U. S. DEPARTMENT OF COMMERCE 

SINCLAIR WEEKS, Secretary 

1..{ WEATHER BUREAU 

' • F. W. REICHELDERFER, Chief 

_!g:Y TO METEOROLOGICAL RECORDS DOCUMENTATION NO. 3 .12 

HISTORY OF OBSERVATIONAL 

INSTRUCTIONS AS APPLIED 

TO THUNDERSTORMS 

i.JASHINGTON, D. C. 1958 

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C. - Price 10 cents 

99736 

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PURPOSE 

The Key to Meteorological Records Documentation Series has been established to pr9vide 

guidance information to research personnel making use of climatological data. 

Frequently users of such data have found it necessary to spend a great deal of time establishing whether the criter~a for observing various elements have changed over the period of record. 

It is therefore hoped that the presentation of this series may not only conserve valuable time 

but may have a direct influence in improving the accuracy of research results. 

GENERAL PREFACE 

This documentation has been prepared to illustrate changes to instructions that have occurred 

over the period of record 1891 to 1957. The history of changes is based upon instructions 

on USWB Forms 1001, 1130 Aer., and to the various WBAN-10 forms. These instructions were 

found to be most nearly complete from a documentation viewpoint with respect to the history 

of observing thunderstorms. The 110riginal Monthly Record of Observations" Form 1001, has 

a period of record commencing with the beginning of Weather Bureau records in 1891 to 1948; 

the 1130 Aer., 1928-1944; followed by the current WBAN-10 series with their instructions 

published as a part of Ci.rcular N. 

Instructions for observing and recording thunderstorms contained with Form 1001 were presented under the headings of Precipitation, Weather, and Footings and Summary through 1944. 

These same headings were .continued in this summary both as a convenience and as a rapid reference to the original instructions. 

On the following pages an attempt has been made to present a chronological sequence of significant changes that have occurred with respect to one element: Thunderstorms. 

H. Baliles 

Office of Climatology 

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PART I - HISIORY OF INSTRUCTIONS FOR RECORDING THUNDERSTORMS IN THE SYNOPTIC 

OBSERVATIONS 

USw""B FORM 1001 

1891 - Weather: 

When a thunderstorm is in progress, or threatening conditions exist over a station with no rain, 

state of weather will be recorded as "cloudy", with "threatening" written immediately after or 

indicated by International Symbol. 

t~en a thunderstorm is prevailing at the moment of observation, or the rain is still falling, 

although neither thunder has been heard nor lightning seen for one hour previous to the observation, the International Symbol for thunderstorm will be entered after "State of Weather" and 

in the same column, and also once for the entire month as a reference mark on the margin. 

1893 - Footings and Summary: 

A day with thunderstorm is one on which thunder is distinc tly heard at the station during the 

day whether or not lightning is seen or rain falls at the station. 

Weather - Same as 1891 

1904 - Precipitation: 

When a thunderstorm occurs the international symbol for thunderstorm will be entered in the 

column for beginning of precipitation. 

Weather: 

When a thunderstorm is prevailing at the moment of obsetvation, or the rain is still falling, 

although neither thunder has been heard nor lightning seen for one hour previous to the observation, the international symbol for thunderstorm will be entered after "State of Weather. " 

Footings and Summary : 

A day with thunderstorm is one on which thunder is distinctly heard at the st ltion whether or 

not lightning is seen or rain falls at the station. 

1905 - Precipitation: 

When a thunderstorm occurs the international symbol for thunderstorm will be entered on the 

right hand margin of page 8. 

Weather: 

Same as 1904 . 

Footings and Summary: 

Same as 1904. 

1906 - Precipitation 

t~en a thunderstorm occurs the international symbol for thunderstorm f' will be entered on 

the right-hand margin of page 8 on each day, midnight to midnight, standard of time in local 

use, on which thunder was heard. 

Weather: 

\ Same as 1904 

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1921 - Weather: 

When a thunderstorm is prevailing at the moment of observation, or the rain is still falling, -:~_) and thunder has been heard or lightning seen within the hour previous to the observation, the 

international symbol for thunderstorm will be entered after "State of Weather. 11 

Precipitation: 

Same as 1905. 

Footings and Summary: 

Same as 1904. 

1936 - Precipitation 

Beginning January 1, 1936, the following symbols for thunderstorms will be entered in the 

right hand margin of page 8, Form 1001; for ·a thunderstorm at the station (thunder, lightning, 

and rainfall),~ ; for a thunderstorm heard or observed, but not actually occurring at the 

station (a distant thunderstorm),~ ; for lightning observed, but thunder not heard {distant 

lightning) , ~ 

Weather: 

Same as 1921. 

Footings and Summary: 

Same as 1904. 

1938 - Footings and Summary: 

A day with thunderstorm is one on which thunder is distinctly heard at the station, whether or 

not lightning is seen or rain falls at the station, or a thunderstorm heard or observed, but 

not actually occurring at the station (a distant thunderstorm). 

Weather: 

Same as 1921 

Precipitation: 

Same as 1936 

1945-l~B - Summary of Temperature and Precipitation: 

Enter a thunderstorm symbol and/or a distant lightning symbol in the proper column when observed in the 24-hour period from midnight to midnight. 

Meteorological Summary: 

Thunderstorm -· A day with a thunderstorm is one in which thunder is heard. 

Distant Lightning: 

A day with distant lightning is one on which lightning is seen, but no thunder is heard on 

account of distance. When distant lightning is observed and thunder is later heard from the. 

same storm, it then becomes a 11 thunderstorm" and should not be listed as "Distant Lightning". 

Form 1001 discontinued at the end of 1948. 

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CJ-
USWB FOm! lOOlC - 733-1 

~-)1949 - Station Meteorological Summary: 

Enter 11T" for thunderstorm or iiDL11 for distant lightning when observed during the period from 

~dnight to midnight. Do not enter more than one T or DL for a day. Enter DL only if thunder 

is not heard, except that if lightning occurs before midnight and thunder is heard after midnight, enter DL for the ·first day and T for the following. Similarly, if thunder occurs be-. 

fore midnight and lightning after. midnight, enter T for the first day and DL for the following 

day. Enter both T and DL if lightning only is dpserved in one storm, and thunder is heard 

froln another. 

NOTE: Prior to 1893, it was the practice of the 1-J'eather Bureau to record a day with thunderstorms only if it was accompanied by rain. This lolas an interpretation on existing instructions. 

W. J~ Humphrey's, Physics of the Air, 1940, Chapter XVII, has noted that this change in regulations was responsible for a phenomenal increase in the annual number of thunderstorms reported in the United States after 1893. Accepted tabulations of thunderstorm frequencies, such 

as W.H. Alexander's, Distribution of Thunderstorms in the United States.,~~ 43, 52, and 63 

rejected the earlier periods of record and even the decade of transition prior to 1904 for 

this reason. 

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PREFACE ·- PART II 

~·lhile instructions for recording synoptic data have remained basically the same over a period J 

of years, the same instructions were unsuited for airways observations. Accumulated experience 

and safety to air travel demanded a more specific type of observation based upon the needs of 

aviation. 

As a means to accomplish the above and as an aid to the observer, ne~~ instructions \Yere written, setting forth certain criteria and rules governing the taking of airways observations. 

These instructions tvere first published as "Instructions for Airways Observers", Circular N. 

1928, by the United States Heather Bureau. 

Thunderstorms were classified as to intensity and given a position of first importance in the 

General Conditions portion of the airways observation. Excerpts pertaining to the observation 

of thunderstorms will follmv chronologically, again ~~ith the endeavor to present an orderly 

sequence of changes in instructions and procedures. 

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) 

PART II - HISTORY OF INSTRUCTIONS FOR RECORDING OF THUNDERSTOm1S FOR THE AIRWAYS 

OBSERVATION 

FORM 1130 AER. 

1928 - Circular N (1st Edition) Airway Observations: 

Thunderstorms will be entered in the General Condition column giving direction of thunderstorm 

in reference to the station; i.e. thunderstorm to the N. W., thunderstorm at station, etc. 

1932 - Circular N - Ain~ay Observations: 

(a) Lightning - Reported under "Remarks'! together with its direction from the reporting 

station. 

(b) Thunder - Reported under "Remarks" together l~ith its direction from the reporting station. 

(c) Thunderstorm - The presence of thunderstorms will be reported as a weather element and 

included in the General Conditions Column, when actually occurring at the station, or in 

such close proximity as to constitute a dominating element in the weather at that time; e.g. 

overcast, thunderstorm approaching from the west; overcast, violent thunderstorm, severe 

lightning, hail, and rain squalls. 

1~1en not falling in the foregoing classification; i.e., when observed at some distance or 

having passed over the station, and thus not constituting a dominating condition, they will 

be reported under "Remarks". For example: Thunderstorm in distant southl~est; mild thunderstorm passed to north half hour ago. 

(note): This is the first recognition of the thunderstorm as a ,.,eather element in the air-

'~ays observation. 

1935 - Circular N: 

Thuoderstorrns shall be reported as the first element under "weather" when occurring at the 

station or in such close proximity thereto as to consitute a dominating e l ement in the weather 

at the station at the time; otherwise they l~ill be reported under "Remarks". If approaching 

the station, the direction from which it is approaching will be given under "Remarks" in all 

cases, A thunderstorm is considered to be in progress whenever lightning, if observed in 

close proximity to the station, or thunder, is heard. 

Thunderstorms will be reported in the following three degrees: 

(1) Mild Thunderstorm: 1-lhen most of the lightning occurs within the cloud; the rainfall 

accompanying it is only light or moderate; no hai 1 occurs; the '~ind occurring at the beginning 

of the storm, if any, does not exceed 25 to 30 miles an hour and is of short duration 3-5 

minutes at most. 

(2) Moderate Thunderstorm: 1-lhen fairly frequent flashes of lightning occur betl-.1een the 

cloud and ground, as well as cloud to cloud; loud peals of thunder occur; moderate to heavy 

rain occurs; an on-rush of wind may precede the storm, reaching velocities 40 HPH; light or 

moderate hail may occur; the storm is distinctly recognizable as well-developed. 

(3) Severe Thunderstorms: When nearly incessant, sharp thunder and lightning occur; heavy 

rain occurs, possibly accom-panied by moderate or heavy hail; wind preceding the stn rm may 

reach velocities in excess of 40 l-1PH and continu·es over a considerable time, as much as 15 

minutes; a rapid drop in temperature, possibly as much as 20° in 5 minutes. 

The "l-.1eather" element of the report is indicated, lmen appropriate, by the following symbols: 

T Mild thunderstorm 

T- Moderate Thunderstorm 

T+ Severe Thunderstorm 

Criteria for Special Observations: A thunderstorm not previously reported is observed or occurs, or one previo~sly reported shows marked increase in intensity. 

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1939 - Circular N - Airways Observations: Rules for reporting thunderstorms: Any thunderstorm in progress at time of observation shall be reported as the first element under 11weathern 

except when a tornado is also observable from or occurring at or near the station, in which 

case the tornado will be reported before any thunderstorm. A thunderstorm is qonsidered to 

be "in progress at the time of observation," if thunder is heard or has been heard within 15 

minutes preceding the time of that.observation. 

It is proper .and desirable that under 11Remarks 11 there shall appear information concerning 

the thunderstorm which will complete the word picture already partly drawn by the elements 

of ceiling, sky, visibility, etc., but such remarks shall be in addition and not in lieu of 

the report of thunderstorm under 11weather11 • If the thunder is but slightly audible and. the 

thunderstorm, therefore, fairly distant from the station, a remark, such as "Thunder distant 

southwest", should be made if the thunderstorm is marked by a limited area of dark clouds, 

rain curtains, and the lightning is near and sharp. This condition will be clear to the 

recipient of the report if the proper degree of severity is shown under 11weather", and a 

remark such as "storm rapidly approaching station", is made. 

If lightning is observed, but no thunder is heard, this condition does 'not constitute a 11 thunderstorm in progress" nor one within the range of the observation station. This condition may be described under "Remarks11 but the distinction must be made that a thunderstorm 

requires that thunder be heard, and a lightning storm, or the mere observation of lightning, 

does not occasion other remarks thcu, those concerned with lightning. 

The direction of movement of thunderstorms is usually difficult to observe, and more difficult 

the nearer the storm. Unless the observer is confident that the direction of movement has 

been accurately determined by him, the reports of the thunderstorm should show, in remarks, 

the direction in which the heaviest thunder is heard, together with any appropriate remarks 

which will indicate the approach or retreat of the thunderstorms as to the station. Individual 

judgment and discretion by the observer will govern the use of the proper remarks. Such remarks as "Thunderstorm in N. W. apparently approaching and increasing in intensity." 

To obviate the difficulty occasionally involved at. times when an observer sees considerable 

lightning fairly near to this station but cannot hear thunder, the use of appropriate remarks 

is proper. Here the description of the storm should never be given to contain the word 

"thunder11 or 11 thunderstorm11 but such descriptive terms as 11 dark11 , '!frequent lightning", 11 thunderheads with lightning11 , 11 lightning and rainfall in ..;-1est 11 , should be used in remarks, and covers 

the cases where the observer strongly suspects a thunderstorm but has not the requfred fact 

of hearing thunder to permit the reporting of a thunderstorm. 

1939 - Circular N - Criteria for Special Observations: A thunderstorm not previously reported 

occurs, or one previously reported shows marked increase in intensity. Also, the cessation of 

a thunderstonn previouslyreported. 

The reporting of the degree of intensity remains the same as in 1935. 

1941- Circular N- Rules of reporting thunderstoms: (primarily same as 1939). 

The specifications of the degree of intensity ..;vere modified to the following - l.fuen a thunder .. 

storm is in progress at the station and the intensity of the thunderstorm is less th·an that 

given for 11heavy11 thunderstorm: The following specifications are given as an aid in determining 

those thunderstorms ..;'lhich will be reported by the term "thunderstorm" or "T11 : (1) Occasional 

or fairly frequent flashes of lightning occur within the cloud, from cloud to cloud,. or from 

<?loud to ground; (2) '1;·7eak to loud peals of thunder occur; (3) rainfall, if any, is light, moderate or possibly heavy; (4) hail, if any, is light or moderate; (5) the wind occurring in 

connection with the passage of the storm does not reach velocities in excess of 40 HPH; (6) 

temperature drop is not as great as in case of heavy thunderstorm. 

Heavy Thunderstorm: (1) l\1hen nearly incessant sharp lightning occurs; (2) loU:d peals of 

almost continuous thunder occur; (3) heavy rain occurs; (4) hail, if any, if light, moderate, 

or heavy, (5) the wind occurring in connection ..;vi.th storm exceeds 40 MPH (6) rapid drop in 

temperature, possibly as mlfch as 20~ in 5 minutes. :':_) 

19l~l - Criteria for Special Observation: - Same as 1939 

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1945 - Form ll30A, WBAN lOA, - Instructions remain the same. 

1: __ .-) 1947 - Circular N .. 11th Amendment - Observations - Revised 1-1-47: "A thunderstorm is re-

,_~ garded as occurring at station when thunder is hea"rd~" 

In addition, a clearer approach for both observing and defining the intensity of the thunderstorm uas presented as follows -

Observation: Not;e the follovting -

1. Occurrence of thunder 

2. Location of storm center with respect to station. 

3. Direction toward which the sOorm is moving. 

4. Vlhether lightning is occurring from cloud to cloud, cloud to ground, or 't'lithin clouds. 

5. Intensity of the storm. 

Determination of Intensity: Classification of a thunderstorm as light, moderate, or heavy 

is·• based upon the appearance of the storm from the point of observation. The thunders term 

may be classified as light throughout: its history as viewed from this point, or it may be 

classified during its pas~age by the station as light, moderate, heavy; and as it recedes, 

moderate, and light. Des-cription of intensity will be based on the following guide. 

Light Thunderstorm: Lightning occurs within the cloud and rainfall accompanying it is light 

or moderate. Small hail may alSo be observed. The thunder is not loud, and lightning occurs 

at intervals of ~ minute or more. The surface ..;dnd speed at the.beginning of or during the 

storm does not exceed 30 HPH, and any sudden increase in speed is of short duration. The 

classification also applies to occas~onal peals of thunder during a general rain storm. 

Moderate Thunderstorm: Loud peals of thunder occur _at brief intervals and frequent flashes 

of lightning occur from cloud to ground, as well as from cloud to cloud; rain is moderate 

or heavy, and small hail is light or moderate. An on-rush of wind may reach speed of 40 MPH. 

Heavy Thunderstorm: Sharp and pronounced thunder and lightning occur almost continuously. 

Heavy rain occurs, sometimes with hail. Hind may exceed 40 HPH. A rapid drop in temperature 

occurs, sometimes as much as 20°F in 5 minutes. Thunderstorms will be reported if 15 minutes 

or less have elapsed since thunder was last heard. Thunderstorms are reported as either 

heavy or moderate; that is, all thunderstorms not classified as heavy are reported as moderate. 

(l<hen using the letter symbols, T, .T +). 

1949 Circular N - Identification Same as 1947 

Intensity: Same as 1947 

~949 - HBAN lOA-B: Enter appropriate symbol T, moderate; T+ heavy. Thunderstorms will be 

reported if 15 min1.1tes or less have: elapsed since thunder 1;'17as last heard. Thunderstorms 

are reported as either heavy or moderate; that is, all thunders terms not classified as heavy 

are reported as moderate. 

lffiAN-lOB- (Col. 82-35): Enter times of beginnings and endings, to the nearest minute, of 

tfiundetstorms, and cnanges in intensity. Intervals of 15 minutes or less bet\veen the time of 

ending and recormnencement need not be recorded unless occurring 1;'17ithin one hour previous to 

the beginning of a 6 hourly Observation, when all beginnings and endings ..;vill be recorded. 

This also applies to changes in intensity that do not last more than 15 minutes. Hhen any, 

phenomenon is occurring at midnight enter 11 cont 11 in column 84 for the day preceding midnight, 

and in column 83 for the day follm<ing midnight. 

\~) 1955 - WBAN-lOA - (Col. 13): 

(a) Enter direction, if observable: (1) with respect to station (2) direction toward whlch 

stonn is moving; e. g., T OVHD HOVG EHD or T SW NOW:IT VPJ3L (omit remark concerning movement 

if movement unknown). 

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(b) Enter the time of beginning and ending, peak speed of gusts, and direction of movement; 

e.g., T+ B34E50 G45 HOVD N, in all observations until it has been transmitted in a record 

observation. 

1957 (WBAN lOA .. (Col 52; Enter appropriate symbol T, moderate; T+ heavy. 

Col. 13: - Enter time of beginning of thunderstorm activity in all observations until appropriately transmitted. 

~mAN- lOB- Climatological data. (Col 33-84): Enter, to the nearest minute: (1) Time of 

beginning and ending, and of changes in intensity. (2) Times r::£ ending should be 15 minutes 

after last thunder. 

1955 - Circular N - Identification: - A thunderstorm is regarded as occurring at the station 

'~hen thunder has been heard '~ithin the previous 15 minutes. If overhead lightning or hail 

has occurred within the previous 15 minutes and the local noise level i s such as might prevent hearing thunder, a thunderstorm '~ill be regarded as occurriDg even t;hough thunder was not 

heard. 

1955 - Intensity: Remains the same as 1947 . 

1957 Revised Circular N - Criteria for Special Observation (ain~ays) Change 

No. 4, 1 /1/57: - Thunderstorm-

(1) Begins - A special observation is no r required to report the beginning of a new thunderstorm if one is currently reported as in progress at the station. 

(2) Increases in intensity. 

(3) Ends_ (Special observation 15 minutes after thunder is last heard at station). 

1957 - Identification: 

(1) Thunder heard '~i thin past 15 minutes or, 

(2) overhead lightning or hail observed '"ithin the past 15 minutes when the local noise 

level is such as might prevent hearing thunder. 

1957 - Intensity: - Remains the same as 1947 

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HISTORY OF CIRCULAR N 

1st ed., 1928 ---------------------------- Instructions for Airway . Observers, Circular N 

2d ed., 1932 ----------------------------- Instructions for Airway Observers, Circular N 

3d ed., 1935 ----------------------------- Instructions for Airtvay Heteorological Service 

Circular N. 

4th ed., 1939 ---------------------------- Iflstructions for Airway Meteorological Service, 

Circular N. 

S.th ed., 1941 ........................................................ Instructions for Airway Heteorological Service 

Circular N. 

6th ed., January 1949 ........................................ tvBAN Nanual of Surface Observations, Circular N 

6th e.d., revised June 1951 --------------- Hanual of Surface Observations (~VBAN), Circular 

N. 

7th ed., January 1955 --------------------Manual of Surface Observations (WBAN), Circular 

N. 

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88736 * U, S, GOVERNMENT PRINTING OFFICE: 1958 0- 45737~