Measurement of Air Pressure
Barometers
• Mercury Barometer
– Atmospheric pressure supports a column
of mercury to a measurable height
• Aneroid Barometer
– Atmospheric pressure forces two sides of
an evacuated bellows together to a
measurable amount
• Electronic Barometers
Pressure
• Observing pressure
is “weighing” the
column of
atmospheric gases
• Mercury barometer
– Atmospheric
pressure supports
the height of a
column of mercury
– Temperature
correction required
http://www.atmos.washington.edu/2005Q3/101/LINKS-html
Conversion and T correction
• (mmHg) * 1.33322 = (hPa)
• Mercury liquid expands when it is heated
• Subtract the correction from the
observation of height of the mercury in mm
Aneroid barometer
– Atmospheric
pressure forces the
two sides of an
evacuated bellows
together
Piezoresistive pressure sensor
•
•
•
•
•
•
•
•
a silicon piezoresistive pressure sensor
provides a highly accurate and linear voltage
output - directly proportional to the applied
pressure.
The sensor is a single monolithic silicon
diaphragm with the strain gauge and a thin-film
resistor network integrated on-chip.
The chip is laser trimmed for precise span and
offset calibration and temperature
compensation.
They are designed for use in applications such
as pump/motor controllers, robotics, level
indicators, medical diagnostics, pressure
switching, barometers, altimeters, etc.
Kestrel: Monolithic silicon piezoresistive
pressure sensor with second-order
temperature correction.
Error 1.5 hPa at 25degC <6000m
Maximum error beyond specified temperature,
+/- 0.09 inHg / 3.0 hPa.
Calibration drift typically -0.03 inHg / -1.0 hPa
per year.
P = a !V + b + drift
a = c + kT + lT 2
drift = "1.0hPa / year
Silicon capacitive pressure
sensor
Capacitive pressure sensors use a thin
diaphragm as one plate of a capacitor. The
diaphragm is exposed to the process
pressure on one side and to a reference
pressure on the other. Changes in
pressure cause it to deflect and change
the capacitance. The capacitance can be
monitored by using it to control the
frequency of an oscillator or to vary the
coupling of an AC signal.
newton.ex.ac.uk/ teaching/CDHW/Sensors/
PMT16A
Vaisala MAWS
http://www.vaisala.com/businessareas/solutions/hydromet/products/
sensors/sensorsformaws
Instrumentation available
• MAWS
– 0.3 hPa -40 to 50 deg C
• WXT 510
– 0.5 hPa 0 to 30 deg C
– 1 hPa -52 to 60 deg C
• Davis
– 1.7 hPa
• Kestrel
– 1.5 hPa at 25degC
*write down
Reporting pressure
Adjustment to Sea Level
• The barometer measures “station pressure ps”
• Direct comparisons of stations at different altitudes
(ie mountains) is not useful for meteorology
• The effect of different elevations must be removed by
“reducing to sea level”
• A fictitious column of air extends from the station to
sea level.
• The sea level pressure is estimated from
*write down
ps = psl exp(!zs / H )
psl = ps exp(zs / H )
zs is the height of the station above mean sea level
where the scale height is H = RT / g
R is the dry air gas constant 287.0J / (kg deg K )
gravitational acceleration g = 9.80145 ± 0.00004m / s 2 at 45.5 o lat
T is the mean temperature of the air
T = ( T+T-12h ) / 2
METAR for KLAF
•
KLAF 40-24-45N 086-56-51W 182M
*
what was the
Conditions at:
KLAF observed 22 August 2007 11:54 UTC
Temperature:
23.3°C (74°F)
station pressure at
Dewpoint:
21.1°C (70°F) [RH = 87%]
11:54 UTC?
Pressure (altimeter): 30.03 inches Hg (1017.0 mb)
[Sea-level pressure: 1016.2 mb]
Winds:
calm
Visibility:
5 miles (8 km)
Ceiling:
at least 12,000 feet AGL
Clouds:
sky clear below 12,000 feet AGL
Present Weather:
BR (mist)
KLAF 221154Z 00000KT 5SM BR CLR 23/21 A3003 RMK AO2 SLP162 T02330211 10250 20228 50014
Conditions at:
KLAF observed 21 August 2007ﾊﾊ23:54ﾊUTC
Temperature:
28.3｡C (83｡F)
Dewpoint:
22.2｡C (72｡F) [RHﾊ=ﾊ70%]
Pressure (altimeter): 29.97ﾊinchesﾊHg (1015.0ﾊmb)
[Sea-level pressure: 1014.2ﾊmb]
Winds:from the SSW (210ﾊdegrees) at 3ﾊMPH (3ﾊknots; 1.6ﾊm/s)
Visibility:10 or more miles (16+ km)
Ceiling:at least 12,000 feet
AGLClouds:few clouds at 10000 feet AGL
Present Weather:no significant weather observed at this time
KLAF 212354Z 21003KT 10SM FEW100 28/22 A2997 RMK AO2 SLP142 T02830222 10306 20261 50000
Altimeter setting
• Aircraft use aneroid altimeters to measure their
height. If the pressure changes because of the
weather, their height measurement would be
inaccurate.
• Air traffic control broadcasts to the pilots their
“altimeter setting” or the representative pressure at
the ground surface on that day.
palt = ps exp(zs / H )
*write down
ps is the observed station pressure
where the scale height is H = RT / g
R is the dry air gas constant 287.0J / (kg deg K )
gravitational acceleration g = 9.80145 ± 0.00004m / s 2 at 45.5 o lat
zs is the height above mean sea level
T = T + !zs
! is the lapse rate for the standard atmosphere =6.5K / km
Altimetry
•
•
Barometers are commonly used to measure height. Aneroid
altimeters are barometers calibrated to read altitude rather than
pressure. A mean temperature is assumed by the manufacturer.
Integrate the hydrostatic balance equation for an isothermal
atmosphere (assuming dry air is an ideal gas)
p ( z ) = psl exp(!z / H )
where the scale height is H = RT / g
R is the dry air gas constant 287.0J / (kg deg K )
T is the mean temperature of the air
psl = pressure at sealevel.
gravitational acceleration g = 9.80145 ± 0.00004m / s 2 at 45.5 o lat
" p (z)%
z = !H ln $
= H ln ( psl ) ! H ln ( p(z))
# psl '&
Adjustment procedure for Kestrel
• Retrieve psl from KLAF
• Retrieve station pressure from Kestrel
• On Altitude screen, adjust pressure to KLAF psl
ps = psl exp(!zs / H )
" p %
zs = !H ln $ s '
# psl &
zs is the altitude derived for output
*Kestrel manual says to
adjust altitude to 0 to see
station pressure
Put 0 in for zs
• Write down altitude
What is station pressure here?
• On BARO screen, adjust altitude to zs
• This refines estimate of psl
psl = ps exp(zs / H )
Altimeter pressure correction
Accuracy requirements
33.86388 ! P (inHg) = p (hPa)
P (inHg) = 0.02953 p (hPa)
dP
(inHg)
= 0.02953
dp
(hPa)
(inHg)
dP = 0.02953
dp
(hPa)
(inHg)
dP = 0.02953
! 0.68 (hPa)
(hPa)
dP = 0.02(inHg)
METAR coding
Visibility > 9500 m
Day of month
Wind dir ddd
(US reports stat miles)
hhmm in UTC
Thunderstorms rain
Wind speed ss
• MYNN 202250Z 15005KT 9999 TSRA
SCT012CB BKN015TCU BKN120 26/24 Q1016
Broken “120”00 ft
Scattered “012”00 ft Cumulonimbus
Altimeter in hPa
Broken “015”00 ft Towering cumulus
Temp/dewpoint C
MTPP
2006/07/16 00:00 UTC MTPP 160000Z 11007KT 3000 TSRA SCT028CB OVC090 24/22 Q1018 A3008
MKJP
2006/08/28 13:00 UTC MKJP 281300Z 04005KT 9000 BKN018 FEW022CB BKN032 29/25 Q1011
MYGF
2006/08/28 13:00 UTC MYGF 281300Z 10004KT 9999 FEW020TCU 30/25 A3001 RMK TCU NE
• Site ID database
* Decode METAR
– http://www.nws.noaa.gov/tg/siteloc.shtml
• Retrieving METARS
– http://weather.noaa.gov/weather/metar.shtml
• Explanation of METAR coding
– http://www.met.tamu.edu/class/METAR/quick-metar.html
Station model
* Draw station model
MTPP
2006/07/16 00:00 UTC MTPP 160000Z 11007KT 3000 TSRA SCT028CB OVC090 24/22
Q1018 A3008
MKJP
2006/08/28 13:00 UTC MKJP 281300Z 04005KT 9000 BKN018 FEW022CB BKN032 29/25
Q1011
MYGF
2006/08/28 13:00 UTC MYGF 281300Z 10004KT 9999 FEW020TCU 30/25 A3001 RMK TCU
NE
Lab report
Introduction –objectives of the laboratory exercise, relevant
background information.
Methodology – description of laboratory setup, steps taken and
activities completed to accomplish objectives, including any
relevant equations.
Environmental conditions –give a summary description of the
conditions during the experiment.
Data – discussion of types of data and sources of data used.
Always include the units. Give uncertainties with all
measurements.
Results – graphs and plots of raw data and processed data. All
graphs must have labels and units on each axis. Tables
must have headings with units.
Discussion – discussion of numerical and/or graphical results.
Include answers to specific questions posed.
Summary/conclusions – summarize work completed and draw
conclusions
Temperature advection
• Temperature advection
# !T
!T
DT
!T
!T &
=
"%u
+v
+w
!t station Dt heatingofaparcel $ !x
!y
!z ('
• Assume the parcel is not
heating/cooling
• Assume vertical wind is zero
Temperature gradient
Trajectory 1
Trajectory 6
CIVL
Trajectory 3
Trajectory 5
Calculate the N-S temperature gradient as the change over the furthest points on
trajectory 1 and 5
Calculate the E-W temperature gradient as the change over the furthest points on
trajectory 3 and 6
Use the wind at the furthest point on Trajectory 1 for the u,v