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How weather and climate affect astronomical viewing and site selection
Dr. Edward Graham, University of the Highlands and Islands
Astroclimatology:
Where is University of the Highlands and Islands (“UHI”)?
2
Scotland !!
E. Graham et al., 2010 3
The Highlands and Islands
University of the Highlands and Islands (“UHI”)
E. Graham et al., 2010 4
University of the Highlands and Islands (“UHI”)
Outline of my presentation today
5
Two parts:
1) General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy
Outline of my presentation today
6
Two parts:
1) General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy
7
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
Definition: Weather
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• 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 »
Definition: Climate
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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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10
Is climate steady?
«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»…
Temperature (red) of last 20,000 years on Greenland ice cap
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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:• 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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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
The Earth’s Energy Balance
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
HL
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.
Helped by fact that air at the equator has greater relative angular velocity (40,000km per day) than air nearer the poles (0km/day).
But there is the CoriolisEffect
HL
HL
The Coriolis Effect:
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°….
Only for air that doesn’t “feel” the Earth’s rotation
For air that “feels” the Earth’s rotation (Geostrophic balance)
Wind and the Coriolis Effect
HL
Wind, the Coriolis Effect, and Friction
But differing amounts of surface friction (land, sea) result in a reduction in speed and a deflection reduced by 10-30°…
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).
Wind and the Coriolis Effect
24
EquatorNorth Pole
-20C
-10C
0C
+10C+20
C
H L
“Slopes” in pressure pattern then cause winds / the jetstream:
The Jetstreams
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
Q: Why do low pressure turn anti-clockwise in the northern hemisphere? (and vice versa…)
1) 2) 3)
L L L
Rotation in weather systems - Lows
Q: Why do low pressure turn anti-clockwise in the northern hemisphere? (and vice versa…)
1) 2) 3)
L L L
Rotation in weather systems - Lows
Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…)
1) 2) 3)
H H H
Rotation in weather systems - Highs
Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…)
1) 2) 3)
H H H
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!
• Especially true near mountains and coasts!
• 1 deg latitude is roughly equivalent to 18km/h (11mph) difference in relative velocity!
HL
HL
Non-Geostrophic Flow: The Sea Breeze
SEA LAND
Warm air over land rises
Pressure difference forces air out to sea
the sea-breeze moves onshore
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)
“Boundary Layer” (Non-Geostrophic)
~300-1000m
But turbulence can still form in “free atmosphere”: Windshear!
“Free Atmosphere” (Geostrophic)
“Boundary Layer” (Non-Geostrophic)
~300-1000m
Windshear
Turbulence in the “free atmosphere”: Instability
“Free Atmosphere” (Geostrophic)
“Boundary Layer” (Non-Geostrophic)
Convection/bouyancy/instability
Turbulence in the “free atmosphere”: Gravity Waves
“Free Atmosphere” (Geostrophic)
“Boundary Layer” (Non-Geostrophic)
~300-1000m
Outline of my presentation today
40
Two parts:
1) General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy
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 andcool 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.
• 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: Astronomical Observation
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…
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
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)
H H
H H
H H
H H
2. A stable atmosphere with little turbulence
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
Atmospheric Constraints on Astronomical Viewing
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)
• 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)
Only locations where mean annual IWV at 700 hPa is less than 4 mm (yellow) and greater than 4 mm (blue)
3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
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
Atmospheric Constraints on Astronomical Viewing
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 %)
• 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
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
Atmospheric Constraints on Astronomical Viewing
5. “Reasonably” low surface and jetstream windspeeds
Sarazin (2004) states 2-9m/sec are ideal surface windspeeds for the VLT
• <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
the stellar image
Isoplanatic Angle (θ): the angle subtended to the telescope becomes smaller as height increases
θ
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
Atmospheric Constraints on Astronomical Viewing
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 !
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
Atmospheric Constraints on Astronomical Viewing
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
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..)
Atmospheric Constraints on Astronomical Viewing
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!
Milliseconds, millimetres (CN
2, CT2
CT2, seeing, r0, τ0)
Decadal cloudiness variability, Jetstream variations, Rossby waves (107m)
1010 differences in scale
71
Thank You (1017 difference in scales!)
Spiral Galaxy, NGC 1232 21 Sep 1998, VLT Paranal (ESO)
Hurricane Epsilon, 3 Dec 2005, NASA
But 1017 times difference in scale!!