James T. ShipmanJerry D. WilsonCharles A. Higgins, Jr.

The Atmosphere

Chapter 19

The Atmosphere

• The Roles of the Atmosphere: • A source of our weather

• 80% of our weather occurs in the lowest 20% of the atmosphere

• An abode for life• 78% Nitrogen• 20% Oxygen• 2% trace gases such as water vapor, CO2, and Methane

Intro

The Atmosphere

• The Roles of the Atmosphere: • A shield to harmful charged particles coming at us

from the Sun• The magnetic field shields us from harmful charged particles

• A blanket • The Greenhouse effect distributes heat from warm areas to

colder ones

• An umbrella • The Ozone layer filters out harmful photons such as X-rays,

Gamma-rays, and UV light coming at us from the Sun and space

Intro

Meteorology

• Meteorology – this term is commonly applied to the study of the lower atmosphere• In the past the term meteorology was used to more

generally describe the study of the entire atmosphere

• The conditions of the lower atmosphere are what we call the weather

• In this chapter we will examine the atmosphere’s composition, properties, and mechanisms

Intro

Sec 19.1 Composition and Structure

• The air of our atmosphere is composed of a mixture of gases and holds varying amounts of suspended liquid droplets and solid particles

• Only two gases, nitrogen and oxygen, make up close to 99%, by volume, of air near the Earth• Both of these dominant gases are diatomic - N2 & O2

• Argon (Ar, 0.9%) and carbon dioxide (CO2, 0.03%) are the other major constituents of air

• Very small quantities of many other gases are found in the atmosphere

Section 19.1

Composition of Dry Air

• Dry air is composed primarily of only two constituents, nitrogen (N2) and oxygen (O2)

Note that the CO2 “slice” is shown much larger than scale for clarity

Section 19.1

Composition of Air

Section 19.1

Atmospheric Variations & Equilibrium

• Carbon monoxide (CO) is largely the product of incomplete engine combustion • Therefore, CO is much more concentrated in areas

with numerous automobiles

• In general, though, the relative amounts of the major atmospheric constituents remain reasonably constant

• Nitrogen, oxygen, and carbon dioxide are all involved in life processes of plants and animals• These gases are constantly being extracted and

replenished during various natural processes

Section 19.1

Atmospheric Equilibrium

• In a process called photosynthesis, plants consume CO2 and produce O2 as a waste-product

• Animals, on the other hand, consume O2 and produce CO2 as a waste gas

• Nitrogen is taken in by some plants and released back into the atmosphere during organic decay

Section 19.1

Density of the Atmosphere

• Due to the gravitational attraction between the Earth and the atmosphere, the density of the air is the greatest near the Earth’s surface

• Over half of the atmospheric mass lies below an altitude of 11 km (7 mi), and 99% lies below an altitude of 30 km (19 mi)

• There is no clearly defined upper limit to the Earth’s atmosphere

• The atmospheric concentrations become less and less, and it simply merges into outer space

Section 19.1

Regions in the Atmosphere

• There are several physical properties of the atmosphere that do show distinct vertical divisions that vary with altitude

• These physical properties include temperature and ozone ion concentrations

Section 19.1

Temperature

• Major recognizable divisions within the atmosphere can be distinguished based on vertical temperature variations

• Vertically, Earth’s atmosphere is divided into four temperature regions• Troposphere – ground to about 16 km (10 mi)• Stratosphere – 16 km to about 50 km (10 – 30 mi)• Mesosphere – 50 km to about 80 km (30 – 50 mi)• Thermosphere – 80 km to outer space

Section 19.1

Troposphere

• Troposphere – contains over 80% of the atmospheric mass and virtually all of the clouds and moisture

• Continual mixing and movement occurs here• The lower troposphere is where most of the

weather occurs• Within the troposphere the temperature of the

atmosphere decreases an average of 6.5o C/km with increasing altitude• The top of the troposphere has temperatures of –45o

to –50o C

Section 19.1

Stratosphere

• The stratosphere and troposphere together account for 99.9% of the atmospheric mass

• The temperature increases in a non-uniform manner as one progresses upward through the stratosphere

Section 19.1

Mesosphere & Thermosphere

• Temperatures once again progressively decrease with altitude within the mesosphere

• Above the mesosphere Earth’s atmosphere become exceedingly thin

• This region above the mesosphere is called the thermosphere

• The thin atmosphere in the thermosphere is intensely heated by the sun

• The thermosphere extends to the outer reaches of Earth’s atmosphere

Section 19.1

Vertical Structure of the Atmosphere

• Major Divisions of the Atmosphere Based on Temperature Variations

• The ‘exosphere’ is beyond the Thermosphere and continues until it merges with space.

Section 19.1

Ozone Layer

• Another way to classify regions in the atmosphere is by ozone concentration

• Ozone (O3) forms from the dissociation and recombining of molecular oxygen (O2)

• O2 + energy O + O & O + O2 O3

• O3 forms most efficiently where there is the proper balance of UV radiation and oxygen molecules – at about 30 km in altitude

• Ozonosphere – a region between 15 – 50 km in altitude characterized by ozone concentration

Section 19.1

Ozone

• The greatest concentration of O3 is roughly equivalent to the stratosphere

• This ozone layer acts as an umbrella to shield life on Earth from much of the harmful UV solar radiation

• The ozone absorption of UV radiation explains the temperature increase in this region

• Near ground-level very little natural O3 is found

Section 19.1

Ionosphere

• Ionosphere – a region between 70 and several hundred km in altitude characterized by a high concentration of ions

• Sub-regions within the ionosphere are labeled the D, E, and F layers

• Radio waves of different frequencies are reflected by these layers

Section 19.1

Vertical Structure of the Atmosphere Based on the concentrations of Ozone and Ions

• Before satellites were available the D, E, and F ion layers were used to provide global radio communications via reflection of specific frequencies

Section 19.1

Global Radio Transmission

Section 19.1

Aurora

• The northern lights or aurora borealis are beautiful displays of lights that are generated in the upper atmosphere over high latitude areas• This phenomena is called aurora australis in the

southern hemisphere

• Solar disturbances provide an abundance of incoming energetic particles that supply energy for ions and electrons to recombine

• When these ions and electrons recombine they emit visible light and other radiation

Section 19.1

Aurora Borealis, or Northern LightsDenali National Park, Alaska

Section 19.1

Sec 19.2 Atmospheric Energy Content

• By far the most important energy source for the Earth and its atmosphere is the Sun

• The Sun is approximately 93 million miles from Earth

• Although the Sun emits a vast amount of energy, the Earth only intercepts a very small portion of this radiation energy

• Insolation – the portion of solar energy that is incident on the Earth’s atmosphere• Insolation = incoming solar radiation

Section 19.2

Fluctuations in Solar Radiation

• Due to the Earth’s 23.5o tilt, the insolation is not evenly distributed over the Earth’s surface• Seasons on Earth are the result of its tilt and orbit

around the Sun

• Over time the top of the Earth’s atmosphere receives a relatively constant amount of solar radiation even though the Sun’s output may vary somewhat

• The surface of the Earth, however, only receives about 50% of this insolation

Section 19.2

Insolation Distribution

• Insolation is affected by a number of different processes as it arrives and transects the Earth’s atmosphere

• Note that only about 50% of the total insolation actually reaches the Earth’s surface

Section 19.2

Albedo

• Albedo – the amount of light a body reflects• Of the insolation received by Earth, about 33%

is returned to space via reflection and scattering• Therefore the Earth has an albedo of 33%

• The moon only has an albedo of 7% due to its dark surface and lack of atmosphere

• As viewed from space the Earth is much brighter and more impressive than the moon

Section 19.2

Planet Earth

• The exact brightness of Earth at any given time and place largely depends on the amount of cloud cover and the ratio of land to water

Section 19.2

Atmospheric Scattering

• Scattering – the absorption of incident light and its reradiation in all directions

• Gas molecules, dust particles, and water molecules in the atmosphere are responsible for the scattering of insolation

• During the scattering of insolation, this radiation may be dispersed back into space or sent to the Earth’s surface

Section 19.2

Rayleigh Scattering

• Named after the British scientist, Lord Rayleigh, who developed the theory

• Rayleigh demonstrated that the amount of scattering by molecular particles is proportional to 1/4

• The longer the wavelength the less the scattering

• Within the visible spectrum, blue light has the shortest , and therefore is scattered more, resulting in the blue sky

Section 19.2

Atmospheric Heating

• Only about 15% of the insolation is directly absorbed by the atmosphere• Most of this direct absorption is accomplished by

ozone, high in the ozonosphere

• Much of the incoming solar spectrum falls within the narrow visible region, and our atmosphere directly absorbs very little energy from this wavelength range

• After reflection, scattering, and direct absorption, about 50% of the total insolation reaches the Earth’s surface

Section 19.2

Direct Heating of the Atmosphere

• Most of the direct heating of the atmosphere comes from the Earth, not from the Sun

• In particular, the troposphere derives most of its energy content from the Earth

• Absorption of Earth energy is accomplished predominantly in three ways:• Absorption of terrestrial radiation• Latent heat of condensation• Conduction from the Earth’s surface

Section 19.2

Absorption of Terrestrial Radiation

• Just like any other warm body, the Earth radiates energy

• The wavelength () of the radiation emitted by Earth is inversely related to its temperature• 1/T

• The Earth primarily radiates energy in the long-wavelength infrared region

• Water vapor and CO2 are the primary absorbers of infrared radiation in the atmosphere

Section 19.2

The Greenhouse Effect

• In general the gases in the Earth’s lower atmosphere readily transmit most of the visible portion of the solar spectrum• This energy goes into terrestrial surface heating

• The warmed Earth emits infrared radiation which is then selectively absorbed, particularly by water vapor and CO2

• This absorbed energy heats the lower atmosphere and helps maintain the Earth’s average temperature

Section 19.2

The Greenhouse Effect

Section 19.2

Latent Heat of Condensation

• A great deal of evaporation occurs globally due to the insolation that reaches the surface

• Recall from chapter 5.3 that 540 kcal of heat is required to change 1 kg of water into vapor

• Therefore, enormous quantities of latent heat are transferred into the atmosphere as this energy is released during the condensation of the gaseous water into clouds, fog, rain, dew, etc.

Section 19.2

Conduction from the Earth’s Surface

• A significant amount of heat from the Earth’s surface is also conducted into the atmosphere

• Air is a poor conductor so this process is only effective in the lowermost atmosphere where the air is directly contacting the Earth

• Due to this phenomena, the temperature of the air tends to be warmer near the surface of the Earth and decrease systematically with altitude

Section 19.2

Sec 19.3 Atmospheric Measurements and Observations

• In order to study and understand the atmosphere’s properties and characteristics, measurements must be taken• Most measurements are taken at least daily and are

compiled for many years

• Meteorologists study these records of measurements to understand the cycles and trends in atmospheric behavior and to predict future changes

Section 19.3

Fundamental Atmospheric Measurements

• Temperature• Air temperature measurements should not be taken

when the thermometer is exposed to direct sunlight

• Pressure• Humidity• Wind Speed and Direction• Precipitation

Section 19.3

Pressure

• Pressure – the force per unit area (p = F/A)• At the Earth’s surface (or wherever we are) we

experience the weight of all the atmosphere above us

• At sea level there is an average pressure of 14.7 lbs/in2 – this is referred to as one standard atmosphere of pressure

• Galileo found that the atmosphere could sustain water up to a height of about 10 m, but no more

Section 19.3

Mercury Barometer

• Invented by Torricelli after Galileo’s work with the column of water

• The column of mercury is supported by the weight of the atmosphere on the surface of the reservoir of mercury

• Therefore, the height of the column of mercury depends on the atmospheric pressure

• One standard atmosphere of pressure will support 76 cm of mercury

Section 19.3

Atmospheric Pressure Units

• One atmosphere of pressure can be expressed a number of different ways (using different units of measurement)• SI 1.013 x 105 N/m2

• British 14.7 lbs/in2

• 76 cm or 760 mm or 30 in. of mercury• 760 torr (1 torr = 1 mm of mercury)• 1013 millibars (1 bar = 105 N/m2)

Section 19.3

Aneroid Barometer

• Due to the danger and awkwardness of handling a column of mercury another type of barometer has been developed

• Aneroid barometer – a mechanical device having a sealed metal diaphragm that is sensitive to pressure• Fair weather is generally associated with a high

barometric pressure• Rainy weather is generally associate with a low

barometric pressure

Section 19.3

Humidity

• Humidity – the measure of water vapor in the air• Absolute humidity – the amount of water vapor

in a given volume of air• In the U.S. absolute humidity is commonly measured

in grains per cubic feet (gr/ft3), where 1lb = 7000 gr

• The most common method of expressing the water vapor content in the air is in terms of relative humidity

Section 19.3

Relative Humidity

• Relative Humidity – the ratio of the actual moisture content to the maximum moisture capacity of a volume of air at a given temperature

• RH = AC/MC x 100%• RH = relative humidity, given as a percentage• AC = actual moisture content of the air• MC = maximum moisture capacity of the air

• Relative humidity is essentially how “full” of moisture a volume of air is at a given temperature

Section 19.3

Maximum Moisture Content Varies with Temperature

• A volume of air at a given temperature can only hold so much water vapor

• The higher the temperature of the air, the more water vapor the air can hold• Warm air has a greater capacity for water vapor than

does cold air

• If the temperature of a sample of air is lowered, its relative humidity will rise and eventually the air will become saturated

Section 19.3

Heat Index

• Hot and humid conditions interfere with the evaporation of perspiration and the cooling of our bodies.

• Therefore the temperature we actually feel may be greater than the measured air temperature.

Section 19.3

Heat IndexSome conditions are not only

uncomfortable but also dangerous

Section 19.3

Dew Point

• Dew point – the temperature to which a sample of air must be cooled to become saturated• At the dew point, the relative humidity is 100%

• Cooling below the dew point results in supersaturation and generally condensation of the moisture

Section 19.3

Psychrometer and Relative Humidity

• A psychrometer consists of two thermometers, a dry bulb and a wet bulb (wet cloth wick)

• The dry bulb measures the actual temperature of the air

• The temperature recorded by the wet bulb is depressed, and is a function of the moisture content of the air

• The lower the humidity, the more evaporation occurs, and the more latent heat of evaporation is removed, thus depressing the wet bulb reading

Section 19.3

Anemometer and Wind Vanes

• The wind speed is measured with an anemometer, consisting of 3-4 cups attached to a rod

• The wind vane indicates the direction from which the wind is blowing

• Wind direction is reported as the direction from whence it comes

Section 19.3

Precipitation

• Typically, rainfall is measured with a simple rain gauge• The U.S. is just about the only country that still

reports rainfall in inches• It is assumed that the rainfall measured in a single

rain gauge accurately measures the rainfall for the surrounding region

• Snow fall is reported as a depth in inches• The actual amount of water received depends on the

density of the snow

Section 19.3

Modern Weather Observations

• Radar – (radio detecting and ranging) is widely used today to detect and monitor precipitation, especially severe storms

• A more advanced radar system, called Doppler radar is now used not only to measure the intensity of precipitation, but also to measure wind speeds

• Doppler, based on the Doppler effect (Chapter 6), can determine the speed of rain drops and therefore gives the speed of the wind

Section 19.3

Satellites

• The first fully operational weather satellite system was put in place by 1966• Compared to the satellite systems of today, these

first satellites were only able to monitor limited areas

• Today, a fleet of GOES (Geostationary Orbiting Environmental Satellites) monitor the globe

• Meteorologists today have a panoramic view of regional and global weather conditions• Satellites collect weather data using a variety of

electromagnetic wavelengths

Section 19.3

Sec 19.4 Air Motion

• Wind – the horizontal movement of air along the Earth’s surface

• Air Currents – vertical movement of air, broken down into updrafts and downdrafts

• Atmospheric gases within the atmosphere are subject to two primary forces• Gravity• Pressure differences due to temperature variations

Section 19.4

Gravity

• The force of gravity is always directed vertically downward

• This force acts on every gas molecule within the atmosphere, resulting in a greater density of air near the Earth’s surface

Section 19.4

Air Pressure

• Air in our atmosphere is a mixture of gases and therefore behaves according to a set of gas laws (Chap. 5) and other physical principles

• Recall that the pressure of a gas is directly proportional to its Kelvin temperature (p T)

• Therefore, if a temperature variation exists between two bodies of air, there will be a pressure difference

Section 19.4

Pressure Differences

• A pressure difference corresponds to an unbalanced force since pressure is equal to force per unit area

• When a pressure difference (and therefore an unbalanced force) exists in the atmosphere, the air moves from high- to low-pressure regions

Section 19.4

Mapping a Pressure Difference

• When a number of barometric readings are taken at different location within a region, the pressure can be mapped using isobars

• Isobar – a line of equal pressures• Air movement does not occur along a given

isobar, since by definition a pressure difference does not exist

Section 19.4

Isobars

Wind direction will always be at right angles to the isobar and in the direction of the lower pressure

Section 19.4

Pressure, Volume, and Density of Air

• Both pressure and volume are directly proportional to Kelvin temperature (pV T)• Therefore, a change in temperature will cause a

change in pressure and/or volume of a gas

• If the volume of a gas changes, there is also a change in its density ( = m/V)• If air is heated, it expands• If air is cooled, it contracts

• Unequal heating of the Earth’s surface leads to thermal circulation

Section 19.4

Daily Convection Cycles Over Land and Water Due to Unequal Heating/Cooling

• Day - the land heats up faster than an adjacent body of water. The air above the land warms, expands, and rises. Air flows in from the sea, and a convection cycle is set up.

• Night – the land cools off faster than the adjacent water. The air above the land cools, contracts, and descends. Air flows from the land to the sea. The day-time cycle is reversed.

Day Sea Breeze

Night Land Breeze

Section 19.4

Monsoons

• Southern Asia – a giant convection cycle brings heavy, summer monsoons in from the ocean.

• Six months later – the cycle reverses and Indonesia and Australia receive winter monsoons.

• Monsoons are analogous to day-night thermal oscillations at an ocean beach.

Section 19.4

Coriolis Force

• The Coriolis force is not a true force, and is the result of an observer on Earth being in a rotating frame of reference

• It is considered a psuedoforce, and helps to explain its effect such that the laws of motion will be consistent

• Projectiles appear to be deflected to the right in the northern hemisphere, and to the left in the southern hemisphere, as observed in the direction of motion

Section 19.4

The Coriolis Force

• An observer in space (over the north pole) watches the projectile go straight.

• But, to an observer on Earth the projectile appears to veer to the right

Section 19.4

Effects of the Coriolis Force on Air Motion in the Northern Hemisphere

(as viewed from above)

• The air in a high-pressure air mass descends, resulting in its clockwise rotation

• The air in a low-pressure air mass rises, resulting in its counterclockwise rotation

Section 19.4

Global Circulation Patterns

• On a local basis there are many factors that affect the lower atmosphere and resulting wind patterns

• From a regional or global scale the air near the Earth’s surface possesses a general circulation pattern

• A number of complicated factors serve to divide the Earth’s lower atmosphere into six general convection cycles, or pressure cells

Section 19.4

Earth’s General Circulation Pattern

• Although many local variations occur within the cells, the prevailing winds of this semi-permanent circulation structure are important in influencing general weather movement around the globe

Section 19.4

Contiguous United States Weather Trends

• Weather patterns within the lower 48 states of the U.S. serve as an example of how the global circulation structure affects a region

• This region lies generally within the westerlies wind zone

• As a result, weather systems almost always move from west to east across the country

Section 19.4

Jet Streams

• Jet Streams – fast-moving ribbons or rivers of air located in the upper troposphere formed along the upper boundaries of warm and cold air masses

• Serious study of these phenomena began during World War II when high-flying aircraft encountered them

• A typical jet stream continuously meanders throughout the year, apparently affecting the severity of winter by their position

Section 19.4

Jet Stream

Section 19.4

Sec 19.5 Clouds

• Clouds – buoyant masses composed of visible water droplets or ice crystals

• Clouds are of particular interest to anyone studying the weather because their size, shape, and behavior form some of the few visible keys to the weather

Section 19.5

Cloud Classification

• Clouds are classified according to their shape, appearance, and altitude

• Four basic root names are used to describe a cloud’s shape and appearance• Cirrus – wispy, fibrous forms• Cumulus – billowy, round forms• Stratus – stratified or layered forms• Nimbus – a cloud from which precipitation is

occurring or threatens to occur

Section 19.5

Cloud Classification - Height

• Clouds can be classified into four groups according to height

• High Clouds – above 6 km• Cirrus, Cirrocumulus, and Cirrostratus

• Middle Clouds – 1.8 to 6 km• Altostratus and Altocumulus

• Low Clouds – ground level to 1.8 km• Stratus, Stratocumulus, Nimbocumulus, Advection

fog, and Radiation fog

• Clouds of Vertical Development – 5 to 18 km• Cumulus and Cumulonimbus

Section 19.5

Vertical Cloud Development

Section 19.5

Cloud Formation

• True water vapor in the air is invisible• For water to be visible it must condense into

droplets• Condensation requires that the given mass of air

must be cooled down to the dew point temperature

• When the dew point temperature is reached, the water vapor will generally condense into fine droplets and form a cloud

Section 19.5

Cloud Formation

• Air within the troposphere is constantly moving, and when an air mass is cooled sufficiently, cloud formation may take place

• Cloud formation is associated with vertical movement of air, because the temperature of the troposphere decreases with altitude

• In general terms, cloud formation is due to vertical air motion (currents), and clouds are shaped due to horizontal air motion (winds)

Section 19.5

Rising Air

• Air masses may rise due to heating, wind action along a high topographic feature, or lifting associated with a weather front

• As warm air is lifted (by whatever means), it cools because it expands• The internal energy of the air mass goes into

expansion, thus cooling the air

• In the troposphere the rate of temperature decrease with height is called the lapse rate• The normal lapse rate in stationary air in the

troposphere is 6.5Co/km (3.5Fo/1000ft).

Section 19.5

Cloud Stability

• When a rising air mass cools to the same temperature as the surrounding stationary air, the densities are now equal

• The rising air mass loses its buoyancy and is said to be in a stable condition

• A stable layer is a layer of air of uniform temperature and density

Section 19.5

Cloud Base and Thickness

• Cloud base – the height at which condensation occurs

• Thickness of the cloud – the vertical distance between the condensation level and the level where stability is reached

Section 19.5