1. Astroclimatology:
How weather and climate affect astronomical viewing and site selection
Dr. Edward Graham,
University of the Highlands and Islands

2. Where is University of the Highlands and Islands (“UHI”)?
Scotland !!

3. University of the Highlands and Islands (“UHI”)
E. Graham et al., 2010

4. University of the Highlands and Islands (“UHI”)
E. Graham et al., 2010

5. Outline of my presentation today
Two parts:
General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy

6. Outline of my presentation today
Two parts:
General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy

7. 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

8. 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 »

9. 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)

10. Is climate steady? 10
Temperature (red) of last 20,000 years on Greenland ice cap
«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»…

11. 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)

12. Intergovernmental Panel on Climate Change (IPCC) scenarios for 21st century

13. Surface air temperature increase ~2090s

14. It’s not just a temperature increase…. An increase in Extremes too!

15. 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

16. 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

17. 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

18. 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…

19. L H Atmospheric Energy Transport: Wind
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.

20. 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).

21. L H L H Wind and the Coriolis Effect 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°…. H L
Only for air that doesn’t “feel” the Earth’s rotation H L
For air that “feels” the Earth’s rotation (Geostrophic balance)

22. L H 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°…

23. 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).

24. The Jetstreams
“Slopes” in pressure pattern then cause winds / the jetstream:
-20C H L
-10C 0C
+10C
North Pole
+20C
Equator

25. Differences in air temperature / air pressure cause the weather patterns:

26. Differences in air temperature / air pressure cause the weather patterns:

27. Latitudional (zonal) air circulation systems

28. Rotation in weather systems - Lows
Q: Why do low pressure turn anti-clockwise in the northern hemisphere? (and vice versa…) 1) 2) 3) L

29. Rotation in weather systems - Lows
Q: Why do low pressure turn anti-clockwise in the northern hemisphere? (and vice versa…) 1) 2) 3) L

30. Rotation in weather systems - Highs
Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…) 1) 2) 3) H

31. Rotation in weather systems - Highs
Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…) 1) 2) 3) H

32. 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.

33. L H 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! H L

34. SEA LAND Non-Geostrophic Flow: The Sea Breeze
Warm air over land rises
Pressure difference forces air out to sea
the sea-breeze moves onshore

35. 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

36. The logarithmic wind profile
“Free Atmosphere” (Geostrophic)
~ m
“Boundary Layer” (Non-Geostrophic)

37. But turbulence can still form in “free atmosphere”: Windshear!
“Free Atmosphere” (Geostrophic)
Windshear
~ m
“Boundary Layer” (Non-Geostrophic)

38. Turbulence in the “free atmosphere”: Instability
“Free Atmosphere” (Geostrophic)
Convection/bouyancy/instability
“Boundary Layer” (Non-Geostrophic)

39. Turbulence in the “free atmosphere”: Gravity Waves
“Free Atmosphere” (Geostrophic)
~ m
“Boundary Layer” (Non-Geostrophic)

40. Outline of my presentation today
Two parts:
General Meteorology & Climatology
BREAK / PAUSE
2) Application of above to Astronomy

41. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

42. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

43. 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
Sea / Sun heating (thermals)‏
Weather fronts (gentle)‏
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.

44. 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

45. 1: Cloud Cover : Contrails

46. 1: Cloud Cover : EUMETSAT satellite (1km nadir)

47. 1: Cloud Cover : UK Met Office African model (12km)

48. 1: Cloud Cover : FriOWL / Re-analyses data
ERA40 reanalyses July total cloud cover (above 2,000m only)

49. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

50. 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…

51. 2. A stable atmosphere with little turbulence
Mean annual ( ) ERA40 vertical velocities exceed 2.5 cm/sec (descent); these are indiciated by green / yellow/ red colours

52. 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

53. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

54. 3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
IWV is extremely height dependent, due to exponential relationship of WV with temperature

55. 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?

56. 3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)

57. 3. Low Integrated Water Vapour (IWV) / Precipitable water (PWV)

58. 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)

59. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

60. 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

61. 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

62. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

63. 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 θ
Isoplanatic Angle (θ): the angle subtended to the telescope becomes smaller as height increases

64. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

65. 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 !

66. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution

67. 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

68. Atmospheric Constraints on Astronomical Viewing
Clear skies / No Cloud
Stable Atmosphere / Little or no turbulence (~10-3 to ~105m)
Low Integrated Water Vapour (IWV) / Precipitable water (PWV)
Low night-time relative humidity (RH)
Gentle to moderate windspeeds, or less, throughout atmosphere
Moderate air temperatures, low variability
Low aerosol contamination
Infrequent or no severe weather (lightning, snow, hail)
Low light pollution & lots of others…. (infrastructure, culture, geology, accessibility, political issues, etc..)

69. 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 !

70. Summary: There are links across a huge range of scales!
Milliseconds, millimetres (CN2, CT2 CT2, seeing, r0, τ0)
Decadal cloudiness variability, Jetstream variations, Rossby waves (107m)
1010 differences in scale

71. Thank You (1017 difference in scales!)
Hurricane Epsilon, 3 Dec 2005, NASA
Spiral Galaxy, NGC Sep 1998, VLT Paranal (ESO)
But 1017 times difference in scale!!