Point Observations

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Chapter 2

• Point Observations

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• Most measurements of weather elements (at least
  in the US) are made by instruments at specified
  locations (point source).
• Some elements may be evaluated by a human
  observer amount of sky cover, intensity of
  precipitation, visibility. (Mostly in other
  countries)
• An instrument is a device for making a
  measurement.
• A measurement is an action intended to assign a
  number as the value of a physical quantity in
  stated units. (E.g., temperature)
• To ensure that the measurement is accurate and
  representative, instruments are evaluated.
• Placement of the instruments are standardized.

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• ACCURACY. Degree to which the response of a
  sensor to its immediate or distant environmental
  signal conforms to the true (but never knowable)
  value of the quantity.
• TRUE VALUE. The value which is assumed to
  characterize a quantity in the conditions which
  exist at the moment when the quantity is
  observed. An ideal value which could be known
  only if all causes of error were eliminated.
• REPRESENTATIVENESS. How well it serves to
  characterize the state of the atmosphere in the
  vicinity of the instrument.
• SITING REQUIREMENTS. WMO establishes the siting
  requirements for observations for each type of
  instrument to ensure, as much as possible,
  standard methods are used to obtain measurements
  of the characteristics of the atmosphere.

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• TEMPERATURE At at a height between 1.25 m and
  2.00 m (standard of 1.5 m) above ground level.
  Over level ground. Screen (not thermometer)
  freely exposed to sun and wind and not shielded
  by, or close to, trees, buildings and other
  obstructions. At a station where snow is
  persistent and of varying depth, it is possible
  to use a support which allows the screen to be
  raised or lowered to maintain the correct height
  above the snow surface.
• The ground cover beneath the screen should be
  grass or, at places where grass does not grow,
  the natural surface of the district. The screen
  should white to reflect radiation and be kept
  clean to permit good airflow and to prevent
  changing the color of the screen.

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• ANEMOMETERS (WIND) An anemometer or vane should
  be exposed at a height of 10 meters (33 ft) above
  the ground or surrounding obstructions over open
  terrain. The distance between the anemometer and
  any obstruction is at least ten times the height
  of the obstruction. Where in open terrain the
  anemometer cannot be exposed at the standard
  height, then the observed wind speeds can be
  adjusted to provide an estimate of the wind at
  ten meters above ground using a mean variation of
  wind speed with height based on the following
  empirical formula
• Vh V100.233 0.656 log10(h 4.75)
• where Vh is the measured wind speed at height
  h meters and V10 is the wind speed, any units, at
  ten meters above the ground.

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• PRECIPITATION The gauge should be situated on
  flat ground and not be located near slopes,
  especially ground that slopes toward the
  direction from which the prevailing wind blows.
  The gauge should never be located on roofs or on
  walls.
• The distance of the gauge from surrounding high
  objects should be not less than twice the height
  of the object above the rim of the gauge. The
  further the distance the gauge is from object,
  the better.

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• Two methods to reduce wind turbulence about
  point-source rain gauges.

Turf wall
Alter type wind screen
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• METAR Observations
• Most surface observations in the US are obtained
  by ASOS or AWOS.
• ASOS - Automated Surface Observing System (NWS).
• AWOS - Automated Weather Observing System (FAA).
• Distributed in a code format called METAR.
• For reporting routine, hourly aviation
  observations. Provision for reporting special
  observations.
• Most located at airports.
• Also
• COOP - Cooperative Observer Network.
• C-MAN - Coastal Marine Automated Network.
• Buoys - Ocean Weather Buoys.

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(4) Graphs in Time

• By having a graph of parameter values with time,
  one can see how changes to that parameter are
  occurring, and thus get an indication of how the
  atmosphere is changing.
• The instantaneous change of a parameter, either
  by time or by distance, or both, is an important
  parameter in many equations used in meteorology
  to express the characteristics of the atmosphere.
  

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• To get the instantaneous rate of change of
  pressure
• Draw a line tangent to the trace at the time for
  which the instantaneous rate of change is
  desired.
• Compute the slope of that line. The slope would
  be

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• The accuracy depends, in part, on how well you
  can draw a tangent line.
• The barograph trace is an example of analog data
  - continuously sensing and recording.

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• METAR observational data is recorded at discrete
  time intervals and reported, typically, every
  hour - a digital data format.
• Meteograms are often used to display this data
  with respect to time.
• Some information may be lost by the digital
  method but usually the atmosphere changes slow
  enough that most of the important information is
  obtained.

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(3) METAR Observations

• The METAR code (FM 15-IX) is the international
  code to report routine, hourly weather conditions
  at air terminals. SPECI (FM 16-IX) is the name
  of the code for an aviation selected special
  weather report. The METAR/SPECI code format as
  used by the United States.
• _CCCC _YYGGggZ__dddff(f)GfmfmKT
  _dndndnVdxdxdx
• _VVVVVSM_
  _ww_
• _TT/TdTd_APhPhPhPh_RMK_(Automated, Manual,
  Plain Language)_(Additive Data and Automated
  Maintenance Indicators)
• A detailed description of the code is found at
• http//www.met.tamu.edu/documentation/metar.html

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• Observation from Madison, Wisconsin
• KMSN 231153Z 19008KT 6SM BR SCT006 BKN110 21/20
  A2986 RMK AO2 SLP108 60009 70275 T02110200 10211
  20206 55003
• From Orly Field, Paris, France
• LFPO 230930Z 01005KT 310V080 4000 -RA BR SCT005
  BKN017 BKN050 17/16 Q1011 TEMPO 4000 BKN010
• A-Altimeter setting in inches mercury.
• Q-Altimeter setting in whole hPa
• SLP-Sea level Pressure to tenth mb

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(5) Synoptic Observations

• Synoptic observations are taken by certain
  stations every 3 hours at 00Z, 06Z, 12Z, and 18Z.
  Intermediate synoptic observations are taken at
  03Z, 09Z, 15Z, and 21Z.
• The information is reported in the Synoptic Code,
  FM-12-IX.
• It consists of a series of 5-digit groups, the
  position of the group and the numbers within the
  groups determines the value of the parameter
  reported.
• Typically, additional information is reported
  which may not be in a METAR observation.

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• 1. Report Format for WMO Region IV (Bulletin
  format)
• MiMiMjMj YYGGiw
• IIiii iRiXhVV Nddff (00fff) 1snTTT 2snTdTdTd
  3PoPoPoPo 4PPPP 5appp
• 
  or 29UUU or 4a3hhh
• 6RRRtR 7wwW1W2 8NhCLCMCH (9GGgg)
• or 7wawaWa1Wa2
• 222// (0snTwTwTw) 1PwaPwaHwaHwa 2PwPwHwHw
  3dw1dw1dw2dw2 4Pw1Pw1Hw1Hw1 5Pw2Pw2Hw2Hw2
  70HwaHwaHwa
• 333 (0CsDLDMDH) 1snTxTxTx 2snTnTnTn 3Ejjj
  4E'sss 5j1j2j3j4 (j5j6j7j8j9) 7R24R24R24R24
  8NsChshs 9SpSpspsp
• 555(national code groups)

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• Synoptic Ob from JFK airport, New York
• 74486 11358 80810 10144 20128 30222 40232
  53005 60117 763// 91151 333 96010 555 92115
• The observation was taken on Aug. 21, 2007.
• The group YYGGiw was 21124 and is found (if not
  in a bulletin) after the first group.

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• Ship Synoptic report
• MiMiMjMj
• DDDD YYGGiw 99LaLaLa QcLoLoLoLo iRiXhVV Nddff
  (00fff) 1snTTT 2snTdTdTd 4PPPP 5appp 7wwW1W2
• or 29UUU
• 8NhCLCMCH 9GGgg
• 222DsVs (0snTwTwTw ) (1PwaPwaHwaHwa) (2PwPwHwHw)
  (3dw1dw1dw2dw2)

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• Qc - quadrant of globe

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• Moored Data Buoy
• MiMiMjMj
• A1bwnbnbnb YYGGiw 99LaLaLa QcLoLoLoLo
  iRiX///
• /ddff 1snTTT 2snTdTdTd (3PoPoPoPo) 4PPPP
• 5appp 9GGgg 22200 0snTwTwTw 1PwaPwaHwaHwa
• 70HwaHwaHwa 333 912ff 555 11fff 22fff 3GGgg
  
• 4ddfmfm 6GGgg dddfff dddfff dddfff dddfff dddfff
  dddfff.
• Very similar to a ship synoptic code.

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• Example of a ship synoptic observation.
• SMVD15 KWBC 230600
• BBXX
• DGMH 23063 99015 30845 41498 62215 10300 20274
• 40120 58005 70322 85180 22254 04320 20404 33300
• 40606 50000 80228

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• Example of a moored data buoy observation.
• SMVD15 KWBC 061200
• BBXX
• 42002 21171 99252 70944 46/// /0708 10297 20269
• 40140 53004 91650 22200 00300 10602 20602 70012
• 333 91210 555 11075 22079 31649 40710 61649
• 073075 075073 072072 072079 070076 072069
• 41 - Southwest portion of North Atlantic
• 42 - Gulf of Mexico
• 43 - West of Mexico
• 44 - North Atlantic from Cape Hatteras north
• 45 - Great Lakes
• 46 - Off the west coast of the US and Gulf of
  Alaska

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(6) Ship and Aircraft Observations

• Ships and aircraft move.
• Therefore, the observational parameters measured
  are not at one location over time, but rather at
  different locations.
• The example states the winds are out of the west.
  This appears to be incorrect, since for the ship,
  the winds are reported as from 020o. The buoy
  winds are 280o.
• However, for both ship and aircraft reports, one
  must remember that the platform is moving, and
  the values obtained will reflect, in part the
  fact that the platform is moving.
• The ship reported a change in pressure over the
  past 3 hours as falling, then steady with a
  change of 0.20 millibars.

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• This is the group 56020 in the previous section.
• The ship was moving southwest at 5 knots. This
  is the group 22251.
• If the ship were moving toward a low pressure
  region, that 0.20 mb change would be an
  overestimate in comparison to a platform not
  moving.
• It would be an underestimate if the ship were
  moving toward a high pressure region.

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(7) Graphs in Space

• Information about the spatial structure of the
  atmosphere can be obtained, to some extent, from
  data collected at different times, if the time
  variation is not too great.
• Knowing the rate of motion of the moving
  platform, and knowing the time the data was
  collected, a determination of location (spatial
  structure) of the data can be made.

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• The example simply converts time of the
  observations into distance from the source, by
  knowing how fast the plane is flying.
• Here, it appears the plane is flying at 1000
  km/hour (or 620 mph).

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Homework Questions

• Do 1, 2, 3, 6, 7