Transcript Name of presentation - Weather and Climate

Lab 6:
Saturation & Atmospheric
Stability
Review Lab 5 – Atm. Saturation
• Relative humidity?
• Mixing ratio / saturation mixing ratio?
– Function of temp.. Clausius-Clapeyron curve
– Sling psychrometer – what does this give us?
• Dew point?
– Looking at RH equation above, when temp is reduced, all
else being equal, what happens to the RH of a sample of
air? Does RH go up or down?
• Air is saturated when RH=100%
Lab 6
• Lab 6: Saturation and Atmospheric Stability
– processes that influence atmospheric saturation – i.e., cause cooling
and/or increase in water vapor content
– atmospheric processes that change either the temp and/or water
vapor content of an air sample
– In this lab, we’ll focus on atmospheric mixing and adiabatic cooling
and some processes that drive these conditions
Saturation & Atmospheric Stability
• Two main ways for air to reach
saturation:
1. Cooling to its dew point temperature (most
common)
2. Increasing water vapor content
Remember
Condensation produces:
1. Fog
2. Dew
3. Clouds
1.
2.
*ALL require saturated air to form!
3.
Atmospheric Mixing
• When two air masses of different temps and
water vapor content mix
• When they mix, the new air mass will change
in temp and water vapor
– resulting in new mixing and saturation mixing
ratios
– Changes relative humidity
Mixing Ratio = SMR * RH
“Saturated air”
100% RH
Assuming the two mixing
air masses are the same
size and you know the
temps and RH find:
1.The new temp of mixed
air mass
2.The new mixing ratio of
the mixed air mass
3.From the above, you can
find the new RH (due to
change in temp and water
vapor)
Adiabatic Cooling
• Adiabatic temperature changes:
– Temperature changes in which heat was neither added nor subtracted
(closed system)
– Average internal energy decreases with expansion – changes in average
kinetic energy
• Compressed air = warm air
• Expanded air = cooler air
NOTE: If a parcel moves ↑, it
passes through regions of
successively lower pressure:
•Ascending air: EXPANDS
•Descending air: COMPRESSES
Saturation & Atmospheric Stability
DRY adiabatic rate: unsaturated
• cools at a constant rate of
10°C/1km of ascent
• warms at constant rate of
10°C/km of descent
WET adiabatic rate: saturated
(has RH 100%)
• Slower rate of cooling
caused by the release of
latent heat
– Rates vary between 5°C &
9°C/1km
» Amount of LH
released depends on
quantity of moisture
in the air
LCL = altitude at which a parcel reaches
saturation & cloud formation begins
Dew Point rate:
• 2°C/1km to the LCL
• At the WALR after the LCL
Saturation & Atmospheric Stability
DALR = 10°C/1km
WALR = 5 – 9°C/1km
Parcel A
Temperature (°C)
Height (km)
Parcel B
Temperature (°C)
5.0
4.5
WALR
Air decreases by
2.5°C
4.0
3.5
3.0
2.5
DALR
Air decreases by
5°C
LCL
10.5
°
13°
2.0
1.5
18°
1.0
23°
0.5
28°
surface
10°
• Lifting Condensation Level (LCL):
– Reached when ascending air cools to its dew point
(saturation = 100% RH) – clouds form
– If it continues to rise:
• Cools at the wet adiabatic lapse rate (between 5°& 9°C)
– Calculated based on:
• Surface temperature & dew point temperature
Part II
Review
• What is adiabatic cooling?
– Wet versus dry
– Thinking about atmospheric saturation, how does
this influence cloud formation (hint: think about
dew point temperature, etc.)
• What is environmental lapse rate?
• Atmospheric lifting forces:
1. Surface heating (air expansion, less dense, rise,
etc..)
2. Two surface air masses colliding (convergence)
3. Contact of dissimilar air masses along warm &
cold fronts (convergence)
4. Topographic barriers (e.g. orographic lift)
5. Upper air divergence
• Rising air doesn’t mix substantially with the surrounding
atmosphere. Once the initial lifting force stops, the
continued rising of an air parcel depends on atmospheric
stability (the state of the atmosphere surrounding the
parcel).
Orographic Lifting
•Air ascends: adiabatic cooling often generates clouds & lots of precipitation
•Air descends: warms adiabatically, making condensation & precipitation less
likely
Incorporating Dew Point
LCL
Td (dew point)
cools at:
• 2°C/km below the
LCL
•The WALR above the
LCL
LCL – T(°C) – Td(°C)/8
25°C – 13°C = 12°C/8 = 1.5 km
LAYERED CLOUDS not much vertical development
• Absolute stability:
– Environmental lapse rate is less than the wet adiabatic rate
(surrounding air cools slower with height)
– Stable air resists vertical movement, and doesn’t want to move. If it
gets forced above LCL it would remain cooler and return to surface
– Note: air parcel cools faster than ELR
VERTICAL CLOUDS potential for
thunderstorms
• Absolute instability:
– Environmental lapse rate is greater than the dry adiabatic
rate (surrounding air cools faster w/ height)
– Unstable air rises because of its buoyancy
– Parcel of air cools slower than ELR
• Conditional stability:
– Moist air has an environmental lapse rate
between the dry & wet adiabatic rates