Wind is measured in terms of velocity, a vector that includes direction and speed. The absence of apparent motion of the air is termed CALM. The direction and speed of the wind should be measured in an unsheltered, unobstructed area. This will avoid, to a large degree, the measuring of wind directions and speeds disturbed by local obstructions and will result in the reporting of winds more representative of the general weather patterns and more representative for aircraft operations.

a.  Wind. As used in this chapter, wind is the horizontal motion of the air past a given point.

b.  Direction of Wind. Wind direction is defined as the direction, in tens of degrees, from which the wind is blowing.

c.  Speed of Wind. Wind speed is the rate of horizontal flow of air past a given point, measured in knots.

d.  Gust. A gust is a rapid fluctuation in wind speed with a variation of 10 knots or more between peaks and lulls. The wind speed data for the most recent 10 minutes shall be examined to evaluate the occurrence of gusts.

e.  Magnetic Variation. Magnetic variation is the difference in degrees between true north and magnetic north. It is either "east" or "west" according to whether the compass needle points to the east or west of the geographical meridian.

f.  Hourly Peak Wind Speed. Peak wind is the highest instantaneous wind speed over 25 knots, recorded since the last METAR report.

g.  Variable Wind Direction. Wind direction is considered to be variable when, during the 2-minute evaluation period, it fluctuates by 60 degrees or more and the wind speed is more than 6 knots. The wind direction may also be considered variable if, during the 2-minute evaluation period, the wind speed is 6 knots or less.

h.  Wind Shift. Wind shift is a term applied to a change in wind direction of 45 degrees or more which takes place in less than 15 minutes and has sustained winds of 10 knots or more throughout the wind shift.

Wind direction, speed, and gusts shall be determined at all stations. All other wind-related parameters shall be determined at designated stations.

The observer shall determine the wind direction by averaging the observed direction over a 2-minute interval when direct-reading dials or recorders are used. Wind direction shall be reported in all observations. In all observations transmitted long-line, direction shall be reported in tens of degrees with reference to true north. The format for reporting wind direction in such observations is given in paragraph 15-11, Wind Group. For local use, wind direction shall be reported in tens of degrees with reference to magnetic north. (Note: Local displays of wind direction are always in reference to magnetic north. Direction must be converted to true for observational purposes.)

At facilities where instruments are not available for determining wind direction, the observer shall estimate the direction by observing the wind cone or tee, movement of twigs, leaves, smoke, etc., or by facing into the wind in an unsheltered area. When estimating wind direction, the observer shall note that even small obstacles may cause variations in the wind direction. The observer shall not use the movement of clouds, regardless of how low the clouds are, in estimating the surface 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. The format for reporting variable wind direction is given in paragraph 15-11b, Variable Wind Direction (Speeds 6 Knots or Less), and paragraph 15-11c, Variable Wind Direction (Speeds Greater than 6 Knots) .

The 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 wind speeds of 10 knots or more. Wind shifts are normally associated with some or all of the following phenomena characteristic of a cold-front passage. These phenomena are:

a.  Gusty winds shifting in a clockwise manner in the Northern Hemisphere

b.  Rapid drop in dew point

c.  Rapid drop in temperature

d.  Rapid rise in pressure

e.  In summer: Lightning, thunder, heavy rain, and hail

f.  In winter:  Frequent rain or snow showers

A SPECI shall be taken after a wind shift occurs. A remark reporting the wind shift, and the time the wind shift occurred, shall be included in the observation. A wind shift shall always be reported when it is observed. When the shift is believed to be associated with a frontal passage, the observer shall report FROPA in the remarks section immediately after the shift begins. When a SPECI report containing a wind shift is not given long-line dissemination, the observer shall include the wind shift data in the remarks section of the next transmitted report. The format for the remark is given in paragraph 15-24, Wind Shift.

If possible, the average wind speed should not be determined during a peak or a lull in gusty winds or squalls. The wind speed shall be determined by averaging the speed to the nearest knot over a 2-minute period. Where direct-reading dials or recorders are used, the observer shall determine the speed by averaging the observed values and applying the appropriate correction from Figure 8-1, Corrections to Indicated Wind Speeds, to the wind speed obtained from direct-reading dials or recorders. Wind speed shall be reported in all observations and shall always be reported in knots. The format for reporting wind speed is given in paragraph 15-11, Wind Group.

The observer shall use the Beaufort scale, Figure 8-2, Estimating Wind Speed, to estimate wind speeds if instruments are out of service.

Figure 8-2. Estimating Wind Speed

The existence of gusts can be determined easily by examining the wind recorder. They will be indicated by fluctuations of the speed by varying distances between mile marks on the recorder. The speed of a gust shall be the maximum instantaneous wind speed recorded during the most recent 10 minutes of observation time. If a recorder is not available, wind character may be determined from direct-reading dials. When a gust is detected within 10 minutes prior to an observation, the character of the wind shall be reported in the body of the observation. The format for reporting wind character is given in paragraph 15-11a, Gust.

The peak wind speed shall be the highest instantaneous speed, greater than 25 knots, observed or recorded since the last routine METAR report. The observer shall determine peak wind data for entry in the remarks section of surface observations.

a.  Peak wind data shall be determined at designated stations with wind speed recorders or by direct observation. If the wind speed record is incomplete, it may still be used, provided there is no indication that the peak wind speed occurred during the period of the missing data.

b.  Peak wind data shall be reported in the remarks section of the next routine METAR report whenever the peak wind speed exceeds 25 knots. The format for the remark is given in paragraph 15-23, Peak Wind.

When no motion of the air is detected, the wind shall be reported as calm; i.e., the direction and speed shall be reported as 00000KT. The format for reporting calm winds is given in paragraph 15-11d, Calm Wind.

To convert wind direction from degrees with respect to true north to degrees with respect to magnetic north, or vice versa, the observer shall obtain the local magnetic variation from an aeronautical chart and proceed as follows:

a.  To convert from true to magnetic wind:

(1)  Add westerly variation to true direction

(2)  Subtract easterly variation from true direction

b.  To convert from magnetic to true direction:

(1)  Add easterly variation to magnetic direction

(2)  Subtract westerly variation from magnetic direction

For example, at ABC Airport, the magnetic variation is 10º West. The local wind indicator is reading 250º (magnetic). When transmitting the wind direction in an observation, because the magnetic variation is 10º West, the observer should subtract 10º to transmit a direction of 240º (true).

(Note: Local displays of wind direction are always in reference to magnetic north. Automated weather observing systems also show direction with respect to magnetic north locally (when the AUX/WX page is displayed), but adjust wind direction to "true" for transmission.)

a.  Priority of Instruments. At facilities having several types of wind equipment, the observer shall use the following priority in selecting the wind equipment to be used.

(1)  Direct-reading recorders

(2)  Direct-reading dials

(3)  Other

b.  Determination of Direct-Reading Dial Indicators or Recorders. Values reported for the speed of a gust shall be the speeds indicated by recorders with the correction applied from Figure 8-1, Corrections to Indicated Wind Speeds. Direct-reading dials or estimation shall be used for determining gusts if a recorder is not available.

When a wind recorder becomes more than 5 minutes in error, the observer shall adjust the chart to the correct time and indicate the adjustment with an arrow and the date and time of adjustment.

The observer shall annotate wind recorder charts as follows:

a.  At the beginning and end of each chart roll, enter:

(1)  The station type (LAWRS, FSS, etc.) and name

(2)  A date/time group to indicate when the trace(s) began/ended

(3)  The chart feed rate if different from normal, or if times are not printed on the chart

b.  Enter other identification as necessary, e.g., when providing the chart for use in special studies or an aircraft accident investigation, enter the station name at the end of the roll and include the runway number to distinguish between chart rolls where multiple recorders are in use.

c.  Indicate time checks by drawing a short line on the chart and entering the time.

d.  Indicate maintenance shutdowns or other inoperative periods by entering date/time groups at the end of one period of operation and the beginning of the next.

e.  Whenever the chart feed rate is changed, enter a time check and an appropriate note; e.g., BEGIN 12 IN/HR, BEGIN 3 IN/HR.

The observer shall change charts at 0000 LST on the first day of each month and at intermediate times, as necessary, to prevent loss of record.

The observer shall replace charts in their original carton, if available, and enter station type, name, and period of record on the end of the carton. Completed charts shall be forwarded to NCDC along with the MF1M-10Cs for the month.

8-19.   CONVERSION OF WIND SPEED

Whenever conversion of wind speed from miles per hour to knots is required, the observer shall use the values given in Figure 8-3, Conversion of Miles per Hour to Knots. Whenever conversion of wind speed from knots to miles per hour is required, the observer shall use the values given in Figure 8-4, Conversion of Knots to Miles per Hour. NOTE: No other tables may be used in lieu of tables in Figures 8-3 and 8-4, below.

8-20.   OPERATION OF EQUIPMENT

Practices and procedures for the operation of wind instruments and related equipment are presented in paragraph 17-91, Wind Retransmitter - Weekly Check, and paragraph 17-92, Wind Retransmitter - Other Checks.