Chapter 1
Introduction to the
Atmosphere
Weather
• Weather is the condition of the atmosphere at a
particular location and moment
• These atmospheric conditions include, among
others, temperature, relative humidity, dew point,
pressure, wind speed and direction, cloud cover,
and precipitation
• What causes weather? Ultimately, the sun
Figure 01: Satellite image of North America
Courtesy of NOAA/NESDIS and SSEC, University of Wisconsin-Madison.
Definitions
• Meteorology is the study of weather variables—
temperature, humidity, pressure, wind speed and
direction, cloud cover, precipitation, the
processes that cause weather, and the
interaction of the atmosphere with the Earth’s
surface, ocean, and life.
Climate
• The climate of a region is the condition of the
atmosphere over many years
• Described by long-term averages of atmospheric
conditions such as temperature and precipitation
• Includes extremes as well as averages
• Climatology is the study of climate
• Climatologists also study changes of climate in
the past and for the future
• How oceans, landforms, living organisms affect
the atmosphere
Figure 02A: Polar stereographic map of the Northern Hemisphere
Figure 02B: Polar stereographic map of the Southern Hemisphere
Meteorology
and Atmospheric Science
• Usually used interchangeably
• Atmospheric science includes not only
meteorology but some other topics as well
– Charged particles and electricity in the ionosphere,
parts of the upper atmosphere
– Atmospheres of other planets
– Includes the study and simulation of climate
– Includes the study of climate change
What is the atmosphere?
• A fluid
• A thin layer surrounding the Earth
• Mainly a mixture of invisible gas with some solid
and liquid particles that stays in place on
account of the force of gravity
Figure CO: Chapter 1, Introduction to the Atmosphere--Clouds over Indian
Ocean
Courtesy of NASA Headquarters—Greatest Images of NASA
(NASA-HQ-GRIN)
What’s in the atmosphere?
• Invisible gases
–
–
–
–
Some are permanent gases, some variable
Some are abundant, some not (trace gases)
Some are greenhouse gases, some not
Some pollutants, some not
• Liquids
– Water: cloud droplets, raindrops, haze, fog
– Pollutants
• Solids
– Water: ice crystals in clouds, snow
– Soil, sand, acid, pollen, other substances
Figure T01: Composition of the Atmosphere
Gases in the atmosphere
• Nitrogen: most abundant (78%), not very
reactive, permanent, not a greenhouse gas,
emitted from volcanoes
• Oxygen (O2: 2nd most abundant (21%),
essential for combustion, respiration, a
greenhouse gas only in the stratosphere and
above (not near the surface), permanent, comes
from plants as a product of photosynthesis
Other gases in the atmosphere
• Argon (1%), permanent, not reactive
• Water vapor, highly variable (0-4%), extremely
important to the weather and essential to life,
most abundant of the greenhouse gases, comes
from volcanoes and maybe comets, hugely
important to climate, invisible, makes air lighter
– Part of a cycle called the hydrologic cycle
– Has a whole chapter, Chapter 4
Figure 05: Hydrologic cycle
Still another gas in the atmosphere
• Carbon dioxide, the 2nd most abundant
greenhouse gas, variable with concentrations
increasing every year and higher now than ever
before in Earth’s history, important for climate
change and global warming
– Has a cycle of sources and sinks called the Carbon
Dioxide (CO2) Cycle
– Photosynthesis is a sink for carbon dioxide
– Burning fossil fuels is an important source of carbon
dioxide
– The oceans are a sink for carbon dioxide
Figure 03: Carbon dioxide cycle
Figure 04: Carbon dioxide measurements at Mauna Loa
Another gas: Methane
– Is another important greenhouse gas
– Concentrations are increasing
– Comes from human activities, including the cultivation
of rice, burning of forests, coal mining, and cattle
raising (digestive processes of domestic animals)
– Also comes from termites
Figure 07: Atmospheric methane measurements
Source: NOAA/ESRL Global Monitoring Division - THE NOAA ANNUAL
GREENHOUSE GAS INDEX (AGGI). (n.d.). . Retrieved from
http://www.esrl.noaa.gov/gmd/aggi/
Ozone (O3)
Is another important greenhouse gas
Forms naturally in the stratosphere from oxygen,
and warms the stratosphere
Allowed life to develop over land
Gets depleted over Antarctica in winter—known
as the “ozone hole”
Is a dangerous pollutant near Earth’s surface
Chlorofluorocarbons (CFCs)
• Are yet more greenhouse gases, and very
powerful
• Do not occur naturally
• Are chemically stable near the surface
• Are broken down in the stratosphere
– Loose chlorine atoms destroy stratospheric ozone
• Are decreasing in emissions rapidly
• Are decreasing in concentrations slowly
Figure 09: CFC concentrations
Source: NOAA/ESRL Global Monitoring Division - THE NOAA ANNUAL
GREENHOUSE GAS INDEX (AGGI). (n.d.). . Retrieved from
http://www.esrl.noaa.gov/gmd/aggi/
Particles of liquid and solid in the
atmosphere
• Are together known as aerosols [ai(e)rborne
solutions]
• Vary in size with the type of substance
• Are measured in units of microns (1/1,000,000 of
a meter or 1/1,000 of a mm)
• Most are invisible because they are so small
• Are more abundant over deserts, less over
oceans
• Sources include wind erosion, volcanoes, fires
and human activity
Figure 10: Aerosol sizes
Figure 06: Global satellite picture of cloud distributions
Courtesy of SSEC, University of Wisconsin-Madison
Particulates
•
•
•
•
Are needed to form clouds
Can influence climate
Can be pollutants
Are anthropogenic when caused by human
activity
Figure 11: Satellite picture of smoke from Santa Ana fires
Courtesy of NASA/MODIS Rapid Response
Pressure and density in the
atmosphere
• Pressure is force per unit area
• Pressure always decreases upward
• Pressure is related to the weight of air in a
column above a particular location
• Density is mass per unit volume
• Density always decreases upward
• Density is related to pressure by the gas law:
pressure = constant x density x temperature
Figure 12: Atmospheric pressure and the density of air
Adapted from Rauber, R.M., Walsh, J.E. and Charlevoix, D.J. Severe &
Hazardous Weather: An Introduction to High Impact Meteorology, Third
edition. Kendall/Hunt, 2008.
Barometric Pressure and Sea-Level
Pressure
• The greater the atmospheric pressure, the
greater the height of the mercury in the tube of a
mercury barometer
– This measures barometric pressure or station
pressure
• Station pressure depends both on weather highs
and lows and on altitude
– Without a correction for altitude a weather map would
look like a topographic map
– Sea-level pressure has been corrected for altitude
Figure 13: Atmospheric pressure summary
Layers of the Atmosphere
• Temperature divides the atmosphere into four
layers
• Closest to the earth, where the temperature
generally decreases upward, is the troposphere
– 80% of the mass of the atmosphere is here
– Most of this book is devoted to the troposphere
– The top of the troposphere is the tropopause
• An upper lid on weather patterns
• Higher in the tropics than at the poles
Figure 14: Layers of the atmosphere
Adapted from Rauber, R.M., Walsh, J.E. and Charlevoix, D.J. Severe &
Hazardous Weather: An Introduction to High Impact Meteorology, Third
edition. Kendall/Hunt, 2008.
The Upper Atmosphere
• Above the tropopause is the stratosphere, where
temperature increases with altitude
– Here ozone is absorbing solar energy
– Here there is a lack of both mixing and turbulence
• Above the stratosphere is the stratopause
• Above the stratopause is the mesosphere,
where temperature decreases with altitude
• The mesopause separates the mesosphere from
the thermosphere, where temperature increases
with altitude again
Figure 15: Satellite picture of aurora
Courtesy of Mark D. Conner and Air Force Weather Agency, Offutt
AFB/U.S. Air Force
An Introduction to Weather Maps
• The surface chart depicts weather at the Earth’s
surface
• Fronts, or boundaries between air masses,
shown using lines with triangles and semicircles
• Isotherms connect observations of the same
temperature
• Isobars connect observations of the same
pressure
• Data from specific locations coded using the
station model
Figure 16: Fronts: cold, warm, stationary, and occluded.
The Station Model
• Is a compressed graphical weather report
• Is coded to display
– Weather conditions at a specific place and time
•
•
•
•
Plus cloud cover
Wind speed and direction
Temperature
Dew point temperature, atmospheric pressure adjusted
to sea level
• Change in pressure over the last 3 hours
• Visibility
• Precipitation amounts
Figure 17: The station model
Figure 18: Surface weather map
Adapted from Plymouth State University Weather Center,
[http://vortex.plymouth.edu/make.html.]. Accessed June 10, 2010
Coordinated Universal Time (UTC)
• Is the reference clock adopted by weather
organizations around the world
• Greenwich, England is the reference time zone
for UTC
• Meteorology also uses a 24-hour military-style
clock
• UTC
EST CST MST PST
• 1200(noon)
0700 0600 0500 0400
• 0000(midnight)1900 1800 1700 1600
Figure 19: Time zone conversion map
Courtesy of The World Factbook, 2009.
Watches, Warnings
• Issued by the National Weather Service
• Watches inform that current atmospheric
conditions are favorable for hazardous weather
– Often issued for large regions
– You should be aware that a weather hazard may
develop in your area
• Warnings inform that hazardous weather will
soon occur in an area
– The hazard is developing in your area
– Take immediate action
Figure T02: Typical National Weather Service Criteria for Issuing Selected
Weather Watches and Warnings
Advisories
• A less urgent statement to bring to the public’s
attention a situation that may cause some
inconvenience or difficulty for people who have
to be outdoors or to travelers
Figure 20: Watches and warnings
Courtesy of SSEC, University of Wisconsin-Madison