GEO 211 1st Edition Exam 3 Study Guide Lectures 15 19 Lecture 15 October 3 What is adiabatic process A process that takes place without a transfer of heat between the system such as an air parcel and its surroundings In an adiabatic process compression always results in warming and expansion results in cooling Rising in air parcel expands and cools Expansion is due to decrease of pressure with altitude Sinking air parcel is compressed and warms As long as the air in the parcel is unsaturated relative humidity is less than 100 the rate of adiabatic cooling or warming remains constant How does Dry Adiabatic Rate differ from Moist Adiabatic Rate DAR the rate of change of temperature in a rising or descending unsaturated air parcel The rate of adiabatic cooling or warming is about 5 5 F per 1000 feet MAR the rate of change of temperature in a rising or descending saturated air parcel The rate of cooling or warming varies but a common value of 3 3 F per 1000 feet is used You should be able to calculate adiabatic cooling and warming of a parcel of air Lecture 16 October 8 How is stability of the atmosphere determined Stability is determined by comparing temperature of rising parcel to that of its surroundings environment The temperature of both the rising air and of its environment must be measured at various level about the earth What do we mean by stable atmosphere unstable atmosphere conditionally unstable atmosphere Absolutely stable exists when environmental lapse rate is less than moist adiabatic rate Rising air parcel is always colder than air surrounding it Absolutely unstable occurs when ELR is greater than DAR Rising air is always warmer than surrounding it Conditionally unstable occurs when ELR is between the MAR and the DAR Atmosphere is stable if rising air is unsaturated but unstable if rising air is saturated How does stable air differ from unstable air Stable air resists upward movement when the rising air is colder than the environment it will be more dense heavier and tend to sink back to its original level Unstable air when the rising air is warmer and thus less dense than the surrounding air it will continue to rise until it reaches the same temperature as its environment What conditions bring about unstable atmosphere 1 Cold air moving over warm surface 2 Daytime solar heating of the surface 3 Radiational cooling of clouds Loss of heat energy to space from clouds What conditions bring about stable atmosphere 1 Warm air moving over cold surface 2 Nighttime radiational cooling 3 Advection of cold surface air Lecture 17 October 10 How do cloud droplets grow larger Droplets with a greater relative humidity than the equilibrium value allow the droplet to grow by condensation Cloud condensation nuclei allow condensation to begin on particles wen the relative humidity is well below 100 percent because they are hygroscopic have an affinity in water vapor What is the ice crystal process Droplet growth in cold clouds with temperatures below 32 F Cloud droplets don t freeze at 0 C freeze at 40 C Is extremely important in middle higher latitudes where clouds extend upward into regions where the air temperature is well below freezing What is collision and coalescence process Droplet growth in warm clouds larger droplets accelerated downwards by gravity collide and merge with smaller droplets Terminal velocity constant speed obtained when upward air resistance balances with downward force of gravity Can play a significant role in producing precipitation The greater the speed the more air molecules the drop encounters each second Larger drops fall faster than smaller drops Coalescence large drops overtake and collide with smaller drops in their path this merging of cloud droplets in collision is called coalescence How does a cold cloud differ from a warm cloud Warm clouds clouds that have above freezing temperatures at all levels precipitation forms by the collision and coalescence process Cold clouds where clouds extend upward into regions where the air temperature is well below freezing Lecture 18 October 13 Know all precipitation types and how each come about Rain falling drops must have diameter of 0 5mm 0 02 inch or greater to be considered rain Less than 0 5mm is drizzle Virga rain that evaporates before reaching the surface observed as streaks of falling precipitation figure 7 14 Snow snowflakes that will reach the surface temperature profile underneath a cloud should be below freezing Rain begins as snowflakes in cold clouds Freeing level is above 12 000 feet in summer when snowflakes melt before reaching the ground freezing level is much lower in winter Sleet translucent balls of tiny ice pellets Form when melting snowflake falls through the deep subfreezing surface layer of air Bounces wen striking the ground figure 7 21 Freezing rain supercooled rain that freezes on contact with the ground Surface beneath a cloud is too shallow to freeze supercooled liquid drops Also called glaze Ice storms when freezing rain creates shimmering landscape Snow pellets small grains of ice formed by accretion in a cloud Doesn t bounce as sleet Also called graupel can easily be crushed unlike a hailstone Hail forms in cumulonimbus clouds updrafts can reach 100mph Form in clouds with abundant supply of supercooled water Lecture 19 October 15 How is precipitation measured 1 Standard rain gauge commonly used Cross section 10 times that of tube Measurable amounts 0 01 inch or greater Less than 0 01 is reported as a trace 2 Tipping buck rain gauge consists of 2 buckets Each time a bucket fills up with 0 01 inch of rain it tips sending an electric signal to the remote recorder 3 Weighing gauge collects rainfall in a cylinder that sits on a sensitive weighing platform When cylinder fills up electrical impulse is transmitted to a pen that records the data