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MET 10 Lecture 6
Atmospheric Stability and Cloud
Development
Chapter 5
Dr. Craig Clements
San Jose State University
Atmospheric Stability
Hydrologic Cycle
Atmospheric stability
We determine the stability of air by comparing the temperature
of a rising parcel to that of its surroundings or
‘environment’.
If the parcel is colder than its environment, it will be more
dense (heavier) and tend to sink back to its original level.
This condition is called stable.
Stable: Air is stable when it resists upward displacement.
If the parcel is warmer than its environment, it will be less
dense (lighter) and will continue to rise until reaching the
same temperature as the environment. This is an example
of unstable air.
Unstable: A lifted parcel of air will be warmer than the
surrounding air and thus will continue to rise upward,
away from its original position.
Atmospheric Lapse Rates
Adiabatic process: when a parcel of air expands and cools, or
compresses and warms, with no interchange of heat with
the outside surroundings.
- As long as the air in the parcel is unsaturated (RH<100%) the
rate of adiabatic cooling or warming is constant
Dry Adiabatic Lapse Rate = 10°C /1000 m (5.5°F /1000 ft.)
- If the rising air cools to its dew-point temperature, the RH
becomes 100%, condensation occurs, a cloud forms, and
latent heat is released. This added heat during condensation
offsets some of the cooling. The air now cools at a lesser rate
Moist Adiabatic Lapse Rate = 6°C /1000 m (3.3°F /1000 ft.)
Dry Adiabatic Lapse
Rate
Stable Atmosphere
The atmosphere is stable when the environmental lapse
rate is small, that is, when there is relatively small
difference in temperature between surface air and the air
aloft.
The atmosphere becomes more stable ( it stabilizes) as
the air aloft warms or surface air cools.
What can cause the surface air to cool?
Becoming more stable
Unstable Atmosphere
The atmosphere is unstable when the environmental
lapse rate steepens, that is, when the temperature of the
air drops rapidly with increasing height.
The atmosphere becomes more unstable (destabilize) as
the air aloft becomes colder or the surface air warms.
What can cause the air to cool aloft?
The surface air to warm?
Becoming more unstable: destabilizes
Convection: Stable or Unstable?
Two lapse rates at once!
Cloud Development and Stability
Most clouds form as air rises, expands, and cools.
-
A majority of the clouds we observe are due to the following
mechanisms:
1. Surface heating and free convection
2. Uplift along topography
3. Widespread ascent due to the flowing together (convergence)
of surface air.
4. Uplift along weather fronts (different air masses).
Convection
Air that is saturated:
number of molecules
escaping the water surface
= amount returning.
At higher air temperatures,
it takes more water vapor
to saturate the air.
Lifted by topography
Convergence
Lifting along weather fronts
Cumulus clouds building on a warm summer afternoon. Each cloud
represents a region where thermals are rising from the surface. The
clear areas between the clouds are regions where the air is sinking.
Orographic uplift, cloud development, and the formation of a rain shadow.
Clouds that form in the wave directly over the mountain are called mountain
wave clouds, whereas those that form downwind of the mountain are called
lee wave clouds
Dew point represents the temperature to which air would
have to be cooled (with no change in air pressure or moisture
content) for saturation to occur.
Dew point is a good indicator of the air’s actual water vapor
content.
High dew points indicate high water vapor content, low dew
points = low water vapor content.
Cloud droplets
Raindrops
Condensation nuclei
Which of the three drops drawn here represents
the real shape of a falling raindrop?
Fig. 4-8a, p. 86
Precipitation Processes
Collision-Coalescence process:
• Larger cloud droplets fall faster than smaller droplets.
• The larger droplets collide with smaller drops in their path.
• This merging of cloud droplets by collision is called
coalescence and occurs in clouds with tops warmer
than -15°C.
• Called warm clouds.
• Coalescence is enhanced if the colliding droplets have
opposite electrical charges.
Dew and Frost
On calm, clear nights, the surface cools
rapidly by what process?
Air near the ground cools to the dew point
quickly, reaching saturation.
Water vapor condenses on blades of grass at
the ground, forming tiny specks of water
called dew.
Precipitation Processes in Warm Clouds
•
Most important factor in the production of raindrops is the
liquid water content of the cloud.
•
1.
2.
3.
4.
Other important factors:
The range of droplet sizes
The could thickness
The updrafts of the could
The electric charge of the droplets and electric field in the
cloud.
Precipitation Processes in Cold Clouds
Ice-crystal process (Bergeron):
• This process proposes that both ice crystals and liquid cloud
droplets must co-exist in clouds at temperatures below
freezing.
• Important in area where clouds can extend upward into
regions where the air temperatures are below freezing.
• These clouds are called cold clouds.
• Water droplets existing at temperatures below freezing are
referred to as supercooled.
The distribution of ice and water in a cumulonimbus cloud.
Frost
The ice-crystal process. The greater number of water vapor molecules
around the liquid droplets causes water molecules to diffuse from the
liquid drops toward the ice crystals. The ice crystals absorb the water
vapor and grow larger, while the water droplets grow smaller.
When the dew point is below freezing (now
called the frost point), frost forms which is
composed of tiny ice crystals.
Water vapor changes directly into ice without
becoming liquid first– called deposition.
Accretion
Splintering
forming graupel
Aggregation
Forming snowflakes
Natural seeding by cirrus clouds may form bands of
precipitation downwind of a mountain chain
The streaks of falling precipitation that evaporate before reaching
the
ground
are called virga.
Cirrus
clouds
The dangling white streamers of ice crystals
beneath these cirrus clouds are known as
fallstreaks.
dendrite snowflakes
Average Annual Snowfall over US
Cumulus clouds. Small cumulus clouds such as these are
sometimes called fair weather cumulus, or cumulus humilis
An aircraft
undergoing de-icing
during inclement
winter weather
HW #4: Due Thursday October 9
1. Identify cloud types for 3 days: Friday, Saturday, and
Sunday would be a good choice.
2. List whether they are high, middle or low clouds.
3. List the time that you observed them.
4. If you can photograph them, put the photos into your
report or find similar photos on the web and include
these into your report.
Components of a standard rain gauge.
Tipping bucket rain gauge
Doppler Radar
Radar- radio detection and ranging
A radar is a remote sensing instrument:
- measuring the atmosphere from a distance
A microwave pulse sent out from the radar transmitter.
The pulse strikes raindrops and a fraction of its energy
is reflected back to the radar unit, where it is detected
and displayed
Doppler radar display showing precipitation intensity
The numbers under the letters DBZ represent the logarithmic scale
for measuring the size and volume of precipitation particles
Doppler radar display showing 1-hour rainfall amounts
Reading and HW assignment:
Due: Monday, Oct. 11
Reading: Precipitation Types. pages: 126-134.
Homework: Chapter 5 problems page 138.
Answer:
4, 20,23, 24, 25