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Chapter 2

Chapter 2. Point Observations. 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)

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Chapter 2

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  1. Chapter 2 Point Observations

  2. 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.

  3. 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.

  4. 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.

  5. 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 = V10[0.233 + 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.

  6. 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.

  7. Two methods to reduce wind turbulence about point-source rain gauges. Turf wall Alter type wind screen

  8. 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.

  9. (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.

  10. 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

  11. 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.

  12. 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.

  13. * (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_ _w’w’_ _T’T’/T’dT’d_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

  14. 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

  15. (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.

  16. 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)

  17. 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.

  18. 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)

  19. Qc - quadrant of globe

  20. 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.

  21. 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=

  22. 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

  23. (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.

  24. 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.

  25. (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.

  26. 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).

  27. Homework Questions • Do: 1, 2, 3, 6, 7

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