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Office HoursTue: 12:30 PM to 2:30 PMWed: 9:00 AM to 10:30 AM & 12:00 PM to 2:00 PMThr: 9:00 AM to 10:30 AM
Course Syllabus can be found at:
http://www.wx4sno.com/portfolio/BSU/Fall_2011/
This lecture will be posted AFTER class at:
http://www.wx4sno.com/portfolio/BSU/Fall_2011/Lectures/
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Lesson 15Adiabatic Processes
Hess, McKnight’s Physical Geography, 10 ed.
138 – 139 pp.
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Adiabatic ProcessesRecall prior to Fall break that as
the temperature of an air parcel decreases, its relative humidity increases.
Once a parcel has cooled to the dew point, it becomes saturated and condensation occurs.
Adiabatic cooling is one of the most common ways clouds form.
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Adiabatic CoolingClouds are the result of
condensationAgain, condensation occurs when
an air parcel’s temperature reaches the dew point
Air parcels are free to flow horizontally or vertically through the atmosphere◦Most commonly, clouds are formed
with vertical movement of air parcels, namely rising air
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Adiabatic Cooling, cont.As an air parcel rises, it becomes
less-pressurizedWith less pressure exerted on the
parcel, it therefore expandsThis expansion causes cooling and increases RH
◦Adiabatic cooling
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Adiabatic HeatingConversely, as a parcel of air
descends it compresses due to higher pressure
This is a warming process, therefore the air warms
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Dry Adiabatic RateConsider an air parcel that is
unsaturated◦RH < 100%
If this air parcel rises, it will cool at what is known as the dry adiabatic lapse rate (DALR)◦10°C per 1000 meters◦10°F per 1000 feet
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Dry Adiabatic Rate, cont.As this air parcel rises its relative
humidity increases (because it’s expanding)
Eventually the air temperature cools and equals the dew point temperature◦ At this point the RH=100%
The elevation that this occurs at is known as the lifted condensation level (LCL)◦ At this point, condensation occurs and cloud
forms
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Moist Adiabatic RateAt this point, the air parcel is
saturated and the RH=100%If the parcel continues to rise, it
will cool even furtherHowever, because it is saturated
(i.e. 100% RH) it will cool slower (MALR)◦6°C per 1000 meters◦3.3°F per 1000 feet
This is the result of latent heat which is released once condensation begins
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Heat Transfer TheoryEvaporation is a cooling process
because liquid water is converted into a gas, which takes heat energy from the environment.
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Final Thoughts…Dry Adiabatic Lapse Rate (DALR):
◦10°C per 1000 meters◦10°F per 1000 feet
Moist Adiabatic Lapse Rate (MALR):◦6°C per 1000 meters◦3.3°F per 1000 feet
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Lesson 16Stability
Hess, McKnight’s Physical Geography, 10 ed.
143– 147 pp.
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Stable vs. UnstableThe atmosphere and individual air
parcels can be characterized as either stable or unstable.
Stability occurs when vertical motion is suppressed.◦ No clouds form (unless they are forced, i.e.
orographic lift)Instability (unstable air) occurs when
air tends to rise on its ownThe temperature of an air parcel
relative to the air which surrounds it determines stability
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Lapse RatesWe know from the previous
lecture there are two types of adiabatic lapse rates◦Dry adiabatic lapse rate (DALR)◦Moist adiabatic lapse rate (MALR)
There is a third lapse rate, known as the environmental lapse rate (ELR)
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Environmental Lapse Rate (ELR)The ELR is basically the actual air
temperature at any height in the atmosphere
It is also known as a temperature profile, vertical temperature gradient, or normal lapse rate
The average value:◦6.5°C per 1000 meters◦3.6°F per 1000 feet
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Types of Stability/InstabilityThere are three types:
◦Absolute stability◦Absolute instability◦Conditional instability
Let’s look at examples of each of these…
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Absolute Stability
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Absolute Instability
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Conditional Instability
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Homework Due FridayPlease omit pages 85, 86, 91,
and 92 from your homework that is due this Friday.