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Astroclimatology:
How weather and climate affect astronomical
viewing and site selection
Dr. Edward Graham,
University of the Highlands and Islands
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Where is University of the Highlands and Islands (“UHI”)?
Scotland !!
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University of the Highlands and Islands (“UHI”)
The Highlands and
Islands
E. Graham et al., 2010
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University of the Highlands and Islands (“UHI”)
E. Graham et al., 2010
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Outline of my presentation today
Two parts:
1) General Meteorology &
Climatology
BREAK / PAUSE
2) Application of above to
Astronomy
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Outline of my presentation today
Two parts:
1) General Meteorology &
Climatology
BREAK / PAUSE
2) Application of above to
Astronomy
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Definition: Weather
Weather is the state of the
atmosphere at any one
particular place at a
particular time.
Two separate places never
have the exactly same
weather, nor does the
weather ever repeat itself
Every moment of weather is
unique in space and time
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Definition: Climate
• Is the « average » of the weather, over « reasonably » long
period of time (e.g. 30 years)
• Actual weather is usually chaotic, but is contained within
certain boundaries, climate is the « average »
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The scale of weather and climate systems
• Weather and climate phenomena
operate over huge temporal and
spatial scales;
• Spatially: 10-3 m (millimetres) to
106m (thousands of kilometres)
• Temporally: 10-3 secs (milliseconds)
to 108 secs (decades)
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Is climate steady?
Temperature (red) of last
20,000 years on Greenland
ice cap
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«Traditional» (deterministic) climatologists (until ~1980s) viewed
climate as being reasonably steady.
Present view is contrary to this: Climate itself may not be stable &
there can be sudden « shifts » or «step-changes»…
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Rate of current climate change…
The rate of global climatic change is much faster than
anything Earth has experienced in at least the last two
millions years… (x 10 times faster)
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Intergovernmental Panel on Climate Change (IPCC) scenarios for 21st century
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Surface air temperature increase ~2090s
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It’s not just a temperature increase…. An increase in Extremes too!
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How does the Climate System work?
Polar regions receive less solar radiation because:
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Ground surface area over which radiation is distributes gets larger
towards poles
Rays have a longer path length through atmosphere
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How does the Climate System work?
Result:
• Unequal heating of the Earth’s surface by the sun, which
varies according to day, season and latitude
• The tilt of the Earth’s axis causes the seasons
• The distribution of continents, mountains and oceans also
play a key role
• Atmosphere is a fluid, but 1000 times less dense than water
• P = ρRT (Ideal gas equationl P=pressure, ρ=density,
T=temperature, R=Universal gas constant)
• Result is heat and moisture transfer towards the poles
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The Earth’s Energy Balance
On average, there’s 342 Wm-2 incident and outgoing
radiation at the top of atmosphere, but clouds & aerosols
alter the balance depending on location
Hence there are energy transfers from equator to poles
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Main methods of Energy Transfer on Earth
The principal mechanisms driving this transfer of energy
from the equator to the poles are the atmosphere and the
oceans…
Both transport about the same, despite sluggishly-moving
ocean currents…
Atmospheric Energy Transport: Wind
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Wind is just air moving from high pressure to low pressure (e.g.
bicycle tyre) i.e. caused by a pressure gradient.
As air gets warmer, it expands, becomes less dense and therefore
pressure decreases.
But there is the Coriolis
Effect
Helped by fact that
air at the equator has
greater relative
angular velocity
(40,000km per day)
than air nearer the
poles (0km/day).
Wind and the Coriolis Effect
The Coriolis Effect:
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Only for air that doesn’t “feel”
the Earth’s rotation
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For air that “feels” the Earth’s
rotation (Geostrophic balance)
The resulting balance between
the Coriolis Effect (due to the
Earth’s rotation) and the
Pressure-Gradient Force is
“Geostropic balance”
It means frictionless airflow is
deflected by 90°….
Wind, the Coriolis Effect, and Friction
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But differing
amounts of surface
friction (land, sea)
result in a
reduction in speed
and a deflection
reduced by 1030°…
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Wind and the Coriolis Effect
Air moving across
latitudes in the
Northern Hemisphere
will swing to the right
(clockwise).
Air moving across
latitudes in the
Southern Hemisphere
will swing to the left
(anti-clockwise).
The Jetstreams
“Slopes” in pressure pattern then cause winds / the
jetstream:
-20C
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-10C
0C
+10C
Equator
+20C
North
Pole
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Differences in air temperature / air pressure cause the
weather patterns:
Differences in air temperature / air pressure cause the
weather patterns:
Latitudional (zonal) air circulation systems
Rotation in weather systems - Lows
Q: Why do low pressure turn anti-clockwise in
the northern hemisphere? (and vice versa…)
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3)
2)
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Rotation in weather systems - Lows
Rotation in weather systems - Highs
Q: And why do high pressures turn clockwise in
the northern hemisphere? (and vice versa…)
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3)
2)
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Rotation in weather systems - Highs
General global pattern of surface air pressure
The locations and
intensities of these
“highs” and “lows”
vary with altitude.
Geostropic flow
around these
weather systems is
permitted for cases
of no friction e.g.
>1km above
surface.
But…. Non-geostropic flow can occur!
• On small scales i.e. local or regional flow may not
be geostrophic!
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• Especially true near mountains and coasts!
• 1 deg latitude is roughly equivalent to 18km/h
(11mph) difference in relative velocity!
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Non-Geostrophic Flow: The Sea Breeze
Pressure difference forces
air out to sea
Warm
air over
land
rises
the sea-breeze
moves onshore
SEA
LAND
The logarithmic wind profile
Where:
u = windspeed (ms-1)
u* = friction velocity (ms-1)
k = Von Karman’s constant (0.4)
z = height (m)
d = zero-displacement height (m)
z0 = roughness length
(after Oke, 1976)
= stability term
The logarithmic wind profile
“Free Atmosphere”
(Geostrophic)
~300-1000m
“Boundary Layer”
(Non-Geostrophic)
But turbulence can still form in “free atmosphere”: Windshear!
“Free Atmosphere”
(Geostrophic)
Windshear
~300-1000m
“Boundary Layer”
(Non-Geostrophic)
Turbulence in the “free atmosphere”: Instability
“Free Atmosphere”
(Geostrophic)
Convection/bouyancy
/instability
“Boundary Layer”
(Non-Geostrophic)
Turbulence in the “free atmosphere”: Gravity Waves
“Free Atmosphere”
(Geostrophic)
~300-1000m
“Boundary Layer”
(Non-Geostrophic)
Outline of my presentation today
Two parts:
1) General Meteorology &
Climatology
BREAK / PAUSE
2) Application of above to
Astronomy
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Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
1. Cloud Cover: Clouds indicate ascending air
It's cooler in the atmosphere as you go up, and cold air cannot
hold as much water vapour as warm air. So, when air is forced to
rise, the excess water vapour (gas) in the air condenses into liquid
droplets.
Three main processes which lift and
cool air to form clouds
1) Sea / Sun heating (thermals)
2) Weather fronts (gentle)
3) Mountains
Overall, the global upward movements
of air are equally balanced by the
downward movements, result is
about 40-50% global cloudiness at
any one time.
1. Cloud Cover: Astronomical Observation
• Clouds occur on the local to synoptic
(national/international) scales i.e. ~102 to ~105m
spatial scale) and on temporal scales of 101 to 105
secs.
• Vertical extent depends on forcing and stability
• Local clouds occur especially daytime over mountain
tops, and night-time in valleys (so good for
astronomical observation)
• Satellite (e.g. EUMETSAT) and climate model data
(“reanalyses”) can be used to estimate cloud cover
1: Cloud Cover : Contrails
1: Cloud Cover : EUMETSAT satellite (1km nadir)
1: Cloud Cover : UK Met Office African model (12km)
1: Cloud Cover : FriOWL / Re-analyses data
ERA40 reanalyses July total
cloud cover (above 2,000m
only)
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
2. A stable atmosphere with little turbulence
• Covered by David / Aziz yesterday….
• Wobbling/scintillation of the stellar image is mostly
due to the vertical temperature gradient i.e. when
dT/dz is large - > unstable -> turbulence
• But also mechanical turbulence due to mountains or
obstacles
• Descending air usually descends gently (unlike most
ascending air, which ascends fast!)
• It so happens that there are preferential zones
zones of gently descending air around the globe…
2. A stable atmosphere with little turbulence
Mean annual (1991-2000) ERA40 vertical velocities exceed 2.5 cm/sec (descent); these
are indiciated by green / yellow/ red colours
2. A stable atmosphere with little turbulence
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ERA40 mid-to-upper tropospheric (775 to 200 hPa) vertical velocities in range 2.5 < >
5.0 cm /sec (i.e. gently subsiding air, turbulence less likely)
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
IWV is
extremely
height
dependent,
due to
exponential
relationship of
WV with
temperature
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
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•
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•
Water vapour is the principal absorbing gas from visible to millimeter
wavelengths in the atmosphere
Also increases the refractive index of air, causing phase distortions
Decreases rapidly with vertical height; 2/3 less by a height of 2.5km
Sarazin (2003) quotes:
• < 5mm IWV is suitable for visible astronomy
• <3mm for infra-red
• <2mm for microwave
Hence “High and Dry” sites are best…. Atacama, Rockies, Hawaii,
Izana (Canarys), Morocco, Sutherland, African?
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Only locations where mean annual IWV at 700 hPa is less than 4 mm (yellow) and
greater than 4 mm (blue)
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
4. Low night-time Relative Humidity (RH %)
•
RH is just the ratio of Vapour Pressure of Water Vapour ÷ Saturated
Vapour Pressure at that same temperature
4. Low night-time Relative Humidity (RH %)
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•
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RH usually reaches a maximum during the night and around dawn
(minima during afternoon)
If RH = 100% -> dew / condensation / mist / frost
Risk of dew/frost on mirror / lenses / optics
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
5. “Reasonably” low surface and jetstream windspeeds
Sarazin (2004) states 2-9m/sec are ideal surface windspeeds for the VLT
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<2m/sec -> no flushing of dome
• >9 m/sec -> shake!
Jetstream:
Sarazin & Tokovinin (2002) show that
the 200hPa jetstream is linearly related
to the speed of turbulent structures on
θ
the stellar image
Isoplanatic Angle (θ): the angle subtended to the telescope becomes smaller as height increases
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
6. Moderate Air Temperatures!
• Differences between dome
temperature and outside
temperature can lead to “dome
seeing”
-> SALT is ventilated to keep dT/dx
differences small!
• Extreme cold /heat can put a strain
on instrumentation, equipment and
personnel !
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution
7. Low Aerosol Contamination
• Aerosols (dust, biomass
burning) contribute to
atmospheric extinction
• On-site wind-blown dust is
a hazard as it degrades
mirrors and optics rapidly
(Giordano & Sarazin, 1994)
• 22-years of TOMS aerosol
data available on FriOWL
Atmospheric Constraints on Astronomical Viewing
1. Clear skies / No Cloud
2. Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
4. Low night-time relative humidity (RH)
5. Gentle to moderate windspeeds, or less, throughout atmosphere
6. Moderate air temperatures, low variability
7. Low aerosol contamination
8. Infrequent or no severe weather (lightning, snow, hail)
9. Low light pollution & lots of others…. (infrastructure, culture,
geology, accessibility, political issues, etc..)
8. Infrequent Severe Weather !
• Much greater exposure to lightning at the top of a mountain…
• But the choice of a dry desert with few storms mitigates against
chance of a lightning hit !
• Engineering needs to allow for specific loadings of snow !
Summary: There are links across a huge range of scales!
Decadal cloudiness
variability, Jetstream
variations, Rossby
waves (107m)
Milliseconds,
millimetres (CN2, CT2
CT2, seeing, r0, τ0)
1010 differences in
scale
Thank You (1017 difference in scales!)
Hurricane Epsilon, 3 Dec 2005, NASA
Spiral Galaxy, NGC 1232 21 Sep 1998,
VLT Paranal (ESO)
But 1017 times difference in scale!!
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