Contour Maps

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# Contour Maps - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about 'Contour Maps' - bonnie

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Presentation Transcript
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
• 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 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 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 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)”]