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Atmospheric stability
Determining stability
Cloud development and stability
Precipitation processes
Precipitation types
Measuring precipitation

Atmospheric Stability
 We know that air rises, cools and condenses into clouds
 Atmospheric stability refers to condition of equilibrium
 We will refer to stable and unstable environments

Atmospheric Stability
 A stable environment is one where the original equilibrium is maintained (things return to where they were
 An unstable environment is one where things move away from their original position
• Stability does not control whether air will rise or sink.
Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

Atmospheric Stability
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Remember, when air rises, it moves into an area of lower pressure, expands, and cools. Sinking air is the opposite
Adiabatic process – a process in which exchanges no heat with its outside surroundings
(how can this happen)
Lapse rate – change of temperature with height
Dry adiabatic lapse rate - 10 °C/1000m or
5.5°F/1000 feet

Atmospheric Stability
 But what happens when air cools (humidity?). If we get condensation, then heat is released and the process is no longer adiabatic.
 Thus, we have the moist adiabatic lapse rate. Is it less or more than the dry adiabatic lapse rate?
 Moist adiabatic lapse rate - 6 °C/1000 m or
3.3°F/1000 feet. Varies greatly with different moisture content.

A Stable Atmosphere
 So how do we determine stability?
 We raise a parcel and compare it to its surroundings. If a parcel is colder than its surroundings, it is more dense and will sink
(stable)
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If a parcel is warmer, it is less dense and will rise
(unstable)
Environmental lapse rate – the rate at which air changes in temperature with height

A Stable Atmosphere
 Absolute stability – when a parcel is colder than the environment at all levels (for both dry and moist adiabats)
 If forced to rise, clouds will have flat tops and be thin

A Stable Atmosphere
 Atmosphere will be stable with the environmental lapse rate is small
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So when the air aloft warms and surface cools
This can happen during radiational cooling
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Influx of cold air
Air moving over cold surface

A Stable Atmosphere
 Most stable around sunrise
 Layer of fog or haze can be evidence of a stable atmosphere
Because stable atmospheres resist vertical movement, they trap pollutants near the ground and can cause dangerous air quality

An Unstable Atmosphere
 Absolute instability – occurs when air parcels are warmer than their surroundings (parcel will rise)
 Warming of surface air helps atmos to becoming unstable

An Unstable Atmosphere
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Unstable atmospheres occur when the environmental lapse rate steepens
Destabilizing processes:
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Solar heating of the surfance
Warm air brought by wind
 Air moving over warm surface
Superadiabatic lapse rate – when environmental lapse rate exceeds the dry adiabatic lapse rate

Conditionally Unstable Air
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Conditional instability – occurs if we force a cool parcel to a part of the environment where it condenses and becomes warmer
Level of free convection – point at which we don’t need to force it up anymore

Conditionally Unstable Air
 Look at the environmental lapse rate and make a hypothesis of what lapse rate you need in order to have conditionally unstable air
 You need an environmental lapse rate between the moist and dry adiabatic lapse rate
Average lapse rate in the troposphere is
6.5
°C/1000 m. What is the average stability?

Convection and Clouds
 Primary ways clouds form:
 Surface heating and convection
 Topographical uplift
 Convergence
 Lifting along a weather front

Convection and Clouds

Convection and Clouds

Topography and Clouds
 Orographic uplift – forced lifting along a topographic barrier
 Rain shadow Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Convection and Clouds

Collision and Coalescence
Process
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How do raindrops become large enough to fall?
Condensation alone is just not enough

Collision and Coalescence
Process
 One process is collision and coalescence
 Occurs in warm clouds (tops warmer than 0 °C
• A typical cloud droplet falls at a rate of 1 centimeter per second.
At this rate it would take
46 hours to fall one mile.

Collision and Coalescence
Process
 Clouds must have varying droplet sizes
 Terminal velocity – the point at which gravity equals the air resistance (falls at constant speed

Collision and Coalescence
Process
 Larger drops merge with smaller drops in a processing called coalescence
 Important factor is how long the droplet stays in the cloud

Stepped Art
Fig. 5-9, p. 116

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Ice Crystal Process
Ice crystal or
Bergeron process
 Occurs in cold clouds – clouds with temperatures that drop to below freezing
 Bergeron process states that liquid and ice droplets must co-exist in clouds

Ice Crystal Process
 Supercooled water droplets – droplets that occur as liquid below freezing
(middle portion of a thunderstorm cloud)
 Occurs when there are few ice nuclei

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Ice Crystal Process
Ice crystals form at the expense of water droplets
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Suppose we have one super cooled droplet and one ice crystal in a saturated environment
Since saturated, number molecules evaporating and condense MUST be equal!
 More vapor molecules over liquid because it is easier to escape liquid to vapor than ice to vapor

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Ice Crystal Process
Since more liquid molecules are going to vapor, it takes more vapor molecules to condense to liquid to keep the equilibrium (saturation)
 Thus, the saturation vapor pressure above water is greater than that above ice
 This causes vapor molecules to move towards the ice
 Removal of water vapor cause water droplet to shrink (not in equilibrium)

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Ice Crystal Process
Ice crystals may also grow larger by accretion
– when ice crystals collide with supercooled dropets

Fig. 5-22, p. 124

Stepped Art
Fig. 5-22, p. 124

Cloud Seeding and
Precipitation
 Cloud seeding – pumping nuclei into clouds to promote precipitation
 Silver iodide – found to be a good ice nuclei
• It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Precipitation in Clouds
 Accretion
 Ice crystal process

Rain
 Rain – falling droplet size greater than or equal to 0.5 mm (0.02 in)
 Drizzle – falling droplet size less than 0.5 mm (0.02 in)
 Virga – rain that evaporates before it hits the ground
 Shower – brief downpours possibility as a result of updrafts

Snow
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Snow – precipitation that falls to the ground as ice crystals. Much precipitation actually starts as snow
Fallstreaks – ice crystals and snowflakes that fall from cirrus clouds
Dendrite – most common type of fernlike snowflake
Blizzard – combination of strong winds, low temperatures, and fine, dry snow
• Snowflake shape depends on both temperature and relative humidity.

Sleet and Freezing Rain
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Sleet – precipitation that has thawed and frozen again
Freezing rain – supercooled droplets that reach the ground and freeze on contact
Rime – accumulation of white ice that occurs when supercooled droplets hit something

Snow Grains and Snow
Pellets
 Snow grains
 Solid equivalent of drizzle
 Snow pellets
 Like grains, but bounce

Hail
 Hail develops as droplets are uplifted in the clouds above the freezing level
 The droplets grows by accretion until it is large enough for gravity to take it to the ground

Stepped Art
Fig. 5-35, p. 134

Instruments
 Standard rain gauge
 Tipping bucket rain gauge
• It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and
Precipitation
 Radar
 Doppler radar
 Can detect precipitation intensity
 Can detect movement away or to the radar

Stepped Art
Fig. 5-39, p. 135