Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

1. Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt Anthony J. Vega

2. Part 1. Energy and Mass Chapter 3. Energy Balance and Temperature

3. Introduction • Solar radiation is the primary heat source for the atmosphere • Most gases are transparent to solar radiation and, instead, absorb terrestrial radiation • Gases are also responsible for scattering incident energy • The balance between incoming solar radiation, the absorption of terrestrial radiation, and outgoing terrestrial radiation describes the global energy budget

4. Atmospheric Influences on Insolation • Radiant energy incident upon the Earth-atmosphere system is either absorbed, reflected, or transmitted by atmospheric gases and/or the Earth’s surface • Energy reflected and/or transmitted (scattered) does not contribute to heating • Absorbed energy encourages direct heating • Absorption • Particular gases, liquids, and solids in the atmosphere absorb radiant energy • Heat increases in the absorber while less energy is transferred to the surface • Although atmospheric gases are rather selective in the wavelengths they absorb, they are overall poor absorbers of energy

5. Reflection • Energy is effectively redirected by objects through reflection • Process does not increase heat in the reflector as energy is not absorbed • In most instances, only a portion of incident energy is reflected • Albedo = the percentage of reflected energy • Specular reflection is reflection of energy as an equally intense energy beam • Energy reflected in such a way as to disperse energy into many weaker wavelengths is diffuse reflection or scattering Scattering of energy

6. Scattering • Gases in the atmosphere effectively scatter radiation • Energy that reaches the surface is diffuse radiation and different in intensity from direct radiation • Characteristics of scattering are dependent upon the size of the scattering agents • Rayleigh Scattering • Involves gases, or other scattering agents that are smaller than the energy wavelengths • Scatter energy forward and backward • Partial to shorter wavelength energy, such as those which inhabit the shorter portion of the visible spectrum • A blue sky results

7. Mie Scattering • Larger scattering agents, such as suspended aerosols, scatter energy only in a forward manner • Larger particles interact with wavelengths across the visible spectrum • Produces hazy or grayish skies • Enhances longer wavelengths during sunrises and sunsets, indicative of a rather aerosol laden atmosphere Longer radiation path lengths lead to an increase in Mie Scattering and reddish skies

9. Nonselective Scattering • Water droplets, typically larger than energy wavelengths, equally scatter wavelengths along the visible portion of the spectrum • Produces a white or gray appearance • No wavelength is especially affected • Transmission • The percentage of energy transmitted through the atmosphere to the surface • Dependent upon the ability of the atmosphere to absorb, scatter, and reflect • Transmission of energy varies diurnally from place to place

10. The Fate of Solar Radiation • Insolation annually varies by 7% • Useful to think of a constant supply of radiation at the top of the atmosphere • Need to account for the relative amount of radiation that is transmitted through the atmosphere, absorbed by the atmosphere and surface, and scattered back to space • A global energy budget • Assume global annual insolation of 100 units of energy • Atmosphere directly absorbs 25 units with 7 absorbed by stratospheric ozone, the rest by gases such as water vapor (absorbs near-infrared wavelengths)

11. Atmospheric reflection averages 25 units, 19 of which are reflected to space by clouds and 6 units which are back-scattered to space from atmospheric gases • Remaining 50 units are available for surface absorption

12. 50 units of energy overall exist at the surface • 5 units reflected back to space • These 5 units combined with the 25 scattered to space from the atmosphere equates to a total planetary albedo of 30% • Remaining 45 units of energy at the Earth’s surface are absorbed • This warms the surface • Earth processes eventually transfer this energy from the Earth system back to space

13. Energy Transfer between the Surface and Atmosphere • Surface-Atmosphere Radiation Exchange • Due to Earth’s low temperatures, terrestrial radiation emitted is primarily longwave • Longwave energy transfer is more complex than solar energy because longwave energy has no obvious beginning or end • Longwave radiation emitted from the surface is largely absorbed by the atmosphere • This increases the temperature of the atmosphere which causes it to radiate more energy outward • Energy is transferred in all direction, including downward • This causes additional surface heating, and the cycle repeats • To describe longwave energy, we begin with 104 units of radiation • 100 units are absorbed by the atmosphere • Water vapor and CO2 are the primary absorbers

14. These, along with other gases, are known as greenhouse gases • A portion of the longwave spectrum can pass through the atmosphere unimpeded • This range of wavelengths, 8-15μm, match those radiated with greatest intensity by the Earth’s surface • This range of wavelengths not absorbed is called the atmospheric window The atmospheric window

15. Although gases do not effectively absorb wavelengths in the atmospheric window, clouds readily absorb virtually all longwave radiation • This is the reason cloudy nights are typically not as cool as clear nights • The result of the reabsorption of energy is that a total of 154 units of energy are reradiated by the surface • Net longwave radiation, the difference between absorbed (100) and emitted (154) longwave radiation equals 54 units • The surface receives 88 units of longwave radiation • Amount is exceeded by the 104 units that are radiation for a net longwave radiation deficit of 16 units

16. Shortwave and longwave radiation undergo different amounts of absorption and reflection • They are not separate entities relative to heating • When either is absorbed, the absorber is warmed • Net all-wave radiation or simply net radiation equals the difference between absorbed and emitted radiation or the net energy gained or lost by radiation • The atmosphere absorbs 25 units of solar radiation but undergoes a net loss of 54 units for a net deficit of 29 units • The surface absorbs 45 units of solar radiation but has a longwave deficit of 16 resulting in a net surplus of 29 units • So, the atmosphere has a net deficit of radiation energy exactly equal to the surplus attained by the surface

17. If radiation were the only means of exchanging energy, the surplus obtained by the surface would result in a perpetual warming while the atmospheric deficit would lead to continual cooling • Our feet would be scorched while our bodies froze • This does not happen because energy is transferred from the surface to the atmosphere and within the atmosphere by conduction and convection • The surplus and deficits offset as a result • Conduction • As the surface warms, a temperature gradient (rate of change of temperature over distance) develops in the upper few cm of the ground • Temperatures are greater at the surface than below • This transfers energy downward • Surface warming also causes a temperature gradient within a very thin sliver of adjacent air called the laminar boundary layer

18. Net radiation Surface/atmosphere offset surplus and deficits

19. Convection • The temperature gradients in the laminar boundary layer induce energy transfer upward through convection • This occurs any time the surface temperature exceeds the air temperature • Normally, this occurs during the middle of the day • At night, the surface typically cools more rapidly than air and energy is transferred downward • Convection can be generated by two processes in fluids • Free Convection • Mixing related to buoyancy • Warmer, less dense fluids rise • Forced Convection • Initiated by eddies and other disruptions to smooth, uniform flow

20. Free Convection Forced Convection

21. Sensible Heat • Heat energy which is readily detected • Magnitude is related to an object’s specific heat • The amount of energy needed to change the temperature of an object a particular amount in J/kg/K • Related to mass • Higher mass requires more energy for heating • Globally, 8 units of energy are transferred from the surface to the atmosphere as sensible heat • Latent Heat • Energy required to induce changes of state in a substance • In atmospheric processes, invariably involves water • When water is present, latent heat of evaporation redirects some energy which would be used for sensible heat • Wet environments are cooler relative to their insolation amounts

22. Latent heat of evaporation is stored in water vapor • Released as latent heat of condensation when that change of state is induced • Globally, 21 units of energy are transferred to the atmosphere as latent heat Heat content of substances

23. Net Radiation and Temperature • Earth’s radiation balance is a function of an incoming and outgoing radiation equilibrium • If parameters were changed, a new equilibrium would be achieved • Balances occur on an annual global scale and diurnally over local spatial scales • Best exemplified by examining radiation and temperature over a cloudless day • Coolest temperatures just after dawn as outgoing radiation is maximized and incoming radiation is weak • As solar declination improves, temperatures increase but maximum solar angle (noon) and max temperatures (2-4 pm) are offset • Diurnal temperature lag caused by afternoon to evening energy surpluses and slow energy transfer mechanisms of conduction, convection, and latent heat • Similar to larger-scale, hemispheric seasonal temperature lags

24. Latitudinal Variations • Radiation equilibrium varies by latitude • Areas between 38oN and S run net energy surpluses • Areas poleward of 38o experience net energy deficits • The margin between net gains and net deficits migrates seasonally • Summer hemispheres • Net energy gains occur poleward of about 15o • Winter hemispheres • Net energy deficits occur poleward of about 15o • Latitudinal imbalances are neutralized through mean horizontal mass advection • This occurs as energy balances create pressure inequalities which result in winds and currents which transport energy latitudinally

25. Annual average net radiation Ocean circulation

26. The Greenhouse Effect • Trapping terrestrial radiation by certain atmospheric gases • Moniker stems from greenhouses which allow solar radiation to enter through glass panes but trap outgoing heat energy • Earth is different from a true greenhouse as greenhouses simply stem heat loss by preventing convection • Without atmospheric gases (namely H2O, CO2, and CH4) trapping outgoing terrestrial radiation, average Earth temperatures would be about -18oC (0oF) • Increases in greenhouse gas concentrations through human activities may lead to future climatic changes A greenhouse stems convection

27. Global Temperature Distributions • There is a general decline of temperatures with increases in latitude • There is a stronger (weaker) latitudinal thermal contrast in the winter (summer) hemisphere • Summertime poleward and wintertime equatorward shift in isotherms over continents • Documents the quick heating and cooling of land surfaces as opposed to water surfaces • Northern hemisphere thermal gradients are more pronounced than in the southern hemisphere due to greater landmass

30. Influences on Temperature • Latitude • Mean annual temperatures decrease with increasing latitude • Due to lower solar angles, and axial tilt influences • Low latitudes enjoy high solar angles and similar lengths of day all year • With increasingly high latitude comes exacerbated insolation extremes as day length shifts seasonally • Altitude • Tropospheric temperatures typically decrease with altitude • Energy enters atmosphere from the surface • Temperatures at high altitudes remain fairly constant • Air at high elevations (but near a surface) undergo more rapid diurnal temperature fluxes than air at lower elevations

32. Atmospheric Circulation • Latitudinal temperature, and pressure, differences cause large-scale horizontal energy transport through advection • Also influences latitudinal moisture regimes and cloud cover which then impact temperatures • Contrasts between Land and Water • Surface composition influences atmospheric heating • Water bodies heat slower than land given similar insolation • Continentality is the exacerbation of seasonal temperature extremes experienced by continental interiors • Maritime locations experience more moderate seasonal temperature extremes due to presence of water bodies which change temperature very slowly • Water heats less due to a higher specific heat, transparency, evaporative cooling, and horizontal and vertical mixing factors

33. Warm and Cold Ocean Currents • Due to ocean-atmospheric circulation coupling, western ocean basins have warm ocean currents while eastern basins maintain cold currents • Coastal air temperatures are affected accordingly • West coast mid-latitude locations are more moderate than east coast locations • Local Impacts on Temperature • Small spatial scale features impact temperatures • Equatorward facing slopes heat more quickly than poleward slopes, forest regions reduce surface insolation during the day and trap radiation at night leading to cooler daytime and warmer nighttime temperatures

34. South-facing slopes are typically more vegetated than north-facing slopes Vegetation reduces surface radiation during the day and traps it at night

35. Measurement of Temperature • Either mercury or alcohol based thermometers are used • A maximum thermometer is used to record daily temperature maximums while a minimum thermometer records minimums • Thermistors are fast response temperature recording devices based on resistance to electrical current • Used mainly in radiosondes • Instrument Shelters • Vented Weather shelters are necessary to accurately gauge temperatures • Painted white to reduce direct insolation gains through higher albedo • Must be 5 ft from a vegetated surface to reduce laminar layer bias

36. Temperature Means and Ranges • Standard averaging procedures used to obtain means • Daily means = average of daily maximum and minimum temperature • Surface heating occurs rapidly in late afternoon, the method induces a bias towards higher daily temperatures over averaging 24 hourly observations • Daily temperature range = daily maximum - daily minimum • Monthly mean = sum of all daily means/number of days • Annual mean = sum of all monthly means/number of months Continuous temperature plot. Note, short period of high temperatures which skew daily averages toward temperatures higher than actually experienced through most of the day

37. Global Extremes • Due to continentality, greatest extreme temperatures occur at continental interiors • World record high = 57oC (137oF) at Azizia, Libya, 1913 • World record low = -89oC (-129oF) Antarctica, 1960

38. Temperature and Human Comfort • Human discomfort due to temperature may be compounded by other weather factors • Wind during cold conditions causes a body to lose heat more quickly • The Wind Chill Temperature Index indicates how cold a particular temperature feels given a certain wind speed • High humidity values cause warm days to feel oppressively hot due to reductions in evaporative power • A Heat Index incorporates the effect of high atmospheric moisture at high temperatures

39. Thermodynamic Diagrams • Thermodynamic diagrams depict the vertical profile of temperature and humidity with height • Extremely important information for forecasting • Enable forecasters to determine the height and thickness of existing clouds and the ease with which air can be mixed vertically • Data obtained from radiosondes carried aloft by weather balloons twice a day at weather stations across the globe • Stuve diagrams plot temperatures as a function of pressure levels through the vertical • Plots are extremely useful for forecasting applications

40. End of Chapter 3Understanding Weather and Climate3rd EditionEdward Aguado and James E. Burt