Consistently superb observing conditions are crucial for achieving the
scientific objectives of a telescope. Submillimeter astronomy is possible only
at a few exceptionally dry sites, notably Mauna Kea, the Antarctic plateau, and
the Chajnantor region in the high Andes east of San Pedro de Atacama in
northern Chile. Long term measurements of 225 GHz and 350 \mu m atmospheric
transparency demonstrate all three locations enjoy significant periods of
excellent observing conditions. Conditions on the Chajnantor plateau and at the
South Pole are better more often than on Mauna Kea. Conditions are better
during winter and at night. Near the summit of Cerro Chajnantor, conditions are
better than on the Chajnantor plateau

Radford, Simon J. E.

English

arXiv

Consistently superb observing conditions are crucial for achieving the scientific objectives of a telescope. Submillimeter astronomy is possible only at a few exceptionally dry sites, notably Mauna Kea, the Antarctic plateau, and the Chajnantor region in the high Andes east of San Pedro de Atacama in northern Chile. Long term measurements of 225 GHz and 350 μm atmospheric transparency demonstrate all three locations enjoy significant periods of excellent observing conditions. Conditions on the Chajnantor plateau and at the South Pole are better more often than on Mauna Kea. Conditions are better during winter and at night.  Near the summit of Cerro Chajnantor, conditions are better than on the Chajnantor plateau

Radford, S. J. E.

Caltech Authors - Main

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RevMexAA (Serie de Conferencias), 41, 87–90 (2011)
OBSERVING CONDITIONS FOR SUBMILLIMETER ASTRONOMY
S. J. E. Radford1
RESUMEN
Condiciones de observacio´n consistentemente excelentes son cruciales para lograr los objetivos cient´ıficos de
un telescopio. La astronomı´a submilime´trica es posible solamente en algunos sitios excepcionalmente secos,
notablemente Mauna Kea, el plateau de Anta´rtida, y la regio´n de Chajnantor en los Andes al este de San Pedro
de Atacama en el norte de Chile. Las mediciones de larga duracio´n de la transparencia atmosfe´rica a 225 GHz
y 350 µm demuestran que las tres localidades cuentan con suficientes per´ıodos de excelentes condiciones de
observacio´n. Las condiciones en el Llano de Chajnantor y en el Polo Sur son mejores con ma´s frecuencia que
en Mauna Kea. Las condiciones son mejores durante el invierno y por la noche. Cerca de la cima del Cerro
Chajnantor, las condiciones son mejores que en el Llano de Chajnantor.
ABSTRACT
Consistently superb observing conditions are crucial for achieving the scientific objectives of a telescope. Sub-
millimeter astronomy is possible only at a few exceptionally dry sites, notably Mauna Kea, the Antarctic
plateau, and the Chajnantor region in the high Andes east of San Pedro de Atacama in northern Chile. Long
term measurements of 225 GHz and 350 µm atmospheric transparency demonstrate all three locations enjoy
significant periods of excellent observing conditions. Conditions on the Chajnantor plateau and at the South
Pole are better more often than on Mauna Kea. Conditions are better during winter and at night. Near the
summit of Cerro Chajnantor, conditions are better than on the Chajnantor plateau.
Key Words: atmospheric effects — site testing — submillimeter: general
1. INTRODUCTION
Star formation occurs deep within in dense in-
terstellar molecular clouds permeated with obscur-
ing dust. Heated by the optical and UV radiation
emerging from nascent stars, the dust reradiates at
longer, submillimeter wavelengths. Submillimeter
observations provide, therefore, key information for
understanding the details of star formation and for
tracing its progress throughout the history of the
universe. In addition to the continuum, there is a
rich line spectrum of molecular rotational transitions
and atomic fine structure lines in the submillime-
ter, which provide essential data for understanding
the cycle of material from the interstellar medium
through stars and back again.
Earth’s atmosphere poses an serious impediment
to submillimeter observations, however, because wa-
ter vapor and other molecules absorb strongly at
these wavelengths. Atmospheric transmission de-
clines dramatically at high frequencies, especially un-
der wetter conditions (Figure 1). Observations are
possible only through discrete windows delineated by
pressure broadened transitions.
1Caltech Submillimeter Observatory, California Institute
of Technology, 111 Nowelo Street, Hilo, HI 96720, USA (srad-
ford@caltech.edu).
Although spacecraft such as Herschel are not lim-
ited by the atmosphere and can provide superla-
tive science return, mission costs are high and the
practical telescope size is limited. Telescopes on the
ground, on the other hand, can be much bigger, are
less expensive, and enjoy easier access but require
consistently superb observing conditions to achieve
their scientific objectives. It is imperative, therefore,
to thoroughly characterize a potential telescope site
before embarking on construction.
2. SITES
Worldwide, the typical precipitable water vapor
(PVW) is about 25 mm (Seidel 2002), high enough
to preclude submillimeter observations. Because of
the uneven distribution of tropospheric water vapor,
however, a few exceptionally dry locations are suit-
able for observatories. Water vapor is concentrated
in the lower troposphere with a typical scale height
of 1–2 km, much less than the pressure scale height of
7–8 km. Deserts, by definition, are arid so accessible
desert mountain summits are good candidates.
Three premiere sites for submillimeter astronomy
are Mauna Kea, the Antarctic plateau, and the high
Andes in northern Chile. Despite its location in the
middle of the Pacific ocean, Mauna Kea enjoys good
observing conditions because a temperature inver-
87
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, 
&
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Sa
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88 RADFORD
sion frequently traps moisture well below the 4100 m
summit altitude. Telescopes on Mauna Kea, includ-
ing the CSO, the JCMT, and the SMA, pioneered
submillimeter astronomy. Even though the air is
close to saturated over the ice surface of the Antarc-
tic plateau, the extreme cold, especially in winter,
means the absolute humidity is very low. Several
telescopes, including the SPT, now carry out mm
and submm observations at the 2835 m high South
Pole. The higher domes C (3230 m), F (3800 m),
and A (4100 m) have been suggested for future ob-
servatories (Spinoglio & Epchtein 2010).
In northern Chile, the Atacama desert is ar-
guably the driest in the world. The combined effects
of a persistent high pressure cell over the eastern
Pacific, a coastal inversion layer created by the cold
Humbolt current, and a coastal mountain range pre-
vent the prevailing westerly winds from transporting
moisture into the region. To the east, the barrier of
the high Andean cordillera hinders the flow of trop-
ical convection from the Amazon. The consequent
lack of glaciers, even on the highest peak in the re-
gion, Volca´n Llullaillaco (6740 m), is unique for these
altitudes and attests to the aridity (Messerli et al.
1993). On the 5000 m high Chajnantor plateau east
of San Pedro de Atacama, several small telescopes
have made important contributions and ALMA is
nearing completion. The candidate site for CCAT
is at 5612 m near the summit of Cerro Chajnantor
overlooking the plateau (Radford et al. 2010).
3. TRANSPARENCY
Atmospheric transparency is the fundamental
site characteristic because astronomical observations
will be futile if cosmic radiation cannot penetrate
the atmosphere. No countermeasures are available
to improve the intrinsic transparency of the atmo-
sphere over a site.
Atmospheric absorption imposes a twofold
penalty. “The degradation in [system] noise tem-
perature due to absorptive elements results from two
effects: reduction in signal ... by ... absorption and
introduction of noise by re-radiation...” (Penzias &
Burrus 1973). This is true for both coherent (het-
erodyne) receivers and background limited bolome-
ters. Because integration time is proportional to the
square of the system noise, a small fractional im-
provement in transparency can yield a large improve-
ment in observing efficiency.
4. ATMOSPHERIC SPECTRUM
At millimeter and submillimeter wavelengths, the
atmospheric spectrum is dominated by strong reso-
nant absorption lines of the major molecular species,
Fig. 1. Model atmospheric transmission spectra at sub-
millimeter wavelengths for different amounts of precip-
itable water vapor (PWV) in temperate conditions (0◦C)
on high altitude (4000–5600 m) mountains. The relation-
ship between PWV and transmission is not applicable
under very cold, i.e., Antarctic conditions. Calculated
with the ATM model (Pardo et al. 2001).
notably H2O, O2, and O3 (Figure 1). Also pseudo-
continuum absorption, which has both wet and dry
components, arises from the far wings of strong in-
frared lines and is stronger at higher frequencies.
Contemporary models, such as ATM (Pardo et al.
2001), am (Paine 2011), and Moliere (Urban et al.
2004), have been verified against spectroscopic mea-
surements (Matsushita et al. 1999; Paine et al. 2000;
Pardo et al. 2005). These models are parameterized
by the barometric pressure, equivalent to the site al-
titude, and the vertical profiles of air temperature
and water vapor. Because the shape of the vertical
profiles usually can be assumed, values of the surface
temperature and the total PVW are sufficient.
It is worth noting the principal submillimeter
windows are delineated by rotational transitions be-
tween low energy states of water vapor (Figure 2).
For a given atmospheric water vapor content, then,
these states will be more populated, and the lines
will be stronger, if the air temperature is very low.
Detailed model calculations bear this out. At 225 K
(Antarctic winter), the ratio of optical depth to
PVW is about twice as high as at 275 K (temperate
locations).
5. TIPPING RADIOMETERS
Microwave sounding of the atmosphere was pio-
neered by Dicke et al. (1946), who inferred the at-
mospheric absorption from measurements of bright-
ness at different zenith angles. This tipping radiome-
ter technique remains standard for monitoring atmo-
spheric transparency. Continuum radiometers mea-
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Sa
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SUBMILLIMETER ASTRONOMY 89
Fig. 2. Energy levels of the lowest rotational states of
water with transitions labeled by the frequency in GHz.
The lines at 557, 752, 988, 1113, and 1229 GHz delineate
the principal submillimeter windows.
Fig. 3. Cumulative distributions of 225 GHz zenith op-
tical depth measured with functionally identical narrow
band heterodyne tipping radiometers on the Chajnan-
tor plateau (ALMA), on the CSO on Mauna Kea, and
at the South Pole (Chamberlin & Bally 1994; Radford &
Chamberlin 2000). At the CSO, τ225 < 0.06 is considered
suitable for submillimeter observations.
sure the total atmospheric absoprtion, including all
components, directly at a wavelength of astronomi-
cal interest.
5.1. 225 GHz measurements
Functionally identical 225 GHz heterodyne ra-
diometers, initially developed by NRAO for charac-
terizing potential mmA sites, have been deployed to
Mauna Kea, to the South Pole, and to the Chajnan-
tor plateau (Figure 3). All three sites enjoy periods
of excellent transparency suitable for submillimeter
observations, τ225 < 0.06. Mauna Kea, however,
experiences these conditions about half as often as
the other two sites. Poor conditions are very rare
at the South Pole. During the best conditions, the
225 GHz transparency at the Chajnantor plateau is
better than at the South Pole.
Fig. 4. Cumulative distributions of 350µm zenith optical
depth measured with functionally identical broad band
tipping radiometers near the summit of Cerro Chajan-
tor (CCAT), on the Chajnantor plateau (ALMA), at the
CSO on Mauna Kea, and at the South Pole. Measure-
ments adjusted for radome transparency (Calisse 2004).
5.2. 350 µm measurements
Because of the uncertainties inherent in ex-
trapolating from 225 GHz measurements to higher
frequencies, broadband tippers were deployed to
directly measure the 350 µm atmospheric trans-
parency. These instruments were developed jointly
by NRAO and Carnegie Mellon University. The dis-
tributions of the 350 µm measurements (Figure 4)
and the 225 GHz measurements (Figure 3) are quite
similar. The first quartile 350 µm transparencies
at the Chajnantor plateau and the South Pole are
roughly equal and noticeably better than at Mauna
Kea.
Seasonal variations are significant (Figure 5). At
all three sites, the transparency is better during the
winter than the summer. At the South Pole, con-
ditions are remarkably consistent from year to year.
The seasonal pattern for Mauna Kea, although evi-
dent, is overshadowed by year-to-year variations. In
the Chajnantor region, conditions are consistently
good from April through December but deterioriate
during the summer months when a shift in atmo-
spheric circulation patterns draws moist air over the
Andes from the Amazon basin. There is considerable
year-to-year variation in the severity of this summer
season. Winter conditions in the Chjanantor region
have less interannual variability than at Mauna Kea.
Both the Chajnantor region and Mauna Kea ex-
perience diurnal transparency variations, with bet-
ter conditions at night. These diurnal transparency
variations lag behind the solar cycle. The best con-
ditions occur around sunrise. At Chajnantor, the
diurnal variations are weaker during the winter than
during the summer.
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90 RADFORD
Fig. 5. Variations of the 350 µm zenith optical depth
measured and on the Chajnantor plateau (ALMA), at
the CSO on Mauna Kea, and at the South Pole. The
error bars show the quartiles for each month or hour.
Fig. 6. Cumulative distributions of 350 µm zenith opti-
cal depth measured simultaneously near the summit of
Cerro Chajantor (CCAT) and on the Chajnantor plateau
(ALMA). This PVW scale is only appropriate for the
Chajnantor region.
6. CERRO CHAJNANTOR
In the Chajnantor region, several peaks rise
more than 500 m above the plateau. Radiosondes
launched from the plateau measured the water va-
por density profiles and have determined the typical
scale height is about 1.1 km. The flights also re-
vealed inversion layers frequently occur, especially at
night, that trap moisture below the below the height
of these peaks (Giovanelli et al. 2001). CCAT is
a proposed 25 m diameter telescope for submillime-
ter astronomy (Radford et al. 2010). The candi-
date CCAT site is a level bench 5612 m high near
the summit of Cerro Chajnantor accessed by a road
constructed by the University of Tokyo. With two
tipping radiometers, one at the summit and the other
on the plateau, the 350 µm transparency was mea-
sured simultaneously. Prior to deployment to Cerro
Chajnantor, the instruments were were operated side
by side on the plateau where they reported identical
results. Observing conditions for CCAT are signifi-
cantly better than on the plateau (Figure 6), offering
more opportunities for observations at short wave-
lengths. The 350 µm optical depth measurements
on the Chajnantor plateau are well correlated with
PWV measurements from a 183 GHz spectrometer
on the APEX telescope. Linear regression then in-
dicates PWV = 0.935 (τ350 − 0.35). This relation is
only appropriate in the environs of Chajnantor.
It is a pleasure to thank the many colleagues who
have contributed to these measurements. Jeff Pe-
terson initiated development of the 350 µm tippers.
CCAT site evaluation is carried out in the Parque
Astrono´mico Atacama in northern Chile under the
auspices of the Programa de Astronomı´a, a program
of the Comisio´n Nacional de Investigacio´n Cient´ıfica
y Tecnolo´gica de Chile (CONICYT). CCAT site eval-
uation received partial support from the National
Science Foundation (AST-0431503). The NRAO is
a facility of the NSF operated under a cooperative
agreement by Associated Universities, Inc. The Cal-
tech Submillimeter Observatory (CSO) is operated
by the California Institute of Technology with fund-
ing from the NSF (AST-0838261).
REFERENCES
Calisse, P. G. 2004, PASA, 21, 252
Chamberlin, R. A., & Bally, J., 1994, Appl. Opt., 33,
1095
Dicke, R. H., et al. 1946, Phys. Rev., 70, 340
Giovanelli, R., et al. 2001, PASP, 113, 803
Matsushita, S., et al. 1999, PASJ, 51, 603
Messerli, B., et al. 1993, Mt. Res. Dev., 13, 117
Paine, S., et al. 2000, PASP, 112, 108
Paine, S. 2011, SMA Technical Memo 152 v. 6.2 (SAO)
Pardo, J. R., et al. 2001, IEEE TAP, 49, 1683
Pardo, J. R., et al. 2005, JQSRT, 96, 537
Penzias, A. A., & Burrus, C. A. 1973, ARA&A, 11, 51
Radford, S. J. E., & Chamberlin, R. A. 2000, ALMA
Memo, 334
Radford, S. J. E., et al. 2010, ASP Conf. Ser. 417, 113
Seidel, D. J. 2002, Water Vapor: Distribution and Trends,
in Encyclopedia of Global Environmental Change
Spinoglio, L., & Epchtein, N., 2010, EAS Pub. Ser., 40
Urban, J., et al. 2004, JQSRT, 83, 529


Observing conditions for submillimeter astronomy

To appear in “Astronomical Site Testing Data in Chile (2011)” RevMexAA(SC)
OBSERVING CONDITIONS FOR SUBMILLIMETER ASTRONOMY
Simon J. E. Radford,1
RESUMEN
Condiciones de observación consistente excelente son cruciales para lograr los objetivos cientficos de un tele-
scopio. Astronomı́a submilimétrica es posible solamente en algunos sitios excepcionalmente seco, notablemente
Mauna Kea, la plateau de Antártida, y la región de Chajnantor en los Andes al este de San Pedro de Ata-
cama en el norte de Chile. Mediciones á largo duración de la transparencia atmosférica á 225 GHz y 350µm
demuestran las tres localidades cuentan con suficiente perodos de excelentes condiciones de observación. Las
condiciones en el Llano de Chajnantor y en el Polo Sur son mejores con más frecuencia que en Mauna Kea.
Las condiciones son mejores durante el invierno y por la noche. Cerca de la cima del Cerro Chajnantor, las
condiciones son mejores que en el Llano de Chajnantor.
ABSTRACT
Consistently superb observing conditions are crucial for achieving the scientific objectives of a telescope. Sub-
millimeter astronomy is possible only at a few exceptionally dry sites, notably Mauna Kea, the Antarctic
plateau, and the Chajnantor region in the high Andes east of San Pedro de Atacama in northern Chile. Long
term measurements of 225 GHz and 350µm atmospheric transparency demonstrate all three locations enjoy
significant periods of excellent observing conditions. Conditions on the Chajnantor plateau and at the South
Pole are better more often than on Mauna Kea. Conditions are better during winter and at night. Near the
summit of Cerro Chajnantor, conditions are better than on the Chajnantor plateau.
Key Words: Atmospheric effects — Site testing — Submillimeter: general
1. INTRODUCTION
Star formation occurs deep within in dense in-
terstellar molecular clouds permeated with obscur-
ing dust. Heated by the optical and UV radiation
emerging from nascent stars, the dust reradiates at
longer, submillimeter wavelengths. Submillimeter
observations provide, therefore, key information for
understanding the details of star formation and for
tracing its progress throughout the history of the
universe. In addition to the continuum, there is a
rich line spectrum of molecular rotational transitions
and atomic fine structure lines in the submillime-
ter, which provide essential data for understanding
the cycle of material from the interstellar medium
through stars and back again.
Earth’s atmosphere poses an serious impediment
to submillimeter observations, however, because wa-
ter vapor and other molecules absorb strongly at
these wavelengths. Atmospheric transmission de-
clines dramatically at high frequencies, especially un-
der wetter conditions (Fig. 1). Observations are pos-
sible only through discrete windows delineated by
pressure broadened transitions.
1Caltech Submillimeter Observatory, California Institute
of Technology, 111 Nowelo Street, Hilo, HI 96720, USA (srad-
ford@caltech.edu).
Although spacecraft such as Herschel are not lim-
ited by the atmosphere and can provide superla-
tive science return, mission costs are high and the
practical telescope size is limited. Telescopes on the
ground, on the other hand, can be much bigger, are
less expensive, and enjoy easier access but require
consistently superb observing conditions to achieve
their scientific objectives. It is imperative, therefore,
to thoroughly characterize a potential telescope site
before embarking on construction.
2. SITES
Worldwide, the typical precipitable water vapor
(PVW) is about 25 mm (Seidel 2002), high enough
to preclude submillimeter observations. Because of
the uneven distribution of tropospheric water vapor,
however, a few exceptionally dry locations are suit-
able for observatories. Water vapor is concentrated
in the lower troposphere with a typical scale height
of 1-2 km, much less than the pressure scale height of
7-8 km. Deserts, by definition, are arid so accessible
desert mountain summits are good candidates.
Three premiere sites for submillimeter astronomy
are Mauna Kea, the Antarctic plateau, and the high
Andes in northern Chile. Despite its location in the
middle of the Pacific ocean, Mauna Kea enjoys good
1
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Fig. 1. Model atmospheric transmission spectra at sub-
millimeter wavelengths for different amounts of precip-
itable water vapor (PWV) in temperate conditions (0◦ C)
on high altitude (4000–5600 m) mountains. The relation-
ship between PWV and transmission is not applicable
under very cold, i. e., Antarctic conditions. Calculated
with the ATM model (Pardo et al. 2001).
observing conditions because a temperature inver-
sion frequently traps moisture well below the 4100 m
summit altitude. Telescopes on Mauna Kea, includ-
ing the CSO, the JCMT, and the SMA, pioneered
submillimeter astronomy. Even though the air is
close to saturated over the ice surface of the Antarc-
tic plateau, the extreme cold, especially in winter,
means the absolute humidity is very low. Several
telescopes, including the SPT, now carry out mm
and submm observations at the 2835 m high South
Pole. The higher domes C (3230 m), F (3800 m), and
A (4100 m) have been suggested for future observa-
tories (Spinoglio & Epchtein 2010).
In northern Chile, the Atacama desert is ar-
guably the driest in the world. The combined effects
of a persistent high pressure cell over the eastern
Pacific, a coastal inversion layer created by the cold
Humbolt current, and a coastal mountain range pre-
vent the prevailing westerly winds from transporting
moisture into the region. To the east, the barrier of
the high Andean cordillera hinders the flow of trop-
ical convection from the Amazon. The consequent
lack of glaciers, even on the highest peak in the re-
gion, Volcán Llullaillaco (6740 m), is unique for these
altitudes and attests to the aridity (Messerli et al.
1993). On the 5000 m high Chajnantor plateau east
of San Pedro de Atacama, several small telescopes
have made important contributions and ALMA is
nearing completion. The candidate site for CCAT
is at 5612 m near the summit of Cerro Chajnantor
overlooking the plateau (Radford et al. 2010).
Fig. 2. Energy levels of the lowest rotational states of
water with transitions labeled by the frequency in GHz.
The lines at 557, 752, 988, 1113, and 1229 GHz delineate
the principal submillimeter windows.
3. TRANSPARENCY
Atmospheric transparency is the fundamental
site characteristic because astronomical observations
will be futile if cosmic radiation cannot penetrate
the atmosphere. No countermeasures are available
to improve the intrinsic transparency of the atmo-
sphere over a site.
Atmospheric absorption imposes a twofold
penalty. “The degradation in [system] noise temper-
ature due to absorptive elements results from two
effects: reduction in signal . . . by . . . absorption and
introduction of noise by re-radiation. . .” (Penzias &
Burrus 1973). This is true for both coherent (het-
erodyne) receivers and background limited bolome-
ters. Because integration time is proportional to the
square of the system noise, a small fractional im-
provement in transparency can yield a large improve-
ment in observing efficiency.
4. ATMOSPHERIC SPECTRUM
At millimeter and submillimeter wavelengths, the
atmospheric spectrum is dominated by strong reso-
nant absorption lines of the major molecular species,
notably H2O, O2, and O3 (Fig. 1). Also pseudo-
continuum absorption, which has both wet and dry
components, arises from the far wings of strong in-
frared lines and is stronger at higher frequencies.
Contemporary models, such as ATM (Pardo et al.
2001), am (Paine 2011), and Moliere (Urban et al.
2004), have been verified against spectroscopic mea-
surements (Matsushita et al. 1999, Paine et al. 2000,
Pardo et al. 2005). These models are parameterized
by the barometric pressure, equivalent to the site al-
titude, and the vertical profiles of air temperature
and water vapor. Because the shape of the vertical
profiles usually can be assumed, values of the surface
temperature and the total PVW are sufficient.
SUBMILLIMETER ASTRONOMY 3
Fig. 3. Cumulative distributions of 225 GHz zenith op-
tical depth measured with functionally identical narrow
band heterodyne tipping radiometers on the Chajnan-
tor plateau (ALMA), on the CSO on Mauna Kea, and
at the South Pole (Chamberlin & Bally 1994; Radford &
Chamberlin 2000). At the CSO, τ225 < 0.06 is considered
suitable for submillimeter observations.
It is worth noting the principal submillimeter
windows are delineated by rotational transitions be-
tween low energy states of water vapor (Fig. 2). For
a given atmospheric water vapor content, then, these
states will be more populated, and the lines will be
stronger, if the air temperature is very low. Detailed
model calculations bear this out. At 225 K (Antarc-
tic winter), the ratio of optical depth to PVW is
about twice as high as at 275 K (temperate loca-
tions).
5. TIPPING RADIOMETERS
Microwave sounding of the atmosphere was pio-
neered by Dicke et al. (1946), who inferred the at-
mospheric absorption from measurements of bright-
ness at different zenith angles. This tipping radiome-
ter technique remains standard for monitoring atmo-
spheric transparency. Continuum radiometers mea-
sure the total atmospheric absoprtion, including all
components, directly at a wavelength of astronomi-
cal interest.
5.1. 225 GHz measurements
Functionally identical 225 GHz heterodyne ra-
diometers, initially developed by NRAO for charac-
terizing potential mmA sites, have been deployed to
Mauna Kea, to the South Pole, and to the Chajnan-
tor plateau (Fig. 3). All three sites enjoy periods
of excellent transparency suitable for submillimeter
observations, τ225 < 0.06. Mauna Kea, however,
experiences these conditions about half as often as
the other two sites. Poor conditions are very rare
at the South Pole. During the best conditions, the
225 GHz transparency at the Chajnantor plateau is
better than at the South Pole.
Fig. 4. Cumulative distributions of 350µm zenith optical
depth measured with functionally identical broad band
tipping radiometers near the summit of Cerro Chajan-
tor (CCAT), on the Chajnantor plateau (ALMA), at the
CSO on Mauna Kea, and at the South Pole. Measure-
ments adjusted for radome transparency (Calisse 2004).
5.2. 350µm measurements
Because of the uncertainties inherent in ex-
trapolating from 225 GHz measurements to higher
frequencies, broadband tippers were deployed to
directly measure the 350µm atmospheric trans-
parency. These instruments were developed jointly
by NRAO and Carnegie Mellon University. The dis-
tributions of the 350µm measurements (Fig. 4) and
the 225 GHz measurements (Fig. 3) are quite simi-
lar. The first quartile 350µm transparencies at the
Chajnantor plateau and the South Pole are roughly
equal and noticeably better than at Mauna Kea.
Seasonal variations are significant (Fig. 5). At
all three sites, the transparency is better during the
winter than the summer. At the South Pole, con-
ditions are remarkably consistent from year to year.
The seasonal pattern for Mauna Kea, although evi-
dent, is overshadowed by year-to-year variations. In
the Chajnantor region, conditions are consistently
good from April through December but deterioriate
during the summer months when a shift in atmo-
spheric circulation patterns draws moist air over the
Andes from the Amazon basin. There is considerable
year-to-year variation in the severity of this summer
season. Winter conditions in the Chjanantor region
have less interannual variability than at Mauna Kea.
Both the Chajnantor region and Mauna Kea ex-
perience diurnal transparency variations, with bet-
ter conditions at night. These diurnal transparency
variations lag behind the solar cycle. The best con-
ditions occur around sunrise. At Chajnantor, the
diurnal variations are weaker during the winter than
during the summer.
6. CERRO CHAJNANTOR
In the Chajnantor region, several peaks rise more
than 500 m above the plateau. Radiosondes launched
4 RADFORD
Fig. 5. Variations of the 350µm zenith optical depth
measured and on the Chajnantor plateau (ALMA), at
the CSO on Mauna Kea, and at the South Pole. The
error bars show the quartiles for each month or hour.
from the plateau measured the water vapor density
profiles and have determined the typical scale height
is about 1.1 km. The flights also revealed inversion
layers frequently occur, especially at night, that trap
moisture below the below the height of these peaks
(Giovanelli et al. 2001).
CCAT is a proposed 25 m diameter telescope for
submillimeter astronomy (Radford et al. 2010). The
candidate CCAT site is a level bench 5612 m high
near the summit of Cerro Chajnantor accessed by a
road constructed by the University of Tokyo. With
two tipping radiometers, one at the summit and the
other on the plateau, the 350µm transparency was
measured simultaneously. Prior to deployment to
Cerro Chajnantor, the instruments were were oper-
ated side by side on the plateau where they reported
identical results. Observing conditions for CCAT are
significantly better than on the plateau (Fig. 6), of-
fering more opportunities for observations at short
wavelengths.
The 350µm optical depth measurements on the
Chajnantor plateau are well correlated with PWV
measurements from a 183 GHz spectrometer on the
APEX telescope. Linear regression then indicates
PWV = 0.935 (τ350 − 0.35). This relation is only
appropriate in the environs of Chajnantor.
Fig. 6. Cumulative distributions of 350µm zenith opti-
cal depth measured simultaneously near the summit of
Cerro Chajantor (CCAT) and on the Chajnantor plateau
(ALMA). This PVW scale is only appropriate for the
Chajnantor region.
It is a pleasure to thank the many colleagues who
have contributed to these measurements. Jeff Pe-
terson initiated development of the 350µm tippers.
CCAT site evaluation is carried out in the Parque
Astronómico Atacama in northern Chile under the
auspices of the Programa de Astronomı́a, a program
of the Comisión Nacional de Investigación Cient́ıfica
y Tecnológica de Chile (CONICYT). CCAT site eval-
uation received partial support from the National
Science Foundation (AST-0431503). The NRAO is
a facility of the NSF operated under a cooperative
agreement by Associated Universities, Inc. The Cal-
tech Submillimeter Observatory (CSO) is operated
by the California Institute of Technology with fund-
ing from the NSF (AST-0838261).
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Water Vapor: Distribution and Trends,

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Observing Conditions for Submillimeter Astronomy

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