Global Energy Balance: Greenhouse Effect Atmospheric Composition & Structure Physical Causes of Greenhouse Effects Chapter 3: 44 48.
Atmospheric Composition
Why does water vapor vary so much? Saturation amount of H 2 O vapor in air at point of condensation (maximum amount of vapor air can hold ) Saturation vapor pressure increases exponentially with T Warmer air has a LOT more water vapor than cooler air Arctic: T ~ 0 C (273 K) Tropics: T ~ 30 C (303 K)
Atmospheric Structure Picture of earth edge taken from the Shuttle over Brazil at sunset
Physical Characteristics Pressure Temperature
Atmospheric Pressure The force per unit area exerted by atmospheric gases or the weight of the atmosphere above given point per unit area. Pa (Pascal), hpa (100 Pa), mb (millibar): 1mb = 1hPa The atmospheric pressure at sea level is 1 atm = 1013 mb = 1013 hpa (i.e., ~15 lb/in 2 )
vacuum h Measuring Atmospheric Pressure: P atm at pt A and B is the same. A B Height of fluid related to balance between gravity and P atm An instrument used to measure atmospheric pressure is called a barometer.
Barometric Law: Atmospheric pressure decreases by about a factor of 10 for every 16-km increase in altitude. e.g., at sea level 1013 hpa 16 km 100 hpa 32 km 10 hpa 48 km 1 hpa
Atmospheric Temperature Lapse rate: Rate at which temperature changes with height in the atmosphere which is a measure of vertical stability
Heat transfer How does heat transfer from one body to another (think earth surface and atmosphere)? Radiation: Emission and absorption Conduction: We call this sensible heat ( sense as in touch ) -- results in warm air near surface Evaporation: We call this latent heat ( latent in the sense that air doesn t heat immediately, but only later when condensation occurs) Convection: Heat transfer through the fluid motion.
Radiation exchange PERSON = 310 K T FIRE = 1500 K
Lapse Rate Stable warm fluid on top of cold fluid Unstable warm fluid below cold fluid
Weather (clouds, rain, etc) happen in troposphere because the troposphere is unstable Why is the troposphere unstable? Because temperature decreases with height But why does temperature decrease with height? Because troposphere is heated from below But how does it happen to be heated from below? Because the atmosphere is nearly transparent to solar radiation!
Solar radiation distribution
Troposphere Troposphere is the layer where the temperature decreases with height, which ranges from the surface to 10-15 km. Troposphere is the layer where weather occurs. The solar radiation heats the surface and the heat is then transported upward into the troposphere by (1) radiation, (2) convection, (3) conduction (i.e., sensible heat), and (4) latent heat. The troposphere is well mixed by convection.
Stratosphere Stratosphere is the layer above the troposphere from about 10-15 km to 50 km. The stratosphere contains most of Earth s ozone. The stratosphere is stable.
Physical Causes of the Greenhouse Effect The greenhouse effect has been attributed to the presence of greenhouse gases, including H 2 O, CO 2, O 3, CH 4, N 2 O, CFC, The defining properties of a greenhouse gas is its ability to absorb/emit infrared radiation. Why do some gases contribute to the greenhouse effect whereas others, such as O 2 and N 2 do not?
The answer lies in our analogy to charges on springs interacting with EM radiation Infrared (IR) radiation carries enough energy to make molecules vibrate and rotate. IR absorption: Make the molecules rotate/vibrate faster IR emission: Slow down the rotation/vibration of the molecules
But a molecule is only radiatively active in the infrared when the positive and negative charges are separated. N 2 : Symmetric structure No separation of positive and negative charge
CO 2 : Symmetric structure and no rotational band Separation of positive and negative charges when vibrating in certain way (e.g., Bending mode)
H 2 O (similar for O 3 ): Asymmetric structure with permanent separation of positive and negative charges. Have both rotational and vibrational bands
Emission Spectrum Taken From Space Emission from cold atmosphere and warm surface Atmospheric Window Spectrum taken over Niger valley, N Africa
Addition of a GHG Absorbing at 11 µm 1. Initial state
Addition of a GHG Absorbing at 11 µm 2. Emission at 11 µm decreases (cold atmosphere)
Addition of a GHG Absorbing at 11 µm 3. New equilibrium: total emission must be same emission at other λ s must increase Earth surface must heat!
Misconceptions (8) Because water vapor absorbs the larger fraction of OLR, reducing CO 2 concentrations will not reduce the Greenhouse Effect.
Summary Atmospheric structure Atmospheric composition and greenhouse gases; barometric law; temperature profile; troposphere and stratosphere; heat transfer (4 ways). Cause of greenhouse effect Separation of positive & negative charges.