ESS15 Lecture 4 Latent heat & getting ready for climate calculations.

Transcription

1 Energy changes makes things happen. ESS15 Lecture 4 Latent heat & getting ready for climate calculations. Review from last lecture Conversions among different kinds of energy power all that happens in the climate. Energy can be stored, moved, and transformed but the total amount of energy never changes. Radiation, convection and conduction are all important ways that energy moves in climate. There is another slightly indirect process through which energy moves around the climate system. ( laboratory demonstration )

2 Latent heat energy Latent heat is a very important form of internal chemical energy molecules store or release energy in their bonds during phase changes. Latent heat energy Condensation vapor turning to liquid water. Clouds forming. Heat is released from H2O molecules! Evaporation liquid turning to vapor water. Heat is stored in H2O molecules! Latent heat is how storms like hurricanes transfer energy from the ocean s temperature into gale force winds! Rule of thumb > when matter switches to a denser phase, energy is released. Hurricane Andrew Devastation in Homestead, Florida August 24, 1992

3 Energy transfer drives the global currents of the ocean and atmosphere (we will learn about this in a few weeks) We see this traced out in the patterns of cloudiness seen from space. Water plays a special role because it can exist in any of its 3 phases within the temperatures and pressures of Earth s atmosphere. Hurricanes and Latent Heat Release Hurricane winds are powered by energy changes involving latent heat release (Satellite reconstruction) An average hurricane has latent heat release = 600 trillion Watts (200 times the world electricity generation) Hurricanes weaken over land because they lose their fuel for condensation

4 Thunderstorms and Latent Heat Release Explosive growth of thunderstorms is also from latent heat release Getting ready for climate calculations with energy!

5 How do you measure energy & its changes? Units, units, units! Energy: Joules (J, kj, MW) 1 kj = 1000 J 1 MJ = 1,000,000 J Changes of energy: Watts (W, kw, MW) 1 W = 1 J/s (Energy per unit time) Food energy is probably the most familiar unit to us. It is measured in weird units of kcal but can be easily converted into Joules Convert to S.I. unit of Joules: 1 kcal = 4184 J 250 kcal = 250 x 4184 J = 1,046,000 J = x 10 6 J ~ 1.05 MJ How much energy does it take to heat stuff up? Specific heat capacity is different for different substances A very important constant: Specific heat capacity of water C ~ 4100 J/kg/K for liquid water Units, units, units: C ~ 4100 J/kg/K for liquid water C ~ 800 J/kg/K for dry soil C ~ 1005 J/kg/K for dry air C measures how much energy it takes to heat a substance up per unit mass and per degree K. If you know how much stuff you have (kg), and how much you want to heat it up (K), C tells you how much energy it takes (in J)

6 How much energy does it take to heat this much water from room temperature to 100 C? C = energy / (mass x (temperature change)) [J/kg/K] = [J] / ( [kg] x [K] ) Density of water = 1 kg/l so mass of 500 ml water = 0.5 kg Temperature change is ~ 75 K C ~ 4100 J/kg/K for liquid water plug n play! 153,000 J! How much energy does it take to vaporize water once it is at 100 C? A very important property of water matter: Latent heat of vaporization: L ~ x 10 6 J/kg Units, units, units: L measures how much energy is absorbed per unit mass when water is evaporated. (Same amount is released when water vapor is condensed) Think: Palm springs pool, steam burn! Once it has reached 100C, how much additional energy would it take to evaporate the water into the gas phase? E = L x m [J] = [J/kg] x [kg] Density of water = 1 kg/l so mass of 500 ml water = 0.5 kg = (2.265 x 10 6 J/kg ) x 0.5 kg = x 10 6 J Latent Heat and Climate Where the atmosphere s vapor exists on average. Surface water vapor content, 40 measured in Kelvin: Source: NCEP Reanalysis

7 Water Vapor and Global Warming With global warming, the atmospheric moisture content is expected to increase Because warmer air can hold more moisture What effects will an increased moisture content have on the Earth s climate? Precipitation Latent Heat and the Global Climate And, How to Keep Your Drinks Cold! A story by Dargan Frierson And Dale Durran Courtesy of the University of Washington, Department of Atmospheric Sciences Hurricanes/supercells/tornados Weather systems Patterns of warming Cloud Models Clouds form by condensation of water vapor And condensation is associated with latent heat release Latent Heating Everyone knows evaporation cools you (how sweat works, etc) Few people know condensation releases heat! Not part of our everyday life Condensation on drinks is an everyday occurrence of latent heating! (in humid climates )

8 How much heating? The Apparatus Let s say 1 millimeter of condensation forms on the outside of a standard 12-oz drink can Math says if all the latent heat of condensation is transferred to the drink, the temperature rise should be 5.0o C (9.0o F)! Let s test this We varied temperatures in this environmental chamber The Moisture Source We varied relative humidity by inputting moisture Chamber Opening We started the experiment with cans just above 0o C

9 Weighing the Condensate Results for 35 C (95 F) 7 o C of temperature change from condensation alone in just 5 minutes! More than half the heating of the can is due to condensation Stella Choi measuring the mass of condensate after an experiment Real-World Context Dhahran, Saudi Arabia: July 8, 2003 Dew point = 35 C (95 F) Where was the most humid day in history? How does that compare to our chamber?

10 Dhahran, Saudi Arabia Results for 40 C (104 F) Can would heat by 14o C (25o F) in just 5 minutes! Of that, 8.5o C (15o F) is due to condensation (60%) World record humidity How to keep cans cold? Koozies are effective both because they protect cans from condensation, and because they insulate the cans

11 Thanks. Next time: Radiation!