Section 10. Wind/Altimeter Information


Air traffic managers must designate in a facility directive which wind sources must be used for operational purposes.

  1. Towers equipped with LLWAS may use direct dial or LLWAS wind information for weather observations, except where automated wind information is available.
  2. Approach control facilities may use direct dial, LLWAS, or automated display wind information for operational purposes.
  3. FSSs must use direct dial or automated display wind information for operational purposes.
  4. Other exceptions must be referred to the Manager of System Safety and Procedures for approval.

All FAA facilities having an associated NWS office or military weather station using the same sensing equipment must compare wind direction and speed indicator readings at the beginning of each work day with those of the NWS or military weather station, keeping in mind that the NWS wind direction equipment are oriented to true north. Apply the magnetic variation to ensure a correct reading. Coordinate the time of the cross-check and the associated procedures with the meteorologist-in-charge or other appropriate officer. Wind instrument errors must be handled as follows:

  1. If an FAA wind direction indicator is out of tolerance with other indicators on the same sensor by 5 degrees, or if the wind speed indicator reveals a disparity of plus or minus 5 knots, notify the appropriate maintenance personnel immediately for corrective action.
  2. If the indicators show an error of over 10 degrees or 10 knots, the equipment must be considered inoperative. In this case, obtain further wind information from other properly functioning wind instruments in the tower, local FSS, the NWS, or military weather office. Notify the appropriate maintenance personnel of all outages.
  1. At least two sources of altimeter setting information or an approved pressure standard are required in a TRACON, Radar Approach Control facility (RAPCON), Terminal Radar Approach Control in tower cab (TRACAB), Combined Center/RAPCON (CERAP), Radar Air Traffic Control Facility (RATCF), Army Radar Approach Control (ARAC), tower cab, and a FSS that takes weather observations and/or provides Local Airport Advisories (LAA). When two or more facilities are located on the same airport, the requirement may be reduced to one source of altimeter setting information per facility. Aircraft altimeters must not be used in reporting altimeter settings.


Stand alone RADAR approach control facilities (TRACON, RAPCON, RATCF, CERAP) not associated with a control tower are only required to maintain altimeter settings for those airports under their jurisdiction.

  1. Each of the following systems is considered to be one (1) source of altimeter setting information for the purposes of this paragraph:
  1. Automated Surface Observing System (ASOS)
  2. Automated Weather Observing System (AWOS)
  3. Stand Alone Weather Sensor (SAWS)
  4. Surface Weather System (SWS)
  5. Digital Altimeter Setting Indicator (DASI)
  6. Altimeter Setting Indicator (ASI)
  1. ASOS, AWOS, SAWS, and SWS systems are considered approved pressure standards for the purposes of this paragraph.
  1. Comparison checks against another source of altimeter setting information are not required for ASOS, AWOS, SAWS or SWS.


ASOS, AWOS, SAWS, and SWS are equipped with a minimum of two (2) and as many as three (3) digital pressure transducers.

  1. Facilities equipped with ASI or DASI:
  1. Compare the reading of each ASI daily with a collocated ASOS/AWOS/SAWS/SWS or with the altimeter setting issued by an associated facility with a commissioned ASOS/AWOS/SAWS/SWS that is located either on the airport or within the distances set forth in subparagraphs c and d.
  2. When the differences between the two altimeter settings exceeds 0.05 in. Hg. at nonprecision approach locations or 0.02 in. Hg. at precision approach locations, remove the instrument from service and notify Technical Operations personnel. When all ASI instruments in the facility are found to exceed the tolerances, report the altimeter setting as missing.
  3. When the difference is less than the tolerances specified in subparagraph 2 above, the value (+ or -) is applied as the correction factor to determine the operational altimeter setting.
  1. On dial-type display ASIs, post the correction factor directly on the face of the instrument. Use the same comparison procedures and determine the correction factor for each instrument in the facility.
  2. On DASI systems, local facility procedures must be developed in coordination with the associated Technical Operations office to make routine comparison checks with ASOS/AWOS/SAWS/SWS and adjust the DASI to display the correct altimeter setting.


Facilities that have DASI equipment that is not FAA owned or maintained must accomplish the procedures in paragraph 2-10-4, b1, b2 and b3(a) monthly.

  1. At ASI or DASI locations that are not collocated with a commissioned ASOS/AWOS/SAWS/SWS, make a comparison against the altimeter setting issued by an adjacent facility with a commissioned ASOS/AWOS/SAWS/SWS.
  1. At locations where precision approaches are conducted, the facility used for comparison must be located within 10 NM, and at both locations the wind speed must be 12 knots or less with no gusts above 15 knots.
  2. At all other locations the distance must not exceed 25 NM, and at both locations the wind speed must be 15 knots or less with no gusts above 20 knots.
  3. The difference in elevation does not exceed 100 feet at precision approach locations and 200 feet at all other locations.
  4. The station's temperature at both locations must be within 30 degrees Fahrenheit of the standard atmosphere temperature for the station's elevation.


The following formula may be used to determine the standard atmosphere temperature for station elevation:

T = Standard Temperature is 59°F

H = Field Elevation.

0.0036 Standard Atmospheric Temperature change per foot.

H x 0.0036 = Standard Temperature for station elevation.


  1. Tower A field elevation 600 feet: 600 x 0.0036 = 2.16°F of change, is rounded to 2°F.
    59°F - 2°F = 57°F standard temperature for Tower A adjusted for elevation.
  2. Tower B field elevation 700 feet: 700 x 0.0036 = 2.52°F of change, is rounded to 3°F.
    59°F - 3°F = 56°F standard temperature for Tower B adjusted for elevation.
    If both sites are between ± 30°F {87°F and 27°F for Tower A and 86°F and 26°F for Tower B} a comparison check is appropriate for temperature.
  1. Do not use altimeter setting values when the difference exceeds ± 0.02 in. Hg. at precision approach locations or ± 0.05 in. Hg. at all other locations.
  1. An approved pressure standard is required for routine altimeter setting comparison checks at all facilities that exceed the requirements of subparagraph c.

ARTCCs having a requirement for interphone delivery of altimeter settings, or changes of report, must make arrangements with FSS/terminals for delivery to associated sector/s.


Terminal and FSS facilities at airports with field elevations of 2,000 feet MSL or higher must broadcast a density altitude advisory to departing general aviation (GA) aircraft whenever the temperature reaches a certain level. These broadcasts must be made on ground control (GC), clearance delivery (CD), airport advisory, or automatic terminal information service (ATIS) as appropriate. Use the following table to determine broadcast applicability: (See TBL 2-10-1.)

TBL 2-10-1

 Broadcast Applicability

Field Elevations (MSL)

Broadcast Advisory When Temperature is

2,000 to 2,999

85°F and higher

3,000 to 3,999

80°F and higher

4,000 to 4,999

75°F and higher

5,000 to 5,999

70°F and higher

6,000 to 6,999

65°F and higher

7,000 and higher

60°F and higher