SURFACE WEATHER MAPS

Objectives:

Weather is the state of the atmosphere at a particular time and place, mainly with respect to its impact upon life and human activity. Weather is defined by various elements including air temperature, humidity, cloudiness, precipitation, air pressure, and wind speed and direction. The surface weather map is a useful tool for depicting weather conditions over broad areas.

After completing this investigation, you should be able to:

• Decode the symbols appearing on a surface weather map and describe weather conditions at various locations.
• Identify fronts appearing on the map, the weather likely to be occurring on either side of a front, and the motion of fronts.
• Describe general relationships between wind patterns and the high and low air pressure centers shown on weather maps.

Introduction:

1. Examine the surface weather map presented in Figure 1 of this investigation. The weather map symbols shown are those commonly seen on the Internet, television, and in newspapers. The H’s and L’s identify centers of relatively high or low air pressure compared to their surroundings. Moving outward horizontally in any direction from the blue H positioned in Texas, air pressure would [(decrease)(remain the same)(increase)].

Figure 1. Idealized surface weather map showing some common map symbols.

2. Moving outward horizontally in any direction from the red L located in Lower Michigan, air pressure would [(decrease)(remain the same)(increase)].

3. The thick curved lines with triangles (spikes) and/or semi-circles on the map are air mass boundaries. In the atmosphere, broad expanses of air with generally uniform temperature, humidity, and therefore density come in contact with other masses of air having different temperature, humidity, and density characteristics. Because air masses of different densities do not readily mix, boundaries separating air masses tend to remain distinct and persistent. These boundaries, called fronts, typically separate warm and cold air—the major determiner of air density differences. The leading edge of advancing cold air is a cold front and, as shown in the map legend in Figure 1, is signified by blue spike symbols which are pointing in the direction toward which the cold front is moving. The leading edge of advancing warm air is a warm front and is signified by red semi-circles on the side of the front’s movement. The front plotted in the Southeastern U.S. is a [(cold)(warm)(stationary)] front.

4. According to the map, persons living in South Carolina can expect [(colder)(warmer)] weather after the front passes.

5. Precipitation is often depicted on weather maps by a variety of symbols as shown on this map including stars or asterisks (*) to represent [(rain)(snow)].

6. Two or three, whole or broken, horizontal lines symbolize [(hail)(fog)(blowing snow)].

Some weather maps display weather conditions at individual weather stations by the use of a station model. Figure 2 below shows the position of weather elements frequently reported in station models.

Figure 2. Surface weather map station model. [Adapted from NOAA]

Temperature: measured by a thermometer, corresponds to the heat energy of the air. In U.S. practice, surface temperatures are given in degrees Fahrenheit. Example above: 57 °F.

Wind: speed measured by an anemometer, movement of air in nautical miles per hour (knots, kts) or land (statute) miles per hour. Long feather represents 10 kts, short feather represents 5 kts. Direction is determined by a wind vane as to where the air is coming from relative to true north. Example above: 15 kts from north-northwest. (North is assumed to be toward the top of the map.)

Sea-level pressure: force of air per unit area due to its weight corrected to equivalent sea level altitude of station. Measured by a barometer in millibars (hectopascals) and plotted in coded form with leading 9 or 10 missing as well as decimal point to show tenths. Typical sea-level air pressures range from 950 to 1049 mb. Example above: 1010.7 mb.

Sky cover: fraction of total sky area covered by clouds of any level. Measured by a ceilometer. Example above: 100% or 8/8 termed overcast.

Dewpoint: measure of the water vapor content of the air. Specifically, the temperature (°F) to which the air at constant air pressure must be cooled to become saturated (begin condensation). Greater values of dewpoint mean greater amounts of water vapor per unit volume of the air. This is one of several measures of atmospheric humidity that may be used. It is preferred in meteorology due to its direct relation to vapor content. Example above:
56 °F. The small difference between the temperature and the dewpoint indicates the air is near saturation or has a high relative humidity. This relation frequently accompanies precipitation or foretells formation of fog.

“Weather”: a symbol for current observed weather conditions, especially those which may affect extent of horizontal visibility and hamper aircraft operations. Example above: moderate rain.

For a more complete listing of weather symbols related to the station model, go to the course website and under Extras, “Weather Map Symbols”, click on Surface Station Model. Refer to the Surface Station Model and related explanations of map symbols to interpret the plotted weather data reported in the red oval of the Figure 3 map below. The station is Tamiskaming Shores, Ontario, Canada.

Figure 3. Station models of surface weather conditions.

7. Temperature: [(16)(21)(33)] °F.

8. Dewpoint: [(6)(10)(14)] °F.

9. Wind direction is shown by the “arrow” shaft drawn into the circle representing the station. North is to the top on the map and east is to the right. Wind is always named for the direction from which it blows. In the above depiction, the wind direction is generally from the [(northeast)(northwest)(southeast)(southwest)].

10. Wind speed is rounded off to the nearest 5 knots (1 knot equals 1.2 miles per hour) and is symbolized by the combination of “feathers” drawn on the clockwise side of the wind-direction shaft. A full feather represents 10 knots and a half feather indicates 5 knots. A pennant flag indicates 50 knots. A wind-direction shaft without feathers depicts a 1-to-2 knot wind and a circle drawn around the station circle signifies calm conditions (0 wind speed). In this case, the reported wind speed is [(5)(10)(20)(50)] knots.

11. Air pressure (adjusted to sea level) is reported in the station model as a coded number to the nearest tenth of a millibar (mb). To decipher the plotted pressure value, first place a decimal point between the second and third numbers from the left. Then add either a “9” or “10” to the left so that the resulting number falls within the range of air pressures that could occur at sea level (almost always between 950 mb and 1049 mb). For example, a plotted value of 126 represents 1012.6 mb and 863 denotes 986.3 mb. The air pressure reported at Tamiskaming Shores is [(11.3)(1011.3)(1130.0)] mb.

12. Sky cover is reported inside the station circle and is expressed as a number of eighths or other descriptors (scattered, broken, overcast, obscured). As examples, an empty circle indicates no clouds, and a half-shaded circle means four-eighths of the sky is cloud-covered. According to the Extras, Surface Station Model, the reported cloud cover at Tamiskaming Shores is [(3/8)(6/8)(overcast)].

13. Current weather is plotted at the “9 o’clock” position on the station model (to the left of the station circle) using a variety of symbols representing the particular weather conditions. According to the Surface Station Model, the two asterisks are symbols of reported current weather being [(rain)(snow)(fog)].

During mid-September, a series of cold fronts advanced southward into the central portion of the U.S. ushering successively cooler air across the north-central and northeastern states. Meanwhile, the southern tier of states remained under the influence of warm and humid weather. Additional heavy rain showers afflicted parts of the western states. Additionally, a tropical weather system was affecting extreme southern Texas.

Figure 4 is the weather map (“Isobars, Fronts, Radar & Data”) for 00Z 16 SEP 2013 (8 PM EDT, 7 PM CDT, 6 PM MDT, 5 PM PDT, etc.), Sunday evening September 15th. The map shows weather conditions at selected stations across the coterminous U.S. The weather data observed at the stations are plotted about circles that represent the locations of those stations. The plotted weather conditions use the coded surface station model of the introductory portion of this investigation.

Figure 4. Surface weather map for 00Z 16 SE 2013.

14. Observe the station model for Huron, in eastern South Dakota. The station model shows a temperature of [(51)(64)(77)] degrees F.

15. The Huron dewpoint was [(41)(48)(52)] degrees F.

16. Winds at stations are symbolized and identified by the direction from which they blow. The wind at Huron was generally from the [(north)(east)(south)(west)].

17. The wind speed, rounded off to the nearest 5 knots, is shown by a combination of “feathers” along the direction shaft where a long feather denotes 10 knots and a short feather is 5 knots. Huron’s wind speed is about [(5)(10)(15)(25)] kts.

18. Wind speeds across the map showed a variety of station model notations for different speeds. Bismarck, North Dakota had one long feather representing 10 knots. Oklahoma City, OK, had a circle around the station circle to denote a wind speed reported as [(calm)(25 knots)(50 knots)].

19. The coded pressure value at Huron was plotted as “243”, meaning the actual atmospheric pressure corrected to sea level was [(24.3)(243.0)(1024.3)(1243.0)] mb.

20. The sky cover (designated by the amount of coverage inside the station circle) at Huron indicated [(clear)(partly cloudy)(mostly cloudy)(overcast)] conditions.

At the “9 o’clock” position alongside the station circle, a weather symbol may be plotted to show present weather conditions. For a listing of the frequently occurring present weather symbols, see the User’s Guide linked from the course website or go to http://www.hpc.ncep.noaa.gov/html/stationplot.shtml (). Figure 4 showed stations displaying several present weather conditions. Figure 2 in the Introductory portion of this investigation also displays a variety of present weather conditions.

21. Casper, in central Wyoming, had two dots that signified [(light rain)(light snow)
(freezing drizzle)(fog)] was occurring. This was also the condition at Seattle where the evening NFL football game was later delayed an hour due to lightning in the vicinity.

22. Wichita, Kansas, and Tallahassee, Florida, had the symbol (), mostly obscured, indicating [(tornado)(blowing snow)(thunder with rain)] was occurring.

In addition to the station model information of Figure 4, the surface weather map contains an analysis of pressure patterns such as the one you made in Investigation 1A. Centers of localized high and low pressures from that computer analysis are also designated. Also, partially obscured, is a red, spiral symbol in the Gulf of Mexico identified with a box giving the position of Hurricane Ingrid. Finally, colored shadings show where a network of National Weather Service (NWS) radars detected precipitation across the U.S.

23. At map time, a high-pressure system of central pressure 1028 mb was centered along the Ontario-Manitoba border just northwest of International Falls, MN. A local low-pressure center was shown on the Kansas-Oklahoma border along a portion of the frontal system from eastern Kansas to northeastern New Mexico having alternating red semicircles and blue triangles on opposite sides of the front. This symbolism marked the front as a [(cold)(warm)(stationary)] front.

24. Bold blue lines with triangles extended from eastern Kansas to northern Ohio and also from Lake Michigan into eastern Canada at the top map border. These symbols indicated the positions of [(cold)(warm)(stationary)] fronts. Several dashed orange lines over the western U.S. designated troughs, extended areas of lower pressure seen along curves of the isobars.

25. Light blue, green, yellow and red shadings indicated precipitation across the U.S. At map time, precipitation was generally closely associated with the [(frontal systems)
(centers of high pressure)]. Precipitation also was found near the troughs, in southern Texas from the hurricane and over the Florida region.

Additional Hs and Ls across the map are the result of localized areas of relatively higher or lower pressure from a larger set of pressure values than those appearing on the Figure 4 map.

Figure 5 is the surface weather map for 00Z 10 SEP 2012, twenty-four hours after Figure 4. By Figure 5 time, the high-pressure center was located over Lake Superior. The cold fronts had advanced generally southward and eastward. A stationary front stretched along the eastern side of the Rocky Mountains between Canada and the Texas panhandle before curving eastward. Together, the fronts formed a boundary around the high-pressure system. Ingrid had weakened to a tropical depression identified by the L in northeastern Mexico.

Figure 5. Surface weather map for 00Z 17 SEP 2013.

26. The wind directions in the several state area surrounding the Lake Superior high-pressure center circulated generally [(counterclockwise and inward)(clockwise and outward)] about the center, consistent with the hand-twist model of a High.

27. The center of the high-pressure system had moved generally toward the [(northeast)(southeast)(southwest)(northwest)].

28. The station model for Des Moines, Iowa showed a temperature of [(44)(62)(83)] degrees F and a dewpoint of 51 °F.

29. The wind at Des Moines was generally from the [(north-northwest)(north-northeast)(east-southeast)(south-southwest)] at about 10 knots (one long “feather” on the direction shaft).

30. The coded pressure value was plotted as “242”, meaning the actual atmospheric pressure corrected to sea level at Indianapolis was [(24.2)(242.0)(1024.2)(1242.0)] mb.

31. The sky cover at Des Moines indicated [(clear)(overcast)] conditions.

32. In Figure 5, Miami, Florida had two dots that signified [(light rain)(light snow)
(freezing drizzle)(fog)] was occurring.

For more practice on deciphering station models and map symbols, go to http://profhorn.aos.wisc.edu/wxwise/AckermanKnox/chap1/decoding_surface.html ().

Suggestions for further activities: Displaying or following a sequence of recent surface weather maps ending with the current map can show the movement of “weather makers” (high and low pressure centers and fronts) and the changes in atmospheric conditions at your location over time resulting from their movements. Practice looking for connections between these weather changes depicted by the map sequence. Noting such connections allows one to make general predictions of local weather for the next half day or so.

You can also observe how station model conditions change as weather systems pass across a region by going to the NWS Surface Analysis link on the course website under Surface. The link takes you to the NWS’ Weather Prediction Center site. Scrolling down allows you to “Create a Surface Analysis Loop” for various regions. Selecting the United States choice, then clicking “Display Loop” will show an animation of the latest day’s series of 3-hourly station models along with fronts and Highs and Lows.

For the complete set of 100 possible weather symbols which can be used in station models, go the NWS JetStream site at: http://www.srh.noaa.gov/jetstream/synoptic/ww_symbols.htm ().