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Lecture_2 - Metr 110 Introduction to Meteorology Lecture 2...

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Unformatted text preview: Metr 110 Introduction to Meteorology Lecture 2 1/23/08 Vertical Structure of the Atmosphere Air molecules are held to the Earth by gravity The combined weight of the air molecules can be measured as air (barometric) pressure Atmospheric pressure always decreases with increasing height Standard pressure is 14.7 psi =1013.25 millibars (mb) = 29.92 inches of mercury within a barometer. One may estimate the mass of the atmosphere by distributing mean sea-level pressure over the surface area of the Earth, accounting for the force of gravity, yielding 5.18 x 10 18 kg ( a billion billion!) Air density/pressure vs. altitude Layers of the atmosphere 4 primary heating sources Infrared emission by the ground and release of LATENT heat by clouds Dissociation of ozone by Ultra-violet (UV) radiation about 40 km above surface in stratosphere Dissociation of molecules absorbing extreme UV at the top of the atmosphere (Oxygen, Nitrogen, Nitric Oxide) (100km-1000km) define lapse r ate as the rate of change of temperature with increasing height Temperature naturally decreases with height according to the universal gas law P V= n R T let n be constant , where R is a gas constant, then the product [ Pressure * Volume ] is proportional to Temperature. This decrease is averages about 6.5 degrees Celsius per km and is The degree of cooling/warming with height determines stability Lapse rates of the Atmosphere Characteristics of the atmosphere The atmospheric layer closest to Earth is called the troposphere Lapse rate is negative, temperature generally cools with height Occasionally the temperature warms with height, this is called an inversion Vertical motion is suppressed within an inversion (said to be statically stable) The stratosphere is statically stable (positive lapse rate) little vertical motion The inversion between the troposphere and stratosphere is termed the tropopause Virtually all the weather takes place within the troposphere where (>99% of the water vapor is located) Atmosphere absorbs potentially damaging solar radiation in thermosphere, mesosphere and stratosphere One of the byproducts is heat Heat Measured by temperature which actually measures the average speed of air molecules Different methods using expansion of liquids or temperature dependent resistance of metals Fahrenheit scale 32 F water freezes, 212 F water boils (at sea level) (~180 increments between them) Celsius scale : 0 C is the freezing point ,100 C as the boiling point (100 increments) ***** To convert F to C use (1.8*C)+32=F ****** Kelvin scale uses absolute zero as it reference point The point at which all molecular motion ceases to exist (never been reached in the lab (cryospheric science) Defines 273 K as the freezing point Uses the Celsius increment K = 273 + C Temperature index comparison Definition: A "parcel" of air is a small representative volume of air (about the size of this classroom) whose properties will change uniformly upon its motion Heat Transfer 3 methods convection, conduction and radiation Conduction Air is a poor conductor, therefore conduction is only important in lowest few centimeters above the surface Hot air rises (buoyancy), cool air sinks and replaces it enhanced by steep lapse rates Ubiquitous within 2km of surface and under certain conditions between 2km and 11km Convection Convection "RISING PARCELS" are warmer/more buoyant than their environment Atmospheric convection & convection beneath the Earths surface (left) both owe their existence to density variations.. Cool matter is more dense and tends to sink Convective clouds form in environments that have steep lapse rates Let's take a 10 minute break Radiation spectrum Wiens displacement law The peak wavelength of emitted electromagnetic radiation is proportional to its temperature of the surface which is emitting (high temperature = short wavelength) (max) = 2897/ T where T is in Kelvin and is in micrometers Longwave vs. Shortwave Emission Amplitudes are normalized Actual amplitudes Factors affecting incoming shortwave energy Atmospheric turbidity (an all emcompassing term) any process that involves a reduction in the transparency to shortwave radiation (i.e cloud cover, absorption, scattering, pollution, aerosol) Zenith and declination angle of the sun Slight variation of the solar constant (~0.1%) but close to 10% for Ultraviolet (UV) radiation The variability in UV could potentially influence climate possibly altering the stratospheric heating by ozone (20-40km above sfc) Example of atmospheric turbidity Longwave radiation Mainly infrared Longwave radiation has 2 components; outbound and inbound or down-welled radiation (linked to greenhouse effect) Kirchoff's law good absorbers are good emitters at a given wavelength Mainly H2O, CO2, N2O, and CH4 (GREENHOUSE GASES) Boundary layer contains the highest concentrations of these gases Difficult to indirectly calculate, best measured with a radiometer Radiation balance at surface Define "albedo" Albedo degree of reflectance = amount reflected / total amount Function of wavelength and surface type Global average is 30%, sum of cloud cover, and reflection by various surfaces Forest -.1 to .2 Short grass -.26 Snow -.4 to .9 (fresh) Ocean - .03 to .1 but up to 1.00 at low zenith angles Dry sand -.35 to .45 Concrete - .17 to .27 At low zenith angles nearly all the sunlight is reflected off of a water surface Shortwave radiation budget Earth-Atmosphere energy balance ...
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