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Composition/Characterstics of the Atmosphere • 80% Nitrogen, 20% Oxygen- treated as a perfect gas • Lower atmosphere extends up to 50 km. • Lower atmosphere most active part of atmosphere where most of the mass and energy transfer (leading to earth’s weather patterns) occurs. It is divided into two parts distinguished by their temperature distribution -- the troposphere and the stratosphere. • Upper atmosphere doesn’t play much of a role in the determination of weather. Characteristics of the Atmosphere 100 km Thermosphere upper atmosphere Mesopause Mesosphere Altitude (km) Stratopause 50 km Stratosphere lower atmosphere sharp change in temp. and pressure produce Jet Streams 8 - 16 km Tropopause poles Troposphere equator -80 -60 -40 -20 Temperature C 0 20 Characteristics of the Troposphere •Variable thickness (8 km poles, 16 km equator). •Decreasing temperature with elevation (linear). •Well defined pressure gradients (max. pressure at bottom). non-linear •Well defined distribution of moisture and suspended particles (max. at bottom) •Sharp air velocity gradient. At earth’s surface velocity is zero (no-slip condition) and velocity increases over 2700 m thick boundary layer according to a logarithmic velocity profile Temperature Distribution • Temperature distribution follows radiation distribution both in time (i.e. daily and seasonally) and space (i.e. distribution over earth). • Daily distribution- Air temperature rises during day and falls at night following solar radiation. Peak temperature lags peak solar radiation (occurs at noon) by several hours due to heating effects on earth’s back radiation which lags solar radiation. • Clouds attenuate diurnal fluctuations by absorbing incoming and outgoing radiation (due to high heat capacity of water) Temperature Distribution • Seasonal Distribution - Air temperatures also follow cycle of incoming solar radiation. In Northern hemisphere peak temperature (July / August) lags peak radiation (June 22) because of effect of earth’s back radiation. • Lag is more significant near oceans because oceans absorb and distribute heat more efficiently than land masses (higher heat capacity and fluid motion) – near oceans in Northern hemisphere: max / min temperatures in Aug / Feb – inland in Northern hemisphere: max / min temperatures in July / Jan Temperature Distribution • Spatial Distribution- Temperatures follow latitude lines that receive equal solar radiation – Highest temperatures just north/south of equator due to extensive cloud cover in this region (intertropical convergence zone). – Similarly, air over oceans tends to stay warmer in winter/colder in summer than air over land due to high heat capacity of ocean ( i.e. same change in heat energy produces a smaller temperature change for oceans versus land) Temperature Distribution • Troposphere shows well-defined linear relationship of temperature with height above earth surface (in an average sense). T ( z ) = To z ambient lapse rate =6 - 10 C/km Z (km) To T(C) • humid cloudy conditions 6 C/km - saturated adiabatic lapse rate • dry clear conditions 10 C/km - dry adiabatic lapse rate (9.8) Temperature Distribution • Ambient lapse rate dictates the stability or instability of air masses. Air can only rise and thus lead to condensation and precipitation if it is warmer than surrounding air. • Get unusually stable weather (i.e., no precipitation) when have a temperature inversion, i.e. when temperature increases with elevation locally. Air can’t rise -- no rainfall thus pollution problems. • Most likely to happen over continental land masses after calm dry nights with clear skies. Land cools faster than upper air. Pressure Distribution • Daily pressure distribution at sea level is variable and unstable. If look at average pressure distributions over long time periods semipermanent patterns emerge: polar easterlies low pressure westerlies high pressure NE Tradewinds low pressure SE Tradewinds high pressure westerlies polar easterlies low pressure Pressure Distribution These pressure belts migrate northward in June/July and southward in Jan/Feb following solar radiation distribution Horizontal pressure gradients are the driving force for winds. Wind direction and circulation however is also affected by: 1. The rotation of the earth which produces the apparent Coriolis force 2. Friction of lower air masses with earth’s surface. Wind Patterns Net affect of pressure distribution, Coriolis force and friction forces: •Convergent equatorial winds of easterly origin (tradewinds or doldrums). Converge in low pressure belt called intertropical convergence zone, ITCZ cloudy, showery weather. •Prevailing westerly winds at mid-latitudes. Associated with high pressure centers, little precipitation. •Highly variable polar easterly winds (not wellcharacterized). •Poleward circulation of air masses is broken up into 3 cells forming banded structure around the earth.