61 Cards in this Set
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Adiabatic process
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A process in which a system (parcel of air) does not exchange heat with its surroundings.
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Rising Air Parcel
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Expansion and cooling occurs because there is less air pressure
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Expansion
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causes rising air to cool in temperature
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cooling of air
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takes place within the air parcel, without loosing its heat energy to the surroundings
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Descending air parcel
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-compresses
-compression causes warming of the descending air
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warming of air
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takes place within the parcel without drawing heat energy from its surroundings
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Dry Adiabatic Lapse Rate
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-rate of temperature in a rising or descending unsaturated air parcel
(5.5 degrees F / 1,000 ft)
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Moist Adiabatic Lapse Rate
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-rate of change of temperature in a rising or descending saturated air parcel
-rate varies, always less than
(5.5 degrees F / 1,000 ft)
-heat added during condensation offsets some of the cooling due to expansion, the air no longer cools at the dry adiabatic rate but at a lesser rate
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Other rates
(not Adiabatic rates)
Environmental Lapse Rate
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-rate of decrease of air temperature (static air) with elevation
(3.5 degrees F / 1,000)
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Dew Point Lapse Rate
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-rate of change of dew point temperature
(1 degree F / 1,000 ft)
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Lapse Rate
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air temperature that changes with altitude
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Environmental lapse rate
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-rate at which the air temperature surrounding us will be changing if we were to climb upward into the atmosphere
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Absolutely stable
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the atmosphere is always absolutely stable when the environmental lapse rate is less than the moist adiabatic rate
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Subsidence inversions
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-atmospheric condition where air becomes warmer with inversions that form as air slowly sinks over a large rate
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Neutral Stability
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-if the lapse rate is exactly equal to the dry adiabatic rate, rising or sinking unsaturated air will cool or warm at the same rate as the air around it.
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Absolutely unstable
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when both air parcels rise, they will continue to rise on their own because the rising air parcels are warmer and less dense than the air around them
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Absolute instability
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results when the environmental lapse rate is greater than the dry adiabatic rate
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Conditionally unstable
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once the parcel is pushed upward, it will tend to move in that direction, the atmosphere is unstable for the saturated parcel
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Conditional instability
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occurs whenever the environmental lapse rate is between the moist adiabatic rate and the dry adiabatic rate. The atmosphere is ordinary at this state
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Causes of instability
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-the atmosphere becomes more unstable as the environmental lapse rate steepens. As the air temperature drops rapidly with increasing height. This may be brought on by either air aloft becoming colder or the surface air becomes warmer
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Cooling of air aloft due to:
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winds bringing in colder air (cold advection)
clouds (or air) emitting infrared radiation to space (radiational cooling)
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warming of surface air due to:
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daytime solar heating of the surface
an influx of warm air brought in by the wind (warm advection)
air moving over a warm surface
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Typical Cloud droplet
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20 um in diameter
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Typical rain drop
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-2000 um (100 times larger than droplet)
-a million cloud drops from a raindrop
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Condensation Nucleus
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-0.2 um
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Precipitation Process
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1. collision- coalescence process
2. Ice-crystal (Bergeron) process- snow, hail, sleet
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Collision- coalescence process
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-droplet growth in warm clouds
-larger droplets accelerated downward by gravity, collide and merge with smaller droplets
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Terminal Velocity
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-constant speed obtained when upward air resistance balances with downward force of gravity
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Formation of raindrops in a warm cloud depends on
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1. liquid water content (major factor)
2. range of cloud droplet sizes
3. Cloud thickness
4. Electric charges and fields that bind droplets
5. Updrafts in the cloud
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The nimbostratus clouds have
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-weak updrafts
-thus: light showers
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Thick cumulonimbus have
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-strong updrafts
-thus heavy showers
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Ice-crystals (Bergeron process)
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Droplet growth in cold cloud with temperatures below 32 degrees F.
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cloud droplets freeze at what temperature?
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-40 degrees C
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Supercooled water
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-liquid water that exists when its temperature is below freezing
-freezes when it impacts an object
e.g. freezing rain
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Glaciated
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cloud top with only ice-crystals
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ice-crystal process
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growth of droplets and ice-crystals
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growth by accretion
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-"falling ice-crystals collide with supercooled cloud droplets and freeze on contact"
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Growth by aggregation
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-"falling ice-crystals collide and stick to other ice-crystals"
-this produces snowflakes
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if a snowflake melts
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it continues its fall as raindrop
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rain in cold clouds begin as
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snow
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Rain
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-falling drops must have diameter of .5mm (0.02") or greater, to be considered rain.
-less than 0.5 mm is drizzle
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virga
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-rain that evaporates before it reaches the surface
-observed as streaks of falling precipitation
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Snow
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-snowflakes that will reach the surface
-temperature profile underneath a cloud should be below freezing
-rain begins as snowflakes in cold clouds
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Freezing level
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-is above 12,000 ft in summer when snowflakes melt before reaching the ground
-much lower in winter
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Sleet
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-translucent balls of ice or frozen raindrops
-occur when sub freezing layer is cold enough to freeze raindrops
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Freezing rain
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-supercooled rain that freezes on contact with exposed objects
-also called glaze or "icing storm" when coating of ice creates shimmering landscapes
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Snow Pellets
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-small round ice particles formed in a cloud mainly by accretion
-remain frozen particles and reach the surface as snow pellets (also called groupel)
-groupel can easily be crushed unlike hailstone
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Accretion
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growth of precipitation particle by the collision of an ice crystal with a supercooled cloud droplet that freezes upon impact
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Hail
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-forms in cumulonimbus clouds where updrafts can reach 100 mph
-occur in clouds with abundant supply of supercooled water
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Measuring precipitation
Standard instruments
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1. standard rain gauge
2. recording gauges
-tipping-bucket
-weighing gauge
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Standard rain gauge
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-routinely used
-measuring amount =0.01" or greater
-less than 0.01"=reported as trace
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Tipping-Bucket gauge
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-consists of two buckets
-each time a bucket fills up with 0.01" of rain, it tips, sending an electric signal to the remote recorder
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Weighing gauge
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-collects rainfall in a cylinder that rests on a sensitive weighing platfrom
-when cylinder fills up, electric impose is transmitted to a pen that records data
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measuring snowfall
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1.depth (with calibrated stick)
2.Water equivalent (general ratio is 10:1)
eg. 10" deep= 1 inch of rainfall
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Cloud seeding
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-silver iodide and dry ice are injected is a cloud to act as condensation nuclei
-works better in cold clouds with supercooled water
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Saturation Vapor
Pressure
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total number of vapor molecules around the droplet remains fairly constant and defines the droplet
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Curvature effect
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smaller cloud droplets exhibit a greater curvature, which causes a more rapid rate of evaporation
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Solute effect
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this condition reduces the equilibrium vapor pressure
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Fall streaks
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ice-crystals and snowflakes fall from high cirrus clouds
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snow squall
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more intense snow shower
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snow grains
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small, opaque grains of ice, equivalent of drizzle
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GEO 211: EXAM 4