The Earths Atmosphere

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The Earths Atmosphere
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Atmospheric Variables
We use a variety of variables to describe the
atmosphere, For example Temperature Pressure
Mixing ratio Discussing the atmosphere requires
an understanding of some important atmospheric
variables
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Temperature
Temperature is a measure of the average speed of
the molecules, faster motion higher
temperature. Temperature is a fundamental
quantity for understanding the weather,
radiation, and chemistry of the
atmosphere. Temperature scales Fahrenheit (F)
water freezes at 32F and boils at 212F Celsius
(C) water freezes at 0C and boils at 100C,
T(F) (9/5) T(C) 32 Kelvin (K) water freezes
at 273.15 K and boils at 373.15 K, T(K) T(C)
273.15
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Pressure
Atmospheric pressure can be thought of as the
weight per unit area of the column of atmosphere
above a given height. Pressure
scales Millibars (mb) Sea level pressure is
1013.25 mb Inches of Mercury (Hg) Sea level
pressure is 29.92 Hg
Barometer
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Pressure continued
Since the number of air molecules above some
altitude decreases with height, pressure likewise
decreases with height. Pressure decreases
exponentially with altitude
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Density, Mixing Ratio, Partial Pressure
Density Air density is determined by pressure and
temperature, d P / RT R is the universal gas
constant Warmer temperatures or lower pressures
correspond to lower air density Mixing Ratio The
abundance of a gas in the atmosphere can be
described by the mixing ratio. Volume mixing
ratio is the volume of gas per unit volume of
air, Q Vg/Vair Partial Pressure The partial
pressure for a given gas is the pressure exerted
by that gas alone. For example we may want to
know the partial pressure of just water vapor
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Composition of the Atmosphere

• The atmosphere is comprised of a variety of
  gases
• Major Constituents (99)
• Nitrogen (N) 78
• Oxygen (O2) 21
• Trace Constituents
• Argon (Ar), about 0.9
• Water vapor (H2O), up to 10000 ppmv
• Carbon dioxide (CO2), 350 ppmv
• Ozone (O3), near zero at the surface, up to 10
  ppmv in the stratosphere
• Methane (CH4), 1.7 ppmv
• and others..
• ppmv parts per million by volume

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Water in the Atmosphere

• Water exists in 3 states solid (ice) liquid
  gas (water vapor)
• The saturation water vapor pressure (es)
  represents the maximum vapor pressure of water in
  air.
• Vapor pressure is determined for equilibrium over
  liquid water or over ice
• es is a function of temperature alone, and
  decreases at colder temperatures.

• Relative humidity is the ratio of the water vapor
  content to the water vapor capacity
• RH 100 x e / es ()
• Dew point is the temperature to which the air
  would have to be cooled to achieve 100 relative
  humidity.

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Vertical Structure of the Atmosphere

• Layers in the atmosphere are defined by
  temperature
• Earth's atmosphere thins out to near nothingness
  several hundred kilometers above the surface
• 99 of the total mass of the atmosphere exists
  below 30 km altitude

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Troposphere and Stratosphere

• Troposphere
• 0 to 15 km altitude
• The lowest region of the atmosphere, where life
  weather exist.
• Temperature decreases with altitude.
• Long-wave radiation emitted from Earth is
  absorbed by the atmosphere, the atmosphere
  becomes less dense with increasing altitude,
  less air to absorb
• Top of the troposphere is known as the tropopause
• Stratosphere
• 15 to 50 km altitude
• Temperature increases with altitude.
• Heating occurs because ozone (O3) absorbs
  ultraviolet radiation from the Sun.
• Top of the stratosphere is known as the
  stratopause

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Mesosphere and Thermosphere

• Mesosphere
• 50 to 90 km altitude
• Temperature decreases with altitude
• The lowest temperatures in the entire atmosphere
  are found at the mesopause during summer at high
  latitudes, 130 K (-226F) can occur
• Top of the mesosphere is known as the mesopause
• Thermosphere
• 90 to 500 km altitude
• Temperature increases with altitude above 90 km,
  and is constant above 200 km.
• This heating is due to absorption of solar
  radiation (wavelengths less than 0.2 microns) by
  molecular oxygen (O2).
• The highest temperatures in the atmosphere can be
  found in the thermosphere, 2000 K can occur

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Atmospheric Circulation
Atmospheric motion, or wind, exhibits a range
of horizontal scales. Planetary scale broadest
features of the global circulation, features
with horizontal dimensions comparable to the size
of continents or oceans, for example the
persistent west to east winds. Synoptic scale
waves with horizontal dimensions on the order of
several hundreds of kilometers, for example high
and low pressure systems. Mesoscale waves
with horizontal dimensions on the order of tens
to hundreds of kilometers, for example mountain
lee waves.
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Global Circulation

• The broadest features of the global circulation
  are driven by the overall temperature
  distribution
• Warm air at the equator rises and flows towards
  the poles
• Cold air at the poles sinks and flows towards the
  equator

The coriolis force turns these winds resulting in
the three cell circulation
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Weather Patterns
Weather patterns are more complex than the global
circulation Areas of high and low pressure change
the weather frequently
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Driving Forces Behind Wind

• Pressure Gradient
• Air flows from high to low pressure (downhill)
• Coriolis
• Caused by the rotation of the earth, wind
  deflects to the right in the northern hemisphere
• Centripital
• Present when winds are in rotation
• Friction
• Air moving along the Earths surface is slowed by
  friction

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Pressure Gradient Force
Air flows from areas of high pressure (density)
to areas of low pressure (density) Pressure on
weather maps is indicated by isobars, or lines
of equal pressure The pressure gradient force is
in the direction from high to low pressure
weak pressure gradient force light winds strong
pressure gradient force strong winds
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Coriolis Force

• The Coriolis force is an apparent force that
  explains the deflection of a body moving across a
  rotating surface.
• The rotation of the Earth causes the wind to
• deflect to the right of its path in the northern
  hemisphere
• deflect to the left of its path in the southern
  hemisphere

High pressure in N hemisphere Add Coriolis bend
to the right
The coriolis force Increases with increasing
wind speed Is zero at the equator and strongest
at the poles
Low pressure in N hemisphere Add Coriolis bend
to the right
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Condensation

• Condensation occurs when the relative humidity
  exceeds 100
• Water only condenses on a surface
• Dew and frost condense on surfaces such as plants
  or windshields
• In the atmosphere water condenses on condensation
  nuclei (CN).

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Condensation Nuclei (CN)

• CN are tiny particles suspended in the atmosphere
• CN stay aloft in the air for many days. They are
  so small that their weight is less than their air
  resistance.
• Radius typically from 0.1 to 1 microns (micron
  10-6 meters)
• Concentrations from 1 to 1000 per cm3 of air
• Not all particles are good CN, effective CN are
• Soluble (for example salt)
• Or wettable (for example clay or minerals)
• But not hydrophobic (for example oils)

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Ice Nuclei

• Water does not always freeze at 32 F
• Water existing at temperatures below freezing is
  called supercooled
• Some particles cause supercooled water to freeze,
  these particles are known as ice nuclei
• Without ice nuclei, pure water would need to be
  40 F to freeze
• Some CN are also good ice nuclei, others are not

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Clouds in the Atmosphere

• Clouds are a collection of water drops and/or ice
  crystals
• Clouds form when water vapor in the atmosphere
  condenses
• Condensation only occurs on CN
• Water vapor condenses when the relative humidity
  exceeds 100
• This can happen if one or both of the following
  occurs
• 1) The air is cooled, reducing the saturation
  vapor pressure
• 2) Water vapor is added to the air
• Rising air expands, expanding air cools, so
  rising air can cause clouds

Most clouds occur in the troposphere There are
exceptions Noctilucent clouds (NLCs) occur in
the mesosphere Polar stratospheric clouds (PSCs)
occur in the stratosphere
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Cloud Formation
Imagine an air parcel, rising upward through the
atmosphere. The air parcel expands as it rises
and this expansion causes the temperature of the
air parcel to decrease.
As the parcel rises, it cools, and the humidity
increases until it reaches 100. When this
occurs, cloud droplets begin forming as the
excess water vapor condenses on CN particles.
Above this point the cloud droplets grow by
condensation in the rising air. If the rising
motion is sufficiently intense and enough water
vapor is present, precipitation will develop.
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Cloud Formation
Why does air rise? An air parcel will rise
naturally if the air within the parcel is warmer
than the surrounding air (like a hot air
balloon). As the earth is heated by the sun,
bubbles of hot air form (called thermals) and
rise upward from the warm surface.
Convergence is an atmospheric condition that
exists when there is a horizontal net inflow of
air into a region. When air converges along the
earth's surface, it is forced to rise since it
cannot go downward.
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Cloud Types

• Clouds are classified into broad categories
• High level
• cirrus clouds
• Altitudes above 20,000 feet
• Composed primarily of ice crystals
• Typically thin and white in appearance
• Mid level
• altocumulus, altostratus
• Altitudes between 6,500 to 20,000 feet.
• composed primarily of water drops, sometimes ice
  crystals

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Cloud Types continued

• Low level Stratus,
• nimbostratus
• Altitudes below 6,500 feet
• Usually composed of water drops
• Uniform, covers entire sky
• Vertically Developed
• cumulus and cumulonimbus (thunderstorms)
• Cloud top heights in excess of 39,000 feet
• Composed of water and ice together, often
  producing hail

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Cloud Types continued
Other cloud types that are uncommon Noctilucent
clouds (NLCs) occur in the mesosphere at polar
latitudes Polar stratospheric clouds (PSCs)
occur in the stratosphere at polar latitudes
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Summary
What is the atmosphere composed of? What are the
layers of the atmosphere, how are they
defined? What are the forces that govern
wind? What is required to form cloud particles?
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The End(Extra slides follow)
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Atmospheric Observations
The most common parameters measured Temperature
Thermometer Pressure Barometer Humidity
Hygrometer Wind speed and direction
Anemometer Precipitation rain gage (rain),
ruler (snow depth) Other parameters
measured Cloud coverage movement Radar,
satellite, human observer Precipitation using
Radar
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Atmospheric Observations

• In Situ Measurements, instrument is in contact
  with the subject
• Surface weather stations in most towns,
  observations every hour, temperature, pressure,
  humidity precipitation, wind, clouds
• Balloon one or two sites per state,
  observations twice a day, temperature, pressure,
  humidity, winds, from the surface to the
  tropopause
• Remote Measurements, instrument is far from the
  subject
• RADAR (radio detection and ranging) 1 or 2 per
  state, clouds precipitation, storm movement
• Satellites cloud images, water vapor
  measurements

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Geostrophic Wind

• Winds aloft (above 1000 m) flowing in a straight
  line, a balance between 2 forces
• Pressure gradient force (PGF)
• Coriolis force (CF)
• A wind that begins to blow across the isobars is
  turned by the Coriolis force until Coriolis
  force and PGF balance

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Gradient Wind

• Winds aloft in rotation, a balance of 3 forces
• Pressure gradient force (PGF)
• Coriolis force (CF)
• Centripital force (Ce)