The Earth’s Atmosphere
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
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 32°F and boils at 212°F
Celsius (C): water freezes at 0°C and boils at 100°C, 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
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
Pressure continued
Since the number of air
molecules above some altitude
decreases with height, pressure
likewise decreases with height.
Pressure decreases exponentially
with altitude
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
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”
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.
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
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
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 (-226°F) 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
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.
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
Weather Patterns
Weather patterns are more complex than the global circulation
Areas of high and low pressure change the weather frequently
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 Earth’s surface is slowed by friction
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
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
The coriolis force:
Add Coriolis
bend to the right
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
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).
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)
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
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
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.
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.
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
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
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
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?
The End
(Extra slides follow)
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
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
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
Gradient Wind
Winds aloft in rotation, a balance of 3 forces
•Pressure gradient force (PGF)
•Coriolis ‘force’ (CF)
•Centripital force (Ce)