Monday Weather Topic in Greater Depth

From: Subject: Monday Weather Topic in Greater Depth Date: Fri, 25 Sep 2009 16:40:41 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/last_week/m_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Monday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Monday, 14 = September=20 2009

Explanation of Hourly Surface Weather Observations = (In Tabular=20 Form)

The near surface weather conditions across the country are monitored = on a=20 nearly continuous basis at several thousand locations as part of a = nationwide=20 network operated by the National Weather Service in conjunction with = other=20 governmental agencies such as the Federal Aviation Administration, the=20 Department of Defense and various state agencies. Many of these weather=20 observation sites are located at airports where the needs of the = aviation=20 industry require a set of weather observations on an hourly or shorter = time=20 interval. With the modernization of the National Weather Service, many = of the=20 weather observations are made by automatic weather stations called=20 ASOS for Automatic = Surface=20 Observation System.

Current hourly weather observations from nearly 1500 stations = throughout the=20 country can be accessed from the DataStreme Atmosphere website = under the=20 heading State Surface Data-Text. Select the desired state using = the=20 appropriate two letter postal abbreviation. Once selected, a tabulation = of the=20 hourly observations from the state will appear. Remember that all times = are=20 listed in UTC or Z time. These files are updated on an hourly basis. =

INFORMATION CONTENTS OF THE TABLE

HH=3DHourly observation time in "Z" or UTC = time
Typically, the=20 displayed weather observations are made within ten minutes before the = top of=20 the hour to permit a timely communication of the observations over the = communication network in a given sequence.=20
STN=3DStation Name
A list of these identifiers appears = in the=20 DataStreme Atmosphere online User Guide, arranged = according to=20 state. These three or four letter identifiers are assigned by the = Federal=20 Aviation Administration. Most of the stations in the continental U.S. = will=20 only have three letters since the leading letter "K" is assumed. Most = of the=20 stations in Hawaii and Alaska will have the letter "P" as the prefix.=20
TMP=3DAir temperature in degrees Fahrenheit = (F)
The air=20 temperature is measured by an electronic thermometer located within = two meters=20 (6 feet) of the earth's surface.=20
DEW=3DDewpoint temperature in degrees Fahrenheit (F)
The = dewpoint=20 temperature, a measure of the water vapor content of the air, is = measured by=20 an electronic device located near the thermometer. Typically, a larger = value=20 of the dewpoint indicates a greater amount of water vapor in the = atmosphere.=20 If the dewpoint were equal to the air temperature, the air would be = saturated=20 and fog most likely would be reported.=20
DIR=3DWind direction in tens of degrees measured clockwise from = true=20 north (360 degrees)
The wind direction is measured by an = instrument=20 called a wind vane located at 10 meters (33 feet) above the earth's = surface.=20 Remembering that in accordance with meteorological tradition, a wind = is named=20 for the direction from where the wind originates, the following = notation is=20 used:
- 09=3D90 degrees=3Deast wind, 18=3D180 degrees=3Dsouth wind, = 27=3D270 degrees=3Dwest=20 wind,
  36=3D360 degrees=3Dnorth wind, and 00=3D0 degrees=3Dcalm =
SPD=3DWind speed in knots
The horizontal wind speed is = measured=20 by a wind sensor called an anemometer located at the same level as the = wind=20 vane. The 1-minute average is reported. By meteorological and aviation = tradition, wind speed is reported as knots, from the nautical mile per = hour.=20 One knot is equivalent to 1.15 mph=20
GST=3DSpeed of the wind gust in knots
A gust is = defined as=20 a sudden brief increase in the speed of the wind of at least 10 knots = between=20 peaks and lulls. Wind gusts are reported when the peak wind speed = reaches at=20 least 16 knots.=20
Sky or Cloud Cover information is listed in three levels:=20
- CLDL, "low" - up to 5000 feet (1524 m)=20
- CLDM, "middle" - from 5000 to 10,000 ft (3049 m)=20
- CLDH, "high" - above 10,000 ft=20
- A three-letter code appears that uses the following convention = based=20 upon the fraction of the local sky hemisphere being covered by = clouds:=20
- CLR=3Dclear (no clouds detected), FEW=3Dfew (1/8 = to 2/8 sky=20 covered); SCT=3Dscattered (3/8 to 4/8 sky cove red); BKN = =3Dbroken=20 (5/8 to 7/8 covered); OVC=3Dovercast and X=3Dobscured = (no clear=20 cloud base observed due to fog).
ALT=3Daircraft altimeter setting in inches of mercury =
This=20 reading is intended for use by pilots when they correct their pressure = altimeters at the station and represents the air pressure corrected to = sea=20 level using a method involving the vertical pressure and temperature=20 variations determined by the US Standard Atmosphere.=20
PMSL=3DSea level corrected pressure in tenths of millibars =
The=20 air pressure is measured by an aneroid barometric pressure sensor and=20 corrected to sea level to remove the effects of the elevation of the = station.=20 In this report the lead "9" or "10" together with the decimal point = are=20 omitted. Thus, "962" is 996.2 mb and "013" represents 1001.3 mb.=20
PTD=3DPressure tendency
given in tenths of millibars, = change over=20 the previous 3 hours. Given at 00Z, 03Z, etc. e.g.. -0.3, pressure now = lower=20 by 3 /10 of mb=20
WTHR=3DSignificant Weather Phenomena
A set of = abbreviations=20 identifies those weather phenomena that could adversely affect = aircraft=20 operations. These phenomena include all types of precipitation, severe = weather=20 events and obstructions to vision, such as fog. Many of the = abbreviations are=20 obvious, such as:
- GF=3DGround Fog; H=3DHaze; R=3DRain, S=3DSnow.=20
- The symbols "-" and "+" indicate light and heavy, respectively. =
PCPN=3Dprecipitation (if any) in hundredths of inches, = given at 00Z,=20 03Z, etc.=20
SNOW=3Dsnowfall (if any) in tenths of inches, given at 00Z, = 03Z, etc.=20

GRAPHICAL DISPLAYS

The weather data appearing in the text format can be displayed = graphically in=20 the traditional surface map format. The weather data for each hour from = the=20 entire country and adjoining portions of Canada and Mexico are collected = and=20 plotted upon weather maps. A more detailed discussion of how the weather = data=20 are plotted on a surface map will be described in another = Supplemental=20 Summary that will appear this week.

Another weather data display option would entail the collection of = the hourly=20 weather data from one particular station for a 24-hour interval. Such a = display=20 is called a meteorogram or metgram. DataStreme Atmosphere = produces=20 meteorograms for a selected set of cities. A future Supplemental = Summary=20 will describe this display.

Return to the Monda= y Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =