Handbook of Meteorology/Anemometers
THE MEASUREMENTS OF WIND VELOCITY:
ANEMOMETERS
Wind direction, wind velocity and the duration of sunshine are usually recorded on the same sheet, each by a separate magneto apparatus. The revolving drum is driven by a powerful clock.
A record of wind direction is required at all observation stations controlled by the various departments of the government. At seaports wind observations are made the subject of public bulletins and shipping interests are furnished not only with information to date but also with forecasts of expected changes. Warnings of dangerous winds are sent broadcast by wireless for the benefit of all vessels that may encounter them.
The requirements of air navigation are even more exacting than those of marine navigation; for, while the marine pilot needs to know the conditions of wind and weather at sea level, the airman must know them from sea level to an altitude of 10,000 feet or more.
Until recently Weather Bureau stations have been equipped with instruments for the study of surface winds only. The research laboratories, however, have made great advances in the study of air conditions at considerable heights, using kites and pilot balloons carrying recording instruments. Ordinarily the observer must depend on wind vanes, smoke columns, flags, dust movements and clouds for the determination of wind conditions.
Wind Direction.—The prevailing directions of the planetary winds are discussed in another chapter. Observers are concerned chiefly with the direction of the wind at the surface. Ordinarily this does not vary materially from the direction of lower cloud movement; sometimes it does vary, however, and when it does the fact should be recorded as cross-winds, the direction of each being noted. If the barometer is steady and the sky is free from clouds, the direction shown by a wind vane may be taken as the direction of the wind to the height of ordinary flight altitudes. Neither the surface winds nor the cloud winds indicate definitely the presence of the updraughts and downdraughts which constitute bumps and air holes.
Robinson anemometer, electrically connected with recording apparatus, P. 239.
The Wind Vane.—The wind vane of the spread-tail pattern, used by the Weather Bureau, is probably the most practical form in use. It holds steadily to the wind; it is sensitive enough to respond to a breeze of 2 miles an hour. The regulation vane is 6 feet over all. The tail is made of very thin board strips, thoroughly weather-proofed; the metal work is rust-proof; the bearings are constructed so that friction is reduced to the minimum. This is the general service vane designed to show wind direction only. For obvious reasons it should be mounted as high above ground as possible, and it should not be in the lee of anything that may affect the wind flow.
The vane used in connection with recording apparatus is 4 feet in length. The axis carries four partly overlapping cam-collars arranged so that at least one collar is in contact with the electrical recording apparatus. The latter prints a dotted line on the record sheet. If two cam-collars are in contact at the same time the intermediate direction of the wind is denoted. Thus, with both north and west cams in contact, and the pen of each recording, the direction of the wind is northwest. The adjustment of the box containing the contact apparatus should be made to the geographic meridian, and to solar and not standard time. Thus, if local time is 20 minutes faster than standard time and the date is May 27, the total correction will be 20 minutes plus 3 minutes. The sun will be on the geographic meridian at 23 minutes before 12:00 o’clock standard time, or 11:37 a.m. The shadow of the wind vane support cast on a horizontal surface will point due north when the sun is on the meridian.
Various devices are used to ascertain wind direction in times of very low velocity. A thread flown at the end of a stick fastened near the wind vane will often enable an observer to discover the direction of the wind when other evidence is absent. An ascending smoke column is swayed by a breeze too light to move a thread. The human face is exceedingly sensitive to the wind. The small boy who wets the ball of his forefinger and holds it against the air is using a method as old as the voyage of Jason in search of the Golden Fleece. Rather more uncertain is the movement of foliage. In many instances a wind vane whittled from a very thin strip of wood and perforated so as to whirl on a pin as an axis has been pretty nearly as serviceable as an expensive vane. Small vanes made of thin aluminium sheet metal, spread-tail in pattern, have answered every purpose required for ascertaining wind direction.
On the other hand, the commercial weather vane on a church steeple or a flagstaff may be an uncertain guide. Years of weathering may have rusted it fast to the spindle; and improper mounting may prevent its coming up to the wind by many degrees.
Obtaining wind direction from the movement of the clouds is frequently misleading as to results. Sometimes it happens that the surface wind blows from one direction, while clouds move toward another. If the clouds are low, the direction whence they come is most accurately obtained by facing them and then turning at a right angle to check the observation.
Cross-winds, that is wind currents of different directions, are more common than is generally known. Airmen have learned their meaning and watch sharply for them. They are apt to occur before and after a storm. At such times they practically mark the advancing or the retreating edge of a cyclonic movement. Billow clouds are the earmarks of cross-winds and such cross-winds are usually at a considerable height. Cross-winds are very common along the coasts of large bodies of water where the land and the sea breeze alternate. These alternating winds are shallow, however, and the airman usually finds the steady prevailing wind at an altitude of half a mile or more. The alternating mountain valley winds are cross-winds of similar character.
Cross-winds are not always discernible to the airman or to the marine pilot. They become visible as to position only when difference in temperature and humidity of the two layers produces cloudiness at the interface.
Wind Velocity.—The velocity of the wind at any locality varies greatly. The dead calm of tropical seas is frequently followed by hurricane winds having a velocity exceeding 100 miles an hour. The hurricane that wrecked Galveston blew with a velocity estimated at more than 125 miles an hour. At Cape Mendocino, California, a velocity of 144 miles was registered, and at Mount Washington a mean hourly velocity of in miles per hour was registered for a whole day.[1] At Battery Park, New York city, the anemometer has registered a velocity of 96 miles; and storm winds along the coast have reached a velocity of 100 miles a dozen times or more. Some of the strongest winds along the Atlantic Coast of the United States are storm winds of a recurved part of West Indian hurricanes.
For the greater part, the mean hourly velocity of the wind at the various stations provided with anemometers varies from 5 miles to 15 miles per hour. On sea and lake coasts it is materi- ally higher; and in a few mountain valleys it is lower than 3 miles.
Throughout the prairie region and the great plains, the wind is apt to be steady, its velocity varying but little during the daylight period. In regions where land breezes alternate with those from the sea, a short period of calm precedes each change.
Dial of the Robinson anemometer.
Each Weather Bureau station is provided with the standard anemometer of the Robinson pattern; most stations are equipped with the Friez triple-magnet register, which records both direction and velocity of the wind. Recording instruments of this character are used at the principal military and naval stations.
The cooperative observation sub-stations, outnumbering the regular stations about twenty to one, are not equipped with recording instruments except as they are procured at private expense. For all observers—volunteer, regular and Beaufort wind scale[2] affords a very good and practical method of approximate determination of wind velocity. The force numbers of the scale adopted by the Weather Bureau are the same as those of the British scale; the velocity in miles per hour corresponding to the force numbers differs considerably.
The table, p. 240, gives the Beaufort number, designation of wind and velocity as adopted by the Weather Bureau. The physical effects are those of the British scale. Cooperative and volunteer observers report merely the prevailing direction of the surface' winds, except as specifically directed. At the regular Weather Bureau stations the direction and velocity of upper winds are noted, a necessary step for the information of the rapidly growing air service. More information concerning the times of the daily maxima and minima of wind velocity is needed for the benefit of air service, and volunteer observers may be very helpful in obtaining this information.
The Anemometer.—For all ordinary purposes in the measurement of wind velocity the Robinson anemometer is almost universally used. For determining the mean velocity of the wind it is the best instrument in meteorological service. Inasmuch as it fails to record momentary gusts of wind perfectly, a Dines pressure anemometer is usually added to the equipment of regular stations. A wind meter of the Biram type is also useful in the measurement of wind gusts; it merely registers wind velocities without recording them.
The Robinson type of anemometer is simple in construction and does not easily get out of order. Four hemispherical cups fastened to arms 6.72 inches from axis to center, made fast to a spindle, communicate their motion to the measuring mechanism. The upper end of the spindle revolves in a sleeve; the lower end rests in an oil cup which also is a bearing. A worm screw thread near the lower end engages a train of wheels. Two of these are registering disks turning on the same axis.
| Beaufort number |
Designation | Physical effects (on land) | Pressure lbs. per sq. ft. |
Meters per second (Br.) |
Miles per hour | |
| Br. | U.S. | |||||
| 0 | Calm | Calm; smoke rises vertically | 0 | Less than 0.3 |
Less than 1 |
0-3 |
| 1 | Light air | Direction of wind shown by smoke drift, but not by wind vanes | .01 | 0.3-1.5 | 1-3 | 3-8 |
| 2 | Slight breeze | Wind felt on the face; leaves rustle; ordinary vane moved by wind | .08 | 1.6-3.3 | 4-7 | 8-13 |
| 3 | Gentle breeze | Leaves and small twigs in constant motion; wind extends light flag | .28 | 3.4-5.4 | 8-12 | 13-18 |
| 4 | Moderate breeze | Raises dust; small branches are moved | .67 | 5.5-7.9 | 13-18 | 18-23 |
| 5 | Fresh breeze | Small trees in leaf begin to sway; crsted wavelets form on inland waters | 1.31 | 8.0-10.7 | 19-24 | 23-28 |
| 6 | Strong breeze | Large branches in motion; umbrellas used with difficulty | 2.3 | 10.8-13.8 | 25-31 | 28-34 |
| 7 | Moderate gale | Whole trees in motion, inconvenience felt when walking against the wind | 3.6 | 13.9-17.1 | 32-38 | 34-40 |
| 8 | Fresh gale | Breaks twigs off trees; impedes progress | 5.4 | 17.2-20.7 | 39-46 | 40-48 |
| 9 | Strong gale | Slight structural damage occurs (chimney pots and slates removed) | 7.7 | 20.8-24.4 | 47-54 | 48-56 |
| 10 | Whole gale | Seldom experienced inland; trees uprotted; considerable structural damage occurs | 10.5 | 24.5-28.4 | 55-63 | 56-62 |
| 11 | Storm | Very rarely experienced; accompanied by widespread damage | 14.0 | 28.5-33.5 | 64-75 | 65-75 |
| 12 | Hurricane | Above 17.0 |
Above 33.6 |
Above 75 |
Over 75 | |
Millibar pressures are approximately one-half as great as pound per square inch pressures.
One of these wheels contains 100 teeth and the other 99. By means of this differential motion the registration of the number of miles up to 990 may be read directly.
A type of Robinson anemometer in which registration is made with index hands is also made. Its mechanism does not differ otherwise from the ordinary type. One of the train of geared wheels in the ordinary form of anemometer is cut with 50 teeth. If replaced with one cut with 62 teeth the disks will record kilometers instead of miles. If an interchangeable gear of this sort is desirable, it is better to have the necessary adjustments made by the manufacturer.
Several types of magnet recorders are used with the Robinson anemometer. In the type most commonly used, an arm carrying the recording pen is attached to the armature of the magnet. The revolving disk carries the studs representing miles of wind movement against the spring which acts as a circuit closer; the recording pen thereupon makes an offset from the straight line on the record sheet. Each offset represents theoretically a mile of wind. As soon as the stud passes the contact spring—a matter of from five to ten seconds—the pen is drawn back to its normal position. The fourth and fifth studs are bridged and the closed circuit makes the offset which covers a theoretical mile of wind movement. This is a convenience which enables the number of miles to be counted in groups of ten. Should any intermediate mile-stud fail to record, the failure will not be lost; it is included in the count.
Caution is necessary at times in reading the closed circuit mile, especially when it represents a high velocity whose measure is to be determined closely. The observer must choose between closing and closing, or between opening and opening of the circuit. Because the opening—the “breaking”—of the circuit is quick and positive, this interval is considered preferable in determinations.
The record sheet is necessary in finding the time of maximum or of minimum velocity of the wind. From it one may also find the velocity for any hour of the day. And inasmuch as such information is frequently required in suits at court, accurate dating and time-checking of the record sheets are essential.
The totals for any specific time may be read from the record sheets. Daily and monthly totals may be read from the dial. Indeed, many observers prefer to make dial readings, except in cases where hour totals are required.
The establishment of the time of the daily maxima and the daily minima is a most useful problem for cooperative and volunteer observers to undertake. This is very easily solved from the daily record sheets, but close observation will enable an observer to get pretty accurate results without the aid of instruments. Daily observations may be summarized in monthly
Wind velocity recording apparatus.
averages, and from these the seasonal averages may be determined.
To obviate the inconvenience of changing record sheets at midnight, they are most commonly changed at noon. A day’s record consists of the two lower lines of one sheet and the two upper lines of the sheet for the day following. The aggregates may be kept in half-day totals, but it is better to carry them over and enter them on the record sheet of the following day. The necessary thing is a definite plan followed with intelligence rather than slavish exactness.
Changing anemograph sheets precisely at noon is highly desirable. If the clock is either fast or slow, its rate is best established at that time. The pressure of the pen on the sheet is bound to vary slightly from day to day, and this in itself is likely to cause the rate of the clock to change. The pen should touch the paper positively but lightly.
The pen requires frequent cleaning; whenever the nib appears clogged it should be wiped with a bit of soft rag. Half a dozen times a year it should be removed and made thoroughly clean, scraping off the sediment that washing will not remove. A clean pen and good ink will leave a record as clear and clean as though the lines had been made with a drawing pen.
The wiring plan for the triple register is set forth in detail in Circular D, Instrument Division, and the details need not be rehearsed here. Wherever wires pass through woodwork, porcelain tube insulators are required by insurance regulations. If the wiring is situated where contact with electric light wires is possible, heavily insulated wires, such as are prescribed by local regulations, should be used. All permanent wire joints and splices should be snugly twisted a length of 2 inches—or, better, soldered—and wrapped with tape. All outside wires should be held by insulators.
Batteries.—In operating a triple register, 10 cells of battery are required; for the wind-direction register, 4 cells; for the anemometer, 3 cells; for the sunshine and rainfall recorder, 3 cells in common. For a 2-magnet register, recording wind-velocity and sunshine, 3 cells in common are sufficient. Unless the electromotive force is strong, however, the sunshine recorder may fail to register when the anemometer contact is on the bridge. This, however, need not lead to error; the bridge contact is always a long offset on the anemograph.
The requirement of battery cells for magnet registers is steadiness rather than strength. Dry cells run down in electro-motive force so quickly that, if the wind is on the bridge for more than a few minutes, the sunshine recorder fails to register unless operated by a separate battery. The ordinary “wet cells” are not much better.
When storage batteries suitable for the work are not available, the cells of the Edison primary type are the best. Such a cell properly charged gives a feeble but steady current on a closed circuit for nearly 400 hours. It will operate a 2-magnet register for more than a year.
- ↑ February 27, 1886. In January, 1878, a velocity of 186 miles was recorded.
- ↑ The scale was devised by Admiral Beaufort in 1805, chiefly to advise sailing-masters of the kind and spread of sail which ships of the line might carry and their probable speed under such sail. Subsequently it was addressed to fishing smacks and trawlers. More recently it was revised for the benefit of weather observers. A few meteorologists have used it; many observers regard it as too complicated to be of practical use. Most observers express their estimate of wind velocity in very few terms: as, breeze, light wind, strong wind, and gale.