TO: All Holders of Operations Manual
SUBJECT: Transmittal Memorandum for Operations Manual issuance 82-9
1. Material Transmitted:
WSOM Chapter B-11, Standards and Procedures for Surface Observing Program.
This chapter provides an updated description of the requirements and responsibilities for meteorological equipment and programs at National Weather Service weather observing land stations.
3. Effect on Other Instructions:
Cancels WSOM chapter B-11, issuance 74-11, dated May 8, 1974, and OML's 21-74, 15-75, 9-81, dated June 20, 1974, July 3, 1975, and October 15, 1981, respectively (filed with B-11).
Richard E. Hallgren
Director, National Weather Service
Issue Date Org. Code NATIONAL WEATHER SERVICE Part Chap.
5-18-82 W521x1 Operations Manual B 11
STANDARDS AND PROCEDURES FOR SURFACE OBSERVING PROGRAM
Table of Contents:
2.1 Instruments for Determining Cloud Heights
2.1.1 Requirement for Ceilometer
2.1.2 Required Backup Equipment for Measuring Cloud Heights
2.1.3 Responsibility for Relocating Ceilometer
2.2 Instruments for Determining Pressure
2.2.1 Requirement for Backup Pressure Sensor
2.3 Temperature Sensors
2.3.1 Requirement for Backup Temperature Sensors
2.4 Wind Measuring Equipment
2.4.1 Requirement for Backup Wind Measuring Equipment
2.5 Precipitation Gages
2.5.1 Requirement for Backup Precipitation Equipment
2.5.2 Snow Shields
2.6 Required Equipment for Measuring Solar Radiation/Amount of Sunshine
2.7 Instrumentation at Automated Stations
3.1 Siting and Exposure of Cloud Height Measuring Equipment
3.1.1 Rotating-beam Ceilometer (RBC)
3.1.3 Ceiling-light Projector
3.2 Siting Criteria for Visual Observations at Airports
3.2.1 Obtaining Location of Airport Reference Point (ARP)
3.3 Siting and Exposure of Visibility Sensors at Automatic Stations
3.4 Siting and Exposure of Pressure Sensors
3.5 Siting and Exposure of Temperature Sensors
3.5.1 Maintenance of Temperature Sensor Site
3.6 Siting and Exposure of Wind Equipment
3.7 Siting and Exposure of Precipitation Gages
3.8 Siting and Exposure of Radiometers and Sunshine Switches
4.1 Procurement of Instruments
4.2 Instrument Documentation
4.3 Determination of Station Elevation (Hp)
4.4 Preparation of Barometer Correction Cards
4.5 Barometer Comparisons
4.5.1 Responsibility for Barometer Comparisons
4.6 Preparation of Pressure-reduction Tables
4.7 Preparation of Cloud Height Tables
4.8 Preparation of Visibility Charts
4.9 Omission of Precipitation Measurements from Station Programs
1.1 Purpose. This chapter describes the requirements and responsibilities for meteorological equipment and programs at National Weather Service (NWS) weather observing land stations. While requirements cannot always be met, they are goals to be achieved as stations are changed, programs are modified, or new stations are established. No change is expected to be made solely to comply with policies described in this chapter.
Section 2 -- Describes the policies of the NWS related to station instrumentation.
Section 3 -- Describes the optimum siting and exposure criteria for the equipment.
Section 4 -- Describes policies related to program management.
Section 5 -- Describes the NWS policy for maintaining the continuity of climatological data and the
procedures used toward that end.
a. Rotating-beam ceilometer (used both day and night).
b. Balloons (used both day and night).
c. Ceiling lights (used at night).
Instructions on the use of these instruments are given in FMH No. 1, Surface Observations.
2.1.2 Required Backup Equipment for Measuring Cloud Heights. All stations will maintain a cloud height measuring capability with appropriate backup. Any combination of instruments listed in section 2.1 above that allows for day and night operations with backup capability will suffice.
2.1.3 Responsibility for Relocating Ceilometer. In general, the responsibility for relocating the ceilometer will rest with the agency necessitating or requiring the move. For example, if the Instrument Landing System (ILS) runway is changed, the agency changing the ILS runway is responsible for relocating the ceilometer.
Aneroid Barometers. These are authorized for use at all classes of stations. Airport stations normally nave an altimeter setting indicator, while other stations may have the precision aneroid. All NWS stations taking surface observations need one or the other.
Mercury Barometer. This is the basic barometer used by the NWS. One mercury barometer is required at all NWS stations making either basic weather or synoptic observations.
Barograph. This sensor provides a record of pressure and may be used at any station to determine station pressure provided it is compared with a mercury barometer at 6-hour intervals in accordance with FMH No. 1. A barograph is required at all stations reporting pressure tendencies.
Aircraft-type altimeters. These instruments are not authorized for use at NWS stations. However, at other stations, they may be used to determine the altimeter setting if routinely compared with other pressure measuring sources in accordance with instructions in FMH No. 9, Aviation Weather Observations.
Hygrothermometers. These are used to determine ambient air, dew point, and maximum and minimum temperatures. Hygrothermometers are used where it is necessary to remote the sensors to obtain suitable sensor exposure.
Liquid-in-glass Thermometers. These are used to determine ambient air, wet-bulb, and maximum and minimum temperatures.
Thermographs. These sensors can be used where temperature data are required when personnel are not on-duty. Thermographs are used to determine air temperature only.
2.3.1 Requirement for Backup Temperature Sensors. All staffed stations should have two systems for determining temperature data. Where the hygrothermometer or similar system is the station standard, it should be backed up with a sling psychrometer or a pair of liquid-in-glass thermometers mounted in a properly exposed instrument shelter. If the standard system is a liquid-in-glass thermometer system, the backup should consist of spare thermometers. The thermograph can also serve as a backup for the liquid-in-glass thermometers if an instrument shelter is available and dew-point temperature is not required.
2.4 Wind Measuring Equipment. For all basic weather observations, the NWS wind measuring system consists essentially of a cup-rotor anemometer and a spread-tail wind direction vane with associated speed and direction indicators.
2.4.1 Requirement for Backup Wind Measuring Equipment. There is no requirement for backup. If instruments are not available, wind direction and speed should be estimated according to directions given in FMH No. 1.
2.5 Precipitation Gages. The 8-inch nonrecording gage is the standard gage used at all NWS stations. However, as a general rule, if 10 percent or more of the annual precipitation (liquid and water-equivalent of solid) occurs as snow and climatic summaries are prepared for the station, the station should be equipped with a weighing gage. If a station is to make basic weather or synoptic observations and is to report precipitation amounts, it should be equipped with a gage capable of determining precipitation amounts to the nearest 0.01 inch.
2.5.1 Requirement for Backup Precipitation Equipment. An 8-inch nonrecording gage should be maintained as a backup for a weighing gage. It should also be available to take core samples for determining the water equivalent of solid precipitation. No backup is required for the 8-inch nonrecording precipitation gage. However, a spare receiver funnel, measuring tube, and stick should be kept on hand.
2.5.2 Snow Shields. Alter-type snow shields should be used on gages in areas where 20 percent or more of annual precipitation falls as snow. However, it is not necessary to shield tipping-bucket rain gages unless the gage is heated.
Pyranometer and Pyrheliometer. These are radiometers used to collect information on the amount and distribution of solar radiation received at the earth's surface.
Sunshine Switch. This instrument is used to determine the duration and time of occurrence of the visibility of the sun's disk.
3.1.1 Rotating-beam Ceilometer (RBC). Locate the RBC either in the vicinity of the ILS middle marker in the approach zone of the primary instrument runway or 3,500 feet from the approach zone of the primary instrument runway. The projector and detector should be as near the middle marker as possible and be parallel to the approach path. However, it may be installed at an angle to the approach path if necessary because of terrain, obstructions, etc. To minimize interference, locate the detector as far as possible from strobe lights, other modulated light sources, radio transmitters, source of vibration, etc. It is desirable, but not necessary, that the detector be located at the same level or in line-of-sight of the projector. If it is not feasible to make the RBC installations at the ILS site, alternate choices are:
A site along the approach path toward the airport from either the middle marker or a point 3,500 feet from the end of the primary instrument runway, or
A site on the airport near the approach end of the primary instrument runway which will not violate obstruction clearance criteria.
If an RBC has a single detector and is to serve an airport permitting aircraft operations with visibilities of less than one-quarter mile, install the RBC on a 400-foot baseline. Otherwise, install single detector RBC's with a baseline between 600 and 1,000 feet, but as near 800 feet as possible. Further detailed siting and installation requirements for the RBC are given in "Instruction Manual Rotating-beam Ceilometer System," May 1967.
3.1.2 Balloons. The inflation building for storing hydrogen or helium and the balloon release site must be readily accessible to the observer. The release site should be relatively free of obstacles that might interfere with the balloon's ascent or obstruct the observer's view of the ascent of the balloon. Except in unusual circumstances, do not use hydrogen to inflate ceiling balloons unless it is also used at the station to inflate raob or pilot balloons. Where ceiling balloons are inflated within the office, use helium. If hydrogen is used, take extreme care when handling and storing this gas since it is extremely flammable. Do not inflate balloons within the office quarters. The same safety rules given in chapter B-45 for RAOB and pilot balloons also apply to the use of hydrogen for ceiling balloons.
3.1.3 Ceiling-light Projector. Before installing the ceiling-light projector, determine the desired baseline, i.e., the distance between the projector and the standpoint of the observer. When determining the baseline, consider the frequency of low visibilities, the most significant cloud heights, and the background lighting. Frequent low visibilities and high background light tend to make a shorter baseline desirable. In general, the baseline should be around 500 to 800 feet. The baseline should be no shorter than one-third the most significant cloud height, and the maximum length should be no more than 1,000 feet. After determining the desired baseline, and with the standpoint convenient to the observer, locate the projector so that there will be a minimum amount of background light and obstructions along the baseline. If possible, install the projector in the direction toward the runway most often used for instrument (or bad weather) approaches. If possible, isolate the projector from vehicle and pedestrian traffic.
3.2 Siting Criteria for Visual Observations at Airports. The site from which visual weather observations are made should ideally be the Airport Reference Point (ARP) to give representative indications of weather conditions in the areas of aircraft approaches, landings, and takeoffs. Since this is not usually practical, the site should be as close to the ARP as possible, except in unusual circumstances, no more than 2 statute miles from the ARP. The site should have an essentially unobstructed view of:
the most frequently used instrument runway and its final approach zone, and
at least half of each quadrant of the natural horizon.
3.2.1 Obtaining Location of Airport Reference Point (ARP). The ARP is indicated on the Department of Transportation/Federal Aviation Administration (DOT/FAA) approved airport layout plan. The coordinates (latitude/longitude) of the ARP for an airport are published on the Department of Transportation/FAA Airport Master Record and in the Airport/Facility Directory (A/FD) published by National Ocean Survey (NOS). Therefore, the published position should suffice for determining the location of the visual weather observation site. The A/FD is readily available from NOS.
Based on FAA Advisory Circular 150/5300-4B dated December 5, 1976, the FAA-approved airport layout plan, containing all proposed developments, is used to determine ARP. Runways to be abandoned or closed are not considered in this determination. The only time that a redetermination of ARP is necessary is when airport developments or changes are proposed.
3.3 Siting and Exposure of Visibility Sensors at Automatic Stations. Expose the visibility sensor so that it will render visibility that is as representative of the prevailing visibility as possible. Avoid placing the instrument in an area that is subject to very localized obstructions to vision or in an area that is frequently free of an obstruction to vision when it is present in the surrounding area. For example, do not locate the sensor:
in a low place where obstructions to vision are trapped,
on a high place from which obstructions to vision will be swept away by the slightest breeze,
near sources of contamination, such as vents, stacks, etc., or
near bluffs, sharp grades, etc., since turbulence and temperature changes will frequently. produce nonrepresentative conditions.
3.4 Siting and Exposure of Pressure Sensors. Locate sensors so they are free of jarring, vibration, and rapid temperature fluctuations. Avoid installing them ln a location where they will be exposed to direct sunlight, drafts from open windows or doors, warm air currents from heaters, or cool air currents from air conditioners. Detailed instructions for installing barometers are given in the Manual of Barometry.
over level terrain (earth or sod) that is typical of the area around the station.
at least 100 feet from any extensive concrete or paved surface and at least 500 feet from any structure, building, or area that might influence readings, e.g., jet-blast areas, cooling towers, etc.
In considering possible sites, avoid areas where rough terrain or air drainage would result in nonrepresentative temperature data. Avoid swampy locations or other areas where water tends to collect and remain after precipitation and areas subject to artificial irrigation. Also avoid areas where frequent drifting of snow occurs. At locations where the temperature sensors are to be mounted within an instrument shelter, position the shelter so that it opens to the north in the Northern Hemisphere with the floor 4 to 6 feet above the ground. In the case of remoted sensors not exposed in instrument shelters, e.g., the hygrothermometer, mount the device so that the air intake is 4 to 6 feet above the ground. If the area is subject to large amounts of snow, install the sensors so that the height can be adjusted to a level 4 to 6 feet above the top of the snow.
If, however, the temperature data are required primarily for a special purpose, that purpose should determine the exposure. For example, if the temperature affecting a low-lying crop is required, the temperature sensor should be exposed at the level of the crop, or if temperature and dew point affecting aircraft operations are required, the sensors may be exposed closer to paved areas than generally desirable.
3.5.1 Maintenance of Temperature Sensor Site. It is essential that the temperature sensors have an unobstructed flow of air. Keep grass or other vegetation within 100 feet of the sensors clipped to a height of less than 10 inches.
3.6 Siting and Exposure of Wind Equipment. The following gives a brief description of the siting requirements for wind sensors installed to satisfy the general requirement for wind data. These siting requirements do not apply to sensors installed for special purposes, e.g., evaporation studies.
If the site is at an airport, it should be located near the center of the runway complex such that wind observations will be representative of conditions in the average lift-off and touch-down areas.
The site should be relatively level. Small gradual slopes are acceptable but avoid ravines, bluffs, ridges, etc., which cause eddy currents. The site should also be as far as practical from and, if possible, climatologically upstream from objects obstructing the free flow of air.
The standard height above the ground for wind sensors is 10 meters (32.8 feet). If local restrictions prevent installing the sensors at the 10-meter standard, install them 20 feet above the ground.
More detailed instructions on the site selection are given in the "Instruction Manual for F420 Series Wind Equipment."
3.7 Siting and Exposure of Precipitation Gages. An exact definition of what constitutes good exposure is difficult to state. Good exposure for measurement of precipitation may be opposite to the conditions that would constitute a good exposure for measurement of areal index for evaporation or wind. The best exposure for a precipitation gage is a location where the gage is protected from wind movements and turbulent wind flow. In general, the gage site should have protection in all directions by objects of uniform height. The height of these objects above the gage should be at least half the distance from the gage, but the height should not exceed their distance from the gage. Isolated or uneven protection near the gage should be avoided because of the variable and unpredictable effect on the gage catch.
3.8 Siting and Exposure of Radiometers and Sunshine Switches. The ideal exposure site for these instruments is one that is free from any obstruction within the field of view of the plane of the sensing element. If compromise is necessary, the site should be as free from obstructions as possible in the sector of the sky traversed by the sun, e.g., from ENE through south to WNW in the Northern Hemisphere. Locate the radiometer and sunshine switch so that:
shadows will not be cast upon them at any time, except for time check purposes,
they are not in close proximity to light colored walls or other objects likely to reflect sunlight directly on the sensor,
they are not subject to radiation from sources other than the sun, earth, or other intended source, and
the underlying terrain conforms (if appropriate) to any requirements for the type of radiometer or sunshine switch or the objective of the measurements being made.
4.3 Determination of Station Elevation (Hp). At new airport stations, the Hp should be equal to the Field Elevation (Ha) rounded to the nearest foot. At nonairport stations, the Hp should be equal to the height of the barometer (Hz) rounded to the nearest foot. At existing stations, the Hp shall be revised in accordance with the above whenever there is some other reason to issue new elevation data for the station and the difference between the old and revised Hp would exceed 50 feet. Changes in Hp are made by W521x1.
4.4 Preparation of Barometer Correction Cards. The regional headquarters (RH) may prepare Barometer Correction Cards for stations in its region. However, upon request, W521x1 will provide the new card. W521x1 will prepare all cards requiring a new gravity correction. To obtain a new card (or revised card) from W521x1, the following information must be provided:
Latitude and longitude (in degrees and minutes),
Hp and Hz (to nearest foot),
Barometer serial number,
Scale error and capillarity, and
4.5 Barometer Comparisons. Pressure measurements at all civil stations over which the NWS exercises quality control of the observation program shall be related to the Primary Standard Barometer (PSB) located at the WSH. This is accomplished through a series of barometer comparisons. Regional Reference Barometers (RRB's) shall be compared to the PSB once every 2 years and assigned Residual Corrections as necessary to adjust their reading to the PSB. The RRB's shall then be compared with the mercury barometers used at field stations at the time of the normal station visitation. NOTE: Residual Corrections approved by the WSH for station barometers are based on the comparisons between the RRB's and station mercury barometers. Therefore, take care that the comparisons are made accurately and that the correction necessary to relate the station barometer to the PSB is reported to WSH on the station inspection report, WS Form B-33.
4.5.1 Responsibility for Barometer Comparisons. W515 is responsible for arranging periodic comparisons between the PSB and the RRB's. The RH's are responsible for comparisons between the RRB's and mercury barometers at field stations over which the NWS exercises quality control. Field station supervisors are responsible for comparing all pressure sensors on station in accordance with FMH No. 1.
4.6 Preparation of Pressure-reduction Tables. W521x1 will prepare these tables for individual stations upon request. Reduction-ratio tables will be prepared unless the station cannot use a pressure-reduction computer. In that case, W521x1 will prepare pressure-reduction tables. To obtain these tables, the following information must be provided:
Station name and type (include a brief description if necessary to distinguish between several stations in the same area, e.g., in POST OfFICE, AT WINTWORTH RESIDENCE, etc.),
Field Elevation (Ha), Station Elevation (Hp), and Height of Barometer (Hz), all to the nearest foot, and
Latitude and longitude (in degrees and minutes).
W521x1 will send three copies of the tables to the RH. Upon receipt of the tables from WSH, the RH will retain one copy, send one copy to the observing stations, and one copy to the office verifying the station's observations.
4.7 Preparation of Cloud Height Tables. Cloud height tables for use with ceilometers or ceiling lights may be prepared either on station or by the RH. In either case, the tables shall be verified by the supervising station or RH. In addition to the relationship between elevation angle and cloud height, the tables should contain:
the identity of the system with which tables are to be used.
the height Or surface upon which the heights are based. At airport locations, this would normally be the field elevation, Ha. At other locations, this would normally be the height of the earth's surface at the station if a field elevation has not been established.
the height difference of the instrument with respect to the field or surface elevation. This is the height of the projector trunnion in the case of rotating-beam ceilometers or the height of the observer's standpoint in the case of ceiling lights.
the length of the baseline. This is the horizontal distance between the detector and projector.
This same information must be included when requesting cloud height tables from the RH. Instructions for preparing these tables are given in FMH No. 1.
4.8 Preparation of Visibility Charts. All fixed stations reporting visibility must have some form of visibility chart to aid in the observation and to document the markers used to determine the visibility. If more than one site is used for visibility observations, a separate chart should be prepared for each site. For this purpose, the phrase "visibility chart" includes any method which clearly documents the identity, location, and distance to the official visibility markers and whether they are daytime or nighttime markers. The visibility charts should be prepared on station, but verified by either the RH or supervising station prior to being used. The official in charge at the observing station is responsible for keeping the visibility charts up to date and advising the RH of any required changes. The RH is responsible for verifying that the visibility charts are current at the time of each station inspection.
4.9 Omission of Precipitation Measurements from Station Programs. When a site cannot be selected that is acceptable to both the Regional Data Acquisition Division and Regional Hydrologist (or the office requiring the data), the RH may authorize a station to omit some or all precipitation measurements from the observational routine of the station. When this is done, record the reason for omitting the data on page 1 of WS Form A-3.
5.1 Determination of Climatological Station Elevation (Hpc). The Hpc is the level to which climatological pressure data for a given station applies. At stations that have been in operation since January 1, 1930, the Hpc is generally equal to the height of the barometer on that date. At stations established after that date, the Hpc is generally equal to the initial Station Elevation. When a station is moved, the WSH in coordination with the Environmental Data and Information Service should determine whether to establish a new Hpc for the station or continue the old Hpc.
5.1.1 Preparation of Hp-Hpc Correction Tables. W521x1 will prepare Hp-Hpc Correction Tables for field stations whenever there is a difference between the two elevations and climatic summaries that include pressure data are prepared for the station. One copy of the correction table will be sent by W521x1 to the National Climatic Center and two copies will be sent to the RH. The RH will in turn forward one copy to the field stations.
5.2 Requirement for Precipitation Comparisons. If practical, NWS stations for which climatic summaries are prepared shall make comparative readings, as described in paragraph 5.2.1 below, whenever a significant change is made in the exposure or location of the station standard rain gage. For the purpose of these comparisons, a change in shielding of the gage does not represent a change in exposure.
5.2.1 Procedure for Making Precipitation Comparisons. Measure the precipitation at the old site or with the old gage daily or after each period of precipitation. Record the amounts determined, clearly identified in block 90 of MF1-10 or in the remarks section if other observational forms are used. Continue these measurements for a period of from 1 to 3 years.
5.3 Requirement for Temperature Comparisons. At NWS stations for which climatic summaries are prepared, pay close attention to changes made in the station sensor that might affect the data. A temperature comparison routine may be required. Changes that can significantly affect temperature data include:
a change in observation site from roof to ground; an urban to rural environment; sheltered location to a runway complex, one airport to another, or
a change from nonaspirated thermometer mounted in a shelter to an aspirated sensor such as a hygrothermometer. NOTE: A comparison is not required if a hygrothermometer replaces another hygrothermometer at the same site.
5.3.1 Procedure for Making Temperature Comparisons. Determine and record the daily maximum and minimum temperatures at the old site or with the old sensors for 1 year. Record these temperatures, clearly identified in block 90 of MF1-10 or in the remarks section if other observational forms are used. If the temperatures are normally read more than once per day, e.g., every 6 hours, the comparative readings should be made at the same time. When comparisons are made because of a change in the location of sensors and an instrument shelter is available at the old location, a thermograph may be used for the comparisons. Details on a temperature comparison routine are given in FMH No. 1.