Tuesday Weather Topic in Greater Depth

From: Subject: Tuesday Weather Topic in Greater Depth Date: Fri, 25 Sep 2009 16:40:50 -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/t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 15 = September=20 2009

READING THE SURFACE STATION MODEL

One of the main points of a surface weather map is that it displays a = considerable amount of information about the weather from a large area = at the=20 same time. Sometimes the word "synoptic" is used to describe a weather = map. This=20 term is derived from the Greek words "syn"=3Dsame or together; = "optic"=3Dvisible;=20 hence, seen together. Since weather systems move and evolve, "snapshots" = in time=20 are needed. As a result, synoptic weather analysis requires the = simultaneous and=20 systematic observation of various weather elements at many widely = located sites=20 using standardized instruments and observation techniques.

The first weather map was drawn in 1819 showing a powerful European = storm in=20 1783. Only after the electric telegraph became widely available in the = mid 19th=20 century could current daily weather maps become a reality. So when you = display a=20 DataStreme Atmosphere map, say for 00Z, all the information that = appears=20 on that chart was observed at midnight Greenwich time, or 7:00 PM EST = (8:00 PM=20 EDT). Only the plotted positions of the fronts may be several hours old. = You=20 should always check the legend to find the observation times and the = time when=20 the fronts were plotted.

Since weather elements are interrelated, the modern surface weather = chart=20 typically incorporates several observed weather elements plotted = simultaneously=20 at each station. These plots would include air temperature, dewpoint=20 temperature, air pressure, sky cover and wind information (wind speed = and=20 direction). To display all the observed weather information for many = locations=20 at one given time would be difficult unless a uniform system of plotting = were=20 adopted. The pictorial presentation and weather data together with an = analysis=20 can be determined at a glance. The location of each reporting station = has been=20 printed on the base maps as a small circle. The data submitted by each = reporting=20 station are plotted around these circles on these base maps in a = particular=20 systematic fashion called a "station model". While data are collected = hourly=20 from hundreds of weather stations, only a very limited number of station = plots=20 appear on the DataStreme Atmosphere maps for ease of interpretation. =

Weather maps used in DataStreme Atmosphere contain abridged = station=20 models, where the following conventions are used.

Using a clock analogy, the arrangement of observed weather elements = plotted=20 around the station model would include the air temperature at the 10 = o'clock=20 position; the sea level corrected air pressure would be located at the 2 = o'clock=20 position and the dewpoint temperature (an indicator of the water vapor = in the=20 atmosphere) is at the 8 o'clock position. If some significant weather = phenomenon=20 were observed, such as precipitation or some obstruction to horizontal=20 visibility (fog or blowing snow), then a special symbol would be plotted = in the=20 9 o'clock position. A list of symbols appears in the DataStreme = Atmosphere=20 Users Guide or you can click on the highlighted "DataStreme = Atmosphere=20 Weather Map Symbols" entry on the DataStreme Atmosphere website. = Inside=20 the circle of the station model, the sky or cloud cover is indicated by = the=20 amount of shading. No shading would mean clear conditions, while a = circle=20 completely shaded indicates overcast conditions.

AIR TEMPERATURE AND DEWPOINT TEMPERATURE CONVENTIONS

In the United States, the current near-surface air temperature and = dewpoint=20 temperature (an indicator of the water vapor in the atmosphere) are = reported in=20 whole (or integer) Fahrenheit degrees. These temperatures are measured = by=20 instruments located in a standard ventilated, shielded enclosure at a = height of=20 approximately 5 feet above the ground. The air temperature is plotted on = the=20 chart to the upper left of the station model, while the dewpoint value = is placed=20 below the temperature, or to the lower left of the station circle. A = negative=20 sign is included when the air temperature or dewpoint is less than 0 = degrees F.=20 The value of the dewpoint may never exceed the air temperature.

WIND CONVENTIONS

Looking at the station model, you will notice that a "wind arrow" = often=20 appears. This wind arrow identifies the observed near-surface wind = direction and=20 wind speed through a combination of wind arrow shaft and wind barbs = plotted=20 around the station model. These wind data are obtained from instruments = mounted=20 at a standard "anemometer height" of 10 meters (approximately 32 feet) = above the=20 ground.

The wind direction is provided by the orientation of the plotted wind = arrow=20 on the map. The wind arrow with feathers can be thought of as the back = portion=20 of an arrow that would "fly with the wind". In other words, the tail is = on the=20 upwind side of the station, while the small circle at the head of the = arrow is=20 located at the station. Thus, the orientation of these wind arrows on = the map=20 indicates the wind direction to the nearest 10 degrees, measured = clockwise from=20 true north (defined as 360 degrees, and located at the top of the = chart). By=20 meteorological convention, the winds are named for the direction from = which they=20 are blowing. Hence, a south wind is from the south. A concentric circle = drawn=20 around the station model with no wind arrow indicates calm conditions = (no=20 perceptible wind).

The number and length of the barbs on the tail of the arrow indicate = the wind=20 speed in knots (nautical miles per hour, which are 15% larger than the = familiar=20 statute miles per hour), i.e. 10 knots is 11.5 mph. Each half barb = portrays the=20 wind to the nearest 5 knots, while each full barb is an increment of 10 = knots; a=20 pennant (rare for surface maps) represents 50 knots. By convention, the = wind=20 barbs and pennants are plotted on the side of the shaft of the wind = arrow=20 pointing toward lower pressure (or to the left of the wind direction in = the=20 Northern Hemisphere). This convention is useful when performing an = isobar=20 analysis.

BAROMETRIC PRESSURE CONVENTION

The current sea level corrected air pressure is plotted on the map to = the=20 upper right of the station model. The numeric pressure entries are in = units of=20 tenths of millibars (a metric unit of pressure, where 1 mb=3D0.0295 = inches of=20 mercury). The barometric pressure measured by a barometer at the station = is=20 adjusted (or corrected) to sea level conditions to eliminate the = variations in=20 reported pressure due to the altitude of the station.

By convention, the lead "9" or "10" is dropped from the reported = value and=20 the decimal point omitted. A sea level pressure report of 995.8 mb would = be=20 plotted as "958", a report of 1002.8 mb would be plotted as "028", and = 1025.8 mb=20 would be "258". Since the sea level pressure usually ranges between 980 = and 1040=20 mb, you should have no problem in determining whether the plotted value = is=20 preceded by a "9" or "10". If in doubt, check the pressure values at = neighboring=20 reporting stations.

WEATHER SYMBOL CONVENTION

A set of unique and international standard symbols would be plotted = directly=20 to the left of the station model (between the air and dewpoint = temperatures) as=20 necessary to indicate the observation of a particular significant = current=20 weather event, such as precipitation or a significant reduction in the=20 horizontal ground level visibility. The abridged list of symbols appears = in the=20 website Users Guide or in "DataStreme Atmosphere Weather Map = Symbols" on=20 the DataStreme Atmosphere website and represents several of the = common=20 symbols that you should recognize:

rain

snow

Drizzle

Precipitation intensities, as ascertained from = observed=20 precipitation rates or reduction in visibility, are indicated by the = repetition=20 of symbols for rain, snow and drizzle.

Two symbols
Three symbols
Four = symbols

Light
Moderate =20
Heavy

SKY COVER CONVENTION

The amount of shading inside the station location circle is used to = depict=20 the total fraction of the local sky hemisphere covered by clouds at the=20 observation time. The following abridged cloud cover symbols:

SKY COVER CODE

CLR
FEW
SCT
BKN
OVC

Clear
Few
Scattered
Broken =
Overcast

0/8 of sky covered by clouds
1/8 through 2/8 of sky = covered=20 by clouds
3/8 through 4/8 of sky is covered by = clouds =20
5/8 through 7/8 of sky is cloud covered
8/8 of sky = is cloud=20 covered

You may wonder about the distinction between a "plot" and an = "analysis". A=20 plot is a chart that contains only the observed data plotted at the = stations. An=20 analysis on the other hand, represents a map product that contains a set = of=20 lines, drawn either by hand or by machine -- essentially a computer = program that=20 uses mathematical formulae to interpolate. These lines, variously called = isolines, isopleths or contours, are used to help visualize patterns in = the=20 geographical distribution of a given weather element more effectively = than can=20 be obtained from a plot of numbers across the map. Last week you gained = some=20 experience drawing isobars (lines of equal air pressure). Fronts are = often=20 placed upon the surface maps when applicable. The frontal symbols = represent the=20 intersection of the boundary between dissimilar air masses at the = earth's=20 surface. Frontal analysis involves inspection of the isotherm (lines of = equal=20 temperature) patterns, how the winds change direction over an area, and = cloud=20 and precipitation patterns.

Return to the Tuesd= ay Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

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