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
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 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!
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.
This document is at
first posted 25 Nov 2013, modified 12 Jan 2014
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