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Name: ______________________ Section/TA: _____________
Atmospheric Sciences 101, Spring 2003
Homework #5–Due the beginning of section Thu/Fri, 15/16 May 2003
1. Using the above sounding answer the following questions:
a. Classify the layers indicated on the sounding as being absolutely stable, conditionally
unstable, or absolutely unstable. Include what factor(s) led you to this conclusion for
each layer.
Layer I: Absolutely Unstable. The environmental temperature cools by 15°C
(from 30° to 15°C) in 1 km, giving an environmental lapse rate of 15°C per km,
which is greater than the dry-adiabatic lapse rate (10°C per km).
Layer II: Absolutely Stable. The environmental temperature cools by only 10°C
(from 15° to 5°C) in 2 km, giving an environmental lapse rate of 5°C per km,
which is less than the moist-adiabatic lapse rate (6°C per km).
Layer III: Conditionally Unstable. The environmental temperature cools by
15°C (from +5° to -10°C) in 2 km, giving an environmental lapse rate of 7.5°C
per km, which is between the moist- and dry-adiabatic lapse rates.
b. Circle the correct choice in the questions below:
The relative humidity increases / decreases moving from 0 km to 1 km.
It is unsaturated (Dew-Point less than Temperature) at the surface, and
saturated (Dew-Point = Temperature) at 1 km.
The relative humidity increases / decreases moving from 1 km to 3 km.
c. At what height would the relative humidity be 100%? How did you conclude this?
At 1 km, where the dew-point is the same as the temperature.
2. Actual vapor pressure is a measure of the actual water vapor content of the air
whereas relative humidity (RH) is a measure of how close to saturation an air parcel
is.
How would you expect the following variables to change if:
a. Amount of water vapor in the air increased at a constant temperature
Relative Humidity
↑ inc
Actual Vapor Pressure ↑ inc
Dew Point Temperature ↑ inc
↓ dec
↓ dec
↓ dec
remain the same
remain the same
remain the same
Air temperature increases without additional evaporation
Relative Humidity
↑ inc
Actual Vapor Pressure ↑ inc
Dew Point Temperature ↑ inc
↓ dec
↓ dec
↓ dec
remain the same
remain the same
remain the same
b. Under what conditions would you find a high relative humidity in a region of low
vapor pressure?
When the air temperature is cold, the saturation vapor pressure is very small.
So, in cold air, you can have high relative humidity with low vapor pressure.
c. If the air is cooled to its dew point temperature what is the RH?
100% The definition of the dew point temperature is the temperature to which
you have to cool air to reach saturation (100% relative humidity).
3. Imagine an air parcel moving up and over a mountain. As the parcel is lifted it cools
until it is saturated (B) at which time precipitation begins. By the time the parcel
reaches the top of the mountain it has lost all its moisture.
a. Between points B and C the parcel is cooling / warming at the moist / dry adiabatic
rate. (circle the correct words)
Between points D and E the parcel is cooling / warming at the moist / dry adiabatic
rate. (circle the correct words)
b. Assuming the temperatures at points A and E are the same, which has a higher
relative humidity? Why?
The RH would be higher at point A. Nearly all of the parcels moisture was
rained out on the way up the mountain, so it has a much lower vapor pressure at
point E, but the saturation vapor pressure is the same (same T).
c. Points A and F are at the same elevation, which location has a higher temperature?
How do you know this?
F will have a higher temperature. The parcel ascends moist adiabatically for
part of the way, cooling only 6°C per km, but it descends dry adiabatically the
entire way, warming at 10°C per km, so it warms more than it cooled.
d. Why are areas of subsidence generally dry?
As the air subsides it warms, increasing the saturation vapor pressure, while the
actual vapor pressure remains what it was when it was higher up and colder. As
a result, the relative humidity will be low, just like it is when you heat air in a
house.
e. The equatorial regions receive the most incoming solar radiation, yet climatically we
see the highest temperatures in the subtropics. Why? (Hint: Think about the Hadley
Circulation.)
Air is generally subsiding in the subtropics. This leads to clear skies (dry air)
and to warm temperatures (the air warms adiabatically).
4. Use the sounding below to answer the following questions. The plot shows the
environmental temperature versus height.
a. There are clouds above the station. The base (or bottom) of the clouds is at 2.5 km.
What type of precipitation is most likely falling from the cloud base? Why?
Snow. They temperature is less than 0°C, and precipitation-sized particles
always freeze at 0°C. Only cloud droplets, which are very small, can be
supercooled.
b. What type of precipitation is most likely occurring at the surface (0 km)? Why?
Sleet. The snow will melt as it falls through the warm layer between 2 and 1 km,
and then refreeze to form sleet as it falls through the lowest 800 m. Sleet usually
forms (instead of freezing rain) when the depth of the near-surface freezing layer
is greater than 500 m.
c. Stratus clouds are normally associated with precipitation in the form of drizzle. Why
don’t stratus clouds form larger rain drops, as can be found in precipitating cumulus
clouds?
Thin clouds produce drizzle at most because collisions occur infrequently
(reducing the efficiency of collision and coalescence growth) and because
droplets fall out of the cloud before they reach raindrop size, and are made even
smaller through evaporation between the cloud and ground. The lack of strong
updrafts (as found in cumulus clouds) in stratus clouds keeps them from being
able to support large drops.
5. Be sure to continue entering your forecasts for the forecasting contest! You must
make at least 14 forecasts to receive credit for the competition, see the course website
for details.