Meteorology: Understanding the Atmosphere            Ackerman and Knox


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.

Table of common isopleths:

Isobar Pressure
Isotherm Temperature
Isotach Wind Speed
Isogon Wind Direction
Isoshear Wind Shear
Isodrosotherm Dew point
Isohyet Precipitation Accumulation
Isohaline Salinity
Isopycnic Density
Isohume Humidity
Isoneph Cloudiness
Isohel Sunshine