Composition and Structure of the Atmosphere - Forsiden · Composition and Structure of the Atmosphere Chapter 1 Lecture Redina L. Herman Western Illinois University Understanding

© 2015 Pearson Education, Inc.

Composition

and Structure

of the Atmosphere

Chapter 1 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo



• Although the density of the atmosphere decreases rapidly with height, the top of the atmosphere is

undefined.

• Overall, the majority of atmospheric mass is contained in a thin layer near the surface.

• The atmosphere still contains an impressive sum of

mass.

Thickness of the Atmosphere


• The atmosphere is a mixture of gases, suspended particles, and liquid/solid water.

• Gases and particles are readily exchanged between

the Earth’s surface and the atmosphere through physical (volcanic) and biological (photosynthesis,

respiration) processes.

• These gases are categorized in two types:

permanent and variable gases.

Factors Affecting the Composition and

Vertical Structure of the Atmosphere


• The Permanent Gases

Composition of the Atmosphere


• Variable Gases






• Variable Gases: Water Vapor

– Water vapor is the most abundant variable gas, as it is

added or removed from atmosphere through the

hydrologic cycle.

– Concentrations exist from nearly 0 percent over desert and

polar regions to nearly 4 percent near tropics.

– Water vapor is a contributor to Earth’s energy balance and

many important atmospheric processes.



• Variable Gases: Water Vapor

– Water vapor image

showing broader

distribution of moisture

than the image of actual

clouds (below).



• Variable Gases: Carbon Dioxide

– Carbon dioxide (CO2) is a trace gas accounting for

0.039 percent of the atmosphere’s volume.

– CO2 is important to Earth’s energy balance.

– CO2 is added through biologic respiration, volcanic activity,

decay, and natural and human-related combustion.

– CO2 is removed through photosynthesis, the process of

plants converting light energy to chemical energy.



• Variable Gases: Carbon Dioxide


The steady increase of atmospheric CO2.




• Variable Gases: Ozone


– Ozone is the tri-atomic form of oxygen and is essential to life

on Earth.

– Ozone near the surface is a pollutant but in the stratosphere

is an essential absorber of ultraviolet radiation.

– Chlorofluorocarbons (CFCs), specifically chlorine atoms,

react with ozone in the stratosphere and destroy ozone.

– Ozone destruction peaks over the southern hemisphere and

persists through spring.


• Variable Gases: Ozone

– Images showing reduction of ozone over Antarctica over time.



• Variable Gases: Methane

– Methane is a variable gas in small but recently increasing

concentrations.

– Released to the atmosphere through fossil-fuel activities,

livestock digestion, and agriculture cultivation (esp. rice).

– Methane works as a very effective absorber of terrestrial

radiation and plays an active role in near-surface warming.


Annual increases in atmospheric methane.



• Aerosols

– Aerosols are any solid and/or liquid particles, other than

water, which exist in the atmosphere (synonymous with

the term particulate).

– Aerosols are both natural (sea spray, dust, combustion)

and human (combustion) produced products.

– Due to their small sizes, they can easily remain in

suspension for longer periods.

– Aerosols contribute to precipitation processes as

condensation nuclei.



• The atmosphere is a mixture of gas molecules, small suspended particles of solid and liquid, and

falling precipitation.

• Meteorology is the study of the atmosphere and the processes that cause what we refer to as “weather.”

• Weather refers to the short-term phenomena.

• Climate refers to the long-term patterns.

• Climatology is the long-term study of the atmosphere and the processes that cause what we

refer to as “climate.”

The Atmosphere, Weather, and Climate


• Density

– Density is mass (kg) per unit

volume (m3).

– Due to compressibility, near-

surface air is more dense than that

above.

– This may be expressed in terms of

the mean free path, or average

distance a molecule travels before

colliding with another molecule.

Atmospheric Density and Pressure

Due to compressibility, atmospheric

mass gradually “thins out” with height.

less overlying

weight

more overlying

weight


• Pressure

– Pressure is a result of the atmosphere’s mass.

– Pressure decreases vertically but not at a constant rate.

Atmospheric Density and Pressure


• Layering Based on Temperature Profiles

• Thermal Layers of the Atmosphere

– Four distinct layers of the atmosphere emerge from

identifiable temperature characteristics with height.

Layers of the Atmosphere


• Troposphere

– The troposphere is the lowest layer, named so as this region

promotes atmospheric overturning.

– Layer of virtually all weather processes.

– Warmed at the surface by solar radiation.

– Identified by a steady temperature decrease with height and as

the thinnest layer (but contains 80 percent of the mass).

– Due to thermal expansion, the tropopause is roughly 16 km

over the tropics but only 8 km at poles.



• Troposphere

Most clouds exist in the troposphere. Occasionally, violent updrafts penetrate

cloud tops into the stratosphere. The flattened top of this cumulonimbus cloud is

in the stratosphere.



• Stratosphere

– The stratosphere is an area of little weather (“stratified”),

and the temperature increases with height.

– Inversion caused by the absorption of ultraviolet radiation

by ozone.

– Although the ozone layer exists through an altitude

between

20 and 30 km (12–18 mi), actual concentration of ozone

can be as low as 10 ppm.



• Mesosphere and Thermosphere

– Mesosphere is characterized by decreasing temperatures with

height and is the coldest atmospheric layer.

– Thermosphere, the upper most layer, slowly merges with

interplanetary space and is characterized by increasing

temperatures with height.

– Temperatures approach 1500oC, however, this only measures

molecular kinetic energy as the sparse amount of mass produces

actual heat content.

– Combining the two layers account for only 0.1 percent of total

atmospheric mass.



• A Layer Based on Electrical Properties: The Ionosphere

– Located within the meso- and thermospheres, the ionosphere

is supplied with ions, electrically charged particles.

– Interactions between the ionosphere and subatomic particles

emitted from the Sun excite atmospheric gases causing the

aurora borealis (northern lights) and the aurora australis

(southern lights)



• Earth’s First Atmosphere

– The early atmosphere was likely composed of hydrogen

and helium.

– Light gases either exited the atmosphere to space through

high escape velocities or through collisions with large

celestial bodies.

Origin and Evolution of the Atmosphere


• Earth’s Second Atmosphere

– A secondary atmosphere formed through volcanic

outgassing and by material, largely water, gained from

comets.

– These “cosmic snowballs” might be the source of most of

the world’s water vapor.



• Further Evolution of the Atmosphere

– Precipitation removed, and continues to remove, excess

water vapor, while high concentrations of CO2 were

replaced by oxygen through photosynthesis and

dissolution in water.

– Due to its stability, nitrogen concentrations slowly grew to

present-day levels.



• Atmospheric Pressure and Wind

– Gas molecules are constantly in motion.

– These molecules exert a pressure (force per unit area)

when they strike a surface.

– Molecules move in all directions, so pressure is exerted in

all directions.

– Air tends to:

• blow away from regions of high pressure to low pressure

(wind).

• rise in areas of low surface pressure and sink in high

pressure. – rising motions favor cloud formation, while sinking promotes

clear skies.

Some Weather Basics


• Mapping Air Pressure

– Pressure equals force/area

– SI Unit: 1Pa = 1 Nm-2

– Atmospheric pressure is plotted on maps using lines,

called isobars

– Pressure often measured using hektopascals

– 1 millibar (mb) = 1 hektopascal (hPa)

Some Weather Basics



• Temperature

– Temperature varies in time and location.

– Drastic temperature differences are usually caused by

fronts.

• Cold fronts

• Warm fronts

• Occluded fronts

– Precipitation is common along fronts, so when a front

approaches, it is not unusual to have rain or snow in

addition to a change in temperature.

Some Weather Basics


• Humidity

• Humidity is the term used to describe the amount of water

vapor present in the atmosphere. Relative humidity: the amount of water vapor present in

relation to the maximum for a given temperature. It is

reported in percentages.

Dew point: the higher the dew point the greater amount of

water vapor in the air.

Some Weather Basics


• Meteorology is the science that studies the atmosphere.

• Weather forecasting is both an art and a science.

Forecasters use computer modeling (science) and their own subjective opinion and knowledge (art).

• There is no precise routine by which a weather forecast is created.

A Brief History of Meteorology


FS file established from:

• ppt lectures

• ppt figures labeled

• ppt clickers

Documents

Composition and Structure of the Atmosphere - Forsiden · Composition and Structure of the Atmosphere Chapter 1 Lecture Redina L. Herman Western Illinois University Understanding