# How to Create an Isobar Map

Written by kelvin o'donahue
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An isobar map is a type of contour map, a map that uses lines to display the distribution of three-dimensional data on a two-dimensional surface. Contours on these maps connect all locations or points at which the measured value is equal to a specific value.

Isobars ("iso" meaning "equal" and "bar" meaning "pressure") connect points of equal barometric pressure. Isobar maps are used in weather forecasting to predict movement of air masses. Winds typically flow from areas of high pressure to areas of low pressure, as shown on isobar maps.

Skill level:
Easy

### Things you need

• Basemap with station locations
• Pressure data for stations in millibars
• Pencil
• Eraser (optional)

## Instructions

1. 1

Post the pressure data at the locations where the measurements were taken. Print the numbers on the map alongside the station.

2. 2

Draw the first contour line, typically the 1000mb (millibars) isobar. This line divides the map into two parts: all pressures that are less than 1000mb lie on one side of the line and all pressures greater than 1000mb lie on the other. Label the line "1000."

3. 3

Estimate the location of the line where it passes between two points by interpolation. The closer the line value is to the point's pressure value, the closer the line approaches the point on the paper. An isobar passes through any point that has exactly its value.

4. 4

Repeat the drawing process to plot a line for the 996mb isobar. By convention, isobar maps are drawn at a 4-millibar contour interval, or with a line every 4mb. This second line will roughly parallel the 1000mb isobar, but must never cross it. Label this line as before.

5. 5

Continue drawing and labelling isobars at 4-millibar increments until you reach the lowest value posted on the map. Repeat the process for isobars with values greater than 1000mb.

#### Tips and warnings

• Isobars can and do close to form circles or ovals on the map. A closed low is a low pressure cell (cyclone) and a closed high is a high pressure cell (anticyclone). These are what makes weather happen.
• Isobars that bunch together indicate a steep pressure gradient, which can cause powerful winds.
• Pressure measurements must be converted to atmospheric pressure at sea level for the map to be accurate.

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