Chapter 3 Atmospheric Energy and Global Temperatures

Energy Pathways & Principles Some insolation (incoming solar radiation) is absorbed by earth, the rest is reflected Absorbed energy is reradiated as longwave energy out from Earth. Longwave radiation absorbed by greenhouse gasses keeps the Earth warm. Without greenhouse gasses, Earth would be much cooler (59 degrees F)*. Going back in time, periods of low atmospheric greenhouse gas concentrations tended to be cooler, and times of high greenhouse gas concentrations were warmer. *Lutgens & Tarbuck (2004). The Atmosphere. Prentice Hall Upper Saddle River, New Jersey. Page 52.

Insolation Figure 3.2

Energy Pathways Figure 3.1

Energy Pathways Radiation balance is composed of inputs, transfers, & outputs. Transfers include convection, conduction, and latent heat of vaporization (explained later). Net radiation: the balance between all incoming radiation (positive quantity) and all outgoing radiation (negative quantity) carried by both shortwave radiation and longwave radiation Transmission: the passage of shortwave and longwave energy through space, the atmosphere, or water Scattering: the deflection and redirection of insolation by gasses, dust, water vapor, clouds, and ice

Latitudinal heat balance: surplus from Equator to 38 degrees N and S.

Energy Pathways (con’t.) Albedo: the % reflectivity of a substance Typical albedos of some surfaces (%) clouds: 15-90 concrete: 17-27 asphalt: 5-15 green crops: 5-25 wet soil: 15-30 fresh snow: 80-90 water: 5-60 (5-10% when solar altitude >30°) Clouds concrete asphalt green crops wet soil fresh soil water

Refraction Figure 3.4

Albedo and Reflection Figure 3.5

Clouds and Albedo Figure 3.6

Heat Transfer Figure 3.7

The “Greenhouse Effect” & Atmospheric Warming The lower atmosphere absorbs heat energy primarily in the form of terrestrial longwave radiation (IR). A real greenhouse traps heat inside by utilizing glass or plastic walls and roof. The atmosphere delays transfer of heat from Earth into space by absorbing IR radiation, and then reradiating some of it back to Earth.

Clouds and Earth’s “Greenhouse” Figure 3.8

Outgoing Shortwave Energy (reflected) Figure 3.9

Outgoing Longwave Energy Figure 3.9

Earth–Atmosphere Radiation Budget Figure 3.10

Diurnal (24 hr.) Radiation and Temperature Change Daily temperature change is a function of the balance between incoming shortwave and outgoing longwave radiation. When incoming energy exceeds outgoing energy, temperature rises. Thus, daily highs are generally reached sometime after solar noon. When outgoing energy exceeds incoming energy, temperature drops. Thus, the lowest temperatures are usually recorded just after sunrise.

Radiation Balance & Seasonal Temperature Change In mid latitudes, warmest time of yr lags after the summer solstice, just as daily highs are after solar noon. (July in OR) In mid latitudes, coldest time of yr (Jan in OR) lags after the winter solstice, just as daily lows are after sunrise. In most tropical areas, the warmest month is determined by the interaction between insolation and cloud cover. In areas of seasonally abundant precipitation, the warmest month can be just before the big rains begin. (next slide) In drier tropical areas, the warmest month may lag the time of solar maximum by a month or two. Everything else equal, coastal areas have longer lags between solar maxima and temperature maxima than interior areas. (San Francisco)

Mexico City and Bangalore http://media.diercke.net/omeda/800/112397E_Klimadia_Mexico_City.jpg http://www.bangalore.climatemps.com/ In tropical/sub tropical deserts, max average monthly temps are July/August. But the onset of rains in non-desert tropics around May/June cools off daily highs. Thus av. temp. peaks earlier ~April/May rather than later. (generalizations apply to Northern Hem.)

Simplified Surface Energy Balance: NET Radiation = + SW (insolation) – SW (reflection) + LW (IR) – LW(IR) Figure 3.13