The surface weather map that you can download from the "Latest Surface Weather Map" link under Atmospheric Information on the DataStreme WES Website is one of the most useful charts for ascertaining current weather conditions just above the surface of the Earth over a broad geographical region. The surface weather maps used in WES focus on the continental United States, using the same scale and projection as in the current Water Vapor and Infrared Satellite images. The map contains plotted weather data, isobar analyses, and an overlay of the current radar echoes. Let's take a tour of a sample surface weather map and examine some of its basic features.
Any time you call up this chart, take a moment to look at the information appearing above the map itself, since some of the information will be needed for proper interpretation.
Look at the date-time group on the upper left-hand corner of the chart to determine the time when the observations were made for this chart. By international agreement, all meteorological observations are taken at the same time according to Universal Coordinated Time (or Greenwich Mean Time, also known as "Z" time). Weather data that are plotted on these maps are based upon hourly surface observations made at many airport weather stations. These observations are made within 5 minutes of the top of the hour.
The frontal analyses, along with the positions of the high- and low-pressure systems, appearing on the surface chart are usually produced at 3-hour intervals (0000 UTC, 0300 UTC, and so forth). Check the time of the frontal analysis on the upper right hand portion of the chart; this time may be several hours earlier than the time of the other observations.
The modern surface weather chart typically incorporates several observed weather elements plotted simultaneously at each station. These plots include air temperature, dewpoint, air pressure, sky cover, and wind information (wind speed and direction). To display all the observed weather information for many locations at one given time would be difficult and confusing unless a uniform system of plotting were adopted. The pictorial presentation and weather data together with an analysis can be determined at a glance. The location of each reporting station is printed on the base maps as a small circle. The data submitted by each reporting station are plotted around these circles on these base maps in a systematic fashion called a "station model." Whereas data are collected hourly from hundreds of weather stations, only a very limited number of station plots appear on the WES weather maps for ease of interpretation.
Weather maps used in DataStreme WES contain abridged station models, where the following conventions are used.
Using a clock analogy, the arrangement of observed weather elements plotted around the station model includes the air temperature at the 10 o'clock position, the sea level adjusted air pressure at the 2 o'clock position, and the dewpoint (an indicator of the water vapor in the atmosphere) at the 8 o'clock position. If some significant weather phenomenon were observed, such as precipitation or some obstruction to horizontal visibility (e.g., fog or blowing snow), then a special symbol would be plotted in the 9 o'clock position. A list of symbols appears on the highlighted Map Symbol Explanation entry on the DataStreme WES Website. Inside the circle of the station model, the sky or cloud cover is indicated by the amount of shading. No shading signifies clear skies (essentially without clouds), whereas a circle completely shaded indicates overcast conditions (where the sky above the station is completely covered with clouds).
In the United States, the current near-surface air temperature and dewpoint are reported in whole (or integer) Fahrenheit degrees. These temperatures are measured by instruments located in a standard ventilated, shielded enclosure (shelter) at a height of approximately 5 ft above the ground. Air temperature is plotted on the chart to the upper left of the station model, whereas the dewpoint is placed below the temperature, to the lower left of the station circle. A negative sign is included when the air temperature or dewpoint is less than 0 degrees F. The value of the dewpoint may never exceed the air temperature.
The station model usually includes a "wind arrow." The wind arrow identifies the observed near-surface wind direction and wind speed through a combination of wind arrow shaft and wind barbs. Wind data are obtained from instruments mounted at a standard "anemometer height" of 10 m (approximately 32 ft) above the ground.
Wind direction is given by the orientation of the wind arrow plotted on the map. The wind arrow with feathers can be thought of as the back portion of an arrow that would "fly with the wind." In other words, the tail is on the upwind side of the station, while the small circle at the head of the arrow is located at the station. Thus, the orientation of wind arrows on the map indicates the wind direction to the nearest 10 degrees, measured clockwise from true north (defined as 360 degrees, and located at the top of the chart). By meteorological convention, the wind is named for the direction from which it blows. Hence, a south wind is blowing from the south and toward the north. A concentric circle drawn around the station model with no wind arrow indicates calm conditions (no perceptible air motion).
The number and length of the barbs on the tail of the arrow indicate the wind speed in knots (nautical miles per hour, which are 15% larger than the familiar statute miles per hour, such that 10 knots is equivalent to 11.5 mph). Each half barb portrays the wind to the nearest 5 knots, while each full barb corresponds to an increment of 10 knots; a pennant (rare for surface maps) indicates 50 knots. By convention, wind barbs and pennants are plotted on the side of the wind arrow shaft pointing toward lower pressure (or to the left of the wind direction in the Northern Hemisphere). This convention is useful when performing an isobar analysis.
The current sea level adjusted air pressure is plotted on the map to the upper right of the station model. The numerical pressure entries are in units of tenths of millibars (a metric unit of pressure, where 1 mb=0.0295 in. of mercury). The air pressure measured by a barometer at the station is adjusted (or corrected) to sea level conditions to eliminate the variations in reported pressure due to the elevation of the station.
By convention, the lead "9" or "10" is dropped from the reported value and the decimal point omitted. A sea level pressure report of 995.8 mb would be plotted as "958," a report of 1002.8 mb would be plotted as "028," and 1025.8 mb would be "258." Because the sea level pressure usually ranges between 980 and 1040 mb, you should have no problem in determining whether the plotted value is preceded by a "9" or "10." When in doubt, check the pressure values at neighboring reporting stations.
A set of unique and international standard symbols is plotted directly to the left of the station model (between the air and dewpoint temperatures) as necessary to indicate the observation of a particular significant current weather event, such as precipitation or a significant reduction in the horizontal ground-level visibility. The abridged list of symbols appears in the Map Symbol Explanation entry linked from the DataStreme WES Website and represents several of the common symbols that you should recognize:
| . |
= Rain |
|
* |
= Snow |
|
, |
= Drizzle |
The amount of shading inside the station location circle is used to depict the total fraction of the local sky hemisphere covered by clouds at observation time. The following is an abridged list of cloud cover symbols:
| CLR |
Clear |
0/8 of sky covered by clouds |
| FEW |
Few |
1/8 through 2/8 of sky covered by clouds |
| SCT |
Scattered |
3/8 through 4/8 of sky covered by clouds |
| BKN |
Broken |
5/8 through 7/8 of sky covered by clouds |
| OVC |
Overcast |
8/8 of sky covered by clouds |
At first glance, the array of data plotted on the map might appear disorganized and overwhelming without some scheme for organizing the data to permit visualization of spatial weather patterns. These maps are called surface analysis charts since they contain fronts and analyzed pressure fields, with the solid lines representing isobars, or lines of equal barometric pressure. Once isobars have been drawn on a surface chart, you can immediately locate regions of high and low atmospheric pressure. The isobars, appearing as thin lines on the analysis charts, connect all points having the same barometric pressure adjusted to sea level. By meteorological convention in the U.S., the isobar spacing is at 4 mb intervals, centered upon 1000 mb; that is, 996 mb, 1000 mb, 1004 mb, and so forth.
High and low pressure centers are indicated by a large block H and L, respectively, together with a set of digits identifying the estimated value of the central pressure. By tradition, H is colored blue whereas L is drawn in red.
A trough of low pressure that contains significant weather phenomena (such as precipitation and distinct wind shifts) may be identified on the map by a yellow dashed line running along the axis of the trough.
The surface analysis may include one or more color-coded lines to identify a front. A front is defined as a narrow transition zone between air masses having dissimilar thermal and moisture properties. Usually, these transition zones are only 50 to 100 km wide, a sufficiently small horizontal distance to permit their representation as lines on a large-scale surface analysis chart. Fronts are classified according to their movement and can be represented on a Surface Analysis chart as follows:
| TYPE |
|
DEFINITION |
|
MAP SYMBOL |
| COLD |
|
A front where cold air replaces warm air |
|
A blue line with blue barbs pointing in direction of cold airflow. |
| WARM |
|
A front where warm air replaces cold air |
|
A red line with red half-moons pointing in direction of warm airflow. |
| STATIONARY |
|
A front with little lateral movement. |
|
A line with alternating red warm front symbols and blue cold front symbols, pointing in opposite directions to symbolize little frontal movement. |
| OCCLUDED |
|
A front where the cold front has overtaken and merged with the warm front. |
|
A front with purple (combined red and blue) half moons and barbs on the same side, pointing toward direction of frontal motion. |
The frontal symbols represent the intersection of the boundary between dissimilar air masses at the Earth's surface. Frontal analysis involves inspection of isotherm (lines of equal temperature) patterns, how the wind changes direction over an area, and cloud and precipitation patterns.
The surface weather map analysis permits one to identify and locate the large-scale features of the sea level pressure field and the surface fronts. Isobars with the lowest value will encircle the region with the lowest value in the pressure field, while the closed isobar with the largest value isolates the highest sea level pressure. The packing of isobars reveals how rapidly the pressure varies with distance in the horizontal direction. Tighter packing indicates a much more rapid horizontal variation of air pressure.
The isobar pattern is also useful for visualizing near surface wind regimes. The winds tend to parallel the isobars, with low pressure to the left of the wind flow in the Northern Hemisphere; a slight cross-isobar deflection of the winds toward lower pressure is often seen. As a result, winds appear to spiral in toward a surface low pressure center in a counterclockwise fashion, and spiral around a high pressure cell in a clockwise outflow regime. Additionally, where the isobars are spaced more closely, the wind speed tends to be greater.
If surface charts are available for the last day or two, you can follow the movement of weather systems over time, based upon continuity principles. You can make a reasonable short-range weather forecast based upon the movement of the low- and high-pressure centers.
A radar image overlay on the surface analysis provides additional information about regions of precipitation. Radar echoes locate areas of precipitation falling between weather stations. The intensity of the precipitation can also be estimated from the radar reflectivity and displayed as a series of 6 color codes from light to extreme.
Return to DataStreme WES website
Prepared by WES Central Staff and Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.