Transcript Chapter 27
Chapter 27 Clouds Use of Power Point design and animations with permission from Dr. Joby Hilliker ( West Chester University, West Chester, PA CHAPTER 6 CLOUDS How is a cloud formed? An air parcel containing water vapor (gas) is cooled to the dewpoint such that the water vapor condenses to form water droplets (liquid). = Liquid water droplets When condensation occurs, the air parcel is ”saturated” (RH=100%) CHAPTER 6 CLOUDS When condensation occurs, the air parcel is ”saturated” (RH=100%) The temp. at which saturation (and condensation) will occur is the dewpoint The farther the dewpoint is below the air temp, the lower the RH, and the less likely clouds will form. CHAPTER 6 RISING PARCELS How can an air parcel cool to the dewpoint? Get it to rise! How can this happen? Density of parcel needs to be less than surrounding air Density of Surrounding Air Density of Main Air Parcel CHAPTER 6 RISING PARCELS Translate density into temperature: Warmer air is less dense; Colder air is more dense A parcel will rise if it’s warmer than surrounding air. “Warm air rises” concept Temp. of Surrounding Air Temp. of Main Air Parcel CHAPTER 6 RISING PARCELS What happens when a parcel rises? It expands Finds itself in an environment with lesser density/pressure It cools adiabatically Takes work (thermal energy) to expand parcel adiabatic = no heat exchange between parcel and environment 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 …at a given altitude to… b. temp. of surrounding parcels (i.e., the atmosphere) which cools at a constant rate changes on a daily basis; is measured RULE #1: As long as the rising parcel is warmer than surrounding air parcels at a given altitude, the parcel will continue to rise If parcel becomes colder, it will sink CHAPTER 6 CASE #1 UNSTABLEATMOSPHERE): Steep lapse rate (_________ Parcel rises to tropopause COLD Result: Thunderstorms! Temperature HOT CHAPTER 6 CASE #2 Steep lapse rate near ground, weak aloft: Parcel rises to a point, then stops Result: Cumulus COLD Temperature HOT CHAPTER 6 CASE #3 STABLE ATMOSPHERE): Weak lapse rate (_________ Parcel rises only slightly COLD Temperature Result: Stratus HOT CHAPTER 6 SUMMARY OF CLOUDS Cloud type reveals: …how quickly temperature is decreasing w/ height CUMULUS Unstable Atmosphere High Lapse Rate Deep, Vertical Development STRATUS Stable Atmosphere Low Lapse Rate Thin, Spread out CHAPTER 6 SUMMARY OF CLOUDS If parcel can rise: Cloud Top = where parcel is no longer warmer than surrounding air Cloud Base = where saturation occurs What could I infer about atmosphere if it is CLEAR? Sinking air Lack of moisture (low dewpoint) CLOUD NOMENCLATURE Height Cloud Base Stable Unstable <8000 feet STRATUS CUMULUS 10000-20000 feet Add prefix “Alto-” >20000 feet Add prefix “Cirro-” With rain? Add “Nimbo-”