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MET 12 Global Climate Change - Lecture 4 Clouds and global climate Shaun Tanner San Jose State University By the end of this chapter you should: Understand the role of water in the Earth system Know the roles of clouds Understand how high clouds differ from low clouds in terms of albedo and absorption 1 Questions What role do clouds play on the Earth’s climate? What would happen to our climate if clouds were to increase/decrease? 2 3 4 Water in the atmosphere Definitions: – Evaporation: Process where a liquid changes into a gas – Condensation: Process where a gas changes into a liquid – Precipitation: Any liquid or solid water that falls from the atmosphere to the ground. (i.e. RAIN!) 5 Condensation The process by which water vapor changes to a cloud droplet Water vapor molecules may ‘stick’ to condensation nuclei and grow (billions) to eventually form cloud droplet. Examples of condensation nuclei include: a. Dust b. Salt c. Smoke 7 Clouds and radiation Cloud - Climate Interactions Albedo effect - COOLING Clouds reflect incoming solar radiation. The cloud droplet size and total water content determine the overall reflectivity. Greenhouse effect - WARMING Clouds are good absorbers (and emitters) of long wave (infrared) radiation. 8 Clouds and day to day temperatures Imagine that you are going camping in the Sierras with your friends. On the first day (and evening) it is cloudy, while on the second day (and evening) it is clear. Based on this information alone: Which day would be warmer? Which evening would be warmer? Explain your answers. 9 10 Low and High clouds Consider two types of clouds: 1. Low levels clouds 2. High levels clouds Q: How is the Earth’s surface energy budget different for low clouds compared to high clouds? 11 Clouds and climate Cloud A: Low level, (dark, thick) Excellent reflector of incoming radiation; good absorber/emitter of infrared radiation Cloud B: High level, light (sun visible or thin) Fair/poor reflector of incoming radiation; good/excellent absorber/emitter of infrared radiation So, clouds both warm and cool the earth. Overall, though, clouds act to cool the earth 12 Changes in clouds Increases in low level clouds will: – Increases in high level clouds will: 13 Changes in clouds Increases in low level clouds will: – cool the surface (cooling outweighs warming) Increases in high level clouds will: – warm the surface (warming outweighs cooling) 14 15 16 17 18 19 20 How would the earth’s climate change as a result of aircraft contrails. 21 22 Definitions Insolation – Incoming solar radiation Solstice – day of the year when the sun shines directly over 23.5°S or 23.5°N Equinox – days of the year when the sun shines directly over the equator 24 25 26 What influences incoming solar energy? The Sun’s angle of incidence: – Lower sun angle, less incoming energy – Higher sun angle, more incoming energy Length of time the Sun shines each day: – Summer season, more sun hours – Winter season, less sun hours 27 What month do you think this graph represents? a) December b) March c) June d) September 28 Review questions On June 21st, at what latitude is the sun directly overhead at noon? On September 22nd, at what latitude is the sun directly overhead at noon? How many hours of daylight are present at the South Pole on February 20th? Where would you expect to have longer days; 45 ° N on June 21st or 50°S on Dec 21st? 30