Energy: The Fuel for the Atmosphere (Text Pg 25-42, Pg 52-57) Learning Outcomes: Describe how energy is transferred as heat in the atmosphere. Describe the factors involved in the balance of energy and how this determines the temperature. ES 304

Energy Energy: conserved, Kinetic: energy of motion or Potential: energy not yet used) can be transferred Heat: energy transfer from one object to another due to the difference in temp between them. Direction: high energy to low energy (hot to cold) Sun: energy source Earth: uneven energy distribution (poles vs equator): transfer energy via ocean and atmospheric transport – Wx and currents ES 304

Temperature and Density average speed of atoms and molecules (Figure 2.1) Celsius: 0 (water freeze pt), 100 C (boiling pt) Absolute: Kelvin (0 K) K = C + 273 warm air, less dense (perfect Gas) cold air, more dense ES 304

Energy Storage: Heat Capacity (pg 70) different substances: different capacities to store energy Specific Heat (energy required to raise the temp of 1 g of substance 1 C) Rock = 0.2 (cal/g x C) Water = 1.0 (cal/g x C) water requires 5 times more energy input than rock to raise its temperature 1 C. to cool 1 C, water requires 5 times more energy lost to the surroundings than rock water/land interface: large temperature differences for same energy input or loss moderating effect of water body on climate. ES 304

Heat Transfer in Atmosphere Direction: Hot to cold Rate of Transfer: depends on temp difference – rate greater for greater difference Conduction : Heat transfer by direct contact of air molecules still air: poor conductor, water 30x better, silver 18,000x better Convection : Heat transfer vertically by vertical movement of warm air (less dense: rises) (Advection): horizontal transport of heat and other properties by wind Radiation : Heat transfer via Electromagnetic Energy ES 304

Convection: Thermal Cell (Figure 2.6) Convective circulation or thermal cell ES 304

Other Heating/Cooling Processes: Phase Change of Water – Latent Heat Latent Heat: heat energy required to change a substance, such as water, from one state to another “Latent” : hidden heat (cannot be sensed) vs. Sensible Heat (heat that can be sensed: thermometer) heat energy is required or released when H2O undergoes a phase change (ice, liquid, vapour) example: evaporation: requires heat energy (600 cal/g): taken from surroundings (if available) becomes latent (or locked up in water vapour molecules): surrounding air cools example: condensation: heat energy latent in water vapour now released to surroundings (600 cal/g): latent heat appears as sensible heat (can be sensed): surrounding air warms ES 304

Heat energy absorbed and released (Figure 2 Heat energy absorbed and released (Figure 2.3) Note terms used to describe phase change Figure 2.4 Latent Heat release with condensation: important in cloud vertical development (inside of cloud warmer than surroundings See video: http://earthobservatory.nasa.gov/Features/GISSTemperature/Images/seviri_water_vapor_720p_best.mov ES 304

Other Heating/Cooling Processes: Expansion/Compression (Figure 1 pg 33) Drop in pressure with height means that as: Parcel of air rises, it expands and cools (why?) Parcel of air sinks, it compresses and warms (why?). ES 304

Summary Energy: fuel for the atmosphere Heat: energy transferred from one object to another due to the temp difference between them: convection (important), and radiation (important) conduction (less important) Rate of transfer depends on temperature difference Other processes involved in heating/cooling air: compression/expansion (important) water phase change (important) ES 304

Radiation – A Closer Look sun’s energy transfer; radiation though electromagnetic (EM) waves of various wavelengths wavelengths measured in microns (m): 1/1000 th of a millimeter radiation from sun, 90% wavelengths are < 1.5 m sun’s radiation wavelength spectrum: UV: UltraViolet < 0.4 m Visible (0.4 - 0.7 m) what we can see (most radiation from the sun is within this range) IR: InfraRed > 0.7 m ES 304

Radiation: wavelength and energy (Figure 2.8) ES 304

Wien’s Law: max = constant / T Radiation: Three Laws Anything with temperature (T > 0 K) emits radiation (E) in the form of electromagnetic waves. The greater the temperature of an object, the shorter the wavelengths, , emitted. Also the peak emission of radiation shifts toward shorter wavelengths Wien’s Law: max = constant / T where max is the wavelength which maximum radiation occurs The greater the temperature of an object, the more energy is radiated (over a given surface area) per second. Stefan Bolzmann Law: E =  T4 ES 304

Sun’s Electromagnetic Spectrum (Figure 2.9) Sun and Earth Radiation Spectrum (Figure 2.10) “Shortwave” Radiant energy from the Sun. “Longwave” Radiant energy from the Earth (IR part of the spectrum) ES 304

What Happens to the Incoming (Shortwave) Radiation? Absorbed: (ground, clouds, atmosphere: warms) Scattered (Diffuse): all directions (clouds, atmosphere) Reflected: back to where it came from (clouds, ground, atmosphere) Albedo: reflectivity of surface for short wave radiation (mirror 100%, snow 75-80%, thick clouds 60-90%) depends on sun angle and surface (see Table 2-2 in Text) Transmitted (Direct): passes through ES 304

Summary Radiation in the form of waves of different wavelengths (radiation spectrum) Anything with temperature emits radiation The higher the temperature: the more energy radiated the shorter the wavelengths Radiation from sun: shortwave (most energy < 0.7 m: UV and visible) Radiation from earth: longwave (IR) Incoming radiation can be absorbed, scattered, reflected (albedo) and transmitted ES 304

Emission, Absorption and Equillibrium Temperature Object radiates more energy than it absorbs: cools Object absorbs more energy than it radiates: warms Object radiates = absorbs: temp constant (radiative equillibrium) why doesn’t earth continue to warm? Blackbody: absorbs all incoming radiation and emits max radiation possible for its temperature Assume Earth a blackbody: equillibrium temp (solar radiation absorbed = radiated IR or “emitted”) = -18 C actual earth ave temp = 15 C (why?) ES 304

The Atmosphere: a Selective Absorber of Radiation Atmosphere (mix of gases): each gas is selective in the wavelengths of radiation it absorbs: water vapor and CO2 do not absorb incoming shortwave (both UV and visible) absorb most outgoing longwave (IR) O3 absorbs most incoming UV (but not visible) does not absorb much outgoing longwave (IR) ES 304

The Atmosphere: A Selective Absorber of Radiation Most incoming radiation (visible and a portion of UV) reaches the surface and is absorbed (warms surface) Earth emits IR which is absorbed by the atmosphere except for wavelengths between 8 – 11 m IR in this wavelength band escapes into space: “Atmospheric Window” absorbed IR by the atmosphere results in warming of atmosphere ES 304

Radiation Absorption by Gases (Figure 2 Radiation Absorption by Gases (Figure 2.11) shaded area is the % of radiation at a particular wavelength absorbed by the gas ES 304

Radiative Properties of Clouds Clouds (tiny liquid droplets) – especially low thick ones: reflect incoming shortwave, good absorber of IR (even absorb IR between 8 – 11 m, clouds close the Atmospheric Window) good emitters of IR upward from top and downward to earth from bottom. During daytime, why are cloudy days cooler? During night (esp during winter), why are cloudy, calm nights warmer than clear nights? ES 304

Atmospheric Greenhouse Effect – Natural and Enhanced water vapour, CO2 (and other trace gases) absorb much of the outgoing IR: atmosphere warms: warm atmosphere radiates energy (IR) back to the earth, which is absorbed: surface warms if no absorption of outgoing IR by these gases, the temp would -18 C (vs current ave temp 15 C the Natural Greenhouse Effect ) Enhancement of the Greenhouse Effect: more Greenhouse Gases (water vapor, CO2, methane, nitrous oxide, CFC’s) → more absorption of outgoing IR (warmer atmosphere), and more IR radiated to the earth and absorbed (warmer surface). More GHG’s?: warmer atmosphere and surface See http://earthobservatory.nasa.gov/Features/WorldOfChange/decadaltemp.php?src=eoa-features ES 304

Figure 2.12 Atmosphere without and with Greenhouse Effect ES 304

Global Climate Change: Feedback Loops Positive Feedback: reinforce effects of initial signal Example: GHG , T , evap , water vapor , water vapor absorbs IR, T of atmos/earth , evap  and so on. Negative Feedback: suppress effects of initial signal Example: GHG , T , evap , water vapor , clouds , more incoming solar radiation reflected back into space, T , evap , water vapor , T of atmos/earth  Positive and Negative Feedback loops on going, simultaneously ES 304

Warming the Lower Atmosphere Air from Below (Fig 2.13) Figure 2.17 How is the Incoming Energy Partitioned ES 304

Figure 2.17 How is the Incoming Energy Partitioned? ES 304

FIGURE 2. 18 The earth-atmosphere energy balance FIGURE 2.18 The earth-atmosphere energy balance. Numbers represent approximations based on surface observations and satellite data. While the actual value of each process may vary by several percent, it is the relative size of the numbers that is important. Figure 2.18: The earth-atmosphere energy balance. Numbers represent approximations based on surface observations and satellite data. While the actual value of each process may vary by several percent, it is the relative size of the numbers that is important.

Summary Incoming shortwave radiation: reflected, absorbed, scattered (diffuse), transmitted (direct). Atmosphere is transparent to most incoming shortwave – reaches the surface CO2 and water vapor: ignore incoming but selectively absorb IR emitted by the earth emit IR back to earth this keeps the earth/atmosphere temp warm increasing GHG’s enhance the greenhouse effect: net effect due to a series of + and - feedback loops. Temperature of air and earth a result of a delicate balance of energy partitioning ES 304

Daily Temperature Variation: Near Surface (Text Pg 56 –62) Atmosphere is not a blackbody: absorbs some wavelengths of radiation and is transparent to others (“selective absorber”) Earth is lie a blackbody: much better absorber and emitter of radiation than atmosphere ES 304

Daily Temperature Variation: Near Surface Day Time Earth (blackbody): absorbs all incoming wavelengths (except that which is reflected) vs. atmosphere absorbs only part of UV surface much warmer than air above. air temp drops rapidly with height within a few meters of the surface. Clouds: reflect incoming (shortwave) back to space, daytime surface temps lower with clouds ES 304

Daily Temperature Variation: Near Surface Night Earth (blackbody): emits IR efficiently vs. air (not efficient IR emitter) surface cools more rapidly than air above under clear skies inversion – warm air over cold (air temp increases with height above ground) “radiation inversions” : clear, calm nights Low Clouds: absorb outgoing IR, emit IR back to earth night time surface temps greater (compared to clear nights) no radiation inversion ES 304

Figure 3.3 Air Temperature Changes with Height Near Surface: Clear Night (Radiation Temperature Inversion) Figure 3.2 Daily Temperature Variation Figure 3.3: On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion. Stepped Art Fig. 3-3, p. 62

Surface Temperature – Daily (Diurnal) Variation Max surface temp mid to late p.m. incoming shortwave radiation max at noon, then falls with lower sun angle but still exceeds outgoing IR radiation Temp still increases Temp drops when outgoing exceeds incoming Min surface temp just after sunrise outgoing radiation all night (no incoming), Temp drops until just after sunrise when incoming exceeds outgoing ES 304