CONTOUR ANALYSIS

To understand atmospheric and oceanic circulations, you must be able to understand how variables (temperature, pressure, winds, humidity, clouds, salinity) are changing in time and how they are changing with respect to one another.  The weather map is a tool that aids this understanding. Various kinds of maps, or charts, are used to graphically depict these variables. A good map allows you to quickly  identify patterns. For example, a weather map of forecasted high temperatures typically available in newspapers indicates the location of warm and cold regions of the country. From these maps you can quickly gage the predicted high temperature for your town.

Maps depicting weather and ocean conditions are drawn based on simultaneous observations made at many places throughout the world. Accurate portrayal of these observations is the key to a correct interpretation of the data.  Meteorologists and oceanographers use a technique called contour analysis to visually explain the information the data is providing.  Contouring data represents an elementary step in data analysis.  Ability to correctly and confidently analyze data is critical to interpreting conditions.  For example, contouring is vital in:

  1. finding the location of atmospheric and oceanic fronts,
  2. locating potential regions of severe thunderstorms,
  3. tracking hurricanes,
  4. tracking the movement of pollutants.
  5. tracing water movement in the oceans
A series of  interactive web exercises follow which enable you to practice contouring. First, some information about contouring.

An isopleth is a line of equal value (a Greek word iso - equal; pleth - value).  Contouring  is the process of drawing isopleths.  A weather map contains isopleths of different weather parameters (Table of common isopleths).  For example, maps of forecasted high temperatures have contours of constant temperature, or isotherms (iso-equal; therm-temperature).  On these maps, anywhere along the 70 degree isotherm the forecasted high temperature is 70 degrees.

Contouring data can be difficult because observations are not made everywhere.  Gaps in the observations exist and we must interpolate between the existing weather observations. For example, the weather map below depicts weather conditions on July 15, 1997 at 1900Z in the southern Wisconsin and the surrounding states.  The red numbers are the observed temperatures, in degrees Fahrenheit.  The city of Madison, WI has a temperature of 85F while Rockford, IL has a temperature of 88F.  If you where to drive from Madison to Rockford on I90 (the yellow line) with a thermometer, along the way you would measure temperatures of 86F and 87 F.  If we were to draw an isotherm of 86F, this isotherm would have to pass between Madison and Rockford, and through the city of Milwaukee, WI where the observed temperature is 86F. The adjacent map analyzes the 86F isotherm as a thick white line. Ocean data is even more sparse than weather data!

 

Contoured maps are common. You see them in the weather section of your newspaper, a map of the Earth often includes two contour analyzes -- one of latitude and the other longitude, and hiking maps depict lines of constant elevation.
 

Follow these general rules of contouring  as you analyze these maps:

  1. isopleths should never cross,
  2. isopleths should not branch or fork,
  3. only the area on the map that has data should be contoured,
  4. you should label your isopleths (in these exercises we will color code them).
When you finish contouring check your results.  One way to do this, is to imagine that you are walking along an isopleth. The values of the data points one side of the line should all be greater than, or all of them should be less than the value of the isopleth.

 Lesson 1: Introduction to contouring

In this first exercise, you will use the mouse to practice contouring.  You might start analyzing a contour by drawing tic marks on the map to mark where the isopleth should be, then connect the dots.  Don't worry about trying to get the lines perfectly smooth.  The purpose is for you to gain an understanding of how contouring is accomplished. Notice that contour lines cannot cross, but they can form closed loops.

Lesson 2: Learning the Basics

In this next exercise, you will again use the mouse to contour a map.  You will be presented with a series of plotted data points that will determine the contoured field.  Contour the field before looking at the answer!  As you go through this first exercise notice how the contour field changes as you add observations to the map.  The more observations you have, the better the field is defined.

Lesson 3: Intermediate Level

To become adapt at contouring requires practice!
When you are done with this map, notice the spacing of the contours. The spacing of the isopleths indicates how rapidly conditions are changing in the horizontal direction.  The gradient of a variable describes how much the variable changes over a given distance.  The more densely packed the isopleths, the larger the gradient.

Lesson 4: Working with Real Data

In this exercise you will be presented with observations of surface dew point temperatures on 1200 UTC, July 18, 1996.  Unlike the previous exercises, all the observations are plotted on the map at once.  But don't let this bother you.  Before drawing your first contour invest some time in preparation.

In real analysis lab, you would be using a pencil instead of a computer mouse so that you can erase the inevitable mistakes.

Lesson 5: Working with Weather Maps
In this exercise you will be presented with a weather map that includes observations of temperature, dew point temperature,  and wind speed and direction made at 1400UTC on July 18, 1996.  On this day a tornado touched in in Oakfield, WI.  The observations are all plotted on the map at once, so don't get temperature confused with the dew point!  Do you notice a relationship between temperature and dew point?  
Lesson 6: Working with Ocean Maps
Oceanography is another science that uses contouring to analyze data.  In this exercise you will be presented with salinity data measured in the Mediterranean Sea.  When compared to weather observations taken over land, there are very few observations made in the oceans.  No matter what the data source, the approach to contouring is the same.
Lesson 7: Contouring Potential Temperatures in the Mediterranean
Potential temperature is an important variable in oceanography and meteorology. It is a conserved variable and is therefore a good tracer.

GENERAL RULES OF CONTOURING:

The more contouring you do the better you will be at interpreting weather maps.  In lab session you will be doing contouring on paper weather maps using a pencil.  A few guidelines to remember when doing the lab exercises:
 
1) Prepare to contour

A. Search for continuity (time and space) on the weather map
B. Eliminate what appears to be faulty, or incorrect, data
C. Locate regions of high and low values
D. Review data to determine isopleth spacing

2) Drawing the map

A. Use a pencil !!
B. Draw smooth lines
C. Interpolate between given values to correctly place an isopleth
D. Isopleths cannot touch or cross
E. Isopleths cannot branch or fork
F. Be neat and accurate
G. Label the isopleths

3) Review your map

A. Do a gross check of isopleths for accuracy
B. If necessary, trace over pencil with final marker (be neat)
C. Check for proper labeling

4) Your map is now ready for interpretation!

 

 
Copyright 1997, University of Wisconsin-Madison, Space Science and Engineering Center.

This contouring exercise was developed by Steve Ackerman and Tom Whittaker. You are free to use it for educational purposes. We would appreciate knowing what you are doing with the module and your suggestions for improvements. You can send me comments.