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Summary of Septembers Weather

• NOWData, Observed Weather, Preliminary
  Climatology Data http//www.weather.gov/climate/i
  ndex.php?wfotfx
• Average Temp 55.9 F, 1.3 F above normal
• 10 days with rain 0.01 in
• 2 days with min. temperature
• 20 clear days
• 9 partly cloudy days
• 1 cloudy day!
• Total HDD (base 65) 266, 60 less than normal
• Total HDD since July 1 302, 199 less than
  normal
• Total CDD (base 65) 0, 14 less than normal
• Total CDD since Jan 1 311, 52 above normal

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Geopotential Heights and Winds

• Can you predict wind directions based on 500 mb
  contours?
• Univ. of Arizona
• 500 mb heights http//www.atmo.arizona.edu/produ
  cts/wximagery/500mbwv.html
• UNISYS
• 500 mb height winds
• http//weather.unisys.com/upper_air/ua_500.html

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Sea level pressure and suf. winds

• Can you relate cyclones / anticyclones with on
  surface winds?
• UNISYS
• surface winds http//weather.unisys.com/surface/
  sfc_con_stream.html
• sea level pressure http//weather.unisys.com/sur
  face/sfc_con_pres.html
• Univ. of Illinois
• sea level pressure (isobars) http//ww2010.atmos
  .uiuc.edu/28Gh29/wx/surface.rxml

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Atmospheric Moisture(GEOG 303, 2 October 2008)

• Water vapor and liquid
• How do we measure water vapor content?
• How does air become saturated?

5
b. Water vapor and liquid water
evaporation condensation saturation
evaporation condensation
No evaporation
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c. How to we measure water vapor?

• HOW?
• 1. Vapor pressure mb
• Partial pressure exerted by
• water vapor
• Varies with temperature
• 1.1 Saturation vapor pressure mb
• Maximum vapor pressure that can exist
• Depends solely on temperature

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c. How to we measure water vapor?

• 1.1 Saturation vapor pressure

• exponential increase with temperature

Saturation vapor pressure (mb)
Condensation occurs
Air unsaturated
Temperature (C)
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c. How to we measure water vapor?

• HOW?
• 2. Specific humidity g/kg
• Mass of water vapor (mv) g existing in a given
  mass of air (m OR mv m) kg
• Does not vary with temperature
• Normally
• 2.1 Saturation specific humidity g/kg
• Maximum specific humidity that can exist
• Depends solely on temperature, and is thus
  directly related to saturation vapor pressure

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c. How to we measure water vapor?

• (1.1) Saturation vapor pressure (2.1)
  saturation specific humidity

Saturation vapor pressure (mb)
Saturation specific humidity (g/kg)
Condensation occurs
Air unsaturated
Temperature (C)
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c. How to we measure water vapor?

• HOW?
• 3. Mixing ratio g/kg
• Mass of water vapor g existing in a given mass
  of dry air kg
• Same properties as specific humidity
• 3.1 Saturation mixing ratio g/kg
• Maximum mixing ratio that can exist

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c. How to we measure water vapor?

• HOW?
• 4. Relative humidity (RH)
• Amount of water vapor in air relative to maximum
  possible at current temperature
• RH (specific humidity / saturation specific
  humidity)

Does not depend on temperature
Depends on temperature
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c. How to we measure water vapor?

• Relative humidity (RH) is a poor measure of water
  vapor content, because it changes with
  temperature

RH ()
RH (specific humidity / saturation specific
humidity)
Saturation vapor pressure (mb)
Saturation specific humidity (g/kg)
T (F)
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c. How to we measure water vapor?

• Relative humidity (RH) is a poor measure of water
  vapor content, because it changes with
  temperature

RH ()
T (F)
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c. How to we measure water vapor?

• HOW?
• 5. Dew point temperature or Frost point
  temperature C
• Temperature at which saturation occurs
• Indicates moisture content

From figure on Slides 7, 9
Mass of water vapor
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d. How does air become saturated?

• Add water vapor to the air
• e.g. Warm shower add vapor to air, bringing it
  to the saturation point condensation forms, and
  then fog develops
• 2. Mix cold air with warm, moist air
• e.g. Contrails behind jets traveling at high
  altitudes steam fog

http//www.crystalinks.com/contrail607.jpg
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d. How does air become saturated?

• Cool the air to the dew point
• to be continued

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COMPLICATING DETAILS

• Effect of curvature of water droplets

• Small drops exhibit greater curvature
• Influences saturation vapor pressure
• Supersaturation may occur (saturation above 100)

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COMPLICATING DETAILS

• Effect of curvature of water droplets
• Small droplets require higher RHs to remain liquid

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COMPLICATING DETAILS

• Condensation nuclei
• natural (salt, dust, ash) and anthropogenic
  (combustion byproducts) particles onto which
  water droplets form
• THUS, water droplets are not pure water!
• Ice nuclei
• Atmospheric water does not freeze at 0C
• Leads to supercooled water
• At or below -40oC (-40oF) spontaneous
  nucleation

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COMPLICATING DETAILS

• Effect of solution
• Evaporation from solutions is less than from pure
  water
• This fact opposes the effects of curvatureto
  condensation usually occur at 100 RH.

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d. How does air become saturated?

• Cool the air to the dew or frost point
• Two ways to do this
• a) Diabatic processes
• energy is added to
• or removed from a
• system (heat moves from
• high to low temps)
• e.g. warming water
• with a stove

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d. How does air become saturated?

• Cool the air to the dew or frost point
• Two ways to do this
• b) Adiabatic processes
• Temperature changes but no heat is added
• Details found in the 1st law of thermodynamics
• Expanding air cools, air undergoing compression
  warms

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d. How does air become saturated?

• b) Adiabatic processes
• Dry air cools at a rate of 1C/100 m (5.5F/1000
  ft), defining the dry adiabatic lapse rate

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d. How does air become saturated?

• Dry adiabatic lapse rate
• 1 C/100 m (5.5 F/1000 ft)
• Bozeman 1360 m (4475 ft)
• Bridger Bowl 1860 m (6100 ft)
• 1860 m -1360 m 500 m (1625 ft)
• 5C (9F) cooling

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d. How does air become saturated?

• Dry adiabatic lapse rate
• 1 C/100 m (5.5 F/1000 ft)
• Seattle to Snoqualmie Pass 914 m (3000 ft)
  9C (16.5F) cooling

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d. How does air become saturated?

• Wet (saturated) adiabatic lapse rate 0.5
  C/100 m (3.3 F/1000 ft)
• Seattle to Snoqualmie Pass 914 m (3000 ft)
  4.6C (10F) cooling

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d. How does air become saturated?

• Environmental lapse rate
• Rate at which ambient temperature decreases with
  height
• Changes diurnally and from place to place

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EXAM 1 next Thursday, 9 Oct.

• Aguado and Burt Chapters 1-5
• Read the text book
• Be able to answer question at end of chapter
• Review your notes
• Review the lectures as needed
• Multiple choice, matching, short answer, be
  familiar with interpreting graphs