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
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
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.
29 Oct 2012
Links to scientific reports about Hurricane Sandy:
Low pressure of 970 mb in Atlantic Ocean, and 1008 mb on coast of New Hampshire.
9 Feb 2013
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
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 produce severe hail, and sometimes tornadoes.
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
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).
Same time as above, but with composite radar image superimposed.
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.
- Richard H. Grumm, Charles Ross, et al.,
"Chesapeake and Ohio Express: The Derecho of 29-30 June 2012,"
- National Weather Service, Mount Holly NJ, "June 29-30, 2012 Derecho,"
Report, 14 pages.
- "Derecho: Behind Washington, DC's Destructive Thunderstorm Outbreak,"
30 June 2012. (meteorology)
- "Violent storm that slammed South Jersey is known as derecho,"
30 June 2012.
- "Derecho Winds Pummel Region,"
in Charleston, West Virginia, 30 June 2012.
- "Thunderstorm warnings issued for NW Ohio,"
1 July 2012.
- "Possibly fatal storm leaves more than 1 million without power"
30 June 2012. (damage reports)
This document is at
first posted 25 Nov 2013, modified 15 August 2014
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