Physical Environment:
Atmosphere Composition and Structure

EVPP 110 Lecture

Fall 2003

Dr. Largen

• Atmosphere
• Composition
• Vertical structure
• Heat transfer
• Atmospheric moisture
• Atmospheric circulation
• Weather and climate

• Earth’s atmosphere

• solar system
• universe
• thin, blanket of air
• presence contributes to physical characteristics
• determines scope of life
• central in physical geography
• heat transfer
• water vapor transport
• weather and climate

• Composition
• Gaseous envelope
• mixture of
• Various gases
• Water vapor and ice crystals
• aerosols

• mixture of gases
• two gases = 99% of volume
• nitrogen (N₂) = 78%
• oxygen (O₂) = 21%
• percentages fairly constant up ~ 80km
• water vapor (H₂O)
• concentrations vary greatly
• vertically and horizontally
• carbon dioxide (CO₂)
• small percent of volume
• ~0.036%
• varies vertically and horizontally
• increasing since industrial revolution

• others
• small amounts
• methane
• nitrous oxide
• chlorofluorocarbons
• sulfur dioxide
• ozone

• Vertical structure of atmosphere
• several characteristics vary altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties
• Vertical structure
• air density and air pressure
• air density
• determined by
• masses of component atoms and molecules
• amount of space between them
• density
• measure of amount of matter in a given volume
• greatest near earth’s surface
• more atoms and molecules in given volume of air
• air pressure or atmospheric pressure
• force exerted by constantly moving air molecules
• air molecules are matter
• occupy space and have mass
• pressure - measured in terms of
• total mass of air above any point
• average or standard atmospheric pressure at sea level is ~14.7 pounds/in²
• 1013.25 mb (millibars)
• 29.92 in. Hg (inches of mercury)
• more air molecules in same column
• = more dense air
• = higher pressure
• fewer air molecules
• = less dense
• = lower pressure
• number of air molecules decreases with increases in altitude
• density decreases with increased altitude
• pressure decreases with increased altitude

• initially decrease rapidly with altitude
• air near earth’s surface is compressed
• ~ 10 mb/100 m increase in altitude
• decrease more slowly with altitude
• 1013 mb at 0 km
• 500 mb at ~ 5.5 km
• 100 mb at ~17.0 km
• 50 mb at ~22.0 km
• 10 mb at ~32.0 km

• air temperature
• vertical profile more complicated than density and pressure

• measure of "heat" of air
• measure of average speed of movement of air molecules
• air temperature
• normally decreases from earth’s surface up to altitude of ~11 km
• atmosphere is heated primarily from below
• transfer of heat energy from surface
• lapse rate
• rate at which air temperature decreases with altitude
• average or standard in lower region of atmosphere is ~6.5°C per 1000 m

• occasionally, temperature may increase with increases in altitude in lower region of atmosphere
• = temperature inversion

• air temperature
• most common parameter used to define atmospheric layers
• troposphere
• stratosphere
• mesosphere
• thermosphere

• troposphere
• from 0 km to ~ 11 km
• characterized by air temperature decreasing with height
• region in which "weather" occurs
• ends at point where temperature stops decreasing with height
• boundary called tropopause

• stratosphere
• from ~11 km to 20 km
• air temperature remains constant with altitude
• from ~20 km to ~50 km
• air temperature increases with altitude
• from ~ - 50°C to ~0°C
• results in temperature inversion
• temperature inversion attributed to ozone
• reaches maximum concentrations in stratosphere
• absorbs energetic UV solar energy
• some of absorbed energy heats stratosphere

• mesosphere
• separated from stratosphere by boundary called stratopause
• from ~50 km to ~85km
• % of N and O ~ same at this level as at sea level
• but much less air
• atmospheric pressure is ~1 mb at 50 km

• mesosphere
• air temperature decreases with altitude
• little ozone to absorb solar radiation
• molecules of air lose more energy than they are able to absorb
• cooling continues up to ~ 85 km
• where temperature reaches its lowest average value, ~ - 90°C

• thermosphere
• separated from mesosphere by boundary called mesopause
• from ~85 km to several hundred km
• temperature increases with altitude
• oxygen molecules absorb solar radiation
• actual temperature varies greatly depending on solar activity

• exosphere
• upper limit of atmosphere
• top of thermosphere
• altitude of ~500 km
• some atoms and molecules from this region
• escape earth’s gravitational pull
• shoot off into space

• Vertical structure
• atmospheric characteristics vary with altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• gaseous composition
• homosphere
• region below thermosphere where gaseous composition remains fairly constant
• from 0 km to ~85 km
• well-mixed region
• ~78% N & ~21% O

• heterosphere
• thermosphere & above (>85km)
• not well-mixed
• heavier atoms and molecules, such as N & O, tend to settle to bottom of layer
• lighter gases, H & He, float to top

• Vertical structure of the atmosphere
• several atmospheric characteristics vary with changes in altitude
• air density and air pressure
• temperature
• gaseous composition
• electrical properties

• Vertical structure
• electrical properties
• ionosphere
• above ~60 km
• electrified region
• fairly high concentrations of ions and free electrons
• atoms lose electrons and become
• Heat transfer
• occurs via a process called
• convection
• transfer of heat by mass movement of a fluid (such as water and air)
• takes place because fluids can move freely and it is possible to set up currents within them
• leads to a cycle of heated air rising and cooled air descending
• called convective circulation
• convective circulation
• certain areas of earth’s surface absorb more heat from sun than others areas
• uneven heating of air near surface
• heated air expands and becomes less dense
• expanded , less dense air rises and transfers heat energy upward
• after warmed, expanded, less dense air rises
• cooler, heavier, more dense air flows toward surface to replace rising air
• upon closer exposure to warm surface
• cool air heats up, expands, becomes less dense and rises
• and cycle is repeated
• vertical exchange of heat called convection
• rising air "bubbles" (or masses of warmed air) known as thermals
• warmed air rises, temperature eventually decreases, sinks to surface where it can replace rising air
• producing a convective circulation or thermal cell
• any air that rises will expand and cool
• creates areas of low pressure
• any air that sinks is compressed and warms
• creates areas of high pressure
• wind
• horizontally moving part of circulation

• Atmospheric moisture
• several processes and principles interact to determine the manner in which moisture enters, moves about in, and leaves the atmosphere
• evaporation of water from surface into atmosphere
• transport of water vapor through the atmosphere
• precipitation, return of water to surface

• Atmospheric moisture
• in lower atmosphere, water exists in all three phases of matter
• liquid - water
• gas - water vapor
• solid - ice
• various atmospheric conditions govern change of water from one phase to another

• water changes phases in atmosphere
• sublimation
• changing from solid (ice) to gas (water vapor) phase without passing through the liquid phase
• evaporation
• changing from liquid to vapor phase
• condensation
• changing from vapor to liquid phase

• Evaporation
• water molecules escape surface & enter atmosphere as water vapor
• energy is required
• comes from radiant energy from sun
• escaping molecules carry heat with them
• evaporation = cooling process
• rate of evaporation is affected by amount of moisture already present in a mass of air
• affected by wind, temperature and humidity

• rate is affected by amount of moisture in a mass of air
• amount of water vapor present in a quantity of air can be specified in several ways
• absolute humidity
• relative humidity

• absolute humidity
• most direct measure of air’s moisture content
• weight of water present in given volume of air
• affected by air temperature
• warm air is able to contain more water vapor per unit volume than cool air

• relative humidity
• ratio of amount of water actually present in quantity of air to amount that could be held by same air if it were saturated
• does not indicate actual moisture content
• change in temperature of moist air = change in its relative humidity
• temperature at which relative humidity becomes 100% = dew point

• Transport
• water is transported by atmosphere in form of water vapor

• Precipitation
• means by which water leaves atmosphere and returns to earth’s surface
• can take form of any of phases of water
• liquid - rain
• gas - fog
• solid - ice
• on average, amount of water that leaves atmosphere = amount that enters atmosphere
• several factors govern the process by which water leaves the atmosphere as precipitation
• temperature
• humidity
• dew point
• presence of condensation nuclei

• condensation
• change from water vapor to liquid water
• occurs when
• moist air is cooled to its dew point
• cooler air can’t hold as much water
• some water vapor condenses to liquid
• dust particles in air serve as condensation nuclei
• collection centers for water molecules
• promote growth of water droplets to a size large enough to be stable
• Adiabatic processes
• combine aspects of vertical structure of atmosphere, heat transfer, moisture content and circulation
• adiabatic cooling
• adiabatic heating
• adiabatic cooling
• temperature of rising mass of air decreases
• air mass heated by earth’s surface, rises
• warm, rising air mass expands
• because air pressure decreases with altitude
• as it expands, it does work against surrounding atmosphere
• expenditure of energy causes temperature to decrease
• water vapor condenses to clouds
• adiabatic heating
• temperature of descending air mass increases
• air mass is cooled adiabatically, descends
• cooled, descending air mass compresses
• because air pressure increases with decrease in altitude
• as it compresses, surrounding atmosphere does work against it, pushing inward
• input of energy causes temperature to increase

• lapse rate
• normally, air temp. decreases with altitude
• ~6.5°C per 1000m increase in altitude
• dry adiabatic lapse rate
• adiabatic cooling in absence of condensation
• ~10.0°C per 1000m increase in altitude
• moist adiabatic lapse rate
• adiabatic cooling in presence of condensation
• varies with moisture content of air
• ~5.0°C per 1000m

The End