The Gas Laws

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• The Gas Laws

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Temperature (T)

• Measure of the kinetic energy of the molecules.
• Average speed of an air molecule at room T ??
• 450 m/sec 1000 mph !
• Heating ? speeding up molecules
• Cooling ? slowing down molecules
• Absolute zero (0 K) No thermal motion

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Charles Law

• V c2 T (at constant pressure)
• V is proportional to T.
• V1/V2 T1/T2
• Example 2 litres/ 3 liters 200 K/ 300 K

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Temperature scales

• Kelvin (K) Centigrade (C) Fahrenheit (F)
• Boiling 373 100 212
• Freezing 273 0 32
• Ab. Zero 0 -273 -459
• Temperature conversions
• K C 273 for example 0 C 273 K
• C 5/9 (F-32) for example 70 F 21 C 32 F
  0 C
• F 9/5 C 32 for example 0 C 32 F 40 C
  104 F
• http//www.eskimo.com/jet/javascript/convert.ht
  ml

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Pressure -- P

• Pressure is defined as force/unit area.
• It is the force exerted by the weight of the
  atmosphere.
• 1 atmosphere 1013 mb 14.7 lbs./in.2
• How much force does the atmosphere exert on your
  body?
• 1000 in.2 x 14.7 lbs./in.2 14,700 lbs. over 7
  tons of force!
• Air molecules at sea level collide 109 times per
  second.
• Air pressure decreases with height (faster in
  cold air).
• 29.92 in. of mercury 34 feet of water
• 1 atmosphere 1013.25 millibars (mb) 1013.25
  hPascals
• This is standard sea level pressure

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An experiment on a bird in the air pump, by
Joseph Wright
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Magdeburg Hemispheres (1657)

• In Germany Otto von Guericke constructed an air
  pump which he used to evacuate two large sealed
  hemispheres. The weight of the air pushing the
  hemispheres together was so great that 16 horses
  were needed to pull them apart.

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Torricellian Experiment (1644)

• Nature does not abhor a vacuum and the air has
  weight.

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• Density -- ?? mass/volume kg/m3
• Dry air is slightly denser than moist air at the
  same T and P

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Boyles Law

• PV constant
• P1V1 P2V2
• Example 1 atm. x 1 liter 0.5 atm. x 2 liters
• In the atmosphere, use density instead of volume.
• P c1 ? (at constant temperature)

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• Ideal Gas Law
• P ? RT
• Where P is pressure, ? is density, R is the gas
  constant, and T is temperature in degrees Kelvin.
• At the same pressure, cold air is denser than
  warm air.

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• Daltons Law of Partial Pressures
• The total pressure of the air is the sum of the
  partial pressures of its component gases.
• Ptotal PN2 PO2 e Ptrace

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Moisture and Clouds
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Moisture

• Moisture is critically important to all sorts of
  weather phenomena.
• If our atmosphere was as dry as Mars, we would
  have winds and sandstorms, and thats about it.
• On Earth, we have dew, fog, mist, clouds, frost,
  rain, snow, sleet, hail, ice, colored sunsets,
  rivers, lakes, oceans, and many other phenomena
  -- all because of moisture.

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Measuring humidity

• Sling psychrometer -- ventilated wet bulb and dry
  bulb thermometers.
• Hair hygrometer -- hair lengthens with humidity
  (about 2.5). Twisted hair generates torsion.
• Dew Cell -- measures humidity by an electrical
  conductor coated with LiCl, a chemical whose
  conductivity varies with moisture.

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Hydrologic Cycle
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Moisture controls the heat budget of the planet

• Water vapor is a major greenhouse gas.
• Clouds, snow and ice reflect sunlight back to
  space.
• Sensible heat is carried by winds and ocean
  currents
• Oceans store heat and moderate coastal climates.
• Snow is an effective insulator.
• Vegetation alters both the heat and moisture
  budgets.
• Latent (or hidden) heat is exchanged when water
  changes from one phase to another.
• Latent heat powers weather systems

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• Ice Liquid Vapor
• highly orderly one free surface random
• lower energy higher energy
• lower temperature higher temperature
• Sublimation, melting, and evaporation all require
  heat from the environment.
• Deposition, freezing, and condensation all
  release heat to the environment.

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• Temperature and phase changes of 1 gram of ice as
  energy is added.

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Relative Humidity

• Vapor pressure, e is that portion of the total
  air pressure exerted by the water vapor. From
  Daltons Law of Partial Pressures, e P (H2O).
• Saturation vapor pressure, es is the maximum
  possible e at a given T. It assumes that the
  water vapor and a plane water surface are in
  equilibrium.
• Relative Humidity RH e/es x 100
• Air does not hold water vapor the dry gases in
  the air coexist with water vapor near the ground.
  
• It is temperature (mainly) that affects the rate
  of water vaporization.
• Imagine water molecules leaving and entering
  Johnson Pond. When they are inequilibrium (for a
  given T) then you measure e.
• As temperatures drop overnight e lowers and RH
  rises
• If a fog covers the pond, then you can assume e
  es

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Dew Point

• Dew Point, Td is the temperature to which air
  must be cooled at constant pressure to reach
  saturation. Dew point indicates the temperature
  at which dew, frost, fog may occur.
• Often the dew point temperature in the evening is
  a good predictor of the overnight minimum
  temperature? Why?
• High Td means high absolute humidity.
• If T and Td are almost the same then RH is high.
• If T Td, then RH 100

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Saturation vapor pressure as a function of T
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Boiling

• The boiling point is defined as the temperature
  at which the saturation vapor pressure of a
  liquid (es) is equal to the surrounding
  atmospheric pressure.
• For water, es 1 atm. at 100 C.
• At pressures greater than 1 atm. (e.g. in a
  pressure cooker), the boiling temperature is
  greater than 100 C.
• At pressures lower than 1 atm. (e.g. on a
  mountain top), the boiling temperature is less
  than 100 C.

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BOILING occurs when es 1 atm.
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Clouds

• Clouds are aesthetically pleasing. They generate
  rainbows, cloudbows, halos, and gorgeous sunsets.
  Cloudscapes are half of all landscapes.
  Clouds indicate important meteorological
  processes such as convection, fronts, and
  mountain waves. Clouds are the source of rain,
  snow, thunder, lightning, hail, and many other
  weather phenomena. The Earth from space appears
  to be about 50 covered by white clouds of all
  shapes.

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Clouds

• Eternal Clouds, let us appear let us arise from
  the roaring depths of Ocean, our father let us
  fly towards the lofty mountains, spread our damp
  wings over their forest-laden summits. . . let us
  shake off the rainy fogs, which hide our immortal
  beauty and sweep the earth from afar with our
  gaze
• -- Aristophanes, The Clouds
• I am the daughter of Earth and Water,
• And the nursling of the Sky
• I pass through the pores of the ocean and shores
• I change, but I cannot die.
• -- Percy Bysshe Shelley, The Cloud

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Fog is a Cloud on the Ground

• How Fog is Formed
• Radiation fog (local)-- Radiational cooling of a
  shallow moist layer with dry layer above it.
  Dissipates with morning sun.
• Evaporation fog (local) -- Cold air in contact
  with a warmer water surface (e.g. lakes in
  autumn).
• Upslope fog (mountains) -- gentle lifting of a
  moist layer.
• Advection fog (regional) -- warm moist air moves
  over a cold surface. E.g. Pacific coast cold
  ocean surface.
• Precipitation fog (regional) -- warm rain falls
  through a layer of cold air or over a snowfield.

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Four Basic Cloud Types

• High, above 6 km
• Cirrus (Ci), Cirrostratus (Cs), Cirrocumulis
  (Cc)
• Middle, 2-6 km
• Altostratus (As), Altocumulus (Ac)
• Low, 0-2 km
• Stratus (St), Stratocumulus (Sc), Fog
• Vertical Development, 1-10 km
• Cumulus (Cu), Cumulonimbus (Cb)
• Luke Howard (1803) Stratus layer, Nimbus
  rain, Alto mid-level, Cumulus heap, Cirrus
  wisp

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• UK Met Office Cloud Guide
• http//www.metoffice.com/bookshelf/clouds/

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• Cloud type information and photos
• Plymouth State University
• Meteorology Program
• Cloud Boutique
• http//vortex.plymouth.edu/clouds.html