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Composition/Characterstics of the
• 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
Altitude (km)
50 km
sharp change in temp. and
pressure produce Jet
8 - 16 km
Temperature C
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).
•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
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
T ( z ) = To  z
ambient lapse rate
=6 - 10 C/km
Z (km)
• 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
high pressure
NE Tradewinds
low pressure
SE Tradewinds
high pressure
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
•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.