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How to Graph Isobars

An isobar is a line on a weather map that represents points of equal atmospheric pressure. When you visually link these places, the result is a series of concentric circles with either increasing or decreasing pressure readings as they progress to the center. Weather maps not only reveal details about the overall areas of high and low pressure, but also superimpose these readings over a geographic area.

Things You'll Need

Tape measure or steel rule
Graph paper
Straight edge
Current weather map

Instructions

- 1
  Lay out the map in front of you and draw a straight line from the eastern edge (right side of map) of the isobar readings straight across the map to the western edge of the weather data. Include only the area of the map that displays the weather data; exclude the map borders.
- 2
  Draw a circle around each junction between the line that you just drew and any intersecting isobar. Write down the numerical value of the isobar of all these points as they occur along the length of the line.
- 3
  Measure the entire distance along the line in inches and note it using whole numbers and decimals. For example, 20 1/2 inches = 20.5 inches
- 4
  Draw an x-axis and a y-axis on graph paper. The x-axis is the horizontal dimension and the y-axis is the vertical. Place your starting point exactly 1 inch up from the bottom and 1 inch over from the left edge of the paper. Then continue both axes all the way to the edge of the paper. Label the x-axis and y-axis in the margins.
- 5
  Measure the distance in inches of the the x-axis from point zero to the right edge of the paper. Record the number in whole numbers and fractions and write the number down next to the map distance from Step 3.
- 6
  Divide the map distance number by the graph distance number and write this number down in a prominent place. Depending on the size of the weather map, this number should have a value between 1 and 5.
- 7
  Measure the distance from the edge of the map to the first point. Divide it by the number you calculated in the previous step. This will be the actual distance along the x-axis (on the graph) where you will mark the point. Mark it very lightly on the graph and write down the corresponding isobar reading right above the point.
- 8
  Repeat the step above for every point along the inscribed line on the map.
- 9
  Look at all the isobar readings and place the highest reading at the top of the y-axis and the lowest reading at the bottom.
- 10
  Transfer each point from Step 7 straight up the graph to its corresponding isobar value as displayed along the y-axis.
- 11
  Connect the dots and label the graph "Pressure Profiles."

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