Interesting Surface Pressure Maps

by Ronald B. Standler

The maps are works of the U.S. Government, and are therefore not protected by copyright. 17 U.S.C. §105.

Table of Contents

Hurricane Sandy 29 Oct 2012
Nor'easter 9 Feb 2013
18 Nov 2013
24 Nov 2013
11 Jan 2014
24 Apr 2014


When I was a child, about 9 y old, I knew that air pressure was lower at high elevations (e.g., on a mountain) than at sea level. So I was perplexed in looking at air pressure maps and seeing no effect by the Rocky Mountains in New Mexico and Colorado. What science textbooks did not make clear was that the air pressure map shows data that has been modified to remove the effect of altitude, so that the land is effectively all at sea level elevation. This modification lets meteorologists clearly see weather systems, without the complication of varying altitude.

Later in life, I noticed that textbook authors and professors often neglect to explain all of the assumptions and limitations, because "everyone knows that" or "it is obvious". As students know, it is not obvious.

Meteorologists measure air pressure in millibars. One millibar is equivalent to 100 Newtons per square meter (equivalent to 100 Pascals). Some meteorologists say "hecto-Pascal" (hPa) instead of "millibar", but the two units are equivalent.

A surface pressure map shows a plot of lines of equal surface pressure, called isobars. Convention is to space the isobars four millibars apart, so one can easily compare maps for different times. Standard sea level pressure is 1013.25 mb. While a barometer filled with mercury typically has a scale in millimeters of mercury, meteorologists convert that measurement to millibars.

On a typical day, the surface pressure map shows a range of pressures from about 1000 mb to 1020 mb. As a general rule, regions of strong high pressure (i.e., more than approximately 1025 mb) are associated with clear skies. Regions of low pressure (i.e., less than approximately 1000 mb) are associated with clouds, and sometimes rain or snow. The farther the pressure sinks below 1013 mb, the more intense the storm. In particular, one measure of hurricane intensity is the minimum surface air pressure in the eye of the hurricane.

The center of a region of low pressure is marked with an L (for low) on a surface pressure map. Note that such L-regions are relative to pressure at surrounding locations. The center of an L-region could have a pressure of 1015 mb (i.e., higher than standard sea-level pressure) if the pressure at surrounding locations is higher than 1015 mb.

When isobars are crowded together, the wind speed is intense, owing to the pressure gradient. The maps shown below contain examples of such large pressure gradients.

Finally, in the Northern Hemisphere, winds rotate clockwise (when viewed from above) around a high-pressure system, and winds rotate counterclockwise around a low-pressure system. Wind blows from high-pressure regions to low-pressure regions, but the rotation of the Earth (i.e., Coriolis force) makes the wind rotate. For that reason, winds are approximately parallel to isobars on the surface pressure map.

Hurricane Sandy
29 Oct 2012

Low pressure of 950 mb in Atlantic Ocean, and 996 mb on coast of Virginia. The center of the storm hit New Jersey, inflicting immense damage there and also in New York City.
Links to scientific reports about Hurricane Sandy:

9 Feb 2013

Low pressure of 970 mb in Atlantic Ocean, and 1008 mb on coast of New Hampshire.
This type of storm is known in New England as an "Nor'easter", because the low pressure to the southeast of New England causes winds to blow from the Northeast. Such storms are significant because they bring moist air from the sea over land, and often create heavy rainfall/snowfall.

18 Nov 2013

Low pressure of 972 mb in Canada, and 1004 mb in lower Michigan.
The high winds associated with this storm interrupted electric power to a half-million customers in Michigan. In Concord, NH, the wind from the south brought warm, moist air: a daily high temperature of +17 celsius (average high for this date is only +9 celsius) and 8 mm of rain.

24 Nov 2013

A low pressure of 965 mb centered over northern Québec province in Canada, with high pressure of 1040 mb centered over Illinois, produced an intense pressure gradient in New England. In Concord, NH, this gradient caused high winds (e.g., gusts of 75 km/h) and low temperatures (e.g., daily high of -4 celsius on 24 Nov, and low of -10 celsius in the early morning of 25 Nov — compared to a 19-year average of +7 and -3 celsius). More than 40,000 utility customers in New Hampshire lost electric power on 24 Nov because high winds had blown trees into overhead electric wires.

NOAA's automated observations at the Concord, NH airport stopped sometime after 13:51 EST on 24 Nov. I used the measurements from the personal weather station in East Concord on the WeatherUnderground website. NOAA resumed automated observations at 08:51 on 25 Nov.

11 Jan 2014

During 7 to 10 Jan 2014, the air temperature at Manchester, NH was consistently below 0 celsius. On 11 Jan, a low pressure system of 984 mb was located to the northwest, which would bring warm air from the south up to New Hampshire (remember winds rotate counterclockwise around a region of low pressure). The map for 21:00Z corresponds to 16:00 EST.

But there was a stationary front stretching east-west across New Hampshire during the afternoon. Temperatures north of the front were near zero celsius, while temperatures south of the front were near +11 celsius. The front traveled north through Manchester, NH between 14:53 and 15:53 EST and the temperature there jumped from 1.7 to 11.7 celsius in one hour. After the front passed to the north, there was a wind from the south with a speed between 10 to 20 km/h, caused by the pressure gradient from the low pressure system to the northwest. The air temperature at Manchester was +11 celsius at midnight on 11 Jan! A typical midnight temperature in January would be -5 celsius.

On the morning of 12 Jan 2014, the low intensified to 975 mb and moved northeast, above Maine. The wind in New Hampshire then shifted to from the west.

24 April 2014

At 06:00 GMT a 987 mb low pressure region south of Nova Scotia, Canada and a 1010 mb pressure in Albany, NY created an intense pressure gradient in New England. In Concord, NH, the surface pressure was 1003.5 mb, with wind gusts of 50 km/h.

At 18:00 GMT, the low pressure region moved northeast of the southern tip of Nova Scotia, and the minimum surface pressure was 984 mb. The pressure was 1006 mb in Concord, NH, with wind gusts to 70 km/h.

This document is at
first posted 25 Nov 2013, modified 25 April 2014

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