pV=nRT How much does all the air in the room weigh? How much does - PowerPoint PPT Presentation

pV=nRT How much does all the air in the room weigh? How much does all the water weigh? What is a hydrogen bond? What do they do?

What is an ice crystal? What is a snowflake? • A collection of organized water • A collection of individual ice molecules, below 0º C crystals falling as a hydrometeor • Crystal lattice of 4 nearest neighbors, • Often consists of 5-100 ice anchored by H-bonds crystals • In the atmosphere, grown from vapor, • Size and crystal numbers depend often but not always by Bergeron- on temperature, crystal habits, Findeisen process and turbulence • Crystallographic symmetry becomes • Snowflakes are not made with expressed macroscopically scissors • Initiated by several ice nucleation mechanisms

Physical Properties of Ice c axis • Density of ice 1h near 0 C: 917kg/m 3 a axes • Heat Capacity: 1960 J/kg*K • Latent Heat of sublimation: basal face • 2.83 x 10 6 J/kg (to break H-bonds) • Ice crystal size: several μ m to ~5 mm • Fall speed: to 80 cm/s • Fastest Growth rate: -14.7 C Unit cells of Ice 1h 10 22 units/cm 3 d=2.76 A c projection view a

Experimental Features • Quadrupole levitation for wall-less environment • Alternate support on fine glass fiber • Vertical flow between 0 and 75 cm/s • Temperature between room and -65 ºC • Total pressure between atmospheric and 50 hPa • Supersaturation controlled between 0 and greater than water saturation

Where In The World ?

Unique physical properties of water: • Very high specific heat capacity • High thermal conductivity for a liquid • Neutral pH • Exists on earth in 3 phases • Ice is less dense than liquid • Nearly universal chemical solvent • High surface tension • Large latent heats (energy of phase change) • Needed as ingredient for beer

• For enthalpy to increase, heat must be absorbed from surroundings. When this energy is removed • In many instances, from the surroundings, it multiple phase changes are can be used to break occurring simultaneously hydrogen bonds. This at one surface. causes the phase change and cools the surroundings • Temperature, pressure, due to heat loss. and humidity conditions determine which • When enthalpy decreases, microscopic process heat is released to the dominates surroundings due to the formation of new • The macroscopic behavior hydrogen bonds. This is the net result of all warms the surroundings processes. We often are concerned only with • The water substance has explaining this net result. constant temperature during a phase change

Phase Diagram for Water: Thermodynamic equilibrium

Clausius-Clapeyron Curve L 1 1 v exp e e 0 R T T v 0 L 1 1 v exp e e 0 R T T v 0

• What is the difference between actual and saturation vapor pressure? • What is steam? • What conditions must be met to allow boiling to occur? • How will the boiling temperature change at high altitude?

Thermodynamics of phase changes in water: • (blue = heat released to surroundings) – condensational warming • (red = heat added (removed) from surroundings) – evaporative cooling L v =2.27x10 6 J/kg • Vaporization/Condensation: L f =3.3x10 5 J/kg • Freezing/Melting: • “Latent Heat” is a measure of enthalpy change associated with hydrogen bonds forming or breaking during a phase change. • Expressions for heating • How long must I microwave the frozen milk?

• Clouds are classified mainly by their visual characteristics and height • They look different because they have different contents • 3 primary types and many sub-types Stratus Cirrus Cumulus

Stratus Clouds Characteristics: • Can be at any altitude – stratus just means that they form a horizontal layer • They are often at low altitude in bad weather (nimbostratus) • Fog is a stratus cloud hugging the ground • They are formed by weak, but widespread vertical motion (~10 cm/s) • The are made of a moderate density of cloud drops , LWC~.1 g/m 3 • Cumulus or cirrus can also form a layer (Stratocumulus and cirrostratus)

Cumulus Clouds Characteristics: • Can be at any altitude – cumulus means “heaping” • They develop more vertically than horizontally. • When they form rain they become cumulonimbus • They are formed by strong vertical motion, sometimes 25 m/s updrafts • Strong vertical motion and cumulus clouds result from free convection that comes from instability • If that vertical motion is deep enough, ice can form in upper part of the cloud • Ice crystals and strong motion -> charge separation ->lightning • They have the greatest LWC: from .5 to 4 g/m 3 depending of updraft rate

Cirrus Clouds Characteristics: • Are composed of tiny ice crystals, not liquid cloud drops • Usually form only when T< -25 C • They are formed by weak vertical motion (~5 cm/s) • The are made of a small density of ice crystals , IWC~.05 g/m 3 • Sometimes generated by jet exhaust (contrail) • Often initiated as anvils of cumulus clouds striking the tropopause-lid • Important effects due to widespread radiative impact

Cloud Height Cloud height Cloud types Low (below 2 km, 6500 ft) Fog Stratus Nimbostratus Stratocumulus Stratus fractus Cumulus humulis Mammatus Funnel Middle(2-6 km, 6500-20000ft) Cumulus humulis Cumulus mediocris Stratocumulus Altostratus Altocumulus High (6+ km, 20000 ft+ ) Cirrus Cirrostratus Cirrus uncinus/fibratus Pileus cloud Large vertical span Cumulus castellanus Cumulus congestus Cumulonimbus

http://apollo.lsc.vsc.edu/classes/met130/notes /chapter5/graphics/drop_cloud_ccn.gif CCN: Cloud Condensation Nuclei • Needed to turn supersaturation into liquid drops (a site is needed for condensation) • This is referred to as “drop nucleation” – a big uncertainty in the science of clouds • CCN are preferentially hydrophillic • Can be composed of dust, bacteria, pollen, pollutants, acid drops, salt, and others • Ice nuclei have slightly different characteristics

Vapor deposition into water drops m = “net” molecules added molecules impinging - molecules vaporizing Will make it in Made it out Will be rejected Cloud Drop ~20 um radius m = - - +

Vapor deposition onto ice surfaces • Average coefficients help determine net mass growth rate • Relative local coefficients determine habit type • Local coefficient is a function of temperature and moisture density • Mechanism for coefficient temperature function is an enduring mystery • Mechanism of incorporating incident molecule into lattice is also unknown On the way out Brief layover Cleared for landing Taxiing to lattice (condensation) (surface diffusion) m= Taking off - (sublimation) Actively growing terrace - + 2D Nucleation

Bergeron-Findeison Process • The saturation vapor over water is greater than ice (see phase diagram) • This is caused by the greater difficulty in breaking 100% vs 80% of H-bonds • Vapor tries to move from high concentration -> low concentration (law of diffusion) • Thus, when water and ice surfaces are nearby, the vapor moves from high concentration (water surface)->lower concentration (ice surface), allowing ice to grow as water evaporates • This is the major form of ice crystal growth in mixed-phase clouds • This process contributes to many stages of the precip. process

Little drops->Big drops: Collision-Coalescence

Riming->graupel->Hail

Frozen precip. scenarios

Crystals photographed in cirrus clouds by aircraft-borne probe

Weather Modification and cloud seeding Witches concoct a brew to summon a hailstorm . “After the bomb, Dad came up with ice” – Kurt Vonnegut, Cat’s Cradle , on the invention of “ice - nine”.