Z, 1200Z, and 1800Z. The TAF utilizes the same descriptors and abbreviations as used in the METAR report. These weather reports can be beneficial to the remote pilot for flight planning purposes. The TAF includes the following information in sequential order:

  1. Type of report—a TAF can be either a routine forecast (TAF) or an amended forecast (TAF AMD).
  2. ICAO station identifier—the station identifier is the same as that used in a METAR.
  3. Date and time of origin—time and date (081125Z) of TAF origination is given in the sixnumber code with the first two being the date, the last four being the time. Time is always given in UTC as denoted by the Z following the time block.
  4. Valid period dates and times—The TAF valid period (0812/0912) follows the date/time of forecast origin group. Scheduled 24 and 30 hour TAFs are issued four times per day, at 0000, 0600, 1200, and 1800Z. The first two digits (08) are the day of the month for the start of the TAF. The next two digits (12) are the starting hour (UTC). 09 is the day of the month for the end of the TAF, and the last two digits (12) are the ending hour (UTC) of the valid period. A forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid period is at midnight UTC, it is annotated as 24. For example, a 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924.
  5. Forecast wind—the wind direction and speed forecast are coded in a five-digit number group. An example would be 15011KT. The first three digits indicate the direction of the wind in reference to true north. The last two digits state the wind speed in knots appended with “KT.” Like the METAR, winds greater than 99 knots are given in three digits.
  6. Forecast visibility—given in statute miles and may be in whole numbers or fractions. If the forecast is greater than six miles, it is coded as “P6SM.”
  7. Forecast significant weather—weather phenomena are coded in the TAF reports in the same format as the METAR.
  8. Forecast sky condition—given in the same format as the METAR. Only CB clouds are forecast in this portion of the TAF report as opposed to CBs and towering cumulus in the METAR.
  9. Forecast change group—for any significant weather change forecast to occur during the TAF time period, the expected conditions and time period are included in this group. This information may be shown as from (FM), and temporary (TEMPO). “FM” is used when a rapid and significant change, usually within an hour, is expected. “TEMPO” is used for temporary fluctuations of weather, expected to last less than 1 hour.
  10. PROB30—a given percentage that describes the probability of thunderstorms and precipitation occurring in the coming hours. This forecast is not used for the first 6 hours of the 24-hour forecast.

Example:

TAF KPIR 111130Z 1112/1212 15012KT P6SM BKN090 TEMPO 1112/1114 5SM BR FM1500 16015G25KT P6SM SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM TSRA BKN030CB FM120400 1408KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM TSRA OVC030CB

Explanation:

Routine TAF for Pierre, South Dakota…on the 11th day of the month, at 1130Z…valid for 24 hours from 1200Z on the 11th to 1200Z on the 12th…wind from 150° at 12 knots… visibility greater than 6 SM…broken clouds at 9,000 feet… temporarily, between 1200Z and 1400Z, visibility 5 SM in mist…from 1500Z winds from 160° at 15 knots, gusting to 25 knots visibility greater than 6 SM…clouds scattered at 4,000 feet and broken at 25,000 feet…from 0000Z wind from 140° at 12 knots…visibility greater than 6 SM…clouds broken at 8,000 feet, overcast at 15,000 feet…between 0000Z and 0400Z, there is 30 percent probability of visibility 3 SM…thunderstorm with moderate rain showers…clouds broken at 3,000 feet with cumulonimbus clouds…from 0400Z…winds from 140° at 8 knots…visibility greater than 6 miles…clouds at 4,000 scattered and overcast at 8,000… temporarily between 0400Z and 0800Z…visibility 3 miles… thunderstorms with moderate rain showers…clouds overcast at 3,000 feet with cumulonimbus clouds…end of report (=).

Convective Significant Meteorological Information (WST)

Convective SIGMETs are issued for severe thunderstorms with surface winds greater than 50 knots, hail at the surface greater than or equal to ¾ inch in diameter, or tornadoes. They are also issued to advise pilots of embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy or greater precipitation that affect 40 percent or more of a 3,000 square mile or greater region. A remote pilot will find these weather alerts helpful for flight planning.

[10-2024]