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
17 Sep 2015 (calm wind)
24 Feb 2016
30 Oct 2017
4 Jan 2018


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 have 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.

17 Sep 2015

map for calm wind
This example shows the surface pressure map for a time when the wind at Concord, NH was calm from 22:51 GMT on 16 Sep continuously through 14:51 GMT on 17 Sep 2015, an interval of 16 hours with calm wind. The surface pressure map is for 06:00 GMT on 17 Sep, approximately in the middle of this period of calm winds. You can see the small pressure gradient in the northeastern USA, which caused the calm wind. At 05:51 GMT (01:51 EDT) on 17 Sep, the surface pressure at Concord, NH was 1022.2 mb and the surface pressure at Albany, NY was 1023.0 mb.

24 Feb 2016

This storm began as a region of low pressure centered on western Tennessee at 01:00 EST on 24 Feb 2016. The pressure at the center was a constant 986 or 987 millibars from 04:00 EST on 24 Feb to 04:00 EST on 25 Feb, as the center moved northeast through Kentucky, Ohio, and New York State.

heavy rain

On the map, notice the squall line that travelled east through Virginia and North Carolina. There is another squall line in Florida.   This storm brought a total of 42 mm of rain to Concord, NH.

As one would expect from the intense pressure gradient, there were high winds across much of the eastern USA on 24 Feb.   The Associated Press and Reuters reported that a total of 8 people died from this storm, half of them in Virginia. There were tornadoes in Virginia and Pennsylvania. More than 1100 airplane flights at two airports in Chicago were cancelled. A maximum of 38 cm of snow fell in northwest Indiana. Chicago Tribune.

I looked at the National Weather Service websites for various cities and found the following information for 24 Feb. Snow with wind gusts up to 79 km/h was recorded at Midway Airport in Chicago. Rain with wind gusts up to 69 km/h at Philadelphia airport, with a total of 48 mm of rain on 24 Feb. Thunderstorm with wind gusts up to 53 km/h at Reagan Airport in Washington, DC.

The Washington Post has a news article that explains some of the meteorology in this storm.

warm front

At about 08:30 EST on 25 Feb, a warm front passed north through Concord, NH. The air temperature jumped from +6 to +14 celsius in less than one hour. The average high temperature for this day is +2 celsius, so the high temperature of +17 celsius was unseasonably warm for Concord, NH.

30 Oct 2017

There was an intense storm with heavy rain and strong winds on 30 Oct 2017 in New England. This storm began with a low-pressure region centered east of North Carolina on 29 Oct 2017. The low-pressure region quickly moved northward as shown in the following list:
intense low pressure
Surface pressure map for 02:00 EDT (06Z) on 30 October 2017.

The weather station at the Manchester, New Hampshire airport recorded wind gusts as high as 95 km/h at 02:00 EDT on 30 October. The strong winds blew tree branches into overhead electric power lines, causing many electric outages. The Union-Leader newspaper reported that this storm caused the fourth-largest electric power outage in New Hampshire history. The Associated Press reported that a total of 1.5 million utility customers in the northeastern USA lost electric power in this storm.

4 Jan 2018

On Tuesday, 2 January 2018, meteorologists predicted that a low-pressure region would form east of Florida, and then rapidly move north along the coast, intensifying to 950 mb on Thursday afternoon. This storm will be essentially a hurricane in January, with snow instead of rain. Some meteorologists referred to this storm with the hyperbole “bomb cyclone”. Washington Post.

The National Weather Service surface pressure map for 18:00 GMT (13:00 EST) on 3 January shows a 1004 mb low-pressure region in the Atlantic Ocean east of Florida. Three hours later, this low had intensified to 997 mb and the center moved to east of the Florida/Georgia border. This storm caused rare snow in Florida and Georgia. New York Times;   Times-Union in Jacksonville, Florida;   Associated Press.

dawn 4 Jan 2018
Surface pressure map at dawn (07:00 EST) on 4 January 2018, showing 957 mb low centered east of Virginia.

10:00 EST, 4 Jan 2018
Surface pressure map at 10:00 EST on 4 January 2018, showing 951 mb low centered east of Maryland. Note that the outer isobars of this storm now cover all of New England.

13:00 EST, 4 Jan 2018
Surface pressure map at 13:00 EST on 4 January 2018, showing 950 mb low centered east of New Jersey. The biggest impact from this storm in New England was coastal flooding as 51 km/h wind gusts combined with an astronomical high tide at 12:48 EST in Portsmouth, NH.

radar 14:48 EST, 4 Jan 2018
Radar base reflectivity mosaic in Northeastern USA at 14:48 EST on 4 January 2018. The echoes from this storm are weak (mostly less than 30 dBZ), as the snow from this storm is dying.

19:00 EST, 4 Jan 2018
Surface pressure map at 19:00 EST on 4 January 2018, showing 952 mb low centered east of Maine. At Concord, NH and Manchester, NH the wind gusts were as high as 74 km/h.

Concord, NH received a total of 33 cm of snow from this storm. However, coastal flooding was the major impact in New Hampshire. Union-Leader.   More on the storm is at the New York Times;   and Boston Globe.


Not all severe weather is associated with concentric isobars on a surface pressure map.

Isolated thunderstorms can occur when heating by sunlight in the morning evaporates water from the ground, the water vapor rises with the warm air, until it condenses and forms a cumulus cloud. If there is enough vertical motion in the atmosphere, the cumulus cloud can develop into a cumulo-nimbus cloud and produce a thunderstorm. Such thunderstorms typically occur between 13:00 and 16:00 local time. As the sun sets, heating of the ground by sunlight stops, and vertical convection stops, which kills development of storms. A single thunderstorm cell may have a duration of approximately 30 to 60 minutes, but will affect one location for only about 5 to 20 minutes, owing to horizontal motion of the storm. Isolated thunderstorms are difficult to predict from looking at only a surface pressure map.

A more complicated phenomena is a line of many thunderstorms on the warmer side of a cold front, called a "squall line". A squall line can occur anytime (i.e., day or night) and is distinguished by high wind speeds. A squall line may also produce severe hail, and sometimes tornadoes.

5 Sep 2014 Cold Front

On the afternoon of 5 Sep 2014, a cold front extended from Missouri, to Illinois, and southern Michigan.
18:00 GMT corresponds to 13:00 CDT in Chicago.

A line of thunderstorms on the warm side of the front caused extensive damage. In northern and western Chicago, 113,000 customers lost electric power, after high winds (gusts to 80 miles/hour) blew trees into overhead electric wires. Chicago Tribune. In the Detroit area, 462,000 customers lost electric power. Free-Press.

At 18:00 GMT, the enlarged map shows temperatures of 20 to 22 celsius (68 to 71 F) on the cold side of the front in Iowa and Wisconsin, and temperatures of 30 to 32 celsius (87 to 90 F) on the warm side in Missouri, Illinois, and Michigan.

Notice the 1023 millibar high in the Atlantic Ocean, and 993 millibar low in Canada. Circulation around these two regions pulled warm, moist air to the northeastern USA, where it was forecast to cause thunderstorms on the afternoon of 6 Sep, when the front moved there.

As the front moved to the east on 5 Sep, thunderstorms occurred in Indiana and northern Ohio. The map for midnight GMT (20:00 EDT) on 5 Sep 2014 is shown below.

There is a squall line (a red line with two red dots) east of the front in Canada, and west of Buffalo, NY. This squall line is not shown on previous or subsequent maps. About 63,000 utility customers in Canada lost electric power owing to this storm. Toronto Star.

radar map

The line of thunderstorms is clearly shown on the map above that superimposes the radar image and fronts at midnight GMT on 5 Sep.

Most of this line of thunderstorms in Indiana and Ohio died soon after 06:00 GMT (02:00 EDT) on 6 Sep. It is common for such a line of thunderstorms to die late at night, and then reappear the following afternoon. The front stalled at western New York state at 5:00 and 08:00 EDT on 6 Sep.

Thunderstorms occurred in Massachusetts, Connecticut, and New Jersey as the front moved through in the afternoon and evening of 6 Sep. About 5300 utility customers lost electric power in Connecticut (Courant), and 32,000 customers lost electric power in New Jersey (NJ101).

30 June 2012 Squall Line/Derecho

On the afternoon (18:00 GMT) of 29 June 2013, one cold front extended from Nebraska to Pennsylvania and up to Maine. A squall line formed and traveled rapidly east from Chicago (at 16:00 GMT) to the Atlantic Coast between Virginia and New Jersey (at 04:00 GMT on 30 June).

At 00:00 GMT, the squall line runs from southwestern Pennsylvania, through the center of West Virginia, and into eastern Kentucky. Looking at the zoom in version of this map, one finds a small surface temperature gradient across the cold front: north of the front, in Pennsylvania, surface temperatures are between 29 and 30 celsius (84 to 86 F); south of the front and east of the squall line, in Virginia, surface temperatures are between 32 and 34 celsius (90 to 94 F).

radar map
Same time as above, but with composite radar image superimposed.

radar map
Three hours later (03:00 GMT), the squall line moved east, and extended from south of the front in Pennsylvania to north-central North Carolina.

References for the 29-30 June 2012 Squall Line: For more citations, use a search engine for the query derecho for dates 29-30 June 2012. In general, see the webpage Derecho Facts by Stephen Corfidi.

This document is at
first posted 25 Nov 2013, modified 5 Jan 2018

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