Ch. 6 & 7//GeoSci 222

At what altitude do you find middle clouds? At what temperature do you usually find these clouds? What are they composed of? What prefix is used with these clouds? What type of precipitation, if any, is associated with these clouds? Are there any special features or characteristics associated with these clouds?

2,000 m - 6,000 m (6,000 ft - 20,000 ft) -20 degrees Celsius to 0 degrees Celsius supercooled water drops and ice crystal combination alto: "cloud streets" snow or rain is possible but not likely

By what processes do cloud drops grow? Where is each of these processes most important?

Bergeron process Collision coalescence

What conditions are necessary for a radiationfog to develop? How do radiation fogs develop? How long do they last?

Conditions: clear skies at night high relative humidity calm or nearly calm winds Due to radiational cooling the ground and the air immediately above it cool rapidly With the high RH, just a little cooling lowers the air temp to the dew point temp Often occurs: after recent rainfall events have added water vapor to the air when chilled air, being denser, flows into low-lying areas such as valleys characteristics: Usually a fairly shallow fog dissipates ("lifts" or "burns off") in 1-3 hrs

What conditions are necessary for an upslope fog to develop? How do upslope fogs develop? How long do they last? What is so special about an upslope fog?

Conditions: relatively humid air moves up a sloping plain or steep hill/mountain With the upward movement, pressure decreases, causing air to expand and cool adiabatically (the only type of fog that forms adiabatically) SO WHY IS IT A FOG?!?!

What are the four mechanisms that cause air to rise naturally or force air to rise?

Convective lifting Orographic lifting Frontal Wedging Convergence

Name the characteristics of rain: How big are the drops? From which type of cloud does it typically originate? How does rain form? How do the drops grow? How does it differ from snow? Describe the temperature profile from the time it forms to the time it reaches the ground.

Falls mainly from nimbostratus and cumulonimbus clouds Usually begins as snowflakes that melt as they descend through warmer air layers ( > 0oC ) Originating in thicker clouds, rain drops continue to grow via C-C after melting of the snowflakes takes place Rain drop diameters are normally in the 1-6 mm range

Describe the properties of water droplets inside a cloud. At what temperatures are the various nuclei active? At what temperatures will liquid droplets exist? At what temperatures will liquid droplets coexist with ice crystals? At what temperatures will ice crystals exist without liquid droplets?

Less than -20 -deposition nuclei -ice crystals Between -20 and -9 -combination supercooled water and ice crystals -freezing nuclei Between -9 and 0 -condensation nuclei -supercooled water dropslets More than 0 -liquid water droplets -condensation nuclei

Name the characteristics of sleet: What is it? How does it differ from freezing rain? Describe the temperature profile from the time it forms to the time it reaches the ground.

Precipitation in the form of frozen raindrops develop in the same manner as freezing rain: snowflakes fall through a warm layer of air ( > 0oC ) and melt, turning into rain... then the rain falls through a thicker layer of subfreezing air (< 0 C ) near the surface and become supercooled to the point of freezing

Name the characteristics of snow: How does snow form? How doe the crystals grow? From which type of cloud does it typically originate? Describe the temperature profile from the time it forms to the time it reaches the ground.

Precipitation in the form of ice crystals At low temperatures, moisture content is low resulting in light, fluffy snow consisting of individual hexagonal ice crystals At warmer temperatures, moisture content is high, the ice crystals stick together, resulting in heavy snowflakes with high moisture Temperatures throughout the atmos are < 0

Name the characteristics of freezing rain: What is it? How does it differ from sleet? Describe the temperature profile from the time it forms to the time it reaches the ground.

Precipitation that forms a coating of ice on the surface and objects at the surface Develops when: snowflakes fall through a warm layer of air ( > 0oC ) and melt, turning into rain... then the rain falls through a shallow layer of subfreezing air ( < 0oC ) near the surface and become supercooled... the supercooled drops then freeze on contact with cold surfaces

What is the relationship between saturation mixing ratio, temperature, and dew point temperature?

RH = MR/SMR mixing ratio-actual amount of water in the air saturation mixing ratio-how much water vapor the air could hold (temperature dependent) lower temp, lower SMR; higher temp, higher SMR the closer the MR and SMR are, the closer the RH is to 100% (dew point temperature)

What is the relationship between actual air temperature, dew point temperature, and relative humidity?

Relative humidity changes when temperatures change. Because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises if no moisture is added to the air. Dew forms when the relative humidity reaches 100%, so if the relative humidity falls, there is less likely to be dew

What causes convective lifting? Give an example. Explain what is occurring physically as it is rises into the atmosphere.

SR heats air at surface and makes it warmer than the surrounding air, in turn lowering its density 1. being less dense, it begins to rise (convection; fluids only) 2. as it rises, pressure decreases 3. as pressure decreases, air expands 4. expansion causes density to decrease more 5. temperature decrease as density decreases (DALR) 6. condensation will occur (releases 600 cal/g) and convective clouds will form; if the saturated air continues to rise it will cool at the MALR Fish tank bubbles unstable air (convection)

Name the characteristics of drizzle: How big are the drops? From which type of cloud does it typically originate? How do the drops grow? How does it differ from rain?

Small water drops, 0.2 to 0.5 mm in diameter Originate in stratus clouds (low and thin clouds) rain drops have limited opportunity to grow by collision-coalescence Slowly drift toward earth Associated with fog and poor visibility Never associated with convective clouds

Describe the process of how the Bergeron Process takes place. What factors must occur in order for the Bergeron Process to take place?

Takes place in cold clouds (below -9oC), clouds composed of a mixture of both supercooled water droplets and ice crystals important in the middle and high latitudes Remember, ice-forming nuclei are not active until the temperature reaches -9 In a cloud at -9, each ice crystal is surrounded by thousands of liquid water droplets air is saturated with respect to liquid water, but supersaturated with respect to ice crystals "saturation" gradient becomes established which is greater around ice crystals (see Table) therefore, the ice crystals collect more water molecules (water deposits on the ice crystals) The ice crystals grow rapidly enough to fall As they fall, the ice crystals enlarge as they intercept cloud droplets When the surface (or layer of air) is warm enough, the ice crystals (snowflakes) melt and fall to the ground as rain

Describe the process of how collision-coalescence takes place. What factors must occur in order for collision-coalescence to take place?

Takes place in warm clouds (those above -9oC), clouds made of liquid water droplets mportant especially in the tropical latitudes Requires droplets that make up the cloud have a range of droplet sizes (droplets of equal size would have the same terminal velocity) Droplets of unequal diameter have differing terminal velocities, so collisions occur more frequently Larger droplets form when "giant" condensation nuclei are present he rate at which droplets fall is size dependent; "giant" droplets fall most rapidly As larger droplets fall, they collide with smaller droplets and coalesce (example, raindrops sliding down a car's windshield) After collecting a million droplets, they are large enough to fall to the surface without evaporating

At what altitude do you find high clouds? At what temperature do you usually find these clouds? What are they composed of? What are the nicknames of these clouds? What prefix is used with these clouds? What type of precipitation, if any, is associated with these clouds? Are there any special features or characteristics associated with these clouds?

above 6,000 meters or 20,000 ft temps generally below -20 degrees Celsius composed almost exclusively of ice crystals (fibrous appearance) cirro: "curl of hair," "mare's tail" no precipitation generally feathery, thin, popcorn-y

What are the characteristics of cloud condensation nuclei? At what point do they become active?

active (promote condensation) at temps above and below freezing water vapor can condense into droplets that remain liquid even when the cloud temp is well below zero, forming super-cooled water droplets (freezing depends on volume)

As moisture is added to/taken away from the air, how does relative humidity change?

adding water vapor increases RH removing water vapor decreases RH

In what two ways are fogs formed?

air is cooled (heat is removed) enough water vapor is added for saturation to occur

What does moist adiabatic lapse rate mean? Under what conditions does it take place?

allows us to predict temp change of saturated air as it ascends and descends cools at a rate of 6 degrees Celsius per 1000 m ascending, warms at a rate of 6 degrees Celsius per 100 m descending

What does dry adiabatic lapse rate mean? Under what conditions does it take place?

allows us to predict temp change of unsaturated air as it ascends and descends cools at a rate of 10 degrees Celsius per 1000 m ascending, warms at a rate of 10 degrees Celsius per 100 m descending

What is the lifting condensation level? What occurs when air reaches the LCL?

altitude at which the dew point temperature is reached, causing condensation and cloud formation

What are the processes of ascending and descending air? Explain.

as air ascends, the pressure decreases and air expands, which decreases density, which cools the air and increases relative humidity; decreases SMR as air descends, air pressure increases, which compresses the air, which causes air density to increase and the air to warm; decreases relative humidity; increases SMR

When do dry adiabatic lapse rates switch over to moist adiabatic lapse rates? Why?

at the LCL-the altitude at which the dew point temperature is reached if cooled enough, the air reaches the dew point temperature and becomes saturated, at which time a cloud will form

At what altitude do you find low clouds? At what temperature do you usually find these clouds? What type of precipitation, if any, is associated with these clouds? Are there any special features or characteristics associated with these clouds?

based is less than 2,000 m or 6,000 ft temperature greater than -5 degrees Celsius liquid water drops, possible supercooled tend to be thicker clouds strata rain or snow comes from these clouds

What are adiabatic lapse rates? How do they take place? Why are they important?

change in air temperature without the addition or subtraction of heat -air ascends, expands, cools -air descends, compresses, warms allows us to predict temperature change of air as it ascends and descends

How can relative humidity be changed?

change water vapor content while keeping temp the same change temp while keeping water vapor content the same

What are the three basic cloud forms that are based on appearance? What are their characteristics?

cirrus: high, thin, wispy, feathery cumulus: individual, globular masses, cauliflower, cloud gazing, flat base with rising dome, vertical development with more rigorous uplift stratus: spread laterally to form sheets or layers called strata, no distinct cloud unit (looks like one cloud over sky)

What happens to water drops when the leave the cloud (grow)?

cloud droplets must grow large enough to overcome the updrafts and make it to the surface without vaporizing It takes about 1 million droplets to form a single raindrop with a 2 mm diameter

What causes frontal wedging? Give an example.

cold front: as a cold front approaches, colder/drier/more dense air behind it wedges under warmer/moister/less dense air ahead of it, forcing warmer, moister air to rise 1. warm air being less dense, it begins to rise over front 2. as it rises, pressure decreases 3. as pressure decreases, air expands 4. expansion causes density to decrease more 5. temperature decrease as density decreases (DALR) 6. condensation will occur (releases 600 cal/g) and convective clouds will form; if the saturated air continues to rise it will cool at the MALR

What conditions are necessary for a steam fog to develop? How do steam fogs develop?

conditions: cool air moves over warm water evaporation from the warm water surface saturates the air immediately above characteristics: normally a very shallow fog re-evaporates quickly in the air above

What conditions are necessary for frontal fog to develop? How does frontal fog develop?

conditions: warm air is lifted over colder air the lifted warm air produces clouds and precipitation the cold air below is near the dew point the precipitation evaporates as it falls, saturating the air below, producing fog often occurs: in spring, associated with warm front precip with gray, gloomy, yucky conditions

What conditions are necessary for an advection fog to develop? How do advection fogs develop? How long do they last?

conditions: warm moist air is advected (moved horizontally by wind) over a cold surface and air is chilled by contact warm, moist air is chilled by the surface below often occurs: along CA coast (warm moist air off Pacific Ocean crosses the CA current) Midwest in spring (warm moist air from Gulf crosses snow-covered surface) characteristics: usually very thick persistent, lasts a relatively long time

As air temperature changes, how does relative humidity change?

decrease temp, decrease water vapor air can hold, increase relative humidity increase temp, increase water vapor air can hold, decrease relative humidity

What is the relationship between relative humidity, temperature, and dew point temperature?

decrease temp-decrease amount of water vapor air can hold, which increases the relative humidity increase temp-increase amount of water vapor air can hold, which decreases the relative humidity dew point temp-temp the air needs to be cooled to in order to reach 100 % relative humidity (the lower the temperature, the easier it will be to reach 100% relative humidity)

How do adiabatic lapse rates affect the temperature and moisture characteristics of a parcel of air?

dry adiabatic lapse rates-unsaturated moist adiabatic lapse rates-saturated DALR-air cools faster MALR-air cools slower cooler the air, the less moisture it can hold, the higher the relative humidity

How does the expansion and compression of air influence temperature and moisture conditions?

expansion: air pressure decreases as air ascends-air expands, density decreases, temp decreases compression: air pressure increases as air compresses; temperatures increase; air pressure increases as air descends

How are fogs similar to clouds? What is the essential difference between them?

fog and clouds are structurally the same; the difference is where they form and how they form fog is merely a cloud with its base very near or at the ground

What are the four classifications of clouds based on altitude?

high, middle, low

What are the characteristics of deposition nuclei? At what point do they become active?

ice forming nuclei are much less abundant than cloud condensation nuclei become active only at temps well below freezing deposition nuclei: causes water vapor to deposit directly as ice, active only at temps below -20 degrees Celsius

What are the characteristics of freezing nuclei? At what point do they become active?

ice forming nuclei are much less abundant than cloud condensation nuclei become active only at temps well below freezing freezing nuclei: causes liquid droplets to freeze, only active at temps below -9 degrees Celsius

What makes air stable and "wanting" to descend?

if a parcel of air is colder than the surrounding environment, it is more dense and will sink if it is sinking, it is not rising and has no potential to bring moisture into the atmosphere

What makes air unstable and "wanting" to ascend?

if a parcel of air is warmer than the surrounding environment, it is less dense and will rise if it is rising, it has the potential to carry moisture from the surface to the atmosphere

What factors determine whether a cloud droplet will fall from a cloud?

if the droplet becomes large enough that its terminal velocity can overcome the updraft

How do clouds of vertical development differ from the other cloud types? How do these cloud types form? What type of precipitation, if any, is associated with these clouds? Are there any special features or characteristics associated with these clouds?

low bases which extend up into mid and high altitudes associated with unstable air (the warmer the air, the more unstable it is) grow high up into the atmosphere rather than spreading across the sky; span all levels of the troposphere and can even rise up into the stratosphere develop by warm air rising from the surface

What causes orographic lifting? Give an example.

mountains act as a barrier to air flow, so air is forced to ascend the mountain slope-most likely stable but could be unstable warm air run into mountain: 1. being less dense, it begins to rise over mountain 2. as it rises, pressure decreases 3. as pressure decreases, air expands 4. expansion causes density to decrease more 5. temperature decrease as density decreases (DALR) 6. condensation will occur (releases 600 cal/g) and convective clouds will form; if the saturated air continues to rise it will cool at the MALR

What fogs form as a result of cooling the layer of air immediately in contact with the surface?

radiation and advection fog

Name the characteristics of hail: What is it? How does it grow? How long will it remain suspended in the cloud? From what type of cloud does it originate?

rounded or jagged lumps of ice develop during intense thunderstorms (cumulonimbus clouds with strong convective updrafts) formation: starts with ice pellets carried high (via updrafts) to the top of a thunderstorm cloud pellets then fall back through the cloud and collect supercooled water droplets pellets are carried back up to the top of the cloud, where the water droplets freeze process continues as long as the updraft is greater than the TV of the hailstone when the hailstone finally gets too heavy for the updrafts to suspend it, it falls to the ground cut the stone, count the rings, to determine how many times the hailstone has traveled through the cloud (just like tree rings) consider the speed of the updrafts, and thus the terminal velocity, that must have been required to keep the largest hailstone suspended

What fogs form as a result of adding moisture to the air through evaporation?

steam fog frontal fog

What is the relationship between dew point temperature and the amount of water vapor in the air?

the higher the dew point temperature, the greater the amount of water vapor is in the air if the difference between the actual air temp and the dew point temp is small, the RH is high

Why do ice crystals grow faster (or at the expense of) liquid droplets?

the lower the temperature the lower the SMR At the same temperature, the SMR around ice crystals is lower than the SMR around supercooled water droplets when air is saturated with respect to supercooled water droplets it is super-saturated with respect to ice crystals

Why are condensation nuclei and ice-forming nuclei needed?

there must be a surface on which water vapor can condense tiny solid and liquid particles on which water vapor can condense can be natural or man made (forest fires, volcanic eruptions, soil particles, salt water spray, domestic and industrial pollutants)

How do dry adiabatic lapse rates differ from moist adiabatic lapse rates? Why?

unsaturated air cools faster than saturated air because after saturation, 600 cal/g of latent heat is released due to condensation

How does a cloud droplet remain suspended?

updrafts are usually strong enough to prevent droplets from leaving the base of a cloud to prevent a droplet from falling, updrafts must be strong enough to counter the droplet's terminal velocity erminal velocity: a balance of forces (upward force is the updraft vs downward force, which is gravity+air resistance) downward forces create a constant downward speed for the droplet the larger the droplet, the greater the downward force the larger the droplet, the stronger the updraft must be to keep the droplet suspended

What forces are at work keeping a cloud droplet suspended?

updrafts countering terminal velocity

Describe the process of how dew forms

usually forms on a clear night more terrestrial radiation emitted by surface than is received from the atmosphere; surface cools if cooled enough, the air at the surface reaches 100% saturation (remember lowering the temperatue lowers the SMR) if temperature is above freezing dew forms; if below frost forms

What happens to water drops when the leave the cloud (vaporization)?

vaporize: If droplets are large enough to overcome the updraft, they usually vaporize in the unsaturated air below the cloud Most cloud droplets are so small that even if there were no updrafts, they would require 24 hours to reach the ground The low terminal velocity of these droplets means that they would evaporate long before then

What causes convergence? Give an example.

wherever air flows to a central point (ex: center of a low pressure) it cannot go into the ground so must rise 1. being less dense, air begins to rise 2. as it rises, pressure decreases 3. as pressure decreases, air expands 4. expansion causes density to decrease more 5. temperature decrease as density decreases (DALR) 6. condensation will occur (releases 600 cal/g) and convective clouds will form; if the saturated air continues to rise it will cool at the MALR