Structure and Composition
Concepts to learn
The general concepts found in this section are:
 The earth's atmosphere is a very thin layer wrapped around a very
large planet.
 Based on temperature, the atmosphere is divided into four layers:
the troposphere, stratosphere, mesosphere, and thermosphere.
 Energy is transferred between the earth's surface and the
atmosphere via conduction, convection, and radiation.
 Ocean currents play a significant role in transferring this heat
poleward. Major currents, such as the northward flowing Gulf
Stream, transport tremendous amounts of heat poleward and
contribute to the development of many types of weather
phenomena.
Atmospheric Properties
 The thin envelope of air that surrounds




our planet is a mixture of gases, each
with its own physical properties.
Two elements, nitrogen and oxygen,
make up 99% of the volume of air.
The other 1% is composed of "trace"
gases, the most prevalent of which is
the inert gaseous element argon.
The rest of the trace gases, although
present in only minute amounts, are
very important to life on earth.
Two in particular, carbon dioxide and
ozone, can have a large impact on
atmospheric processes.
Another gas, water
vapor, also exists in
small amounts. It
varies in concentration from being
almost non-existent
over desert regions to
about 4% over the
oceans. Water vapor is
important to weather
production since it
exists in gaseous,
liquid, and solid
phases and absorbs
radiant energy from
the earth.
Atmospheric
 The atmosphere is
divided vertically into
four layers based on
temperature: the
troposphere,
stratosphere,
mesosphere, and
thermosphere.
 In this portion of the
unit we'll focus primarily
on the layer in which we
live - the troposphere.
Troposphere
 The word troposphere comes from





tropein, meaning to turn or change.
All of the earth's weather occurs in the
troposphere.
It extends from the earth's surface to an
average of 12 km (7 miles).
The pressure ranges from 1000 to 200
millibars (29.92 in. to 5.92 in.).
The temperature generally decreases
with increasing height up to the
tropopause (top of the troposphere);
this is near 200 millibars or 36,000 ft.
The temperature averages 15°C (59°F)
near the surface and -57°C (-71°F) at the
tropopause.
The layer ends at the point where
temperature no longer varies with
height. This area, known as the
tropopause, marks the transition to the
stratosphere.
 Winds increase with height up to





the jet stream.
The moisture concentration
decreases with height up to the
tropopause.
The air is much drier above the
tropopause, in the stratosphere.
The sun's heat that warms the
earth's surface is transported
upwards largely by convection and
is mixed by updrafts and
downdrafts.
The troposphere is 70% N2 and 21%
O2.
The lower density of molecules
higher up would not give us enough
to survive
Troposphere
Interactions - Atmosphere and Ocean
 Water is an essential part of the earth's system.
 The oceans cover nearly three-quarters of the earth's surface and play an
important role in exchanging and transporting heat and moisture in the
atmosphere.
 Most of the water vapor in the atmosphere comes from the oceans.
Most of the precipitation falling over land finds its way back to oceans.
About two-thirds returns to the atmosphere via the water cycle.
 You may have figured out by now that the oceans and atmosphere interact
extensively. Oceans not only act as an abundant moisture source for the
atmosphere but also as a heat source and sink (storage).
Heat transfer
 The exchange of heat and moisture has profound effects on
atmospheric processes near and over the oceans.
 Ocean currents play a significant role in transferring this heat
poleward.
 Major currents, such as the northward flowing Gulf Stream, transport
tremendous amounts of heat poleward and contribute to the
development of many types of weather phenomena.
 They also warm the climate of nearby locations. Conversely, cold
southward flowing currents, such as the California current, cool the
climate of nearby locations.
Energy Heat Transfer
 Practically all of the energy that
reaches the earth comes from
the sun.
 Captured first by the
atmosphere, a small part is
directly absorbed, particularly
by certain gases such as ozone
and water vapor.
 Some energy is also reflected
back to space by clouds and the
earth's surface.
Conduction vs. convection
 Energy is transferred between the earth's surface and the





atmosphere via conduction, convection, and radiation.
Conduction is the process by which heat energy is
transmitted through contact with neighboring
molecules.
Some solids, such as metals, are good conductors of heat
while others, such as wood, are poor conductors. Air and
water are relatively poor conductors.
Since air is a poor conductor, most energy transfer by
conduction occurs right at the earth's surface.
At night, the ground cools and the cold ground conducts
heat away from the adjacent air.
During the day, solar radiation heats the ground, which
heats the air next to it by conduction.
Convections
Convection transmits heat by transporting groups of
molecules from place to place within a substance.
Convection occurs in fluids such as water and air, which
move freely.
 In the atmosphere, convection includes large- and smallscale rising and sinking of air masses and smaller air
parcels.
 These vertical motions effectively distribute heat and
moisture throughout the atmospheric column and
contribute to cloud and storm development (where rising
motion occurs) and dissipation (where sinking motion
occurs).
Convection
 To understand the convection cells that distribute heat
over the whole earth, let's consider a simplified,
smooth earth with no land/sea interactions and a slow
rotation.
 Under these conditions, the equator is warmed by the
sun more than the poles.
 The warm, light air at the equator rises and spreads
northward and southward, and the cool dense air at
the poles sinks and spreads toward the equator.
 As a result, two convection cells are formed.
Coriolis effect
 Meanwhile, the slow
rotation of the earth
toward the east causes the
air to be deflected toward
the right in the northern
hemisphere and toward
the left in the southern
hemisphere.
 This deflection of the wind
by the earth's rotation is
known as the Coriolis
effect.
Radiation
 Radiation is the transfer of heat energy without the
involvement of a physical substance in the
transmission.
 Energy travels from ________________
the sun to the earth by means of
electromagnetic waves.
 Most of the sun's radiant energy is concentrated in the
visible and near-visible portions of the spectrum.
Shorter-than-visible wavelengths account for a small
percentage of the total but are extremely important
because they have much higher energy. These are
known as ultraviolet wavelengths.
PART I – SUN’S RADIATION AND HEATING OF THE
ATMOSPHERE , Pre-Lab
 How is the surface of
 Solar radiation is energy that is released by the sun in
the form of particles or electromagnetic waves, and
there is no need of a substance for transport.
 All forms of radiation coming out from the sun can be
seen on the electromagnetic spectrum.
Electromagnetic spectrum
 Visible light energy coming from the sun is the only solar
radiation wave that we can see naturally, and is made up of
all colors (Red, Orange, Yellow, Green, Blue, Indigo, and
Violet or ROYGBIV).
 43% of the solar radiation output is visible light, 49% is
infrared radiation, and 7% is ultraviolet radiation. The
remaining 1% is in the form of x-rays, gamma rays, and
radio waves.
 radiation is absorbed by the ozone layer. Some of the
infrared radiation gets absorbed by the clouds and other
atmospheric gases.
 Therefore most of the energy that reaches the Earth’s
surface is in the visible part of the electromagnetic
spectrum.
Electromagnetic
 The Earth’s surface absorbs the radiation, and then re-
emits the radiation in the form of long-wave infrared. The
infrared radiation that the earth emits is a longer
wavelength of infrared than what the sun emits.
 So the earth emits long-wave radiation (long-wave
infrared) and the sun emits short-wave radiation
(ultraviolet, visible, and short-wave infrared).
Electromagnetic
 The radiation emitted by the earth, can then be absorbed





by clouds and other atmospheric gases.
The two primary gases that absorb long-wave radiation in
the lower atmosphere are water vapor and carbon
dioxide.
Methane, ozone, and chlorofluorocarbons can also absorb
some of the long-wave radiation.
The gases can then re-emit the energy again and send the
energy back down towards the Earth’s surface.
The emitting long-wave radiation from the earth and gases
heats our planet from the surface up into the atmosphere.
The sun’s short-wave energy DOES NOT directly heat up
the atmosphere. If that were the case, then outer space
would be very warm and not extremely cold.
PART II – ALBEDO
 Reflection is when
energy (radiation) is
bounced off of an object
at the same angle and
intensity.
 Albedo is the percent of
radiation that is
reflected by a surface.
 Surfaces that reflect a
lot of energy have a high
albedo.
Albedo
 Energy that is reflected
does not get absorbed by
the surface and is not
changed into the longwave infrared radiation.
 Therefore, the gases in the
atmosphere can’t absorb it
and the temperature
remains cool.
 Surfaces that absorb a lot
of energy have a low
albedo.
 The energy is re-emitted as
long-wave infrared
radiation which heats the
atmosphere
Albedo
Albedo = % incident energy reflected by a body
 Fresh snow:
75 – 95%
 Old snow: 40 – 60%
 Desert:
25 – 30%
 Deciduous forest, grassland: 15 – 20%
 Conifer forest: 5 – 15%
 Camera light meters set to 18%
Global Albedo
Albedo
 Albedo is calculated as the amount of energy reflected from a surface divided by the
total amount of incoming energy to the surface, multiplied by 100 to get a
percentage.
Albedo (%) = Reflected energy x 100
Incoming energy
 For example: if the amount of energy hitting a surface is 645 units and the amount of
energy being reflected by that surface is 135 units, then the albedo of that surface
would be:
135 units x 100 or 0.209 x 100 = 20.9%
645 units
 This means that 20.9% of the energy that is hitting the surface is getting reflected
back into the atmosphere, while 79.1% (100-20.9%) of the energy is being absorbed
by the surface. The earth’s surface heats up, emits the long-wave infrared radiation
which the gases absorb and radiate energy back to us. This surface will have a warm
air temperature over it.
 EXERCISE 1: Calculate the albedo of the surfaces listed on the worksheet. Rank the
surfaces from 1 to 3, with 1 being the surface with the warmest air temperature above
it and 3 being the surface with the coolest air temperature above it. Write your
answers on the pre-lab worksheet.
PART III – DAILY MEAN TEMPERATURE AND
THE DAILY RANGE
 you will be asked to calculate the daily mean
temperature and the daily temperature range. The
daily mean temperature or average temperature is
calculated by averaging the 24 hour readings from each
hour of the day or by averaging the high and low
temperature throughout the day.
Maximum Temp. + Minimum Temp.
= Daily Temperature Mean
2
 The daily temperature range is simply the difference
between the high and low temperatures. Under normal
conditions, the smaller the temperature range the more
humid an observing station is. The greater the
temperature range, the less humid an observing station is.
Exercise 2
 EXERCISE 2 – Calculate the daily mean temperature
and the daily range for each of the three cities listed on
the worksheet. Rank the three cities from 1 to 3, with 1
being the most humid and 3 being the least
humid. Write your answers on the pre-lab worksheet.
Temperature Controls
 Cloud Cover and Albedo
• remember that Albedo is the fraction of total
radiation that is reflected by any surface.
• Many clouds have a high albedo and therefore
reflect back to space a significant portion of the
sunlight that strikes them.
Clouds Reflect and Absorb Radiation
Clouds Reflect and Absorb Radiation
Clouds
•On Earth, water naturally occurs in all 3 phases
or states of matter (gas, liquid, solid)
•Clouds are composed of tiny liquid water
droplets or tiny ice crystals.
–Clouds are not made of water vapor (Otherwise, we
wouldn’t be able to see them!)
•In nature, clouds form when the temperature of
air is lowered to its dewpoint temperature.
Local Temperature Variations
Atmosphere
VOCABULARY
isotherm
A contour line
drawn on a
weather map
through places
having the same
atmospheric
temperature at a
given time.
The intensity of insolation (measure of solar
radiation) depends upon the angle at which
sunlight strikes Earth’s surface. The intensity is
greatest at low latitudes, during the summer,
and around noon.
The angle of sunlight varies with
latitude.
Tilt of Earth’s Axis
reaches Earth.
Insolation:
The solar (energy)
radiation that
reaches Earth.
**The Axial Tilt
of the Earth is the
cause of the
seasons.
Tilt of Earth’s Axis
Altitude of sun affects amount of energy
received at surface because
lower angle -> more
spread out and less
intense radiation
(as for flashlight
beam).
lower angle -> more
of atmosphere to
pass through, and
hence more chance to
be absorbed or
reflected
(can look at sun at
sunset).
SUN’S
RAYS
June 21
Dec. 21
During the summer, the Northern Hemisphere is tilted
towards the Sun. Here, locations receive the Sun’s most
direct rays, and have longer periods of daylight hours.
During the winter, the Northern Hemisphere is tilted away
from the Sun. Periods of daylight are shorter, and the Sun’s
rays are less direct.
Solstices and Equinoxes
During the vernal
(spring) and
autumnal (fall)
equinoxes, neither
hemisphere is tilted
towards or away
from the Sun. On
these dates, every
location on Earth
receives 12 hours of
daylight and 12
hours of darkness.
Atmosphere Characteristics
 Solstices and Equinoxes
• The summer solstice is the solstice that occurs on
June 21 or 22 in the Northern Hemisphere and is
the “official” first day of summer.
• The winter solstice is the solstice that occurs on
December 21 or 22 in the Northern Hemisphere
and is the “official” first day of winter.
Atmosphere Characteristics
 Solstices and Equinoxes
• The autumnal equinox is the equinox that occurs
on September 22 or 23 in the Northern
Hemisphere.
• The spring equinox is the equinox that occurs on
March 21 or 22 in the Northern Hemisphere.
Temperature Controls
 Earth’s Motions
• Earth has two principal motions—rotation and
revolution.
Rotation = 24 hours
Revolution = 365 days
 Earth’s Orientation
• Seasonal changes occur because Earth’s position
relative to the sun continually changes as it travels
along its orbit.
Temperature Controls
 Factors other than latitude that exert a strong influence
on temperature include heating of land and water,
altitude, geographic position, cloud cover, and ocean
currents.
 Geographic Position
• The geographic setting can greatly influence
temperatures experienced at a specific location.
 Land and Water
• Land heats more rapidly and to higher temperatures
than water. Land also cools more rapidly and to lower
temperatures than water.
Temperature Controls
What determines temperature?
 Latitude: locations at lower latitudes typically
experience higher temps year-round than higher
latitude locations, because the lower latitudes
receive more solar energy
 Proximity to water: locations near water, especially
a cool ocean current, have smaller annual temp
ranges than landlocked locations
 Elevation: locations at higher elevations (altitude)
usually have cooler conditions than locations at
lower elevations
Observing the Atmosphere
 Air Pressure: force per area exerted by the mass of air
above a point
 Measured in:
 Inches of mercury (in. Hg)
 Millibars (mb)
 Average sea-level pressure = 1013.25 mb or 29.92 in. Hg
 Air pressure is Measured using a barometer
Mercury Barometer
Barometer - apparatus used
to measure pressure; is
derived from the Greek "baros"
meaning "weight"
Aneroid Barometers: (without liquid)
Air Pressure
 By lucky coincidence, earth’s atmospheric pressure is





approximately neat round numbers in metric terms
14.7 pounds per square inch, PSI (1 kg/cm2)
Pressure of ten meters of water
Approximately one bar or 100 kPa
Weather reports use millibars (mb)
One mb = pressure of one cm water
The Effect of the Ocean on Annual
Temperature
Dew Point
 The dew point is the temperature below which the
water vapor in a volume of humid air at a constant
barometric pressure will condense into liquid water.
 Condensed water is called dew when it forms on a
solid surface.
 The dew point is a water-to-air saturation
temperature.