• Barometry objectives:

– Learn some of the methods used

to measure the static pressure
exerted by the atmosphere

– Understand the

advantages/disadvantages to each
of the methods

– Experiment with exposure and

calibration issues related to
measuring atmospheric static
pressure

ATMS 320 –

Meteorological

Instrumentation

• A review on

pressure

ATMS 320 – Barometry

http://www.godchecker.com/pantheon/greek-mythology.php?deity=ATLAS

Static pressure- Force/Area against
any surface in the absence of air
motion.

Dynamic pressure- Force/Area due
to air motion.

• The Earth’s

atmosphere exerts a
static pressure on
the surface of the
Earth equal to the
weight of a vertical
column of air of unit
cross-section. . .

ATMS 320 – Barometry







0

)

(

)

(

)

0

(

dz

z

z

g

p



W&H f2-1 here

Since air is a fluid, this pressure, or force, is exerted
equally in all directions.

• If the wind is

blowing, it exerts
a dynamic
pressure which
introduces a
static pressure
error…

http://news.bbc.co.uk/1/hi/in_depth/photo_gallery/3085722.stm

ATMS 320 – Barometry

2

2

1

V

C

p







• Physical

principles
employed for
measuring
atmospheric
pressure:

– Direct techniques
– Indirect

techniques

http://www.rube-goldberg.com/html/pencil_sharpener.htm

ATMS 320 – Barometry

• Direct – mercury

barometers:

– Balance the force

due to
atmospheric
pressure against
the weight of a
column of
mercury

ATMS 320 – Barometry

• Direct – mercury

barometers
(cont.)

– Difficult to

automate

– Not suitable for

field experiments

– Health risk

ATMS 320 – Barometry

• Direct – mercury

barometers (cont.),
why mercury?

– High density (small

column height)

– Low vapor pressure

(little evaporation
into vacuum)

– Chemically stable
– Liquid for a wide

range of
temperatures

ATMS 320 – Barometry

http://www.gormangiftgallery.com/tormerbar.html

ATMS 320 – Barometry

h

g

p

m





1

mercury

of

mass



c

m

A

h



a

m

F 

c

A

F

p 

1

Mercury barometer calibration equation

• A nice web page

to help with the
understanding of
how mercury
barometers
work…

ATMS 320 – Barometry

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/Barometer/Barometer.html

•

Main sources of error in

a mercury barometer:

– Dynamic wind pressure

(alleviate via a static port)

– Density of mercury (and of

glass tube) are functions

of temperature

– Local gravity must be

known accurately (a

function of latitude)

– Presence of air or water

vapor in “vacuum”

– Surface tension effects

– Barometer must be kept

vertical

– Impurities in the mercury

ATMS 320 – Barometry

• Advantages of

mercury
barometers:

– Simple in concept

(can visualize how
it works)

– Easy calibration

ATMS 320 – Barometry

ATMS 320 – Barometry

G

s

C

p

p





2

T

x

C

C

p

p







1

2

Thermal correction equation

Altitude and latitude correction equation

Getting the atmospheric pressure reading “right”…

Brhf2-4 here…

• Direct – aneroid

(without fluid)
barometers:

– Balance the force

due to
atmospheric
pressure against
the restoring
force of an
“elastic” material
(e.g. metal)

ATMS 320 – Barometry

• Aneroid

barometers:

– By changing the

shape of the
material surface
used in the
aneroid barometer,
we can improve
the sensitivity of
the barometer at
high atmospheric
pressures

ATMS 320 – Barometry

Brf2-6 here





3

1

0

r

r

y

c

y

c

p





r

y

Q

p 

Curve A or B??

Curve A or B??

dp

dy

r



y

sensitivit

static

A

B

y

r

y = deflection of diaphram center, t = diaphram thickness

• Other aneroid

barometers

:

– Stacked aneroid cells

(2-7)

– Aneroid capsule (2-8)
– Silicon diaphragm (2-

9)

– Bourdon tube (2-10)

ATMS 320 – Barometry

• Main sources of error in

aneroid barometers:

– Same exposure errors as

mercury barometers (e.g.

dynamic pressure)

– Temperature-induced

error

– Error arising from defects

or irregularities in the

diaphragm material

and/or shape

– Sensitivity to pressure is

non-linear

– Diaphragm “creep”

(causes drift, a long-term

change in the sensor

sensitivity)

ATMS 320 – Barometry

• Advantages of

aneroid barometers:

–

Very small size

–

Readily automated

–

Insensitive to
orientation, motion,
and shock (portable)

–

No gravity
correction required

–

Users not exposed to
toxic materials

ATMS 320 – Barometry

• Indirect* –

hypsometers:

– Pressure sensor

that utilizes the
property of the
decreasing boiling
point of a liquid
with decreasing
pressure in order
to determine
pressure

ATMS 320 – Barometry

*A pressure measurement technique is call indirect if it does not
respond directly to the force due to atmospheric pressure but,
instead, responds to some other variable that is a function of
pressure.

• Hypsometers:

– Must somehow

provide heat to
get liquid to boil

– If the liquid has a

boiling point
below the air
temperature, a
heater is not
required (Freon-
13; 191.75 K)

ATMS 320 – Barometry

http://www.chefscatalog.com/store/catalog/silo.jhtml?itemId=cat000106&parentId=cat000000

ATMS 320 – Barometry





2

0

/

ln

T

R

L

dT

p

p

d



Clausius-Clapeyron equation…

Hypsometer equations:

Calibration equation…

Transfer equation…





























T

T

R

L

p

p

1

1

exp

0

0

















0

0

0

ln

1

p

p

L

T

R

T

T

ATMS 320 – Barometry

The static sensitivity of hypsometers

dp

T

d

changes over the range of typical
atmospheric pressures. If large static
sensitivity is good, at what range of
pressures do hypsometers perform
“good”?

at HIGH pressure or at LOW pressure ?

Cast your votes now…

ATMS 320 – Barometry

A pressure observing
network
1-2 punch!!

(1) hypsometer on a
radiosonde

(2) aneroid barometer at low altitude

• Main sources of

error in
hypsometers:

– Sensitive to

orientation of
instrument

– Extreme non-

linearity at sea-
level pressure

ATMS 320 – Barometry

• Advantages of

hypsometers:

– Small size
– Can be automated
– Reasonably portable
– No gravity or

temperature
correction required

– Simple physical

concept (does require
careful
implementation)

– No drift

ATMS 320 – Barometry

• All barometers are

subject to dynamic
wind effects (e.g., air
flow, building air
conditioning or
ventilation). A static
port is designed to
reduce dynamic
error for barometers
located inside
shelters.

ATMS 320 – Barometry

Static port:

Must be located outside of the significant pressure field caused by the

shelter. Field impacts 2.5 – 10 times shelter height.

Should be kept at a tilt angle of less than 10 degrees.

wind vector

tilt angle

+Z

ATMS 320 – Barometry

• Barometer

project

– Which type of

barometer in
Chapter 2 is most
like your
spaghetti sauce
jar barometer?

BAROM

ETER

http://www.atomicmuseum.com/tour/manhattanproject.cfm