MANOBS Manual of Surface Weather Observations: wind

7.1 General

Wind is defined as air in motion. It represents the horizontal flow of air at a height of 10 m. Wind information is included in all surface weather reports and shall contain both velocity (direction and speed) and character. For meteorological purposes, the wind direction is reported with respect to true north and the speed is expressed in nautical miles per hour (knot).

7.1.1 Direction

The wind direction is the direction from which the air is moving. It represents the average direction during the two-minute period ending at the time of observation.

7.1.2 Speed

The wind speed is the rate of airflow past a fixed point. It represents the average speed during the two-minute period ending at the time of observation. The term "calm" is used to describe winds less than 2 kts.

7.1.3 Character

Wind character is reported when there are significant variations in the wind speed, during the ten-minute period ending at the time of the observation. The character will be reported as a "gust" or "squall" depending on the magnitude and duration of the variation.

7.1.3.1 Gusts

Gusts are sudden, rapid, and brief changes in the wind speed. They are characterized by the more or less continual fluctuations between the high (peak) and low (lull) speed.

7.1.3.1.1

These criteria for reporting gusts are the same as criteria used by autostations. Gusts shall be reported when:

  1. The highest peak speed is at least 5 kts higher than the current two-minute average, and
  2. The highest peak is at least 15 kts.

7.1.3.2 Squalls

Squalls are essentially gusts with a longer duration of higher speeds. Squall speeds are reported only by stations with a recording wind instrument and then only when the following criteria are recorded:

  1. The wind speed increases by 15 kts or more over the two-minute average speed that preceded the increase;
  2. The duration of the peak speed period is at least 2 minutes;
  3. The wind speed attains a one-minute mean of at least 20 kts, during the peak speed period;
  4. The wind speed diminishes by at least 5 kts.
7.1.3.2.1

The speed to be reported is the highest one-minute mean speed.

7.1.3.2.2

When the Beaufort scale is used for estimating wind speed, (see 7.4) the following criteria should be used for the reporting of squalls: "A sudden increase in wind speed, by at least three stages of the Beaufort scale, the speed rising to force 6 or more, lasting for at least two minutes, and then diminishing by at least one stage or more."

7.1.4 Wind Shift

Wind shift is a definite change in the general direction from which the wind is blowing.

7.1.4.1

A wind shift shall be recorded when all of the following occur:

  1. The direction from which the wind is blowing changes by 45° or more;
  2. The change in direction takes place in less than 15 minutes; and
  3. The average wind speed at the completion of the shift is 10 kts or more.

7.1.4.2

The time of the wind shift shall be the time at which the wind begins to shift.

Note: A change in wind speed from calm to 10 kts or more is not considered a wind shift.

7.1.5 Variations in wind direction

Wind direction is defined as variable when the wind direction varies by 60° or more and less than 180°, in the 10-minute period preceding the observation, and the mean wind speed is 3 kts or greater.

7.1.5.1

When direction is variable, the two extreme directions shall be reported in a clockwise fashion.

7.2 Wind instruments

The standard instrument for measuring the wind is the anemometer, which is normally exposed at the internationally agreed height of 10 m. Several types of anemometers are used in Canada. They are described in detail in the appropriate instrument manuals and briefly in the following pages.

7.2.1 The U2A system

This system consists essentially of two parts: a wind speed measuring system and a wind direction measuring system.

7.2.1.1

The wind speed measuring system comprises a small direct current generator driven by a cup wheel, which in turn is driven by the force of the wind. The output voltage of the generator is proportional to the speed of rotation of the cup wheel, and therefore proportional to the wind speed. The output of the wind speed detector is connected to a remote dial indicator by means of cables.

7.2.1.2

The wind direction measuring system comprises a precision positional motor or synchro, controlled by a wind vane, which in turn reacts to the ambient wind direction. The output of the detecting positional motor is connected to a receiving positional motor by means of cables. The receiving positional motor is fitted with a pointer and housed in a remote dial indicator.

7.2.1.3

The outputs from the wind speed and wind direction detectors may also be connected to a remote strip chart recorder, which provides a continuous analog record of the wind speed and wind direction at the station.

7.2.1.4

The wind speed system is self energized and requires no external power supply.

7.2.1.5

The wind direction system requires a power supply of 32 v or 115 v, 60 Hz connected through a suitable transformer and normally applied at an indicator panel.

7.2.1.6

The U2A anemometer has a fairly low lag factor and produces more or less instantaneous values of speed and direction. By examining the chart record for the appropriate period or by observing the dial indicators over a period of time, mean values of wind speed and direction can be approximated and gust or squall conditions determined.1 To obtain mean values, the dial indicators or the recorder chart traces should be observed for the required period. The position on the dial or chart over which the indicator or recorder pen moved for the major part of the time shall be determined, neglecting brief movements above and below this position. The mid-point of this position shall be taken as the mean value, for example, if the dial indicator or recorder pen moved mainly in the range 12 to 18 kts, with gusts up to 28 kts, and lulls down to 8 kts, the mean wind speed shall be taken as 15 kts. The same method shall be followed in determining the mean wind direction.

1 Note: See 7.5.3 and 7.6.4 for order of preference when both U2A dial and U2A recorder equipment are available.

7.2.2 The 78D anemometer system

The basic system consists of a 78D anemometer and a display. The anemometer is a cup and vane anemometer using U2A cups, vanes, and housings. The 78D anemometer uses low power, high reliability, optoelectronic transducers and has a built-in microcomputer to sample and calculate five-second vector components of wind. Every five seconds a wind message is transmitted to the display unit which provides further averaging for periods of two and ten minutes as well as displaying these values and determining and displaying significant wind gusts. The display unit operates on 115 VAC and supplies the 12 VDC necessary for the anemometer. The observer can select the two or ten-minute average wind on the display by means of a simple switch. Different configurations possible with the 78D system include the addition of an analog type chart display and multiple displays on a single anemometer.

7.2.3 Collocated automatic weather stations

Wind direction and speed may be derived hourly from transmitted reports of collocated automatic stations. Automatic stations currently in use, AWOS use a variety of sensors, generally exposed at the standard 10-metre height. The wind velocity is averaged over a two-minute period and the wind speed is reported in knots. Hourly wind reports from an AWOS station are averaged over a two-minute period; synoptic wind reports from an AWOS station are averaged over 10 minutes. Wind directions from all of the above autostations are reported in tens of degrees.

7.3 Anemometer unserviceable due to ice accretion

During periods of freezing precipitation (i.e. freezing rain, freezing drizzle or prolonged periods of freezing fog), wind sensing equipment is subjected to ice accretion which may reduce or cause complete failure of anemometer performance. The nature of this problem is that ice accretion on the wind speed detectors slows or totally impedes the rotation of the sensing cups, resulting in wind speed readings which are lower than actual wind speeds or reading "calm" when in fact they are not. Also, erroneous wind direction readings can occur due to ice accretion on the wind direction vane. Ice loading on the vane will reduce the efficiency of direction sensor movement or totally restrict movement, resulting in unreliable wind direction readings. As erroneous anemometer readings result in a real hazard to aviation, the observer must use extreme care when determining wind data during conditions of ice accretion.

7.3.1

The observer shall determine if ice accretion is resulting in an unserviceable anemometer and if so, estimate the wind direction and/or speed, see 7.4 to 7.4.3.

7.3.2 Determination of reliable wind data

In determining the reliability of wind data the observer should consider the following:

  • Visual ice accretion on detectors
  • Ice accretion rate on ice accretion indicator
  • Duration of freezing precipitation
  • Intensity of freezing precipitation
  • Comparison of wind data prior to ice accretion
  • Wind data inconsistent with windsock, flags or other visual reference
  • Erratic direction readings, sluggish response to direction variability
  • Speed readings lowering, sluggish response to gusts
  • Comparison of measured wind with sensation of actual wind on ones face or hands
  • Wind data in relationship to pressure system
  • Comparison of wind data with other stations in proximity

Note: Stations equipped with only 78D digital or U2A dial and no wind recorder may find it more difficult to recognize decreasing anemometer performance.

7.4 Estimation of wind

When suitable instruments are lacking or when the instruments are not in operating condition, the wind direction (to eight points of the compass) and the wind speed and character shall be estimated. This can be done with a fair degree of accuracy by observing the common effects of the wind.

7.4.1

The direction of the wind may be determined by watching a wind vane, windsock or the drift of smoke.

7.4.2

The speed may be estimated by using the Beaufort Scale of Winds which relates common effects of the wind and equivalent speeds in knots (see 7.7). Care must be taken when applying scale specifications (twigs, branches) as they may also be affected by ice accretion resulting in lower estimated wind speed.

7.4.3

If winds are estimated due to ice accretion, the following remark shall be included in the report: WND ESTD DUE ICE ACCRETION

7.4.4

If winds are estimated for reasons other than ice accretion, the following remark shall be included in the report: WND ESTD

7.5 Reporting wind - hourly observations

7.5.1

The wind direction and speed reported in the hourly observation shall be a two-minute mean. The direction shall be determined to the nearest ten degrees and the speed to the nearest knot. The occurrence of gusts, squalls and wind shifts shall be reported. The time of a wind shift shall also be recorded in Remarks (see 10.2.19.6).

7.5.2

The two-minute mean may be readily determined at stations equipped with dial or digital indicators or with a U2A recorder or other analog chart recorders.

7.5.3

When there is more than one type of wind instrument installed at a station, the following order of preference shall apply in determining both direction and speed.

  1. U2A or other analog chart recorder
  2. 78D display or voice generated module output
  3. U2A indicator (dial)
  4. Collocated Automatic Station (to obtain an estimated two-minute mean)

7.6 Reporting wind - synoptic observations

7.6.1

The wind direction and speed reported in the synoptic observation shall be a ten-minute mean. If the ten-minute interval prior to the observation includes a discontinuity in the wind data (i.e., an abrupt change in direction or speed or a break in the record), only data occurring after the discontinuity shall be used for obtaining mean values: Hence the time interval in these circumstances will be correspondingly reduced. The wind direction shall be reported in tens of degrees, using WMO code 0877 (see 12.3.2.2) and the speed shall be reported in knots.

7.6.2

The wind direction shall be observed to the nearest ten degrees or to the nearest 16 points of the compass, depending on the type of anemometer available or to eight points of the compass when it is necessary to estimate the wind direction. For record purposes, the wind direction shall be recorded to the nearest ten degrees.

7.6.3

Wind speeds in knots shall be determined from the anemometer. If speeds from the anemometer are obtained in miles per hour, they shall be converted to knots, using the "Conversion of miles per hour to knots" table, see 12.3.2.3.

7.6.4

When there is more than one type of wind instrument installed at a station, the following order of preference shall apply in determining both direction and speed:

  1. U2A recorder
  2. 78D display
  3. U2A indicator (dial)
  4. Collocated Automatic Station

7.7 Beaufort scale of winds

Beaufort Scale of Winds table providing descriptive terms, Beaufort force, speed range, knots average and specification for estimating speed
Force Knots average Knots Descriptive term Effects observed at sea Effects observed on land
0 0 Less than 1 Calm Sea surface like a mirror, but not necessarily flat. Smoke rises vertically.
1 2 1 - 3 Light air Ripples with the appearance of scales are formed, but without foam crests. Direction of wind shown by smoke drift, but not wind vanes.
2 5 4 - 6 Light breeze Small wavelets, still short but more pronounced. Crests do not break. When visibility good, horizon line always very clear. Wind felt on face. Leaves rustle. Ordinary vane moved by wind.
3 9 7 - 10 Gentle breeze Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered whitecaps. Leaves and small twigs in constant motion. Wind extends light flag.
4 14 11 - 16 Moderate breeze Small waves, becoming longer. Fairly frequent whitecaps. Raises dust and loose paper. Small branches are moved.
5 19 17 - 21 Fresh breeze Moderate waves, taking a more pronounced long form. Many whitecaps are formed. Chance of some spray. Small trees with leaves begin to sway. Crested wavelets form on inland waters.
6 25 22 - 27 Strong breeze Large waves begin to form. The white foam crests are more extensive everywhere. Probably some spray. Large branches in motion. Whisteling heard in telephone wires. Umbrellas used with difficulty.
7 31 28 - 33 Near gale Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. Whole trees in motion; inconvenience felt walking against wind.
8 37 34 - 40 Gale Moderately high waves of greater length. Edges of crests begin to break into the spindrift. The foam is blown in well-marked streaks along the direction of the wind. Breaks twigs off trees. Generally impedes progress. Walking into wind almost impossible.
9 44 41 - 47 Strong gale High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility. Slight structural damage occurs, e.g. roofing shingles may become loose or blow off.
10 52 48 - 55 Storm Very high waves with long overhanging crests. Dense white streaks of foam. Surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock-like. Visibility affected. Trees uprooted. Considerable structural damage occurs.
11 60 56 - 63 Violent storm Exceptionally high waves. Sea completely covered with long white patches of foam. Visibility affected. Widespread damage.
12 - 64+ Hurricane Air filled with foam and spray. Sea entirely white with foam. Visibility seriously impaired. Rare. Severe widespread damage to vegetation and significant structural damage possible.

7.7.1 Northern Beaufort scale and wind effects on land

Northern Beaufort Scale and Wind Effects on Land table
Beaufort number Speed range Knots average Effects
1 1 - 3 2 No noticeable wind. Smoke rises nearly vertically.
2 4 - 6 5 Wind felt on face, leaves rustle.
3 7 - 10 9 Hair is disturbed, clothing flaps.
4 11 - 16 14 Dust and loose paper raised, hair is disarranged.
5 17 - 21 19 Force of wind felt on body. Limit of agreeable wind on land.
6 22 - 27 25 Some inconvenience in walking.
7 28 - 33 31 Difficulty when walking against wind.
8 34 - 40 37 Difficulty with balance in walking.
9 41 - 47 44 Danger in being blown over.
10 48 - 55 52 Trees uprooted; considerable structural damage.
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