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Contour Maps. Lots of weather observations plotted on a map look like a sea of numbers—hard to see patterns. Solution: draw contour lines or isolines to help us visualize the patterns.

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contour maps
Contour Maps
  • Lots of weather observations plotted on a map look like a sea of numbers—hard to see patterns.
  • Solution: draw contour lines or isolinesto help us visualize the patterns.
  • “Iso” = “same” (from Greek). A contour line or isolineconnects places on the map that all have the same value of some quantity.
  • For example:
    • line of same or constant pressure is an isobar
    • line of same or constant temperature is an isotherm.
    • line of constant wind speed is an isotach.
drawing contour lines on a map
Drawing Contour Lines on a Map
  • For more complete picture of a pattern:
    • draw multiple contour lines
    • each line has a different “contour value”
  • Infinite number of possible values--need to choose a few!
  • Decide which contour values to use:
    • Pick a reference value to start (e.g., 1000 mb, or 0°F, etc.)
    • Pick a contour interval. What’s reasonable? Use a standard value, or:
      • Find the highest and lowest values on your map
      • Calculate the range of values on your map (range = highest – lowest)
      • Divide range by 10
      • Round up or down to a nice, round number (e.g., 1, 2, 4, 5, 10, etc.)
      • Result: ~ 10 contour lines—not too many, not too few
      • On sea-level pressure map of North America, standard choice is 4 mb.
    • Generate list of candidate values:
      • Values at intervals = contour interval, above and below the reference value
drawing contour lines on a map1
Drawing Contour Lines on a Map
  • Which contour line should I draw first?
    • first candidate contour value above the lowest value on the map; or
    • first candidate contour value below the highest value on the map
  • Where do I start drawing my first line?
    • If lucky, a station reports exactly that value. Start there.
    • If no luck, must interpolate values between stations.
      • Find two adjacent stations, one reporting a value higher than your contour value, the other reporting a lower value.
      • Estimate where between stations the value equals your contour value.
      • Put a dot there. Your contour line passes through that point.
drawing contour lines on a map2
Drawing Contour Lines on a Map
  • I have a dot. How do I make a line?
    • Find two more, adjacent stations nearby, one reporting a value higher and the other lower than your contour value.
    • Estimate where between them the value equals your contour value.
    • Put a dot there.
    • Repeat for two more, adjacent stations nearby that new dot. Keep repeating, until you reach edge of reported observations or end up where started.
    • Connect the dots.
drawing contour lines on a map3
Drawing Contour Lines on a Map
  • Label the contour line with its value
  • Repeat the preceding for the next contour value on your candidate list. Repeat until list is exhausted.
  • At spots where values are lower than surrounding areas, write an “L”. Where values are higher than surrounding areas, write “H”.
  • Label the map:
    • The quantity that you’re contouring (e.g., “Sea-Level Pressure”)
    • The contour interval [e.g., “(4 mb intervals)”]