Surface Map Analysis example

Surface Weather Map Analysis Example

The map that we will be analyzing is shown below (you'll be surprised at how much order we will be able to bring to what now looks like just a mess or lines and numbers)

Our first job will be to draw in some isobars. To make that job easier the map has been redrawn below with just the pressure data shown.

Remember you need to add either a 9 or a 10 and a decimal point to the data plotted on the map. The 070 plotted at lower left in the map is either 907.0 mb or 1007.0 mb. You pick the value that is closest to 1000 mb, average sea level pressure. In this case 1007.0 mb is the correct sea level pressure value.

The highest pressure value on the map is 1009.6 mb, the lowest is 994.0 mb. Four of the allowed isobar values (the red values) fall in this range: 994.0 mb 996 1000 1004 1008 1009.6 mb. Isobars will pass through any station with a value that is exactly equal to the isobar's value. Otherwise isobars pass between pairs of stations: one with a pressure greater the other with a pressure smaller than the isobar's value.

On the map below stations with a pressure less than 996 mb have been shaded purple. Stations with pressures between 996 mb and 1000 mb have been shaded blue. The 996 mb isobar will separate the purple stations from the blue stations. Stations with pressures between 1000 mb and 1004 mb have been colored green. The 1000 mb isobar will separate the blue from the green stations.

Next we color stations with pressure between 1004 and 1008 yellow. The 1004 mb isobar will separate the yellow and green shaded stations. Note there is one station in the upper right corner with a pressure of exactly 1004.0 mb, the 1004 mb isobar will pass through that station.

One station with a pressure greater than 1008 mb has been colored orange. The 1008 mb isobar will separate these station from the other stations on the map. Note that only portions of the 1004 mb and 1008 mb isobars have been drawn. Once you reach the edges of the map and run out of plotted data you really have no idea where those isobars should go.

The isobaric analysis is now complete. We transfer these isobars onto the original weather map below.

Also on this map an attempt has been made to identify air masses with different temperatures. The warmest air with temperatures in the mid and lower 60s has been circled in orange.

In the next figure we will locate the cold front at the western boundary of this warm air where cold air is pushing down from the northwest. We will locate the warm front at the northern edge of the cold air mass.

You should verify that some of the other criteria used to locate fronts (changes in wind direction, changes in moisture content, clouds and precipitation, pressure tendency) confirm these frontal locations. Note that the warm and cold fronts both originate in the center of low pressure. With time these fronts would rotate in a counterclockwise direction around the low pressure center. At the same time the low pressure center would most likely be moving to the east or northeast.