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)
– 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.
• 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.
• 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.
• 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.
• 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.
• Two methods to reduce wind turbulence about pointsource rain gauges.
Turf wall
Alter type wind screen
• 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.
(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.
• 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 Change of pressure or P
Change of time

t
1013.1mb  1012.5mb 0.6mb

 0.1mb / hr
12 : 00hrs  6 : 00hrs
6hrs
• 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.
• 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.
(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.

METAR
• or
_CCCC _YYGGggZ__dddff(f)GfmfmKT _dndndnVdxdxdx

N s N s N s h s h s h s
SPECI
_VVVVVSM_


RD R D R /VR VR VR VR
or

RD R D R /VNVNVN VNVVX VX VX VX
_w’w’_

or
VVh s h s h s
or

SKC/CLR
_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
*
• 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
(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.
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)
• 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.
Ship Synoptic report
M iM iM jM j
DDDD YYGGiw 99LaLaLa QcLoLoLoLo iRiXhVV Nddff
(00fff) 1snTTT 2snTdTdTd 4PPPP 5appp 7wwW1W2
or 29UUU
8NhCLCMCH 9GGgg
222DsVs (0snTwTwTw ) (1PwaPwaHwaHwa) (2PwPwHwHw)
(3dw1dw1dw2dw2)
Greenwich (Prime) Meridian
• Qc - quadrant of globe
N
Qc = 7
W
Qc = 1
Equator
E
Q c= 5
Q c= 3
S
Moored Data Buoy
M iM iM jM j
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.
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=
• 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
(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.
• 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.
(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.
• 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).
Homework Questions
• Do: 1, 2, 3, 6, 7