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Isobars And Air Pressure Homework Answers

A line drawn on a weather map connecting points of equal pressure is called an isobar. The isobars are generated from mean sea level pressure reports and the pressure values are given in millibars.


The diagram below depicts a pair of isobars. At every point along the top isobar, the pressure is 996 mb and at every point along the bottom isobar the pressure is 1000 mb.

Any point in between these two isobars will have a pressure somewhere between 996 mb and 1000 mb. Point A, for example, has a pressure of 998 mb and is therefore located between the 996 mb isobar and the 1000 mb isobar.

Sea level pressure reports are available every hour, which means that maps of isobars are likewise available every hour. The solid blue contours are isobars and the numbers along particular contours indicate the pressure value of the isobar.

Surface maps of isobars are useful for locating areas of high and low pressure, which correspond to the positions of surface cyclones and anticyclones. A map of isobars is also useful for locating strong pressure gradients which are identifiable by a tight packing of the isobars. Stronger winds are associated with larger gradients in pressure.


Using a black colored pencil, lightly draw lines connecting identical values of sea level pressure. Remember, these lines, called isobars, do not cross each other. Isobars are usually drawn for every four millibars, using 1000 millibars as the starting point. Therefore, these lines will have values of 1000, 1004, 1008, 1012, 1016, 1020, 1024, etc., or 996, 992, 988, 984, 980, etc.

Begin drawing from the 1024 millibars station pressure over Salt Lake City, Utah (highlighted in blue). Draw a line to the next 1024 value located to the northeast (upper right). Without lifting your pencil draw a line to the next 1024 value located to the south and then to the one located southwest, finally returning to the Salt Lake City value. Remember, isobars are smooth lines with few, if any, kinks.

The result is an elongated circle, centered approximately over Eastern Utah. The line that was drawn represents the 1024 millibars line and you can expect the pressure to be 1024 millibars everywhere along that line. Repeat the procedure with the next isobar value. Remember, the value between isobars is 4 millibars. Since there are no 1028 millibars values on the map, then your next line will follow the 1020 millibars reports. Then continue with the remaining values until you have all the reports connected with an isobar.

Label each isobar with the appropriate value. Traditionally, only the last two digits are used for labels. For example, the label on the 1024 mb isobar would be 24. A 1008 mb isobar would be labeled 08. A 992 mb isobar will be labeled 92. These labels can be placed anywhere along the isobar but are typically placed around edges of the map at the end of each line. For closed isobars (lines that connect) a gap is placed in the isobar with the value inserted in the gap.

Your map should look like this.

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