IntroductionIn order to be able to predict the conditions of the atmosphere, the "weather," a meteorologist must first understand what is producing the current weather conditions. The meteorologist does this by obtaining a "picture" of the atmosphere based on value of certain weather elements. The elements used to describe the condition of the atmosphere are: air temperature at various levels up through the atmosphere; wind speed and direction at various levels; moisture content; presence of clouds, their type and altitude; whether it is raining, snowing, foggy, hazy, etc., or none of the "present weather" phenomena, pressure, etc..
To get the best "picture" of the atmosphere at a particular time, the value of various weather elements needs to be collected from as much of the atmosphere surrounding the whole earth as possible. Thus, tremendous amounts of data must be considered by a meteorologist when looking at the "picture" of the atmosphere. One way to display this data in an easy to understand format is to depict the distribution of the elements on maps, charts and diagrams. The objective of the use of these in meteorology is to represent the state of the atmosphere as a function of space, either at fixed intervals of time or at varying intervals of time. A three-dimensional representation drawn to scale would be the most accurate and natural form, but it would present numerous difficulties in plotting the weather elements at their proper location, and in interpreting the information.
That part of the atmosphere with which meteorologists
are most directly concerned is a thin spherical shell with lateral dimensions
thousands of times its vertical depth. The United States measures roughly
13,860,000 feet from the east coast to the west coast along latitude 39N.
The troposphere, the lowest layer of the atmosphere in which the greatest
amount of "weather" occurs is, on the average, 11 kilometers (7 miles or
36,960 feet) thick. To depict the atmosphere over the United States in
a three dimensional representation drawn to scale in which the horizontal
length was 2 feet would require a vertical distance of only 0.00534 feet
(0.064 inches). Not very practical!
A "diagram" is defined as a line drawing made for mathematical or scientific purposes; a graphic design that explains rather than represents. Graphs typically are used to relate one element to another, or several other elements, or an element to altitude.
Typically a chart or map displays information in the horizontal direction; north-south and east-west, like a road map which shows you where to go.
MapsMaps in meteorology are used to display the element characteristics of the atmosphere, as measured by individual stations, such that the stations are in the proper horizontal relationship to each other. The map then provides a means to analyze the measured elements so the meteorologist can understand the "picture" of the atmosphere.
This image shows the condition of the atmosphere as measured at or viewed from near ground level by a weather observer.
Open the image timezone.gif in the Atmo 202 folder. As you can see from the table below, and this image, the local time in Texas (Central Standard Time) is 6 hours behind the local time in Greenwich, England, (UTC), unless Texas is on Daylight Saving Time, during which period the local time in Texas is only 5 hours behind Greenwich, England.
With just the air temperature values plotted, it is hard to see any pattern to the temperature of the near-ground air across Texas. However, if this temperature field is analyzed, then a pattern emerges.
This map shows an analysis of temperatures across the United States, not just for Texas. The area between the analysis lines has been colored to aid in interpretation. This map has been analyzed at an interval of 5F degrees. NOTE: The analysis lines are the dividing lines between the colors and represent specific temperature values. The scale at the bottom shows the temperature value occurring at the dividing line between the colors. These temperature values are also plotted at some locations on the map.
The lines drawn on the map are called isolines, or isopleths, which is a line along which the value of some element is everywhere the same. NOTE: In the temperature analysis, the isolines are the dividing lines between the different colors. The map which has these isopleth lines drawn is said to be isoplethed, or more commonly, to be analyzed or sometimes contoured.
It is very important for a meteorologist, especially those involved in synoptic meteorology and weather forecasting, to see "at a glance" how the various elements vary in two- and sometimes three-dimensional space. This plotted map represents a field of the variable (or element), and can be applied to any element which has a continuous distribution in at least two dimensions. Temperature and pressure are the two most common elements in meteorology to which this is done. Having an analyzed field to interpret is much preferable to having to interpret only the numbers, as in a table, from which these fields are drawn. However, since there are many elements which may be plotted on weather maps, and analyzed by such isopleth lines, specific names are given to the various kinds of isopleths. The following gives some of the more common isopleths used on weather maps.
You can close the temperature analysis now.
Rules for Drawing IsoplethsFor meteorologists throughout the world to be able to understand the analyzed maps developed in other counries, there must be guideline to analyzing data so one does not get lost in the forest of information plotted on weather maps. The procedure and guidelines for analyzing meteorological data on maps has been standardized and agreed upon by all nations which are members of the World Meteorological Organization. Following are some of these rules.
The following figure illustrates these rules. The numbers represent a value of some measured weather element; e.g. temperature. For the isopleth whose value 5, the isopleth is drawn at the location between stations where a value of 5 is most likely to occur, and through the station where the plotted value at the station is exactly the same as the isopleth value.
Helpful Hints for Drawing Isopleths
Plotted Data on Weather MapsAs shown on the first figure you opened, the one with weather observation data plotted for Texas, weather plotting charts are printed with a small circle at the geographic location of the major reporting stations. The image below is the model for plotting surface data.
The station circle is shown as the circle with capital N in the center.
Stations are identified on the printed weather map by their assigned three-digit station number, or a three-letter identifier, next to (usually beneath) the station circle.
Meteorological data from the weather observation is plotted about the station circle in specified locations which must not be violated. If any mandatory plotted element is garbled, or partly missing, an "M" should be plotted in its place. Some data is not mandatory for reporting or plotting so if it is missing, the space for it is simply left blank and no "M" is plotted.
The surface data plotting model figure, shown again below, shows the location of data plotted about a station. The figure includes the location of data reported by ship stations as well as by land stations. The following defines the symbols of the data (as plotted on National Weather Service surface analyses) and indicates whether the data is mandatory or not.
The direction, North, is identified as a wind from 360 degree. A direction of zero degrees is used to represent calm winds. The wind direction shown in the Surface Plotting Model image is for a wind from 330 degrees. Wind direction is not a mandatory element.
"ff" represents the wind speed. Wind speed is plotted as barbs and/or flags extending from the wind direction shaft.
Barbs and flags should extend to the left of the shaft when looking along the shaft toward the station circle. Wind speed is a mandatory element if wind direction is reported. If wind direction is given but wind speed is missing, place an X at the end of the wind direction shaft rather than an M.
"TTT" represents Air Temperature. Although the standard for most of the world is to plot air temperature to the nearest tenth of a degree Celsius, the National Weather Service plots air temperature to the nearest whole degree Fahrenheit. Air temperature is a mandatory element.
"VV" represents the Prevailing Visibility. In the United States, prevailing visibility is plotted in whole statute miles and/or fractions, just as it is reported. Visibility is an mandatory element.
"ww" represents Present Weather. A symbol is plotted which represents the type of present weather occurring. Each symbol is associated with a two-digit number which is reported in the weather observation message. Table 1 of Exercise 1 of your Laboratory Manual shows the symbols associated with each present weather number. Present weather is not a mandatory element.
"TdTdTd" represents Dew Point. The reported dew point is plotted in the same manner as air temperature, rounded to the nearest whole degree Fahrenheit for stations in the United States. Dew point is a mandatory element.
"Cl" represents the type of Low level Cloud. In the weather observation report, the type of cloud is reported as just a number. The symbol associated with the reported number is plotted. See table 2 in your Laboratory Manual for the symbols associated with each type of cloud reported. Low cloud type is not a mandatory element.
"Cm" represents the type of Middle level Cloud. The symbol for the middle level cloud type is plotted just above the station circle and the various symbols are found on table 2. Middle level cloud type is not a mandatory element.
"Ch" represents the type of High level Cloud. The symbol for the type of high level cloud is plotted just above the middle level cloud symbol. The various symbols are found on table 2. High level cloud type is not a mandatory element.
"PPP" represents the Sea-Level Pressure plotted in tens, units, and tenths of hectopascals (or millibars which is the same as a hectopascal). Sea-level pressure is a mandatory element. Sea-level pressure values generally fall between 950 and 1050 hectopascals. A pressure of 1012.5 hPa would be plotted as 125, without the decimal point. A pressure of 996.4 hPa would be plotted as 964. Since only the tens, units and tenths digits are plotted, then to decode the plotted values and obtain the actual sea-level pressure, either a 9 or a 10 must be prefixed to the plotted values and the decimal point placed between the middle and last digit of the plotted values. If prefixing a 9 to the plotted digits makes the result lower than 950.0 hectopascals, then you should prefix a 10 rather than a 9. If prefixing a 10 makes the result greater than 1050 hectopascals, then you should prefix a 9 rather than a 10. This works for most sea level pressure values.
"ppp" represents the Amount of Pressure Change that occurred during the last three hours at the station. The tens, units and tenths of hectopascals is plotted without the decimal point. Thus, a pressure change of 02.4 hPa would be plotted as 024. The amount of pressure change is plotted in conjunction with the pressure tendency symbol and both are mandatory elements.
"a" represents the Pressure Tendency, sometimes called the barometric trace characteristic. It shows how the pressure behaved during the past 3 hours. Look under the column labeled "a" on table 2 in exercise 1 of your Laboratory Manual. This shows the symbols to be used to indicate the pressure tendency. Notice that the symbols for numbers 0 to 3 all end at a higher point than they begin. This means that the sea-level pressure is now higher than it was 3 hour ago. Notice that the symbol for number 4 is a straight line. This means the pressure is now the same as it was 3 hours ago. Note, it may have increased, then decreased, or decreased, then increased, but it is now the same as 3 hours ago. For numbers 5 to 8, the symbol ends at a point lower than it began. This means the sea-level pressure is lower than 3 hours ago. When plotting the pressure tendency and amount of pressure change, the first symbol to the right of the station circle will be either a + (when using symbol 0, or 1, or 2, or 3), a blank (when using symbol 4), or a - (when using symbol 5, or 6, or 7, or 8). Next is plotted the amount of pressure change, ppp, with no decimal point. Next is plotted the pressure tendency symbol.
"W" represents the Past Weather, the weather that occurred during either the past 3 hours or the past 6 hours depending on the time of the observation. Look under the column labeled "W" on table 2 in exercise 1 of your Laboratory Manual. This shows the symbols to be used to indicate the past weather. Notice that the symbols follow a similar pattern as the symbols on the rows of table 1 for present weather. The plotting model provides for plotting the two most significant past weather events; however, past weather is not a mandatory element and there may be no past weather or only one type of past weather. if past weather is not reported, no symbol is plotted. If past weather is reported, the proper symbol is plotted to the lower right of the station circle.
This is a composite map composed of a satellite image, plotted surface observations for selected stations, an analysis of sea level pressure with fronts and information from weather radars which show the location and intensity of precipitation.
At the bottom of the map are intensity levels (from 1 to 6) which show the relative intensity of precipitation, six being the heaviest precipitation. The clouds are shown as white areas across the map. The dark, blue, thin lines are isobars (lines of equal pressure). The heavy blue lines are cold fronts and the heavy red lines are warm fronts. Alternating blue and read mean a stationary front and a heavy purple line is an occluded front. Centers of high pressure are indicated with a H and centers of low pressure are indicated with a L .
Study the composite map and answer the following questions.
Plotting Upper-Air Observation Elements.
This image is the model for plotting data about a station circle on a constant-pressure map. The maps of upper-air observations are called constant-pressure maps because the air temperature, dew point temperature, the wind speed and direction plotted on a particular map; e.g., a 500-hPa (or 500-mb) map, are all measured at the level above each station where the pressure is 500 hectopascals (or millibars). The height at which a particular pressure value is found is different for each station. Therefore, instead of doing an analysis of pressure on an upper-air map, (constant-pressure map), an analysis of height is done, much as a topographic map.
Common upper-air (constant-pressure) maps which you will see on the hallway map display are: 850-mb, 700-mb, 500-mb, 300-mb, 250-mb, 200-mb. The following describes the procedure for plotting the observed data.
TT represents Air Temperature. Air Temperature is plotted to the nearest whole degree Celsius.
hhh represents the Height of the Constant-Pressure Surface above sea level. The height of the constant-pressure surface is plotted to the nearest whole meters on the 850- and 700-hPa map and to the nearest tens of meters on the 500-hPa map and those maps aove 500 hPa. Only three digits are plotted. On the 850- and 700-hPa map, the thousands digit is dropped. on the 500-hPa map and above the units digit is dropped.
DD represents the Dew Point Depression. The dew point depression is the difference between the air temperature and the dew point. The dew point is always less than, or the same as the air temperature. The dew point depression is plotted to the nearest whole degree Celsius. If the dew point depression is equal to 5 or less, the station circle is shaded.
hchc represents the12-hour Height Change. The 12-hour height change is the difference between the height of the particular pressure surface 12-hours ago and the current height.
This image shows the latest analysis for upper-air conditions at a level of 850 hPa, approximately 1500 meters (5000 feet) above sea level. The black lines are contours, or isoheights. The, dashed lines are isotherms.
Data is plotted about the station circles in blue. Note: Contrary to the standard format given above, on this image map and the next map for 500 millibars the height value is plotted using four digits rather than three. Normally, for plots of the height at which 850 hPa is found, a height of 1468 meters would be plotted as 468, however on this image, all four digits are plotted as 1468.
Close the 850 hPa map when finished with problem 16.
This next image is an example of an analyzed 500 hPa map. The standard method is for the units digit of the height of 500 hPa to be rounded up or down to the nearest tens of meters when plotted on the map. Thus, a height of 5854 meters is normally plotted as 585 to the upper right of the station. However, on this image, the complete height, including the units digit, is plotted. Like the 850 hPa map, the black lines represent isoheights (contours), lines of equal height; the dashed, lines represent isotherms, lines of equal temperature. Wind speed, wind direction, air temperature, dew point and height are all plotted in blue about the station circle. The center of low height centers and high height centers are indicated by an L or H, respectively.
Close the 500-hPa map when finished with problem 17.
The table of problem 18 provides data obtained from rawinsonde observations.
The table below gives the station name, the five-digit identifier for the station, (called the block and station number), the temperature, the dew point, the height at which the measurement was made and the wind direction and wind speed at this height. We are using data only for the level at which a pressure of 500 hPa is found. As you can see, a pressure of 500 hPa is found at different heights in the atmosphere. A pressure of 500 hPa is approximately 7.25 pounds per square inch.
You have looked at the latest 850 mb and the latest 500 mb upper-air analyses and seen how the height of these pressure surfaces is represented by contour lines, much as the contour lines on a topographic map. You should have noticed that the contour lines nearly parallel the direction of the plotted wind barbs. This is an aid when analyzing upper-air maps.
Copyright © 1996-2007 Texas A&M University, Texas A&M Meteorology Department and Marion Alcorn.