8. A stable atmosphere. An absolutely stable
atmosphere exists when a rising air parcel is
colder and heavier (i.e., more dense) than
the air surrounding it. If given the chance
(i.e., released), the air parcel in both
situations would return to its original
position, the surface.
An unstable atmosphere. An absolutely
unstable atmosphere exists when a rising air
parcel is warmer and lighter (i.e., less dense)
than the air surrounding it. If given the chance
(i.e., released), the lifted parcel in both
situations would continue to move away
(accelerate) from its original position.
16. • Result from radiational cooling of the ground
• Occur on cloudless nights – nocturnal
• Typically surface based
• Are intensified in river valleys
• Cause pollutants to be “trapped”
Fig 3.3
What happens to inversion when sun rises?
تشعشعی وارونگیRadiation Inversion
17. فروکشی وارونگیSubsidence Inversion
• Associated with high-pressure systems
• Inversion layer is formed aloft
• Covers hundreds of thousands of square kms
• Persists for days
هست در هوا ، است معروف ها سیکلون آنتی به که فشار پر سیستم با است مرتبط که فروکشی وارونگیبه آن مرکزی ه
در باال سمت به شود می دیده شکل در که گونه همان بیرونی های لبه در هوا ولی بوده حرکت در پایین سمتمی حرکت
باشد.
18. • Breakup after sunrise
• Breakup results in elevated ground level
concentrations
• Breakup described as a fumigation
تشعشعی وارونگیRadiation Inversion
22. Inverted Lapse Rates (Strongly Stable)
• Characterized by increasing temperature with height
Does it occur during the day or at night?
Is it associated with high or low pressure systems?
Does it improve or deteriorate air quality?
Inversion
26. The inversion layer prevents pollutants from
escaping into the air above it. If the inversion
lowers, the mixing depth decreases and the
pollutants are concentrated within a smaller
volume.
The smoke from the shorter stack is trapped
within the inversion, while the smoke from
the taller stack, above the inversion, rises,
mixes, and disperses downwind.
28. This is an example of a
generalized temperature
profile for a surface
inversion.
Temperature-profile
changes in bottom diagram
after the sun has heated
the surface.
29. A temperature inversion, in which a warm air layer sits atop a cooler air layer, can take place in
either of two sets of topography and weather conditions. Air pollutants can build to harmful
levels during an inversion, which can occur during cold, cloudy weather in a valley surrounded
by mountains (left). Frequent and prolonged temperature inversions can also occur in an area
with a sunny climate, light winds, mountains on three sides, and the ocean on the other
(right). A layer of descending warm air from a high-pressure system prevents ocean-cooled air
near the ground from ascending enough to disperse and dilute pollutants. Because of their
topography, Los Angeles, California (USA), and Mexico City, Mexico, have frequent
temperature inversions, many of them prolonged, during the summer and fall.
30. Thermal (temperature) inversion
• Usually, tropospheric air temperature
decreases as altitude increases.
– Warm air rises, causing vertical
mixing.
• Thermal inversion: a layer of cool air
occurs beneath a layer of warmer air
– Inversion layer: the band of air in
which temperature rises with
altitude
– Denser, cooler air at the bottom of
the layer resists mixing.
– Pollutants are trapped at ground
level.
31. Comparison of environmental lapse rates with the adiabatic
lapse rate with relationships to stability conditions.
ناپایدار خنثی
پایدار پای بسیاردار
32. Lapse Rate: Negative of the temperature
gradient in the atmosphere
Concept that describes (non-)movement of air near the surface
Dry adiabatic lapse rate ()
= 0.976 oC/100 m ~ 1 oC/100 m
International standard lapse rate
= 0.0066 oC/m (avg mid-latitude)
Q: Which one has a larger temperature
change for the same change in elevation?
Why?
Q: Does lapse rate have anything to do with
air quality?
33. Neutral Lapse Rates
• Temperature decreases are similar to the adiabatic lapse rate
• Results from:
– Cloudy conditions
– Elevated wind speeds
– Day/night transitions
• Describes good dispersion conditions
Isothermal Lapse Rates (Weakly Stable)
• Characterized by no temperature change with height
• Atmosphere is somewhat stable
• Dispersion conditions are moderate