Write full Lecture 6 content on clouds, weather, and climate, add 7 figures

- 8 sections (~6500 words): cloud formation, Clausius-Clapeyron derivation,
  clouds across the solar system, atmospheric dynamics, geostrophic balance,
  weather/storms, faint young Sun paradox, carbonate-silicate cycle
- 6 named equations including boxed Clausius-Clapeyron result
- 7 figures from NASA and Wikimedia Commons (Venus UV, Jupiter cloud layers,
  Hadley cells, Jupiter GRS, Mars dust storm, Saturn hexagon, C-Si cycle)
- 5 new BibTeX entries: Walker1981, Feulner2012, SanchezLavega2011,
  Showman2020, Ingersoll1969
- Update L6 status to "Draft complete (figures added)" and correct
  blackboard derivation topic to Clausius-Clapeyron

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timlichtenberg

book/06_atmospheres_2/atmospheres_2.md

@@ -602,3 +602,60 @@ @article{Wordsworth2022
     year    = {2022},
     doi     = {10.1146/annurev-astro-052920-125632}
 }
+
+% ==========================================
+% Lecture 6: Atmospheres II — Clouds, Weather, & Climate
+% ==========================================
+
+@article{Walker1981,
+    author  = {Walker, James C. G. and Hays, P. B. and Kasting, James F.},
+    title   = {A negative feedback mechanism for the long-term stabilization of {Earth}'s surface temperature},
+    journal = {Journal of Geophysical Research: Oceans},
+    volume  = {86},
+    number  = {C10},
+    pages   = {9776--9782},
+    year    = {1981},
+    doi     = {10.1029/JC086iC10p09776}
+}
+
+@article{Feulner2012,
+    author  = {Feulner, Georg},
+    title   = {The faint young {Sun} problem},
+    journal = {Reviews of Geophysics},
+    volume  = {50},
+    number  = {2},
+    pages   = {RG2006},
+    year    = {2012},
+    doi     = {10.1029/2011RG000375}
+}
+
+@book{SanchezLavega2011,
+    author    = {S{\'a}nchez-Lavega, Agust{\'\i}n},
+    title     = {An Introduction to Planetary Atmospheres},
+    year      = {2011},
+    publisher = {CRC Press},
+    address   = {Boca Raton},
+    doi       = {10.1201/9781439894668}
+}
+
+@article{Showman2020,
+    author  = {Showman, Adam P. and Tan, Xianyu and Zhang, Xi},
+    title   = {Atmospheric Circulation of Brown Dwarfs and Jupiter- and Saturn-like Planets: Zonal Jets, Long-term Variability, and {QBO}-Type Oscillations},
+    journal = {Space Science Reviews},
+    volume  = {216},
+    number  = {8},
+    pages   = {139},
+    year    = {2020},
+    doi     = {10.1007/s11214-020-00758-8}
+}
+
+@article{Ingersoll1969,
+    author  = {Ingersoll, Andrew P.},
+    title   = {The Runaway Greenhouse: A History of Water on {Venus}},
+    journal = {Journal of the Atmospheric Sciences},
+    volume  = {26},
+    number  = {6},
+    pages   = {1191--1198},
+    year    = {1969},
+    doi     = {10.1175/1520-0469(1969)026<1191:TRGAHI>2.0.CO;2}
+}

book/06_atmospheres_2/figures/carbonate_silicate_cycle.jpg

@@ -127,7 +127,7 @@ Each week consists of:
 
 - Cloud formation: condensation, nucleation, and cloud types across the solar system
 - Venus: runaway greenhouse, sulfuric acid clouds, super-rotation
-- **Blackboard derivation (~10 min):** Derive the planetary effective temperature from energy balance (absorbed stellar flux = emitted thermal flux), then add a single-layer greenhouse atmosphere to show how T_surface > T_eff
+- **Blackboard derivation (~10 min):** Derive the Clausius-Clapeyron equation from thermodynamic phase equilibrium — starting from equal Gibbs free energies on the coexistence curve, obtain P_sat(T) = P_ref exp[−L_v/R_v(1/T − 1/T_ref)], and apply to predict cloud condensation across the solar system
 - Mars: thin CO₂ atmosphere, dust storms, seasonal CO₂ cycle
 - Titan: methane hydrological cycle, organic haze
 - Giant planet atmospheres: banded structure, zones and belts, composition
@@ -394,7 +394,7 @@ All homework sheets are **ungraded formative practice**. Each contains 4–6 pro
 | 3 | Planetary heat & energy transport | Draft complete (figures added) | High |
 | 4 | Chemical differentiation & magnetospheres | Draft complete (figures added) | High |
 | 5 | Atmospheres I | Draft complete (figures added) | High |
-| 6 | Atmospheres II | Not started | High |
+| 6 | Atmospheres II | Draft complete (figures added) | High |
 | 7 | Planetary surfaces | Not started | High |
 | 8 | Planetary interiors | Not started | High |
 | 9 | Rocky planets: Earth & Venus | Not started | Medium |