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Lecture 13 – Humidity and Cloud Formation

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Lecture 13 – Humidity and Cloud Formation Lecture 13 – Humidity and cloud formation Reading for today: P.335-340 Quiz 4 review Learning Outcomes • Humidity on Earth. Be able to: – explain/read graphs/carry out calculations of how we quantify atmospheric water vapor (dew point temperature and relative humidity) • Cloudiness on Earth. Be able to: – describe/explain/draw diagrams to explain the processes involved in cloud formation – draw feedback loops to explain the effects of clouds on climate – explain why clouds don’t always lead to precipitation Atmosphere “the gaseous envelope that surrounds a planet or other celestial body” Weather – the state of the atmosphere a given time & place: • temperature • air pressure • humidity • cloudiness • wind speed & direction Climate – average weather over a long period of time Evaporation and Condensation Vapor pressure: pressure of a gas above a liquid Equilibrium: rate of evaporation equals rate of condensation Saturation: maximum amount of water in gaseous phase Saturation Vapor Pressure: maximum vapor pressure at a given temp How we quantify atmospheric moisture Indices of water vapor content: 1. Vapor pressure 2. Relative humidity Indices of water vapor content 1. Vapor pressure What happens if temperature increases? = Undersaturated (more evaporation can take place if water available) What happens if temperature decreases? = Supersaturated (condensation will take place until reach saturation) What happens if you add more water vapor? = Supersaturated (condensation will take place until reach saturation) Indices of water vapor content 1. Vapor pressure At what temperature would this air become saturated? a) -10 oC b) 10 oC = dew point temperature c) 25 oC d) 40 oC Temp of saturation for a parcel of air = dew point temperature How we quantify atmospheric moisture Indices of water vapor content: 1. Vapor pressure 2. Relative humidity – amount of water in air expressed as a percentage of the maximum amount possible – Relative humidity = actual vapor pressure x 100% saturation vapor pressure Indices of water vapor content 2. Relative humidity Relative humidity = actual vapor pressure x 100 % = 10 mb x 100 % = 50 % saturation vapor pressure 20 mb Saturation vapor pressure Actual vapor pressure iClicker Question The parcel of air is at 30 oC and vapor pressure is 10mb. What is the relative humidity? a) 5 % b) 25 % c) 33 % d) 50 % Why does high humidity feel uncomfortable? • Evaporation of sweat requires energy so absorbs energy from your skin, cooling you down (latent heat flux) • At high relative humidity, evaporation is very slow so you cannot cool down so quickly • At the same temperature, places with low humidity (e.g. Irvine) feel nicer! Atmosphere “the gaseous envelope that surrounds a planet or other celestial body” Weather – the state of the atmosphere a given time & place: • temperature • air pressure • humidity • cloudiness • wind speed & direction Climate – average weather over a long period of time How to saturate air with water vapor 1. Add water vapor to the air (i.e. increase vapor pressure) 2. Decrease the temperature of the air How to change air temperature 1. Absorption/loss of energy (by radiation, conduction, sensible heat) 2. Mixing warm and cold air (by convection) 3. Changing the phase of water (e.g., condensing water vapor releases latent heat to surrounding molecules of air in parcel and warms them) = Diabatic processes – energy is removed from or added to a system 4. New category of process that change temperature… = Adiabatic processes – energy is NOT added or removed, involve changes in pressure Compressing air heats up Expanding air cools down Demonstration of adiabatic processes! Review: Pressure decreases with altitude • Rising air expands so cools down • Sinking air compresses so warms up Cloud formation Adiabatic lapse rate = change in temperature with altitude -15oC As unsaturated air rises… -10oC 1.Temperature drops at: -5oC Dry adiabatic lapse rate = 10 oC/km 2.Relative humidity increases 3.Air becomes saturated (dew point) Saturated adiabatic 4.Condensation starts at this altitude lapse rate 5 oC/km 5.Condensation releases latent heat 6.Heat released decreases rate of cooling: Saturated adiabatic lapse rate = 5 oC/km Where does the rising air in this photo reach its dew point temperature? A B C Natural mechanisms for lifting air Cloud types - Lots of different cloud types - For now we only care that some are high and some are low. Clouds and global temperature • Clouds reflect solar radiation effectively (i.e. increase albedo) – thicker clouds reflect more so cool • Clouds absorb and emit terrestrial (infrared) radiation – how much radiation a cloud absorbs and emits depends on its height • High clouds result in net warming, low clouds result in net cooling. Water vapor, clouds and global climate Temperature Absorption of Albedo outgoing terrestrial radiation Clouds Water vapor in An increase in clouds will result in: a)Increased global temperatures atmosphere b)Decreased global temperatures c)Ummm – difficult to say… On Wednesday: Global circulation patterns Reading: P. 355-361 (ignore “the Basics: Air Masses”), 364-365 NASA.
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