Outline

1. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (1 of 14) Further Reading: Chapter 04 of the text book Outline - global radiative energy balance - insolation and climatic regimes - composition of the atmosphere

2. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (2 of 14) Introduction • Last time we discussed radiation and distinguished between solar (or shortwave) and thermal • (or longwave) radiation • In addition, we discussed how temperature affects the intensity and wavelength of radiation • Today we want to look at how radiation can affect the temperature of a body • In particular, we want to: • Discuss how radiation from the sun determines the temperature of the earth’s surface based upon a very simple model for the global earth system • Look at how solar radiation varies with latitude and time of year • We are also going to use this lecture to introduce some basic concepts used in atmospheric • sciences to describe the composition and state of the atmosphere

3. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (3 of 14) Thermal Equilibrium • Amount of energy absorbed equals the amount of energy released (or radiated away) • If the input of energy exceeds the output, energy is added to the system and it will heat up • If the input of energy is less than the output, energy is removed from the system and it will • cool down • Hence, thermal equilibrium implies that there is no net heating of the system

4. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (4 of 14) Global Radiation Energy Balance • The flux arriving at the top of the atmosphere is constant • This constant input of energy goes to heating the earth • Heating of the earth raises the temperature • As the temperature of the earth increases, this increases amount of longwave radiation it • gives off • Eventually, the temperature is just right so that the amount of longwave radiation given • off exactly balances the incoming solar radiation

5. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (5 of 14) Solar Constant • Energy flux arriving at the top of the atmosphere measured • perpendicular to Sun’s rays at mean Earth-Sun distance • Equal to 1370 W/m^2 (Watts per square meter)

6. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (6 of 14) Total Incident Solar Energy • Energy flux arriving at the top of the atmosphere • Es = 1400 W/m^2 (approx. to the solar constant) • Interception area of Earth = pi*r^2 = 129,000,000,000 m^2 • Ein = 1.8 x 10^17 Watts

7. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (7 of 14) Total Solar Energy Absorbed • Not all energy reaching the earth’s atmosphere is absorbed • About 33% is reflected back to space • Clouds • Ice and Snow • Plants • 33% reflected away  Esurf = 1.2 x 10^17 Watts

8. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (8 of 14) Radiation Energy Balance • Esurf(incoming) = Esurf(outgoing) • Esurf(outgoing) = sT^4 times Area of Earth • T^4 = (1.2 x 10^17/Area)/s • T(earth) = 253 K (degrees Kelvin) • Water freezes at 273 K; The actual T(earth) is 290 K • That is because of the natural greenhouse effect

9. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (9 of 14) Insolation • So far we have been talking about global balances; this ignores regional and seasonal effects • For a particular point, as opposed to the globe, we refer to “insolation” • For a given point, insolation depends on the • angle of the incoming sunlight and the duration • of sunlight • Angle of sun (solar zenith angle): if this is large, the energy of the sun is spread over a larger area and insolation goes down • Duration: the longer the point is exposed to the sun, the more radiation it receives, hence its insolation goes up • For daily insolation, maximum occurs at the • poles during the solstice because of duration • effects (i.e. the day is 24 hours long) • However, it also has the lowest insolation during • winter, hence it has a large annual temperature • range

10. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (10 of 14) Annual Insolation • Over the year, however, equatorial regions have the highest insolation because of • the consistently low solar zenith angles • Because of this, the tropics have the highest average annual temperatures • The earth’s tilt results in • Substantial increase in mean insolation at high latitudes • Slight decrease in insolation at low latitudes

11. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (11 of 14) Climatic Regimes Areas of the globe defined based upon the annual insolation they receive (13 regimes) Explain first order control on climate Higher insolation and warmer climate at low latitudes (and vice versa) Climates at higher latitudes characterized by strong seasonality

12. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (12 of 14) Radiation and Temperature • Up until now we have looked at the general relationship between solar energy, the earth’s orbit • and global and latitudinal temperatures • For instance, we know that in the tropics its warm and has a constant temperature through the • seasons; for the polar regions, it is cool and the temperature changes throughout the seasons • NOTE: this doesn’t always hold. For instance London at 51N has cooler summers and • warmer winters than Boston at 42N • Now we want to discuss heterogeneity in temperatures. This deals with how energy interacts • with the cryosphere, oceans, biosphere, lithosphere, and atmosphere at different points on the • globe

13. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (13 of 14) Composition of the Atmosphere • Starting with the atmosphere • First, we are interested in how energy is transferred through the atmosphere • This depends on its composition • The earth is 128,000km across • The atmosphere is gaseous envelope held • close to earth by gravity • 97% of atmosphere is within 30km of the • surface • Originally the gases from the atmosphere seeped out from volcanoes • 10% was CO2, 85% was H2O; No O2; it was also much warmer (100C) • As the earth cooled, the H2O condensed into clouds, rain, oceans • Photosynthetic organisms evolved, converting CO2 into O2 • Evolution of land plants led to rapid conversion of CO2 into O2 • Today we find most is N with O2 second • The most highly variable constituent is H2O: from 0.25-2%; depends on • temperature, location, dynamics

14. Natural Environments: The Atmosphere GG 101 – Spring 2005 Boston University Myneni Lecture 06: Radiation and Temperature Feb-02-05 (14 of 14) State Variables • Temperature:Average kinetic energy of air molecules • Highly variable (-60 -> 50 C) • Study is called “thermodynamics” • Pressure:Average mass of air molecules above a given point • Related to potential energy • Study is called “dynamics” • Humidity:Average number of water molecules in a give volume of air • Water is one of the most important constituents of the atmosphere - influences • energy balance, water balance, and dynamics • Study is called “hydrodynamics”