Lesson 3: Avoiding Thunderstorms in the Terminal Area


Welcome back to lesson three. This lesson is about avoiding thunderstorms in the terminal or airport area. While we will cover a lot, we won't cover it all. We will actually concentrate on the wind shear and microburst hazards in the terminal area in lesson five. For our international students, this lesson is about thunderstorm detection, reporting, and forecasting in the National Airspace System (NAS) of the United States. Several weather organizations from around the world will join us later on in the course to supplement this material with the weather services in other countries.

It is my privilege to introduce today's "guest speaker," Mr. Jack May, the Acting Director of the National Weather Service's Aviation Weather Center (AWC) in Kansas City, Missouri.

When I learned to fly nearly thirty years ago, my instructor gave me this maxim: It's better to be on the ground wishing you were in the air than in the air wishing you were on the ground. Your safe flight depends on your ability to discern the weather hazards that may lie on your route and at your destination. Your safety and the safety of your passengers depend on your ability to recognize the weather hazards related to your flying experience and the equipment on your aircraft.

There is one hazard, however, that is dangerous to all pilots and all types of aircraft -- thunderstorms, especially at the time of take off and landing. The violent updrafts and downdrafts of a thunderstorm are deadly to an object dependent on the smooth flow of air for controlled flight.

The National Weather Service WSR-88D weather radar network has improved our ability to observe thunderstorms, their development, and their movement. The research community has improved our ability to predict thunderstorms. But thunderstorms are just as dangerous as they were thirty years ago. In fact, several major accidents have helped us understand thunderstorm dangers that were unknown thirty years ago.
We at the Aviation Weather Center in Kansas City are pleased you have taken the time to learn more about the dangers of thunderstorms in the terminal area.

Jack May, Acting Director
National Weather Service Aviation Weather Center

And now on to the lesson...How do we as aviators and operations personnel know about thunderstorms in the terminal or airport area? In this lesson will cover the basic ways an aviator will be told about the weather either in written or verbal form.

All aviation weather begins at the airport.

The first step in the process of creating aviation weather is the observation. This report is also one of the first clues that the airport you are using has a thunderstorm impacting it. Before we even worry about the encoding or decoding language, we have to understand the things a observer looks for when an observation is taken. Only then can we understand exactly what the observer is trying to communicate when he or she mentions gusting winds, falling pressures, and the other thunderstorm clues. The following sections come directly from the Federal Meteorological Handbook Number 1, the Weather Observers ultimate guide.
5.4.1 Wind Direction. The wind direction shall be determined by averaging the direction over a 2-minute period. When the wind direction sensor(s) is out of service, at designated stations, the direction may be estimated by observing the wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the wind in an unsheltered area.

5.4.2 Variable Wind Direction. The wind direction may be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less. Also, the wind direction shall be considered variable if, during the 2-minute evaluation period, it varies by 60 degrees or more when the average wind speed is greater than 6 knots.

5.4.3 Wind Speed. The wind speed shall be determined by averaging the speed over a 2-minute period. At designated stations, Table 5-1 shall be used to estimate wind speeds when instruments are out of service or the wind speed is below the starting speed of the anemometer in use.

5.4.4 Wind Gust. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts. Gusts are indicated by rapid fluctuations in wind speed with a variation of 10 knots or more between peaks and lulls. The speed of a gust shall be the maximum instantaneous wind speed.

5.4.5 Peak Wind Speed. Peak wind data shall be determined with wind speed recorders. The peak wind speed shall be the maximum instantaneous speed measured since the last routine observation.

5.4.6 Wind Shifts. Wind data shall be examined to determine the occurrence of a wind shift. A wind shift is indicated by a change in wind direction of 45 degrees or more in less than 15 minutes with sustained winds of 10 knots or more throughout the wind shift.

5.5.5 Peak Wind Data. The peak wind shall be reported in the remarks section whenever the maximum instantaneous speed in knots (since the last observation) is greater than 25 knots
Visibility Parameters

The visibility parameters are:

Prevailing visibility. The visibility that is considered representative of visibility conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.

Sector visibility. The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.

Surface visibility. The prevailing visibility determined from the usual point of observation.

Tower visibility. The prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.

Table 6-2. Summary of Visibility Observing and Reporting Standards and Procedures
Type of Station
Surface Represents 10-minutes of sensor outputs Visual evaluation of visibility around horizon
Variable Reported when the prevailing visibility varies by 1/2 mile or more and the visibility is less than 3 miles. 
Tower Augmented Reported at stations with an ATCT
Sector Not reported Reported at all stations
Present Weather
Rain. Precipitation, either in the form of drops larger than 0.02 inch (0.5 mm), or smaller drops which, in contrast to drizzle, are widely separated.

Hail. Precipitation in the form of small balls or other pieces of ice falling separately or frozen together in irregular lumps.
Small Hail and/or Snow Pellets. Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08 to 0.2 inch (2 to 5 mm).

Squall. A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained at 22 knots or more for at least one minute

Funnel Cloud (Tornadic Activity) (1) Tornado
. A violent, rotating column of air touching the ground. (2) Funnel Cloud. A violent, rotating column of air which does not touch the surface. (3) Waterspout. A violent, rotating column of air that forms over a body of water, and touches the water surface.

Present weather qualifiers fall into two categories: intensity or proximity and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.

Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.
Intensity of Precipitation.
When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the intensities determined shall be no greater than that which would be determined if any forms were occurring alone.
The intensity of precipitation shall be identified as light, moderate, or heavy in accordance with one of the following:

Intensity of Rain or Ice Pellets. The intensity of rain and ice pellets shall be based on the criteria given in Table 8-1, Table 8-2


Table 8-1. Intensity of Rain or Ice Pellets Based on Rate-of-Fall
Light Up to 0.10 inch per hour; maximum 0.01 inch in 6 minutes
Moderate 0.11 inch to 0.30 inch per hour; more than 0.001 inch to 0.03 inch in 6 minutes.
Heavy More than 0.30 inch per hour; more than 0.03 inch in 6 minutes.


Table 8-2. Estimating Intensity of Rain
Light From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
Moderate Individual drops are not clearly identifiable; spray is observable just above pavements and other hard surfaces.
Heavy Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to height of several inches is observed over hard surfaces.

Descriptors. Descriptors are qualifiers which further amplify weather phenomena and are used with certain types of precipitation and obscurations. The descriptor qualifiers are: shallow, partial, patches, low drifting, blowing, shower(s), thunderstorm, and freezing.

Shower(s). Precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.

Thunderstorm. A local storm produced by a cumulonimbus cloud that is accompanied by lightning and/or thunder.

Present weather is reported when it is occurring at, or in the vicinity of, the station and at the time of observation. Unless directed elsewhere in the Handbook, the location of weather phenomena shall be reported as:
• "occurring at the station" when within 5 statute miles of the point(s) of observation.
• "in the vicinity of the station" when between 5 and 10 statute miles of the point(s) of observation.
• "distant from the station" when beyond 10 statute miles of the point(s) of observation.

Thunderstorm. A thunderstorm occurring with or without accompanying precipitation shall be reported when observed to begin, to be in progress, or to end. In addition to reporting a thunderstorm in the body of the observation, remarks may be added to report the time, location, and movement of the storm

Beginning of Thunderstorm
. The beginning of a thunderstorm shall be reported as the earliest time:

(1) thunder is heard;

(2) lightning is observed at the station when the local noise level is sufficient to prevent hearing thunder; or

(3) lightning is detected by an automated sensor.

Ending of Thunderstorm. The ending of a thunderstorm shall be reported as 15 minutes after the last occurrence of any of the above criteria.

Beginning/Ending Times of Precipitation, Tornadic Activity, and Thunderstorms.

a. Precipitation. At designated stations, the time precipitation begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in the next observation after the event. Beginning and
ending times for separate periods shall be reported only if the intervening time exceeds 15 minutes

b. Tornadic Activity. At designated stations, the time tornadic activity begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in a SPECI and the next METAR after the event

c. Thunderstorm. At designated stations, the time thunderstorm(s) begins or ends shall be reported to the nearest minute. The beginning and ending times shall be reported in a SPECI and the next METAR after the event. Beginning and ending times of separate thunderstorm(s) shall be reported only in a METAR if the intervening time exceeds 15 minutes


Pressure Change (Rising/Falling). At designated stations, the pressure calculated for each report shall be examined to determine if a pressure change is occurring. If the pressure is rising or falling at a rate of at least 0.06 inch per hour and the pressure change totals 0.02 inch or more at the time of the observation, a pressure change remark shall be reported

So, these are the criteria for reporting weather phenomena associated with thunderstorms. So how will the observer make the observation? And how will the observer report the weather to you?

There are actually several different ways.

Many of us in the civilian world of the United States have an automated observing system (AOS) at our airport. Of course there are different systems out there and there are different uses for these systems. But no matter which type, there are many with limitations right now for sensing thunderstorms in the terminal area.

The FAA and NWS have several different types of AOS. There are AWOS and there are ASOS.

Go to the FAA automated observing website and find out what each of these systems can or can't do about thunderstorms. Both AWOS and ASOS are covered here. What kind of system do you have at your airport?

For more information on the ASOS and its capability to report the weather, look at the Pilots' Guide to ASOS by the NWS. You can also see a picture of the sensor package of the ASOS.

Not every ASOS produces the same kind of observation. At the most congested and the ones most impacted by weather, humans provide the missing information like thunderstorm locations and movements. There are four levels of augmentation. What are the levels and what kind of augmentation is at your airport? Also check out this link. You can also read about it in the FAA's AIM in the Resource Menu.

For the military (but this is changing) and some small airports, human observation is still the only report you can get. Did you ever wonder how a machine compares to a human? So did the pilots unions, industry, and the FAA. Here are the results of one study.

Some important differences between just machine and just human...

The human observes the whole sky dome and reports on it. The machine with a "cloud height" indicator ONLY looks up every minute to describe the sky. The human looks at a distant object for visibility, the machine uses light scattering over two foot distance to determine prevailing visibility on the whole airport. The human requires 20 minutes to do a whole observation while the machine can do one in several minutes.

Now lets give you a chance to review or learn the written weather code that comes from an observation, and is used to describe the forecasted weather for an airport.

Before we begin, please let me address the question of why pilots need to know how to read METAR/TAF code. Granted that the original reason for encoded weather reports was because of limited ability to transmit observations by teletype. Yes, the computer has made that reason obsolete when it comes to giving you the information you need to fly. But there is still limited bandwidth on the lines that collect the observations from all over the United States and the world. In the foreseeable future that will require METAR code for weather folks. Even more important to you, the pilot, is the fact that we will soon be data linking weather information directly to aircraft. Almost all commercial carriers in the United States use the ACARS radio system which can provide digital character messages to airborne or taxiing flights. Right now the United State's air traffic control system is preparing to provide flight-information services (FIS) like weather via digital means. The USAF will be datalinking weather information in the future. There is only so much radio bandwidth available and costs of these services are based on the number of characters sent. If you need one more reason, how about the fact that reading encoded observations is the most efficient means of gleaning a word-picture of the status of the airport. Take it from someone who reads digitally presented ATIS messages for a living, it is a lot easier to find out one bit of information (like the code TS in its specific place) if it is presented in a logical sequence with the minimum number of characters rather than have to read words until you come across the word "thunderstorm" imbedded in the message. By the way, if you want to get a United States Air Transport Pilot (ATP) rating, the Federal Aviation Regulation (FAR) says that you will need to be familiar with "all codes in the Federal Meteorological Handbook."

Okay, you get to choose. Please read one or more of the following presentations of the METAR/TAF codes and look at the ways that are used to describe the phenomena we learned about earlier in this lesson. When it comes to reading the TAF forecasts, PLEASE use the study questions on the left for this lesson to make sure you know some of the finer details of the code.

You can read about it in the Pilot's Handbook of Aeronautical Knowledge. Look in Chapter 11.

You can look in the Federal Meteorological Handbook.

You can review the quick reference on METAR/TAF codes.

Allright, can you decipher these codes? If not, go back to the quick reference card above.

PL (This is a change from PE which was deleted several years ago...)

By the way, what criteria will change an observation? In the US there are several...


Ceiling changes between the values of 3,000 - 1500 - 1000 - 500 - Lowest approach mins

Volcanic eruptions

Sky condition that obscures descends below 1000 ft

Aircraft mishap

Wind shift (do you remember the definition?)

Visibility changes between the values of 3 miles - 2 miles - 1 - lowest approach min (1/2 mi)

RVR (runway visibility range) goes above or below 2400 feet visibility

Tornado seen or disappears

Thunderstorm begins or end

Precipitation (hail, freezing precip, ice pellets) begin or end

INTERESTING FACT: The ASOS CANNOT make a change to the observation (make a SPECIAL OBSERVATION) for any reason between 46 and 53 minutes past the hour. This is the time it is doing a regular observation. Even if you had a tornado heading straight for the airport or on the airport, there is no way for anyone to override this process to update the observation or broadcast the fact over the ATIS.

So now the observation is made and augmented. At FAA controlled towers it is sent for review and addition of the airport information (when controller workload allows) and added to the ATIS for your consumption. For advanced students who ever wondered why an ASOS at an uncontrolled tower can update its weather every minute, while at controlled airports the ATIS is only updated every hour--the answer is the desire of the FAA tower to control the weather information we get around the airport. There can only be ONE OFFICIAL AIRPORT WEATHER and ATIS wins.


Come from National Weather Service (NWS) forecast offices located in the fifty states. Most are NOT on the airport.

Please link to the NWS Organization Page to learn how the service is divided into regions and centers. Find out which region you fly in.

Now go to the region homepage for your area at the NWS main website. Find your forecast office. Some of them offer online tours. Take the tour and see all the products that come from your forecast office. Its not just for aviation.
If you can't find a tour for your office, take the tour at mine, Washington DC/Baltimore MD.

Airlines that are designated as EWIN (Enhanced Weather Information) certified are able to make their own forecasts based on observations. Most of the times the offices making these forecasts are away from the actual airport.

TAF code informaton has forecasts usually for 24 hour periods. There is the main body forecast which will describe the weather which must exist for at least half the time and have a greater than 50 percent chance of happening. THIS ONLY COVERS FIVE MILES AROUND THE AIRPORT.

Operationally significant weather should be forecasted including thunderstorms, wind shear, freezing precipitation, moderate or greater rain, accumulating snow, winds, and wind changes and gusts.

There are also conditional statements which can describe weather changes or intermittent conditions.

TEMPO - Greater than 50 percent chance of happening for less than one hour and cover less than half the forecast period

PROB - 40 or 30 indicates numerical chance of occurring. This is sometimes translated as "chance" and "slight chance."

Can you read a TAF?

Let's change formats for a while and look at the bigger picture. Now that the observations are done and the forecasts for the terminal area are made, someone is creating warning messages about thunderstorms for you to pay attention to. Let's learn who they are.

Where does aviation weather come from?

USAF and US Army personnel taking this course will want to link to a lesson that teaches where their weather information comes from.

The National Weather Service's Aviation Weather Center is the hub of activity for aviation specific planning and warning information. We should probably look at the organization and just what it does about keeping pilots aware of the hazards of thunderstorms. This portion of the lesson comes from one of the co-chairs of the National Weather Association's Aviation Committee, Ms. Carolyn Kloth. Many of you will recognize her name from the SIGMETS produced by the AWC.
Where do aviation weather forecasts come from?

Since the passage of the Air Commerce Act of 1926, the National Weather Service (NWS), and its forerunner the Weather Bureau, has been responsible for providing aviation weather forecasts in support of air commerce.
In the succeeding decades, there have been a number of administrative and bureaucratic changes within the U.S. government (see historical outline). However, two facts remain essentially unchanged to the present day: 1) the Federal Aviation Administration (FAA) is responsible for regulating air commerce and managing the flow of traffic within the National Airspace System (NAS), and 2) the National Weather Service (NWS) is responsible for providing weather
forecasts in support of aviation and the mission of the FAA.

One of the main providers of this weather information is the Aviation Weather Center in Kansas City, MO.

Who or what is the Aviation Weather Center?

The Aviation Weather Center (AWC) is one of 9 National Centers for Environmental Prediction (NCEP) within the NWS.
NCEP and the AWC were established in October 1995 as part of the NWS Modernization and Reorganization.
The AWC operates 24 hours a day, 365 days a year issuing forecast products exclusively for the aviation community, both domestic and international. Of all the aviation weather forecasts issued by the NWS, the AWC accounts for approximately two-thirds of them.

The AWC staff consists of 54 full-time employees, 47 of which are degreed meteorologists, plus a number of contract personnel.

What does the AWC do for aviation?

AWC operations are divided into two parts, the Domestic Branch and the International Branch.
Products issued by the Domestic Branch include:

Convective SIGMETs (WSTs) for thunderstorms

Non-convective SIGMETs (WSs) for volcanic ash, severe icing, and severe or greater turbulence

AIRMETs (WAs) for moderate icing, moderate turbulence and Low Level Wind Shear (LLWS), IFR conditions and mountain obscuration

Area Forecasts (FAs)

Low Level Significant Weather Prognostic Chart (LoLvl SIGWX Prog)

Collaborative Convective Forecast Product (CCFP) — a planning tool for air traffic control which begins when meteorologists from the AWC, the National Weather Service Laboratories, Air Traffic Control facilities and the airlines produce the best forecast for thunderstorms in the continental United States and adjust the ATC routes accordingly.
Although thunderstorms are mentioned in the FA and are depicted on the LoLvl SIGWX Prog, the primary product containing information on thunderstorms of concern to aviation is the Convective SIGMET.

What is a Convective SIGMET?

In the jargon of aviation weather, a SIGMET is a SIGnificant METeorological message that contains information about phenomena that are hazardous to aviation operations. A Convective SIGMET is a message that contains information specifically about thunderstorms that, in the judgement of the forecaster, are hazardous to aviation operations.
The Convective SIGMET product consists of three parts, or bulletins, each containing one or more individual Convective SIGMET advisories for a particular part of the country. The East bulletin (WSTE) covers that part of the U.S. east of 87 degrees West longitude. The West bulletin (WSTW) covers the U.S. west of 107 degrees West longitude. And the Central bulletin (WSTC) covers the middle part of the country between 87 degrees West longitude and 107 degrees West longitude (see Inflight Advisory Plotting Chart).

In-Flight Advisory Plotting Chart

The three Convective SIGMET bulletins are transmitted every hour at h + 55. The individual Convective SIGMET advisories are valid for 2 hours. Each Convective SIGMET advisory is identified by a discreet number and letter, the letters corresponding to the bulletin in which the thunderstorm is occurring (i.e., E for the East bulletin, C for the Central, etc.). The numbers are incremented sequentially, and are reset to 1 (one) every day at 00Z in each bulletin.
Each individual Convective SIGMET advisory is defined using the points contained on the Inflight Advisory Plotting Chart. These points are mainly high-altitude VORs that are evenly distributed across the continental U.S., and should be found on both high-altitude (IFR) and low-altitude (sectional) navigational charts.

What’s the difference between a Convective SIGMET and a non-convective SIGMET?

Convective SIGMETs are issued specifically for thunderstorms that will impact aviation operations. They are issued hourly and are valid for 2 hours. The word "convective" refers to thunderstorms.
Non-convective SIGMETs are issued for the non-convective hazards of severe icing, severe or greater turbulence, and volcanic ash. These SIGMETs are issued only as necessary, and are valid for 4 hours at a time. Each non-convective SIGMET is identified by a phonetic name (e.g. Alfa, Bravo, etc.) and number, beginning with 1 (one). If the hazard for which a non-convective SIGMET is issued lasts for more than the initial 4 hours, then the SIGMET is continued under the same phonetic name but with the number incremented by one. Also, a non-convective SIGMET must be formally canceled when the phenomenon ends or weakens to less-than-severe intensity.
Most importantly, a Convective SIGMET for thunderstorms implies the presence of the associated hazardous phenomena of severe icing, severe or greater turbulence, and Low Level Wind Shear (LLWS), as well as possible IFR conditions.

What determines when a Convective SIGMET is needed?

A Convective SIGMET is issued when any of the following minimum criteria are met:

Severe thunderstorms (SEV). A severe thunderstorm is defined as one containing hail ³ 3/4 inch in diameter, winds ³ 50 knots, and/or a tornado.

Embedded thunderstorms (EMBD). A thunderstorm is defined as embedded when it occurs within a larger area of rain or rain showers, or is hidden in multi-layered clouds and/or IFR conditions, or is otherwise obscured such that its presence is not visually apparent.

Lines of thunderstorms greater than 60 nm long with greater than 40% coverage of significant radar echoes (i.e., those ³ VIP 4 or _ 40 dBz).

Areas of thunderstorms greater than 3,000 square nm in size with greater than 40% coverage of significant echoes (see above).

In the judgement of the forecaster, the SIGMET-ed thunderstorms pose a threat to aviation operations.

If no thunderstorms meet the above criteria, the bulletin is still transmitted at h+55, but it will contain a negative message

It should be noted that a negative message does not indicate the absence of thunderstorms. It indicates only that any thunderstorms that may be occurring do not meet the above minimum criteria.

Who determines whether a Convective SIGMET should be issued?

The Convective SIGMET Unit at the AWC is responsible for monitoring the weather across the continental U.S. (CONUS) plus coastal waters for thunderstorms that may impact aviation operations. The five meteorologists who staff the unit all have a minimum of a Bachelor of Science degree in meteorology, and are trained specifically to forecast thunderstorms. The unit operates around the clock, with one meteorologist on duty at any given time.
The forecaster uses a wide variety of data sources to monitor the atmosphere and assess whether a Convective SIGMET is needed or not. Satellite, radar, and lightning data are all essential for monitoring the current state of the atmosphere. Other data sources include hourly surface observations, twice-daily rawinsonde data, and a variety of computer-generated forecasts that mathematically mimic the atmosphere. All of this information must be synthesized by the forecaster on duty in order to determine if a) conditions are right for thunderstorm development, and b) a given thunderstorm or group of thunderstorms meets the minimum Convective SIGMET criteria.
Pilots can rest assured that a trained forecaster who specializes in thunderstorms is always on duty at the AWC, monitoring the skies for thunderstorms of concern to aviation operations.

Link to an example of a Convective SIGMET product

This lesson we've covered some important aspects of knowing that thunderstorms are impacting an airport. In lesson 4 we will cover radar as we discuss enroute problems. In lesson 5 we will look at microburst and wind shear detection systems which give near-real time warnings while we are flying in the terminal areas.

Finally, thunderstorms pose hazards at the airport before we are even airborne. Last year, a ramp worker was killed when he was standing beside an aircraft during a thunderstorm. Please go to the 45th Weather Squadron's lightning safety page for tips on protecting yourself while on the ground.

Thank you for continuing to participate in the National Weather Association's "Thunderstorm and Flying" internet course. We're halfway through right now and have three more lessons to go.
Study Questions:

1. What is the name of the observer's handbook?
2. How is the observation wind defined?
3. What are the requirements for a variable wind?
4. When did gusty winds have to take place to be reported?
5. What is a wind shift?
6. How fast must the wind speed be to be reported as a "peak"?
7. How long does an automated observing platform have to work to create the surface visibility?
8. What is a squall?
9. When a thunderstorm has an intensity what is it refering to?
10. What is a thunderstorm?
11. What is the definition of "vicinity" and "distant" thunderstorms?
12. Can a thunderstorm be reported without precipitation?
13. When does a thunderstorm begin?
14. When does a thunderstorm officially end?
15. How many minutes must seperate thunderstorms in order to have multiple events at the airport?
16. What is the definition of "pressure falling rapidly"?
17. Which of the automated observing systems can report a thunderstorm?
18. What is the difference between augmentation levels A, B, C, and D for an ASOS?
19. What is a CB?
20. What is +TSRA refer to?
21. What are the codes for lightning?
22. What are the codes for pressure rapidly falling?
23. What does AUTO mean?
24. What conditions force a new observation to be made and hense change the ATIS?
25. What does TEMPO refer to?
26. What does PROB 40 mean?
27. What are some of the products of the domestic AWC division?
28. What is a severe thunderstorm according to the NWS?
29. How long are SIGMETs valid?
30. What hazards does a convective SIGMET imply?
31. What is an embedded thunderstorm?
32. If a convective SIGMET message is negative, does that mean there are NO thunderstorms