How weather and climate affect astronomical viewing and site selection Dr. Edward Graham,

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 The Highlands and Islands

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

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

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

7. Definition: Weather Weather is thestate of the atmosphere at any one particular place at a particulartime. Twoseparate places never have the exactlysameweather, nordoes the weathereverrepeatitself 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) • Actualweather is usuallychaotic, but is containedwithin certain boundaries, climate is the « average »

9. The scale of weather and climatesystems • Weather and climatephenomenaoperateover huge temporal and spatial scales; • Spatially: 10-3 m (millimetres) to 106m (thousands of kilometres) • Temporally: 10-3secs (milliseconds) to 108 secs (decades)

10. Is climatesteady? Temperature (red) of last 20,000 years on Greenland ice cap «Traditional» (deterministic) climatologists (until ~1980s) viewedclimate as being reasonablysteady. Presentview is contrary to this: Climateitselfmay not bestable & there can besudden « shifts » or «step-changes»… 10

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 temperatureincrease ~2090s

14. It’s not just a temperatureincrease…. An increase in Extremestoo!

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’saxis causes the seasons • The distribution of continents, mountains and oceansalsoplay a key role • Atmosphere is a fluid, but 1000 times less dense thanwater • P = ρRT (Ideal gas equationlP=pressure, ρ=density, T=temperature, R=Universal gas constant) • Resultis heat and moisturetransfertowards the poles

17. The Earth’sEnergy Balance On average, there’s 342 Wm-2incident 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. AtmosphericEnergy Transport: Wind H L 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. 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 “Geostropicbalance” It means frictionless airflow is deflected by 90°…. H L H L Only for air that doesn’t “feel” the Earth’s rotation For air that “feels” the Earth’s rotation (Geostrophic balance)

22. Wind, the Coriolis Effect, and Friction H L 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) 3) 2) L L L

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

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

31. Rotation in weather systems - Highs Q: And why do high pressures turn clockwise in the northern hemisphere? (and vice versa…) 3) 1) 2) H H 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. 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 deglatitude is roughly equivalent to 18km/h (11mph) difference in relative velocity! H L H L

34. 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

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) ~300-1000m “Boundary Layer” (Non-Geostrophic)

37. But turbulence can still form in “free atmosphere”: Windshear! “Free Atmosphere” (Geostrophic) Windshear ~300-1000m “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) ~300-1000m “Boundary Layer” (Non-Geostrophic)

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

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. ~102to ~105m spatial scale) and on temporal scales of 101 to 105secs. • 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…