Moisture, Optics, and Instrumentation

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Moisture, Optics, and Instrumentation

• Lynn Gribble
• 5/28/09

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Stability review

• RECALL
• The environment is the skew-t
• The parcel is either wet or dry
• We are concerned about the condition of the
  PARCEL relative to the environment (warmer,
  cooler)

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Stability Review

• Absolutely unstable
• Gwet (6)ltGdry (10)ltGenv
• ie whether moist or dry, the parcel is ALWAYS
  warmer than the environment, therefore it WILL
  rise.
• Absolutely stable
• GenvltGwet (6)ltGdry(10)
• ie whether moist or dry, the parcel is ALWAYS
  cooler than the environment, therefore it WILL
  NOT rise.
• Conditionally unstable
• Gwet(6)ltGenvltGdry(10)

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Adiabatic Review

• Q W U Latent Heat
• Q Heat
• W Work (depends on volume and pressure)
• U Internal Energy (depends on temperature
• Latent heat is released during condensation
• Adiabatic means that there is no exchange of heat!

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Practice

• So what is the Stability of an Environmental
  lapse rate of 5C/km?
• 15C/km?
• For a given set of atmospheric conditions, we can
  say that clouds will form at 5C. The surface
  temperature is 30C. Assuming dry adiabatic
  assent until cloud formation, what height will
  clouds form?

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Temperature Inversions

• On a summer morning, do your feet ever feel
  colder than your head?
• On still nights, it is possible for the surface
  to be colder than the air above it. (Nocturnal
  Inversion)

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Temperature Inversions
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Temperature Inversion

• Inversions are extremely stable
• They stop air (and anything in it) from mixing
• Situations where inversions are likely to occur
• Clear, calm nights
• Long nights (winter)
• Snow cover
• Topographic forcing (valleys)

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Wind Chill

• Wind Chill describes the increased loss of heat
  due to movement of the air. (Fxn of T and wind
  speed)
• Basically takes into account how quickly wind
  removes the small layer of body heat warmed air
  surrounding your body.

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Eco-tip 3 Walk it out

• Up to half of journeys made by car in developed
  countries are for trips of less than 2 miles.
• Try to walk, bike, or take public transport (CDTA
  is free for SUNY students!!) for short trips.
• This is not only good for the environment
  (reduced CO2 and ozone emissions, less fossil
  fuel usage), but its great for us too (lets keep
  heart disease down)!

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Evaporation

• Water can account for up to 4 of the mass of the
  atmosphere.
• Evaporation is the primary source of water vapor
  in the atmosphere
• Transpiration also injects moisture into the
  atmosphere.
• Saturation no net change

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Expressing Amounts of Moisture

• Mixing ratio mass water/total mass of air (g/kg)
• absolute measure of vapor content. (Does not
  change when ?T, ??)
• Vapor Pressure the portion of atmospheric
  pressure that is contributed by vapor
• Saturation Vapor Pressure the maximum vapor
  pressure for a given temperature (ie when the
  atmosphere is saturated, it is holding the most
  water that it can)

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Expressing Amounts of Moisture

• Relative Humidity
• 100(vapor pressure/saturation vapor
  pressure)
• dependant on what?
• Dew point/Frost point The temperature to which
  air must be cooled, so that it becomes saturated
  at constant pressure. (ie when dew will form)
• If saturation temperature is below freezing,
  frost will form.
• D\Media\Active_Figures\45_Water_Vapor\T_45.html

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Atmospheric Moisture and Health

• Heat stroke and exhaustion
• Sweat cools our bodies through evaporation
• If the air is nearly saturated, evaporation
  happens slowly
• If our bodies dont cool fast enough, organs can
  fail and death can occur
• Heat Index gives an idea of how hot it feels
  based on heat and humidity

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Atmospheric Moisture and Health
Box 4-1
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5 Minute Break

• Reminders
• 1st homework passed out today
• 1st Quiz Monday

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Cloud Formation

• Solute effect It is easier for water to condense
  on salt molecules than other water molecules.
• Condensation can occur _at_ RH lt 100
• Curvature effect It is harder for water to
  condense onto a curved surface than on a flat
  one.
• Condensation occurs _at_ RHgt100
• These effects counteract each other.

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Cloud Formation

• Nucleation the formation of cloud droplets
• Homogeneous Nucleation
• (Only occurs _at_ Tlt-40C)
• Heterogeneous Nucleation
• Cloud condensation nuclei
• Hygroscopic attract water
• Salt, soot ash, dust
• Ice Nuclei silver iodide
• Same crystal structure as ice

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Cloud Formation

• Ice Nuclei
• Super cooled water less than 0C (freezing
  nucleation)
• Condensation Nuclei
• Hydrophobic repel water
• Hygroscopic attract water
• Examples see previous

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Cloud Formation (Lifting Mechanisms)

• Most often clouds dont form by free convection
  (recall lapse rates and stability)
• Clouds form by various types of forced lifiting.

Figure 4-13
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Cloud Classification

• Clouds are classified in several ways
• Layered (strato-) vs Convective (cumulo-)
• Precipitating (Nimbo)
• Altitude of occurance
• High Cirro-
• Middle Alto-

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Cloud Classification

• Figure 4-16

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Fog

• Radiation Fog (autumn, low lying areas)
• Advection Fog (California Coast)
• Evaporation/Frontal Fog
• Upslope Fog
• Basically you can get fog whenever changes in
  temperature or moisture content causes surface
  air to become saturated.

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Precipitation Processes
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So how do cloud droplets grow?

• Warm Clouds Collision-Coalescence

Figure 4-31
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How cloud droplets grow cont.

• Accretion/Aggregation (ice and water/ice and
  ice)
• Bergeron Process ice crystals grow at the
  expense of super-cooled water droplets due to the
  greater saturation vapor pressure over water than
  over ice.

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Bergeron Process
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Precip Types
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Precip Types

• Rain Tgt0 for majority of atmospheric column
• Snow Tlt0 for entire atmospheric column
• Freezing Rain start as ice but Tgt0 everywhere
  except the surface
• Sleet Tgt0 for a portion of the column
• Hail Portion of column below freezing, strong
  updrafts (T-storms)

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Optics

• Atmospheric Refraction and Scattering create many
  of the interesting optical effects that we
  witness here on earth. (Figure 5.29)

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Optics

• Mirage formed by the refraction of light
• Strong refraction occurs with large temperature
  gradients (hot asphalt for example)

Figure 5.24
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Optics

• Halos a whitish rainbow that encircles but does
  not touch the moon or sun, caused by refraction
  of light through ice crystals.

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Optics

• Green Flash caused by green being refracted more
  than red, but not scattered as much as blue and
  violet.

Figure 5-30
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Optics

• Rainbows refraction by raindrops, 40 arc

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Instrumentation

• Official Surface Measurements are made by the
  Automated Surface Observing System (ASOS)
• Temperature resistance thermometer
• Humidity dewpoint hygrometer
• Pressure aneroid barometer
• Wind wind vanes and cup anemometers
• Precipitation heated tipping cup rain gauge
• Caveats?

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Instrumentation (Indirect Methods)

• Satellite
• Visible
• IR infra red
• WV water vapor
• RADAR
• Velocity
• Reflectivity
• See weather.gov