Chapter 27 - Clouds Use of Power Point design and animations with permission from Dr. Joby Hilliker ( West Chester University, West Chester, PA

â How is a cloud formed? ê An air parcel containing water vapor (gas) is cooled to the dewpoint such that the water vapor condenses to form water droplets (liquid). CHAPTER 6 - CLOUDS = Liquid water droplets â When condensation occurs, the air parcel is ”saturated” (RH=100%)

â The temp. at which saturation (and condensation) will occur is the dewpoint é The farther the dewpoint is below the air temp, the lower the RH, and the less likely clouds will form. CHAPTER 6 - CLOUDS

â How can an air parcel cool to the dewpoint? é Get it to rise! â How can this happen? é Density of parcel needs to be less than surrounding air CHAPTER 6 - RISING PARCELS Density of Main Air Parcel Density of Surrounding Air

â Translate density into temperature: é Warmer air is less dense; Colder air is more dense â A parcel will rise if it’s warmer than surrounding air. é “Warm air rises” concept CHAPTER 6 - RISING PARCELS Temp. of Main Air Parcel Temp. of Surrounding Air

What happens when a parcel rises?  It expands þ Finds itself in an environment with lesser density/pressure  It cools adiabatically þ Takes work (thermal energy) to expand parcel þ adiabatic = no heat exchange between parcel and environment þ parcels cools at the dry adiabatic lapse rate (DALR), which is 5.5°F / 1000 feet CHAPTER 6 - RISING PARCELS

To determine if a parcel will rise, compare: a. the parcel’s temperature which cools at a constant rate …at a given altitude to… b. temp. of surrounding parcels changes on a daily basis; is measured (i.e., the atmosphere) RULE #1: â As long as the rising parcel is warmer than surrounding air parcels at a given altitude, the parcel will continue to rise é If parcel becomes colder, it will sink CHAPTER 6 - RISING PARCELS

â Steep lapse rate (_________ ATMOSPHERE): Parcel rises to tropopause Result: Thunderstorms! CHAPTER 6 - CASE #1 TemperatureHOTCOLD UNSTABLE

â Steep lapse rate near ground, weak aloft: Parcel rises to a point, then stops Result: Cumulus CHAPTER 6 - CASE #2 TemperatureHOTCOLD

â Weak lapse rate (_________ ATMOSPHERE): Parcel rises only slightly Result: Stratus CHAPTER 6 - CASE #3 STABLE TemperatureHOTCOLD

â Cloud type reveals: …how quickly temperature is decreasing w/ height â CUMULUS STRATUS Unstable Atmosphere Stable Atmosphere High Lapse Rate Low Lapse Rate Deep, Vertical Development Thin, Spread out CHAPTER 6 - SUMMARY OF CLOUDS

â If parcel can rise: é Cloud Top = where parcel is no longer warmer than surrounding air é Cloud Base = where saturation occurs â What could I infer about atmosphere if it is CLEAR? é Sinking air é Lack of moisture (low dewpoint) CHAPTER 6 - SUMMARY OF CLOUDS

Height Cloud BaseStableUnstable <8000 feetSTRATUSCUMULUS feetAdd prefix “Alto-” >20000 feetAdd prefix “Cirro-” With rain?Add “Nimbo-” CLOUD NOMENCLATURE