Tools Of The Trade

Surface Observations

In order to forecast the weather a meteorologist must start out with a proper diagnosis of current weather conditions. One of the tools that helps us diagnose the atmosphere is a surface observation. In the past meteorologists took observations manually. Recently, with the modernization of the National Weather Service, these observations are produced automatically by remote sensing units called the Automated Surface Observing Station (ASOS). ASOS units are outfitted with an array of instruments that automatically report observations as frequently as once per minute. The unit can detect most of the basic elements of a standard weather observation. Listed below are the weather elements it can detect and how it measures the atmosphere:

1) Sky Condition - a vertically pointing laser beam sends out a beam of energy which intersects cloud particles aloft. Some of this energy is scattered back to the sensor. The time it takes for the beam to leave the sensor and return equates to a cloud height: it takes longer for the beam to intersect high clouds than low clouds. ASOS can measure cloud heights between the surface and 12,000 feet and cloud amount is determined by an average number of "hits" the beam makes as it travels through different cloud layers. Factors such as precipitation, haze, virga (precipitation falling out of the cloud but evaporating before it reaches the ground) complicate the automated measurement of cloud heights and amounts but sophisticated algorithms (computer programs) are run to overcome these challenges. Reportable values are:

SKC - sky clear below 12,000 feet

FEW - few clouds (<1/8 to 2/8 coverage)

SCT - scattered clouds (3/8 to 4/8 coverage)

BKN - broken clouds (5/8 to 7/8 coverage)

OVC - overcast (total coverage)

2) Visibility - a photometer sensor consisting of a Xenon flash lamp and a detector measures the visibility every minute based on the amount of light scattered forward by the hydrometeors (fog droplets, haze particles, snow, rain, etc.). The past 10 minutes of these average observations is then input into an equation to calculate the mean visibility. Reportable values are in 1/4 mile increments up to 2 miles, every half mile from 2 to 3 miles, every mile from 3 to 5 miles, and at 7 miles. ASOS does not make a distinction between visibilities >10 miles. If the visibility is <7 miles, fog is reported when the temperature and dewpoint temperature difference (called the dewpoint depression) is less then 4 degrees. When the dewpoint depression is >4 degrees haze is reported.

3) Rain and Snow - An infrared beam continuously measures changes in the scintillation pattern produced as precipitation passes through the beam. This pattern is uniquely tied to the particle size and fall velocity: large, rapidly falling particles produce a different pattern than small, slowly falling particles. Most of the time the sensor can measure the difference between rain and snow and the intensity of rain and snowfall. Reportable values are:

-RA (light rain) -SN (light snow) <1/2" accumulation/hour

RA (moderate rain) SN (moderate snow) " to 1" accumulation/hour

+RA (heavy rain) +SN (heavy snow) >1" accumulation/hour

UP is reported when precipitation type cannot be determined. This occurs during a light fall of mixed precipitation (rain and snow). At this time ASOS cannot measure drizzle, showers, ice pellets (sleet), hail, thunder, and freezing rain or drizzle. One of the challenges we face in the Northern Plains during winter is that, under certain conditions, ASOS will report precipitation when skies are clear. This can occur when blowing snow moves through the infrared beam and causes a change in the scintillation pattern which mimics that of falling snow.

4) Precipitation Amount - a heated tipping bucket (to melt snowfall during winter and provide a liquid water equivalent) measures precipitation with a resolution of .01" and an accumulation rate of >.01" to 10" per hour. When rain falls into the funnel a tipping mechanism fills with .01" of liquid. When this happens the weight of this water tips the mechanism and triggers an electrical signal which counts the number of tips each minute.

5) Temperature and Dewpoint temperature - temperature is measured by the electrical resistence through a platinum wire. Since the amount of electrical resistence is uniquely tied to the ambient air temperature this sensor can accurately measure outside temperature. The dewpoint is measured by optical techniques and is based on the fact that as the air temperature cools to its dew point moisture begins to form on solid objects (much like on the outside of a cold glass of water on a hot, humid day). The sensor works by a chilling a mirror so that moisture (dew or frost) begins to condense on its surface. When this happens a beam of light shined at a 45 angle onto the mirror and a detector on the other side of the mirror measure the amount of scattered light from the dew (or frost) particles on the mirror. As the amount of cloudiness on the mirror increases this sensor receives more light. The mirror is then heated so that dew particles are evaporated. This heating and cooling cycle is repeated rapidly until there is a balance between condensation on the mirror and not having condensation. The temperature of the mirror in this balanced state is the dewpoint temperature.

6) Pressure - One of the most important measurements of the atmosphere for meteorologists and pilots is pressure. It must be very accurate for safe and reliable aircraft operations. Pressure has always been measured carefully and routinely compared to a reference standard. Because of this critical nature, ASOS takes 3 separate measurements of pressure using different sensors. These barometers are not the classic mercury or aneroid barometers you may be used to. They consist of digital pressure transducers and advanced microcomputer based electronics to provide accurate measurements. The pressure sensor is considered to be the most reliable instrument on the ASOS unit.

7) Wind Speed and Direction - an anemometer and weather vane are used to measure these quantities. Electro-magnetic signals are generated by the rotating anemometer cups and the wind vane and these signals are directly translated into reportable values. These values are computed by averaging measurements over a 2 minute period. Wind speeds are reported from 0 to 125 knots and direction in tens of degrees from 0 to 360 degrees.

One of the best advantages to the ASOS unit is that it can be located in remote areas where, in the past, the Weather Service did not have a reporting stations to take observations. This has filled in large gaps that existed in the data network across the country giving forecasters a more uniform distribution of surface observations. Also since ASOS operates round-the-clock we have more observations at night than in the pre-ASOS days when many manual reporting stations would cease operations.

Once the observation is taken it is encoded in a special format and sent out through a national network. Below is an example of a standard observation for Sioux Falls and a table which explains the special codes used for all surface observations:

METAR KFSD180946Z 05018KT 1 1/2SM -SN BR SCT009 BKN020 M03/M04 A3027 RMK A02 SNB26 SLP273 T10281039 PNO

U.S. METAR/SPECI CODE FORMAT WITH REMARKS
METAR/SPECI_ CCCC_ YYGGggZ_AUTO_COR_dddff(f)Gf_mf_m(f_m)KT_d_nd_nd_nVd_xd_xd_x_ VVVVVSM_[RD_RD_R/V_RV_RV_RV_RFT or RD_RD_R/V_nV_nV_nV_nVV_xV_xV_xV_xFT]_w'w'_[N_sN_sN_sh_sh_sh_s or VVh_sh_sh_s or SKC/CLR]_T'T'/T'dT'd_AP_HP_HP_HP_H_RMK_(Automated, Manual, Plain Language)_(Additive and Automated Maintenance Data)
Body of Report: PARAMETER	DESCRIPTION
Type of Report (METAR/SPECI)	METAR is the routine (scheduled) report. SPECI is the non-routine (unscheduled) weather report.
Station Identifier (CCCC)	ICAO station identifier. Consists of four alphabetic characters, e.g., KABC.
Date/Time (YYGGggZ)	Day of the month, followed by the actual time of the report or when the criteria for a SPECI is met or noted. Group ends with Z to indicate use of UTC. For example, 251456Z.
Report Modifier (AUTO or COR))	AUTO indicates a fully automated report. No human intervention. COR indicates a correction to a previously disseminated report.
Wind (dddff(f)Gf_mf_m(f_m)KT) (d_nd_nd_nVd_xd_xd_x)	True wind direction in tens of degrees using three digits. Speed is reported in whole knots (two or three digits). Gusts (G) are appended to the speed if required. Group ends with KT to indicate knots. For example, 23018G26KT. If wind direction varies by 60 or more and speed is > 6 knots a variable wind group is also reported, e.g., 180V250. Direction may be reported VRB (variable) if speed is 6 knots, e.g., VRB05KT. Calm winds are reported 00000KT.
Visibility (VVVVVSM)	Surface visibility reported in statute miles. A space divides whole miles and fractions. Group ends with SM to indicate statute miles. For example, 1 1/2SM. Auto only: M prefixed to value < 1/4 mile, e.g., M1/4SM.
Runway Visual Range (RD_RD_R/V_RV_RV_RV_RFT or RD_RD_R/V_nV_nV_nV_nVV_xV_xV_xV_xFT)	10-Minute RVR value: Reported in hundreds of feet if visibility is one statute mile or RVR is 6000 feet. Group ends with FT to indicate feet. For example, R06L/2000FT. The RVR value is prefixed with either M or P to indicate the value is lower or higher than the RVR reportable values, e.g., R06L/P6000FT. If the RVR is variable during the 10-minute evaluation period, the variability is reported, e.g., R06L/2000V4000FT.
Present Weather (w'w')	Present weather (other than obscurations) occurring at the station are reported in the body of the METAR/SPECI. Obscurations are reported if visibility < 7 miles. VA may be reported with any visibility. BCFG and PRFG may also be reported if visibility 7SM. Some present weather and qualifiers may be reported if In-the-Vicinity (not at point-of-observation), e.g., TS, FG, SH, PO, BLDU, BLSA, BLSN, SS and DS. Weather is reported in order of decreasing dominance. Maximum of three groups reported (precipitation included in one group; separate groups for other weather). Automated stations can only report RA, SN, UP, FG, BR, FZFG, HZ, and SQ without augmentation. See table on reverse for more information on qualifiers and weather phenomena.
Sky Condition (N_sN_sN_sh_sh_sh_s or VVh_sh_sh_sor SKC/CLR)	Automated stations truncate to three layers up to 12000 feet; if no layers are detected CLR is reported. At manual stations up to six layers can be reported; if no layers observed SKC is reported. Each layer contains the amount (FEW, SCT, BKN, OVC) immediately followed by the height using three digits, e.g., FEW015 BKN030. Any layer containing CB or TCU (manual only) the contraction is appended to the layer height, e.g., FEW015TCU. All layers are considered opaque. Vertical visibility (VV) is reported in hundreds of feet for an indefinite ceiling, e.g., VV002. Surface obscuration (manual only) reported using amount (FEW, SCT, BKN), followed by "000," e.g., SCT000; remark required.
Temperature/Dew Point (T'T'/T'_dT'_d)	Temperature and dew point are reported to the nearest whole degree Celsius using two digits, e.g., 17/13. Sub-zero values are prefixed with an M, e.g., 03/M02.
Altimeter (AP_HP_HP_HP_H)	Altimeter is prefixed with an A indicating altimeter in inches of mercury. Reported using four digits; tens, units, tenths, and hundredths of inches of mercury, e.g., A2990.
Remarks (RMK) -- Divided into two categories: 1. Automated, Manual (Augmented), Plain Language (Manual Only), 2. Additive and Automated Maintenance Data. The following describes the order in which remarks are reported.
Automated, Manual, Plain Language	Volcanic Eruption, Tornadic Activity (B/E_(hh)mm_LOC/DIR_(MOV), Type of Automated Station (AO1, AO2), Peak Wind (PK_WND_dddff(f)/(hh)mm), Wind Shift (WSHFT_(hh)mm_FROPA), Tower Visibility (TWR_VIS_vvvvv), Surface Visibility (SFC_VIS_vvvvv), Variable Prevailing Visibility (VIS_v_nv_nv_nv_nv_nVv_xv_xv_xv_xv_x), Sector Visibility (VIS_[DIR]_vvvvv), Visibility at 2nd Location (VIS_vvvvv_[LOC], Lightning ([FREQ]_LTG[type]_[LOC]), Begin/End Pcpn (w'w'B(hh)mmE(hh)mm), Begin/End Thunderstorm (TSB(hh)mmE(hh)mm), Thunderstorm Location (TS_LOC_(MOV_DIR)), Hailstone Size (GR_[size]), Virga (VIRGA_(DIR)), Variable Ceiling Height (CIG_h_nh_nh_nVh_xh_xh_x), Obscurations (w'w'_[N_sN_sN_s](h_sh_sh_s), Variable Sky Condition (N_sN_sN_s(h_sh_sh_s)_V_N_sN_sN_s), Significant Cloud Types, Ceiling Height at 2nd Location (CIG_hhh_[LOC], Pressure Rising/Falling Rapidly (PRESRR, PRESFR), Sea-Level Pressure (SLPppp or SLPNO), Aircraft Mishap (ACFT MSHP), No SPECI ReportsTaken (NOSPECI), Snow Increasing Rapidly (SNINCR_[inches-hr/inches on ground]), Other Significant Information (agency specific, e.g., LAST)
Additive and Automated Maintenance Data	Hourly Precipitation Amount (Prrrr), 3- and 6-Hour Precipitation Amount (6RRRR), 24-Hour Precipitation Amount (7R₂₄R₂₄R₂₄R₂₄), Snow Depth on the Ground (4/sss), Water Equivalent of Snow on Ground (933RRR), Cloud Types (8/C_LC_MC_H), Duration of Sunshine (98mmm), Hourly Temperature and Dew point: 0.1C (Ts_nT'T'T's_nT'_dT'_dT'_d), 6-Hour Maximum Temperature: 0.1C (1s_nT_xT_xT_x), 6-Hour Minimum Temperature: 0.1C (2s_nT_nT_nT_n), 24-Hour Maximum/Minimum Temperature: 0.1C (4s_nT_xT_xT_xs_nT_nT_nT_n), 3-Hour Pressure Tendency (5appp), Sensor Status Indicators: RVRNO, PWINO, PNO, FZRANO, TSNO, VISNO_LOC, CHINO_LOC, Maintenance Check Indicator: $
If an element or phenomena does not occur, is missing, or cannot be observed, the corresponding group and space are omitted (body and/or remarks) from that particular report, except for Sea-Level Pressure (SLPppp), and 3-, 6-, and 24-Hour precipitation groups. At designated stations, SLPNO shall be reported in a METAR when the SLP is not available.

NOTATIONS FOR REPORTING WEATHER PHENOMENA

QUALIFIER

Intensity or Proximity

Light

no sign

Moderate

Heavy

In the Vicinity

Descriptor

Shallow

Partial

Patches

Low Drifting

Blowing

Shower(s)

Thunderstorm

Freezing

WEATHER PHENOMENA

Precipitation

Drizzle

Rain

Snow

Snow Grains

Ice Crystals

Ice Pellets

Hail

Small Hail/Snow Pellets

Unknown Precipitation (auto; no intensity)

Obscuration

Mist

Fog

Smoke

Volcanic Ash

Widespread Dust

Sand

Haze

Spray

Other

Well Developed Dust/Sand Whirls

Squalls

Funnel Cloud(s) (Tornado, or Waterspout)

Sandstorm

Duststorm

REPORTABLE CONTRACTIONS FOR SKY COVER

REPORTING OF LAYERS AUTOMATED STATIONS

Reportable Contraction

Meaning

Summation Amount of Layer

Priority

Layer Description

Vertical Visibility

8/8

lowest few layer

SKC or CLR

Clear

lowest broken layer

FEW

Few

<1/8 - 2/8

overcast layer

SCT

Scattered

3/8 - 4/8

lowest scattered layer

BKN

Broken

5/8 - 7/8

second lowest scattered layer

OVC

Overcast

8/8

second lowest broken layer

SKC is reported at manual stations when no clouds are observed.

CLR is reported at automated stations when no clouds are detected at or below 12000 feet.

highest broken layer

highest scattered layer

REPORTABLE VISIBILITY VALUES -- Automated

REPORTABLE VISIBILITY VALUES -- Manual

M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, 6, 7, 8, 9, 10

0, 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 7/8, 1, 1 1/8, 1 1/4, 1 3/8,

1 7/8, 2, 2 1/4, 2 1/2, 2 3/4, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 20, 25, 30, 35, 40, etc., in 5 mile increments.

FORMAT AND ORDER OF CODED REMARKS -- Times of Transmission

Synoptic Cloud Types, 8/C_LC_MC_H (manual)

Snow Increasing Rapidly, SNINCR [inches/hr]/[inches on ground] (manual)

Hourly

Depth of Snow on the Ground, 4/sss (manual)

X¹

X²

X¹

X²

Water Equivalent of Snow on the Ground, 933RRR (manual)

Duration of Sunshine, 98mmm (manual)

0800 UTC

Hourly Precipitation Amount, Prrrr (automated stations only)

Hourly

3- and 6-Hour Precipitation Amount, 6RRRR

X³

X⁴

X³

X⁴

X³

X⁴

X³

X⁴

24-Hour Precipitation Amount, 7R₂₄R₂₄R₂₄R₂₄

Hourly Temperature and Dew Point, Ts_nT_aT_aT_as_nT'_aT'_aT'_a

Hourly

6-Hour Maximum Temperature, 1s_nT_xT_xT_x

6-Hour Minimum Temperature, 2s_nT_nT_nT_n

24-Hour Maximum/Minimum Temperature, 4s_nT_xT_xT_xs_nT_nT_nT_n

Midnight Local Standard Time (LST)

Pressure Tendency, 5appp

1 - included whenever there is more than a trace of snow on the ground

2 - included whenever there is more than a trace of snow on the ground and more than a trace of precipitation occurred within the past 6 hours

3 - 6-hour precipitation amount

4 - 3-hour precipitation amount

Surface observations are taken simultaneously all across the country so that forecasters can depend on a new set of observations each hour. From these observations forecasters can construct surface weather maps. For a guide on how to interpret these maps click here.

Forecasters can then analyze these charts for a variety of weather elements - pressure, temperature, dewpoint, wind speed, cloud heights, and pressure tendency to focus in on or highlight an important weather feature that can help them in their decision. To see an analysis of surface temperature and dewpoint based on these plots click here. When successive surface charts are produced during a shift we can track weather systems moving across the country, moisture moving northward that can indicate a better chance of precipitation, or rapid pressure falls that can signify a developing surface low.

Examination and analysis of good, reliable surface observations is a crucial first step for any meteorologist to prepare a forecast. In order to forecast what will happen we must first get a good grasp on what is happening now. The high density of ASOS units and frequent reports of surface observations supply the forecaster with a wealth of information to help in making that diagnosis. To learn more about ASOS and the METAR observation code you can access the following home pages:

ASOS: http://www.nws.noaa.gov/asos/toolkt.htm

METAR code: http://www.nws.noaa.gov/oso/oso1/oso12/metar.htm

NOAA's National Weather Service Weather Forecast Office

Tools Of The Trade

Surface Observations