Assignment 3a Solutions
1. Why does a "parcel of air" cool as it rises? Warm as it descends?
This is really a three-part answer─two parts for a "parcel of air" as it rises and
cools at the dry adiabatic rate and the wet adiabatic lapse rate (A and B), and
one part for a "parcel of air" as it descends and warms at the wet adiabatic
lapse rate (C).
A. As a "parcel of air" rises from the ground to the lifting condensation
level (LCL), it cools at the dry adiabatic lapse rate (DALR), which is a
fixed rate of 5.5ºF/1000 ft. The “parcel of air” cools when it rises at this
rate because there is a decrease in pressure and a decrease in
temperature at higher altitudes.
B. At the Lifting Condensation Level, which is the altitude at which the
“parcel of air” becomes 100% saturated, the rate changes to the Wet
Adiabatic Lapse Rate (WALR). Where as DALR is a fixed rate of
5.5ºF/1000 ft, the WALR varies according to pressure and temperature
at different altitudes. The diagram above shows the WALR at
2.7ºF/1000 ft. Air continues to cool for the same reasons it did at the
DALR (temperature and pressure decreases), but now because the
“parcel of air” is 100% saturated, it has to give off moisture, called
latent heat release, as the temperature drops. This moisture can form
clouds, which begin to appear at the LCL.
C. As a "parcel of air" descends on the leeward side of the mountain, it
descends at the dry adiabatic rate because the “parcel of air” is
warming at it descends in altitude (due to pressure and temperature)
and does not give off moisture to the outside air. A wetter climate
prevails on the windward side of a mountain than on the leeward
(downwind) side as moisture is removed by orographic precipitation,
leaving drier air on the descending (generally warming), leeward side
where a rain shadow is observed.
2. What can cause a cloud to appear? What can cause a cloud to
disappear?
A. A cloud appears when a “parcel of air” ascends to the Lifting
Condensation Level (LCL), which is the altitude at which the “parcel of
air” becomes 100% saturated, and begins to expel moisture into the
outside air, forming a cloud. The altitude of the LCL can vary from
ground level (fog) to several thousand feet.
B. Clouds can disappear when the temperature of the “parcel of air”
increases (warms) allowing the existing moisture in the “parcel of air”
to become less than 100%. Watch “puffy” cumulus clouds on a sunny
day when the sun warms the clouds, increases the temperature, and
the clouds begin to break up and disappear.
3. What causes some clouds to ascend above a mountain, and others hover
below the mountain peak?
Stability. Clouds that ascend above a mountain are Absolutely unstable (see
definitions below for Question #5.) Clouds that hover below the mountain
peak are absolutely stable.
4. What causes the temperature of a "parcel of air" to COOL at TWO
different rates while ascending a mountain, but WARM at only ONE rate
while descending a mountain?
Altitude. The higher the altitude the less the pressure and the cooler the
temperatures. Parcels of air cool at the dry adiabatic lapse rate (DALR) and
the wet adiabatic lapse rate (WALR) while ascending the mountain, but only
at the DALR why descending the mountain.
5. What causes some air parcels to be stable (not rising on their own
accord), and others unstable (rising on their own accord)?
Stability. Clouds can be absolutely stable, absolutely unstable, conditionally
unstable, or neutral.
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Absolutely Stable - The external forces on a “parcel of air” try to return it
to its original level. Regardless of moisture content. We often see no
cloud, fog or stratus or high cirrus clouds under these conditions.
Absolutely Unstable - The displaced parcel keeps going, faster and
faster. No matter how moist or dry it is. This is virtually nonexistent but it
would give different types of clouds. We would get strong cumuloform
clouds. Big piles.
Conditionally Unstable - Stability depends on whether it is dry or
saturated. We also call this potential instability. This condition gives us
most of our clouds.
Neutral - This time the air's temperature remains the same as the
surrounding air temp.; It depends on water content as well. We don't see
much cloud at this time either.
6. What can cause a change in the actual humidity of a "parcel of air"?
Actual humidity is the quantity of moisture a “parcel of air” DOES hold. The
outside (environmental) air and a “parcel of air” do not mix. Therefore, when a
“parcel of air” ascends, it contains a fixed quantity of moisture (actual
humidity), which remains the same fixed quantity of moisture until it reaches
the LCL. Above the LCL the parcel begins to give off moisture to the
environment because it has reached 100% saturation─ Relative Humidity =
actual humidity / Saturation Vapor Density, or in simpler terms,
The quantity of moisture a parcel of air DOES hold
Relative Humidity = ___________________________________________
.
The quantity of moisture a parcel of air CAN hold
Consequently, the quantity of actual humidity in a “parcel of air” decreases in
altitudes above the LCL.
7. What can cause a change in the capacity of a "parcel of air"?
Temperature. An increase in temperature, increases capacity. A decrease in
temperature, decreases capacity.
8. How is "saturation of humidity" related to the production of rain, snow,
sleet?
A “parcel of air” is saturated (100% relative humidity) when the quantity of
moisture it DOES hold equals the capacity of moisture it CAN hold. The
capacity of a “parcel of air” is directly related to temperature. Increases in
temperature, increases capacity; decreases in temperature, decreases
capacity.
Once a “parcel of air” reaches “saturation of humidity” at the Lifting
Condensation Level (LCL), it begins to release moisture to the outside
environmental air, producing rain at temperatures above 32ºF, or snow or
sleet at temperatures below 32ºF.