Cloud Development and Cloud Development and PrecipitationPrecipitation

Prof. John Toohey-Morales, Prof. John Toohey-Morales, CCMCCM

St. Thomas UniversitySt. Thomas University

Miami Gardens, FloridaMiami Gardens, Florida

Adiabatic Cooling and Adiabatic Cooling and WarmingWarming• Rising air expands & Rising air expands &

cools, while sinking cools, while sinking air is compressed air is compressed and warmsand warms

• Adiabatic process is Adiabatic process is one in which there one in which there is no heat exchange is no heat exchange with the with the surrounding airsurrounding air

Stability: Key to Cloud Stability: Key to Cloud DevelopmentDevelopment

• The stability of a layer in the atmosphere The stability of a layer in the atmosphere is determined by comparing the is determined by comparing the temperature of a rising parcel of air (which temperature of a rising parcel of air (which cools at the cools at the adiabaticadiabatic lapse rate) to that of lapse rate) to that of its surroundings (which cool at the its surroundings (which cool at the environmentalenvironmental lapse rate) lapse rate)– Dry adiabatic rate of cooling is 10°C per 1 kmDry adiabatic rate of cooling is 10°C per 1 km– Moist adiabatic rate is 6°C per 1,000 metersMoist adiabatic rate is 6°C per 1,000 meters– Environmental rate Environmental rate averagesaverages 6.5°C per 1 km 6.5°C per 1 km

Stability: Key to Cloud Stability: Key to Cloud Development (question)Development (question)

• If a parcel of unsaturated air with a If a parcel of unsaturated air with a temperature of 30° C rises from the surface temperature of 30° C rises from the surface to an altitude of 1000 m, the unsaturated to an altitude of 1000 m, the unsaturated parcel temperature at this altitude would be parcel temperature at this altitude would be aboutabout

a. 10° C warmer than at the surface.a. 10° C warmer than at the surface.

b. 10° C colder than at the surface.b. 10° C colder than at the surface.

c. 6° C colder than at the surface.c. 6° C colder than at the surface.

d. impossible to tell from the data givend. impossible to tell from the data given

Stability: Key to Cloud Stability: Key to Cloud Development (question)Development (question)

• If a parcel of unsaturated air with a If a parcel of unsaturated air with a temperature of 30° C rises from the surface temperature of 30° C rises from the surface to an altitude of 1000 m, the unsaturated to an altitude of 1000 m, the unsaturated parcel temperature at this altitude would be parcel temperature at this altitude would be aboutabout

a. 10° C warmer than at the surface.a. 10° C warmer than at the surface.

b. 10° C colder than at the surface.b. 10° C colder than at the surface.

c. 6° C colder than at the surface.c. 6° C colder than at the surface.

d. impossible to tell from the data givend. impossible to tell from the data given

Stable AirStable Air

• An absolutely An absolutely stable atmosphere stable atmosphere exists when a exists when a rising air parcel is rising air parcel is colder and heavier colder and heavier (more dense) than (more dense) than the surrounding airthe surrounding air

• If released, the air If released, the air parcel returns to its parcel returns to its original positionoriginal position

Factors Affecting StabilityFactors Affecting Stability

• A stable layer results when there is a A stable layer results when there is a relatively small difference in temperature relatively small difference in temperature between the surface air and the air aloftbetween the surface air and the air aloft

• If air aloft warms or surface air cools, the If air aloft warms or surface air cools, the atmosphere stabilizesatmosphere stabilizes

• Some examples of processes leading to Some examples of processes leading to surface cooling aresurface cooling are– Nighttime radiational coolingNighttime radiational cooling– Influx of cold airInflux of cold air– Air cooled by a colder surfaceAir cooled by a colder surface

Stable Air (question)Stable Air (question)

• If an air parcel is given a small push If an air parcel is given a small push upward and it falls back to its original upward and it falls back to its original position, the atmosphere is said to beposition, the atmosphere is said to bea. stable.a. stable.b. unstable.b. unstable.c. isothermal.c. isothermal.d. neutral.d. neutral.e. adiabatic. e. adiabatic.

Stable Air (question)Stable Air (question)

• If an air parcel is given a small push If an air parcel is given a small push upward and it falls back to its original upward and it falls back to its original position, the atmosphere is said to beposition, the atmosphere is said to bea. stable.a. stable.b. unstable.b. unstable.c. isothermal.c. isothermal.d. neutral.d. neutral.e. adiabatic. e. adiabatic.

Unstable AirUnstable Air

• An absolutely An absolutely unstable unstable atmosphere exists atmosphere exists when a rising parcel when a rising parcel of air is warmer and of air is warmer and lighter (less dense) lighter (less dense) than the air than the air surrounding itsurrounding it

• If released, the air If released, the air parcel would parcel would continue to risecontinue to rise

Factors Affecting InstabilityFactors Affecting Instability• An unstable layer results when the An unstable layer results when the

environmental lapse rate steepens or the environmental lapse rate steepens or the temperature of the air drops quickly with temperature of the air drops quickly with heightheight

• If air aloft cools or surface air warms it If air aloft cools or surface air warms it becomes unstablebecomes unstable

• Some examples of processes leading to Some examples of processes leading to surface warming aresurface warming are– Daytime solar heatingDaytime solar heating– Influx of warm airInflux of warm air– Air warmed by a hotter surfaceAir warmed by a hotter surface

Unstable Air (question)Unstable Air (question)

• Which of the following environmental lapse Which of the following environmental lapse rates would represent the most rates would represent the most unstableunstable conditions in a layer of unsaturated air?conditions in a layer of unsaturated air?

a. 1° C per 1000 ma. 1° C per 1000 m

b. 3° C per 1000 mb. 3° C per 1000 m

c. 6° C per 1000 mc. 6° C per 1000 m

d. 9° C per 1000 md. 9° C per 1000 m

e. 11° C per 1000 me. 11° C per 1000 m

Unstable Air (question)Unstable Air (question)

• Which of the following environmental lapse Which of the following environmental lapse rates would represent the most rates would represent the most unstableunstable conditions in a layer of unsaturated air?conditions in a layer of unsaturated air?

a. 1° C per 1000 ma. 1° C per 1000 m

b. 3° C per 1000 mb. 3° C per 1000 m

c. 6° C per 1000 mc. 6° C per 1000 m

d. 9° C per 1000 md. 9° C per 1000 m

e. 11° C per 1000 me. 11° C per 1000 m

Ways Clouds FormWays Clouds Form

Way Clouds Form (question)Way Clouds Form (question)

• An example of orographic clouds would An example of orographic clouds would bebe

a. clouds forming over a warm ocean current.a. clouds forming over a warm ocean current.

b. clouds forming on the windward slope of a b. clouds forming on the windward slope of a mountain.mountain.

c. clouds forming behind a jet airplane.c. clouds forming behind a jet airplane.

d. clouds formed by surface heating.d. clouds formed by surface heating.

Way Clouds Form (question)Way Clouds Form (question)

• An example of orographic clouds would An example of orographic clouds would bebe

a. clouds forming over a warm ocean current.a. clouds forming over a warm ocean current.

b. clouds forming on the windward slope of a b. clouds forming on the windward slope of a mountain.mountain.

c. clouds forming behind a jet airplane.c. clouds forming behind a jet airplane.

d. clouds formed by surface heating.d. clouds formed by surface heating.

Rain ShadowRain Shadow• Air dries on the leeward side of mountains Air dries on the leeward side of mountains

because of water vapor condensing and because of water vapor condensing and remaining as cloud droplets and precipitation on remaining as cloud droplets and precipitation on the windward sidethe windward side

• Rain shadow is the region on the lee side of the Rain shadow is the region on the lee side of the mountain with low precipitation and dry airmountain with low precipitation and dry air

Producing Rain: Collision and Producing Rain: Collision and CoalescenceCoalescence

• Cloud droplets forming on Cloud droplets forming on condensation nuclei are extremely condensation nuclei are extremely small with a diameter of 0.02 mm so small with a diameter of 0.02 mm so they stay suspended by slight they stay suspended by slight upwards air currentsupwards air currents

• Some larger droplets may form Some larger droplets may form through random collision or by through random collision or by forming on larger condensation nucleiforming on larger condensation nuclei

Producing Rain: Collision and Producing Rain: Collision and Coalescence (continued)Coalescence (continued)

• Larger drops fall Larger drops fall faster than smaller faster than smaller drops, overtaking, drops, overtaking, colliding and colliding and coalescing with coalescing with smaller droplets, smaller droplets, eventually growing eventually growing too heavy and too heavy and falling to the falling to the ground due to ground due to gravitygravity

Producing Rain: Collision and Producing Rain: Collision and Coalescence (continued)Coalescence (continued)

• Warm clouds with Warm clouds with strong updrafts strong updrafts (like Cumulus (like Cumulus Congestus) can Congestus) can sustain the weight sustain the weight of more larger of more larger drops, leading to drops, leading to bigger raindrops bigger raindrops and heavier rainand heavier rain

Producing Rain: Collision and Producing Rain: Collision and Coalescence (question)Coalescence (question)

• The merging of liquid cloud droplets The merging of liquid cloud droplets by collision is calledby collision is calleda. coalescence.a. coalescence.

b. riming.b. riming.

c. accretion.c. accretion.

d. deposition.d. deposition.

e. condensation.e. condensation.

Producing Rain: Collision and Producing Rain: Collision and Coalescence (question)Coalescence (question)

• The merging of liquid cloud droplets The merging of liquid cloud droplets by collision is calledby collision is calleda. coalescence.a. coalescence.

b. riming.b. riming.

c. accretion.c. accretion.

d. deposition.d. deposition.

e. condensation.e. condensation.

Ice-Crystal ProcessIce-Crystal Process

• Most common type of rain formation at our Most common type of rain formation at our latitudes (mid to high latitudes)latitudes (mid to high latitudes)

• Small Small supercooledsupercooled water droplets can exist water droplets can exist at below freezing temperaturesat below freezing temperatures

• Ice-Crystals are also present, but in Ice-Crystals are also present, but in smaller quantities (because ice nuclei are smaller quantities (because ice nuclei are scarce)scarce)

• Bergeron process: Ice-Crystals grow at the Bergeron process: Ice-Crystals grow at the expense of the surrounding water droplets expense of the surrounding water droplets through accretion, with the liquid water through accretion, with the liquid water freezing on contact with the icefreezing on contact with the ice

Ice-Crystal Process Ice-Crystal Process (question)(question)• Most rain at middle latitudes is Most rain at middle latitudes is

produced by the ice crystal process. produced by the ice crystal process. This is becauseThis is becausea. ice crystal nuclei are more plentiful than a. ice crystal nuclei are more plentiful than

condensation nuclei.condensation nuclei.b. most clouds form in cold regions of the b. most clouds form in cold regions of the

atmosphere.atmosphere.c. ice crystals evaporate more slowly than c. ice crystals evaporate more slowly than

water water droplets.droplets.d. most rain occurs during the winter.d. most rain occurs during the winter.

Ice-Crystal Process Ice-Crystal Process (question)(question)• Most rain at middle latitudes is Most rain at middle latitudes is

produced by the ice crystal process. produced by the ice crystal process. This is becauseThis is becausea. ice crystal nuclei are more plentiful than a. ice crystal nuclei are more plentiful than

condensation nuclei.condensation nuclei.b. most clouds form in cold regions of the b. most clouds form in cold regions of the

atmosphere.atmosphere.c. ice crystals evaporate more slowly than c. ice crystals evaporate more slowly than

water water droplets.droplets.d. most rain occurs during the winter.d. most rain occurs during the winter.

Snow to RainSnow to Rain• Ice-Crystals grow, fall, collide, and stick to Ice-Crystals grow, fall, collide, and stick to

one another forming a snowflakeone another forming a snowflake• If snowflake melts before hitting the If snowflake melts before hitting the

ground, it falls as a raindropground, it falls as a raindrop• Most rain begins as snowMost rain begins as snow

Snow to Rain (question)Snow to Rain (question)

• Which of the following statements is Which of the following statements is notnot correct?correct?a. Generally, the smaller the pure water a. Generally, the smaller the pure water droplet, droplet, the lower the temperature at the lower the temperature at which it will which it will freezefreezeb. Ice nuclei are more plentiful in the b. Ice nuclei are more plentiful in the atmosphere than condensation nucleiatmosphere than condensation nucleic. Much of the rain falling in middle northern c. Much of the rain falling in middle northern

latitudes begins as snowlatitudes begins as snowd. Ice crystals may grow in a cold cloud even d. Ice crystals may grow in a cold cloud even

though supercooled droplets do not.though supercooled droplets do not.

Snow to Rain (question)Snow to Rain (question)

• Which of the following statements is Which of the following statements is notnot correct?correct?a. Generally, the smaller the pure water a. Generally, the smaller the pure water droplet, droplet, the lower the temperature at the lower the temperature at which it will which it will freezefreezeb. Ice nuclei are more plentiful in the b. Ice nuclei are more plentiful in the atmosphere than condensation nucleiatmosphere than condensation nucleic. Much of the rain falling in middle northern c. Much of the rain falling in middle northern

latitudes begins as snowlatitudes begins as snowd. Ice crystals may grow in a cold cloud even d. Ice crystals may grow in a cold cloud even

though supercooled droplets do not.though supercooled droplets do not.

Sleet and Freezing RainSleet and Freezing Rain

Go to Visualization

Sleet & Freezing Rain Sleet & Freezing Rain (continued)(continued)

• A raindrop which freezes before A raindrop which freezes before reaching the ground is calledreaching the ground is calleda. snow.a. snow.

b. graupel.b. graupel.

c. sleet.c. sleet.

d. glaze.d. glaze.

Sleet & Freezing Rain Sleet & Freezing Rain (continued)(continued)

• A raindrop which freezes before A raindrop which freezes before reaching the ground is calledreaching the ground is calleda. snow.a. snow.

b. graupel.b. graupel.

c. sleet.c. sleet.

d. glaze.d. glaze.

HailHail• Produced by accretionProduced by accretion• Up to 7” in diameterUp to 7” in diameter• Causes hundreds of Causes hundreds of

millions of dollars in millions of dollars in damage annuallydamage annually

• For a hailstone to grow For a hailstone to grow to golf ball size it must to golf ball size it must remain in the cloud for remain in the cloud for 5 to 10 minutes5 to 10 minutes

• The hailstones may The hailstones may cycle within a cycle within a Cumulonimbus, or be Cumulonimbus, or be too heavy and droptoo heavy and drop

Go to Visualization

Measuring and Detecting Measuring and Detecting PrecipitationPrecipitation

• A rain gauge is used to collect and measure A rain gauge is used to collect and measure precipitation starting at 0.01” (less is called precipitation starting at 0.01” (less is called a “trace” of rain)a “trace” of rain)

• 10” of snow will melt to 1” of water (total 10” of snow will melt to 1” of water (total precipitation)precipitation)

• Radar (radio detection and ranging) shows Radar (radio detection and ranging) shows where precipitation is and its intensitywhere precipitation is and its intensity

• Doppler Radar allows us to observe rotating Doppler Radar allows us to observe rotating winds inside T-Storms to detect tornadoeswinds inside T-Storms to detect tornadoes