Wicked Weather

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Wicked Weather

• WHAT YOU LEARN
• How we measure humidity.
• How fog, frost, and dew form.
• Why and how clouds form.
• Adiabatic Cooling and Warming.
• How clouds are classified.
• How orographic, convectional, and cyclonic
  precipitation occur

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WATER VAPOR IS THE MOST IMPORTANT ATMOSPHERIC
GAS. WHY?

• Water vapor is the source of moisture for clouds,
  rain, frost, etc.
• Water vapor releases latent heat when it
  condenses.
• Water vapor is a heat-absorbing gas.
• Water vapor influences our sense of temperature.

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WHAT IS HUMIDITY?

• Humidity is the amount of water vapor in the air.
  
• Specific humidity is the weight of water vapor in
  grams contained in a kilogram of air.
• Relative humidity is the ratio of the water vapor
  present to its potential water vapor capacity.

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WHAT IS SATURATION?

• Some water molecules leave (evaporate), others
  return to the water.
• Evaporation increases Vapor Pressure of air .
• Eventually water molecules leaving equals the
  amount returning.
• This is saturation (filled to capacity).

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http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/atmospheric_moisture/concept_of_saturation.h
tm
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WATER VAPOR CAPACITY

• The airs capacity to hold water vapor varies
  with temperature.
• Warm air can hold more water vapor than cold air.
  
• Air with a specific humidity of 7 grams/kg at 10
  C is saturated (100 RH).
• The same air with 7 grams/kg at 20 C has a
  relative humidity of ________

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SATURATION

• If air can no longer hold water vapor without
  condensing, it
• Is saturated.
• Has a relative humidity of 100.
• Has reached its dew point temperature.

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CONDENSATION

• Dew point temperature and relative humidity tell
  how close an air mass is to saturation.
• Air is saturated when the dew point temperature
  and the air temperature are the same.

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MEASURING HUMIDITY

• Relative humidity and dew point temperatures can
  be determined using a sling psychrometer.
• The difference between the wet and dry bulb
  temperatures is applied to a sliding scale or
  published tables.

http//images.usatoday.com/weather/photos/sling2.g
if
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HOW DOES DEW AND FROST FORM?
Moist air in contact with a cold surface

• Moist air in contact with a cold surface
  condenses into liquid water droplets. This is
  dew.

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• Moist air condenses on a surface below freezing,
  ice crystals form. This is frost.
• Dew and frost occur on cool, calm, clear nights
• Why? Radiation cooling

http//www.malawicichlidhomepage.com/macro_nature/
frost01.jpg
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WHAT IS FOG?

• A cloud on the ground
• Fog in California forms in two major ways
• Radiation cooling
• Advection cooling

http//www.photohome.com/pictures/sunrise-sunset-p
ictures/pier-trail-path-fog-1a.jpg
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RADIATION FOG

• Radiation to space on cold, calm, clear nights
  chills a deep layer of air below dew point.
• Water vapor condenses onto condensation nuclei.
• Tiny water droplets form radiation fog.

http//www.rjma.com/flight/airwaves/PH02068J.jpg
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WHY AND HOW CLOUDS FORM?

• Three things are required for clouds to form
• Water vapor in the air.
• Condensation nuclei.
• A cooling process.
• Adiabatic cooling forms clouds

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THE ADIABATIC PROCESS

• Adiabatic temperature change is cooling or
  warming caused by expansion or compression of a
  gas.
• No heat is added or subtracted.

http//www.kidsgeo.com/images/adiabatic.jpg
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HOW ARE CLOUDS CLASSIFIED?

• Clouds are classified by height and shape

http//www.enchantedlearning.com/cgifs/clouds.GIF
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PRECIPITATION

• Not all clouds produce precipitation.
• For precipitation large masses of air must be
  forced upward and cooled adiabatically.

http//imagecache2.allposters.com/images/pic/IMCca
rd/KD6061Raining-Cats-Dogs-Posters.jpg
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LIFTING MECHANISMS

• Orographic lifting - Moist air is lifted over
  mountain barriers

http//upload.wikimedia.org/wikipedia/commons/thum
b/b/b2/Orographic_lifting_of_the_air_-_NOAA.jpg/80
0px-Orographic_lifting_of_the_air_-_NOAA.jpg
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Convectional lifting - Unequal heating produces a
rising current of air.
http//geography-info.com/physical/images/6/convec
tional.gif
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Cyclonic or Frontal lifting - Warm air rises in
contact with a cold air mass.
http//upload.wikimedia.org/wikipedia/commons/thum
b/0/03/Cyclone_Larry_2006.jpg/600px-Cyclone_Larry_
2006.jpg
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Convergence - Air flows together
http//bimedia.ftp.clickability.com/wshmwebftp/Web
Stuff/Convergence_Divergence.jpg
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CONVERGENCE LIFTING

• Air masses converge.
• Air forced upwards.
• Common near Equator at Intertropical Convergence
  Zone (ITCZ).

http//ww2010.atmos.uiuc.edu/guides/mtr/cld/dvlp
/gifs/cnvrg1.gif
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LIGHTNING

• Lightning is associated with convectional lifting
  and cumulonimbus clouds.
• Rapid updrafts and downdrafts generate
  electricity which is discharged as thunder and
  lightning.

http//www.starstore.com/acatalog/lightning-bolt-p
oster.jpg
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cited

• College of Alameda Physical Geography,2004,
  Moisture in the Atmosphere Learning Module
  (internet).
• All photos cited on powerpoint.