Surface Observations Study Guide
and METAR Reference

This page can help you study for the standard National Weather Service (NWS) test that one must take in order to become a certified surface observer in the United States. It is also a handy reference for understanding American METAR reporting and coding practices.

I compiled this guide while studying for my own certification test, but that was at a point in my career where I was already accustomed to reading and using METAR observations. Thus I left a lot of information that I already knew out of the guide. I am working on revising the guide to be comprehensive, not glossing over the more commonly known information. If you are already a student of meteorology, you probably do not need what I left out, and you may not even need all of the material that I did include. Nonetheless, this page is best used as a companion to the source material (see below).

The official study guide for the test is the "blue book," the NWS Training Guide in Surface Weather Observations. That book does not actually contain all the answers to its own questions, which is silly. It references the NWS Observing Handbooks, numbers 7 and 8, for answers to some questions. In the time since the training guide was last updated in 1998, these handbooks have been superseded by FAA Order 7900.5B. My guide (this page) was derived from the blue book and the FAA document. (Please see the disclaimer.)

This guide offers three levels of "difficulty:"

The hover feature in Level 1 relies on support for CSS opacity, but that should not be a problem for modern browsers. If you switch to it and you see all the answers, try a newer browser.

General rules

What is the order of elements in a METAR report?Type (METAR/SPECI), Site ID, date, modifier (AUTO/COR), wind, visibility, weather, sky condition, temperature, altimeter setting, remarks
How often must one do time (accuracy) checks on the official station clock?For part time stations: daily; for full-time: once per shift.
How long must the station retain carbon copies of forms?90 days (unless specified by local guidelines)
What if SPECI criteria are met at the time of a normal METAR?Code it as METAR anyway.
Where does the station ID go in the report?Second, after the type
What determines the time used in the METAR report?The time the last element was observed
What determines the time of a SPECI report?The time that the SPECI criteria were met
What is the maximum allowable time before the scheduled observation time that one can begin taking the ob?15 minutes
What time format is used in column 2 of the form?24-hour time. UTC for LAWRS; LST for others.

Wind

What numeric value is used to code north?360
How is speed coded on the manual form?No less than 2 digits (use leading zero if necessary)
How would this be coded: Wind direction indicator says 250°, and air movement is detected, but there is no reading on the speed indicator25001KT, 25002KT, or 00000KT
How would this be coded: Wind direction is varying from 200 to 300 deg, with a speed of 4 knots?VRB04KT
What are the criteria to record direction as variable?Less than or equal to 6 kt
What is the minimum fluctuation to declare a wind gust?10 kt
How is direction encoded?Tens of degrees, 360 is north (always 3 digits)
What is the criterion for recording a Peak Wind?Speed of more than 25 knots
What is the format of an encoded Peak Wind (in remarks)?PK WND dddff/(hh)mm where ddd is direction, ff is speed, hh is the hour and mm is the minute. Hours may be omitted if the report comes out during the same hour of day as the Peak Wind.
What type of north is used for wind direction?True north
What are the criteria for dddVddd format variable wind?Wind direction varies by 60° or more and wind speed is more than 6 kt
What is the main difference in squalls and gusts?The duration of increased speed
What defines a squall?A sudden increase in average speed of 16 kts or more, sustained at 22 kts or more, and maintained for 1 minute or more during the 10 mins before the ob.
What defines a wind shift?Wind direction changes by 45° or more in less than 15 minutes, with sustained speed of 10 kts or more throughout this period. The time of the shift is when it began.

Visibility

Define visibility.The greatest distance at which selected objects (markers) can be seen and identified
Define prevailing visibility.The maximum vis equaled or exceeded over 180° of horizon
When observing visibility, from how many locations should one take an observation?As many locations as practicable
What are the best objects to use for nighttime visibility markers?Unfocused lights of medium intensity
What are the reportable values for a manual observing station?From 0 to 3/8SM: 1/16SM increments; from 3/8 to 2SM: 1/8SM increments; from 2 to 3SM: 1/4SM increments; 3 to 15SM: 1SM increments; 15SM and up: 5SM increments
When is it necessary to report sector vis?When vis is not uniform in all dirs and/or vis is less than 3SM in a sector
How are sector visibilities to be reported?Clockwise: VIS N 1 E 1 1/2 S 1/2 NW 1 1/4 (include all sectors differing from prevailing by a reportable value)
What are the criteria for variable vis? (i.e., when should one not average)When prevailing vis changes between reportable values during the ob, averaging less than 3SM and the variation is 1/2SM or more
How is variable vis reported?Use the average in the body of ob, then in remarks, 'VIS xVy'
When is the tower vis to be reported?If it is less than 4SM, and it is put in the body of the ob if it is less than the surface vis
How is tower vis reported?In the body of report if it is less than the surface vis, otherwise put in remarks, 'TWR VIS x'; if it is less than surface vis, put the surface vis in remarks as 'SFC VIS x'
Define "runway visual range." (here abbreviated as RVR)The maximum distance at which the runway, its markings, or lights can be seen from a position above a specified point on its center line
Where does RVR go in report?After the visibility
When is RVR reported?When prevailing vis is 1SM or less, or RVR is 6000FT or less. RVR equipment must be installed at the station in order to report a measurement.
What is the METAR format of RVR?Rnn/vvvFT, where nn is the runway number (possibly suffixed with L, C, or R), and vvv is the observed range. vvv may be prefixed with 'M' (P) for range values below (above) the minimum (maximum) reportable value. Either 3 or 4 digits are used for the value.
What are the reportable RVR values?In 100s from 100 to 1000FT, in 200s from 1000 to 3000FT, in 500s from 3000 to 6000FT; M = below minimum possible, P = above maximum possible values.
Can variable RVR be reported?Yes. If RVR varies over the 10 minutes before the ob, the range value (vvv in the example above) is coded as xxxVyyy, e.g., R12L/1000V1600FT.
What should the observer do if there are problems with the RVR sensor?If there are any problems, do not record the values, and put 'RVRNO' in the ob remarks.

Weather

Define "light rain."At a minimum, scattered drops that do not completely wet an exposed surface. At a maximum, a condition where individual drops are easily seen.
Define "moderate rain."Individual drops are not clearly identifiable; spray is observable just above hard surfaces
Define "heavy rain."Rain falls in sheets; individual drops are not identifable; heavy spray occurs several inches above hard surfaces
Define "light ice pellets."Scattered pellets do not completely cover exposed surfaces; visibility is not affected
Define "moderate ice pellets."Slow accumulation occurs on the ground; ice pellets reduce visibility to less than 7SM
Define "heavy ice pellets."Rapid accumulation occurs on the ground; pellets reduce visibility to less than 3SM
Determine -RA or -PL from accumulation rateup to 0.10 in/hr, or 0.01 inch in 6 min
Determine RA or PL from accumulation rate0.11 to 0.30 in/hr, or 0.01 to 0.03 inches in 6 min
Determine +RA or +PL from accumulation ratemore than 0.30 in/hr, more than 0.03 inches in 6 min
Determine -SN or -DZ from visibilitygreater than 1/2SM
Determine SN or DZ from visibilitygreater than 1/4SM but less than or equal to 1/2SM
Determine +SN or +DZ from visibilityless than or equal to 1/4SM
What defines the beginning of a thunderstorm?When thunder is heard, overhead lightning is observed and the local noise level is such as might prevent hearing thunder, or lightning is detected by an automated sensor within ten miles of the airport.
What defines the end of a thunderstorm?15 minutes after the last occurrence of any of the criteria that determine the beginning
When obscuring phenomena or other precipitation is also present, how should the intensity of SN, DZ types be reported?The intensity assigned shall be no greater than that determined using visibility criteria if any of the SN or DZ types were occurring alone
What is the maximum allowable visibility for heavy snow?1/4SM
What is the maximum allowable visibility for moderate snow?1/2SM
Give the proper order of weather types in the report.Tornado, thunderstorm, precipitation, obscurations, other weather (mnemonic: TSPOO)
List weather types that cannot be assigned intensity.TS, IC, GR, GS, obscurations, PO, SQ
When is lightning reported in present weather?Never!
How does one note the direction from the station where vicinity phenomena are observed?List the VC type followed by the directions where observed. E.g.: RMK (VCSH, VCFG, etc.) (dirs e.g. N-NE; report clockwise starting from north)
How would one code "vicinity patchy fog?"VCFG is any type of fog 5-10 SM from the station.
When is the presence of volcanic ash reported?Always--whenever it occurs
What are the criteria to report BR, DU, FU, SA, HZ, PY?When these phenomena are presently occurring at the station, at the time of the observation, and reducing vis to less than 7SM
Define "shallow" or "partial" fog.Fog with little vertical extent; visibility in the fog is less than 5/8SM. PRFG is fog of depth greater than or equal to 6 feet but less than 20 feet deep; MIFG is less than 6 feet deep.
Define "drifting" as it applies to snow, dust, etc.Particles are raised to less than 6 feet.
Define "blowing" as it applies to snow, dust, etc.Particles are raised to greater than or equal to 6 feet.
Define "showers."Rapid changes in precipitation intensity
What produces a thunderstorm?A cumulonimbus cloud
Define "freezing precipitation."Precipitation that forms a glaze on the ground
What visibility criteria are associated with an obscuration?less than 7SM
How would one code "thunderstorm with showers?"TS SHRA, not TSSHRA
Give the criteria defining a sandstorm or duststorm.visibility less than 5/8SM but greater than or equal to 5/16SM.
Give the criteria defining a heavy sandstorm or heavy duststorm.visibility less than 5/16SM
Snow and blowing snow are occurring. You can't tell if snow is actually falling. What do you report?BLSN only
How does one determine the intensity of snow occurring alone (without other weather)?By its effect on visibility
When may FG be reported only with qualifiers?When visibility is less than 7SM but greater than or equal to 5/8SM. WARNING! There are contradictory definitions of this rule in the different sources.
What are the temperature range and visibilities associated with fog?less than 5/8SM and greater than -0.1°C
What are the temperature range and visibilities associated with freezing fog?less than 5/8SM and less than 0°C
What are the differences in reporting hail with GR versus GS?GR: Largest stone greater than or equal to 1/4 inch, and it is acceptable to put the size in remarks. GS: Largest stone less than 1/4 inch, and no size remark is allowed.

Sky

Define "ceiling".The lowest layer aloft reported as BKN or OVC, or if the sky is totally obscured, the vertical visibility (VV) is the ceiling.
What fraction of sky cover is coded VV?8/8; sky is totally obscured
What fraction of sky cover is coded SKC?0/8; no clouds
What fraction of sky cover is coded FEW?any clouds less than 1/8 up to 2/8
What fraction of sky cover is coded SCT?3/8 to 4/8
What fraction of sky cover is coded BKN?5/8 to anything less than 8/8
What fraction of sky cover is coded OVC?8/8
When an obscuration layer is present, what remark is included in the ob?Indicate what layer is associated with the phenomenon. E.g.: RMK (wx type) (encoded layer description), RMK FU BKN002
What should one do if ceiling height is variable?The ceiling is the average of the observed values. Then, RMK CIG xxxVyyy (where xxx and yyy are in hundreds of feet).
What should one do if sky coverage is variable?RMK CCC[hhh] V CCC, where CCC is the sky cover contraction, and hhh is included if it is necessary to discern between different layers with the same contraction.
What is the maximum number of sky cover layers that can be reported?6 for manual stations, 3 for automated stations
What is the transmissivity requirement of sky cover layers?They must be opaque
Define "obscuration."The portion of sky that is hidden by a weather phenomenon, either surface-based or aloft.
What if a surface-based obscuration covers 8/8, but the sky can be seen through it?Report sky cover as SKC.
What if an obscuration aloft covers 8/8, but the sky can be seen through it?Report sky cover as OVC.
What if a surface-based obscuration covers 8/8, but the sky is hidden?Report sky cover as VV.
What if an obscuration layer aloft covers 8/8, and hides 6/8 of the sky?Sky cover is 8/8; report OVC
What is the rule on coding the layer of a surface-based obscuration?Never use OVC000. The use of FEW000, SCT000, or BKN000 is acceptable, but OVC000 is not.
In what order are sky cover layers reported?ascending
What is the resolution to which sky cover layer heights less than or equal to 5000 ft are reported?Nearest 100
What is the resolution to which sky cover layer heights of at least 5000 ft, but less than or equal to 10000 ft are reported?Nearest 500
What is the resolution to which sky cover layer heights of at least 10000 ft are reported?Nearest 1000
When a sky cover value falls halfway between two reportable amounts, what is reported?Report the lower of the two amounts
What is the coded height of a layer 50 feet or less above the surface?000
How would one use the observed height where a ceiling balloon disappears?Report the next lowest reportable height WARNING! This answer is unverified. Stations that use ceiling balloons are rare nowadays.
When several layers are reported as broken, what is the ceiling?Ceiling is the lowest of the broken layers
What are the criteria for variable ceiling?A ceiling less than or equal to 1000 ft must vary by greater than or equal to 200 ft; a ceiling greater than 1000 ft but less than or equal to 2000 ft must vary by greater than or equal to 400 ft; a ceiling greater than 2000 ft but less than or equal to 3000 ft must vary by greater than or equal to 500 ft; ceilings greater than 3000 ft cannot be variable.
How does one code a sky cover layer at an elevation below that of the station?///

Temperature and dewpoint

Temperature follows which element in a METAR/SPECI?Sky condition
How is a missing temperature observation coded?Leave out the T/Td group
Temperature and dewpoint are determined to what precision?0.1°C
When ventilating a psychrometer, when is the dry-bulb thermometer read? Are there exceptions to this rule?Read it when the lowest wet-bulb temp has been reached. Exceptions: when there is driving rain or snow, or if there is frost on the dry bulb at the time of observation.
When is dewpoint reported with respect to ice?Never.
When is it not necessary to use the wet-bulb temperature in order to determine the dewpoint?When freezing fog is present, or the dry bulb temp is -30°F or below
How should 24-hour maximum and minimum temperatures be recorded in columns 57-58 of the MF1M form?To the nearest 0.1°C
Describe the rules for rounding and coding negative temperatures and dewpoints.If the fractional part of the value is greater than 0.5, the preceding digit shall be decreased by one. So -1.5 becomes -1 and is coded as M01; -0.6 becomes -1 and is coded as M01; -0.5 becomes 0 and is coded as M00.
When reading thermometers, which of the wet/dry bulbs are read first?Wet first. The dry bulb is read after the wet bulb stops dropping with ventilation.

Pressure

If two aneroid instruments are used to determine altimeter setting, how often should they be compared to each other? When should the comparison be entered in column 65 of the manual form?Whenever altimeter setting is determined; enter the comparison in column 65 at least 2 times daily.
When two instruments are read to determine the altimeter setting, and the readings are different, which reading do you use as altimeter setting? Why?Use the lower of the two; the lower reading would cause an altimeter to read a lower altitude, which gives pilots an extra margin of safety.
When using an aircraft-type altimeter to determine the altimeter setting, to what height should you set it to before reading it? How often should you reset the indicated height?Set it to indicate the actual height of the station. Reset it each time the altimeter setting is determined.
When pressure values fall in between reportable values, how should one round to arrive at a reportable value?Down
How is the 12-hour mean temperature derived for the purpose of pressure calibration?From the current temp and the temp 12 hours before
What are the criteria to include a pressure change remark?0.06 inches per hour, with a total change of at least 0.02 inches
What units are used for sea-level pressure?hPa (not mb!)
The barograph correction value should be entered in the MF1M form to the nearest...0.005 inHg
Station pressure is determined by adjusting the corrected barometric pressure to compensate for the difference between the height of the barometer and what?The designated station elevation

Remarks

What is the order of remarks?volcanic eruption, funnel cloud/tornadic activity, peak wind, wind shift, tower/sfc vis, variable prevailing vis, sector vis, ltg, precip begin/end times, TS b/e time, TS loc, hailstone size, virga, vble cig hgt, obsc, vble sky cond, sig cld types (CB etc.), PRESRR/FR, SLP, ACFT MSHP, NOSPECI, SNINCR, other sig info
What is the order of the additive data?3/6-h prcp, 24-h prcp, snow depth, snow water equiv, cld types, duration sunshine, hrly T and Td, 6-h Tx, 6-h Tn, 24-h Tx/Tn, 3-h prs tend
What does NOSPECI mean when present in an encoded observation?At staffed stations where SPECI's are not taken, the remark NOSPECI shall be coded to indicate that no changes in weather conditions will be reported until the next METAR.
How would one code obscuration clarifications? E.g., how would one indicate in remarks that snow obscures 5/8 sky?SN BKN000
When is snow depth entered in column 61 of the MF1M form?1200 UTC, or at a time designated locally
When is a 24-hour precip total group included?1200 UTC, if a trace or more of precipitation has fallen in the previous 24 hours
When is a snow depth (4/) group included?0000 and 1200 UTC, when more than a trace of snow is on the ground. Also, at 0600 and 1800 UTC if more than a trace is on the ground and more than a trace of precip occurred in the previous 6 hours.
What is the format of a pressure change remark?5appp; a = 1-3 if higher, 6-8 if lower, 0/4/5 if same than 3 hours ago; ppp = change in tenths of hPa
What is the format of a "snow increasing rapidly" remark?SNINCR (accumulation rate in whole inches per hour)/(depth at time of observation in whole inches), e.g., SNINCR 2/7
What are the three elements of a lightning remark?frequency, type, location (in that order)
What are the abbreviations and definitions of lightning flash frequencies for a lightning remark?OCNL: less than 1 flash per minute; FRQ: 1-6 flashes/minute; CONS: greater than 6 flashes/minute
What is the format of a peak wind remark?PK WND dddff(f)/(hh)mm, where ddd is direction, ff(f) is the 2 or 3 digit speed, and (hh)mm is the time of the reading
When does the timestamp on a remark have hours and minutes?When the hour is different than the hour in which the observation was taken

Remarks order

Remark mnemonic (syllables)VoTo WiViLi PreTh HaViCe ObSk CloPress AM No Snow
1Volcano (Plain text)
2Tornado (B or E)
3Wind (PK WIND) (WSHFT)
4Vis (TWR VIS or SFC VIS) (VIS xVy) (sectors)
5Lightning
6Precip (B or E)
7Thunderstorm (B or E) (location)
8Hail
9Virga
10Ceiling (CIG hhhVhhh)
11Obscuration
12Sky Cond (ccc V ccc)
13Cloud types
14Pressure (PRESRR or PRESFR) (SLPxxx)
15Aircraft mishap
16NOSPECI
17Snow (SNINCR)
18Other

Additive data order

Additive mnemonicPaul Simon Craves Shirley Temple's Meringue Pie
1Precip (6) (7)
2Snow (4/) (933)
3Clouds
4Sunshine
5Temps (T)
6Max/min temps (1) (2) (4)
7Pressure

SPECI criteria

Wind shiftgreater than or equal to 45° in 15 min and wind speed is greater than or equal to 10 kt during the shift
VisibilityDrops below (or rises to or above) 3SM, 2SM, 1SM, or minimum instrument approach procedure minimum per NOS (or 1/2SM)
Runway visual rangeHighest RVR decreases below 2400 ft (or rises to or above 2400 ft) in the past 10 minutes
Tornado, funnel cloud, waterspoutIs observed or disappears from view or ends
ThunderstormBegins or ends (no SPECI is needed if one begins while one is already occurring)
HailBegins or ends
Freezing precipitationbegins, ends, changes intensity
Ice pellets (sleet)begins, ends, changes intensity
Squall (also give definition)Wind speed suddenly increases by greater than or equal to 16 kt and is sustained at 22 kt for greater than or equal to 1 minute
CeilingHeight of base of cloud covering 5/8 or more forms or dissipates below, decreases to less than, or increases to equal or exceed: 3 kft, 1.5 kft, 1 kft, 500 ft, 200 ft
Sky conditionCloud layer or obscuration aloft is present below 1000 feet but not present in preceding observation.
Volcanic eruptionAlways
Aircraft mishap (always, unless...)there was an intervening ob
Any meteorological condition which the observer finds importantWhenever

last updated 2012.04.04 :: return home

Though all efforts have been made to present correct information, the information provided here is given without any warranty or guarantee of accuracy. The author of this website, Jake Wimberley, will not be held responsible for any errors in the content of this page, or for any failures by the user in passing any certification exam the user may take. Use at your own risk.