ATM 10 Severe and Unusual Weather Prof Richard Grotjahn http atm ucdavis edu grotjahn course atm10 index html Lecture topics Moisture Mixing ratio Vapor pressure Relative humidity Saturation vapor pressure Dew point temperature Cloud types Four categories of common clouds Unusual types 9 different variables 1 absolute humidity 2 specific humidity 3 mixing ratio 4 vapor pressure 5 saturation vapor pressure 6 relative humidity 7 wet bulb temperature 8 dew point 9 frost point 10 etc etc etc 5 primary moisture variables 1 2 3 4 5 We can whittle this list down The most important variables from the list for this class are these 5 mixing ratio w vapor pressure e saturation vapor pressure es relative humidity rh and dew point Td Water in the Atmosphere Water molecule is 2 H O atoms Water can exist in atmosphere as any of 3 states Fig 5 1 gas vapor liquid drops and droplets and solid ice Fig 5 2 Ice in the Atmosphere part 1 Water in solid state combines to form hexagonal 6 sided shapes Fig 5 1 Fig 5 2 Ice in the Atmosphere part 2 Ice has several forms in the atmosphere all of which have 6 sided symmetry Fig 5 2 4 Transitions and Saturation Water molecules are constantly being exchanged across an interface Example air liquid water air Evaporation liquid to vapor Condensation vapor to liquid Sublimation solid ice to vapor Deposition vapor to solid ice Saturation where number of molecules going from one state equals number going the opposite way Saturation versus Temperature Before a cloud can form air must become saturated That maximum amount water vapor in the air depends on the temperature A parcel of warm air can hold more water vapor than a cold air parcel Note the mass of air in the parcel that is not water is the same Important implications for hurricanes Describing Atmospheric Moisture Atmospheric water vapor has been defined several different ways These terms include absolute humidity specific humidity mixing ratio vapor pressure and relative humidity Figure 5 6 Mixing Ratio w Figure 5 8 Specific humidity equals the mass of vapor divided by total mass of air in a parcel and is not affected by changes in parcel volume Mixing Ratio w is the mass of water vapor divided by the mass of DRY air in a given parcel of air w is not affected by changes in parcel volume a Saturation mixing ratio ws Clouds For air parcel w is constant whether you warm the air parcel or change its altitude The saturation mixing ratio ws is the mixing ratio saturated air would have Ws changes with T and with P A cloud forms when a parcel of air changes T and or P until the mixing ratio equals the saturation mixing ratio Vapor pressure e Recall Pressure is force per unit area Actual vapor pressure is only that force exerted by the water vapor molecules in a parcel of air so it is independent of the surrounding dry air pressure As temperature goes up e increases When more water molecules are present e increases Saturation Vapor Pressure es Recall Pressure is force per unit area Saturation vapor pressure is only that force exerted by the water vapor molecules in a parcel of air when the air is saturated As temperature goes up es increases When es P or greater then water boils Figure 5 10 Relative Humidity rh Measures ratio of number of molecules of water vapor present divided by the number needed for saturation and expressed as a Saturated air as rh 100 rh 100 w ws and rh 100 e es Unlike mixing ratio rh varies as a parcel changes its T P and or altitude Like mixing ratio rh increases when water molecules added to the air Dewpoint Td Temperature at which air becomes saturated without changing the water vapor present or changing the pressure Important uses Good indicator of water vapor content of air When T and Td are similar rh is high Td used on charts to see important properties of the air and air parcels Test your understanding Patagonia T 280 Td 280 LaCampana T 320 Td 290 Both sites are near sea level 1 Which has higher B mixing ratio 2 Which has higher vapor pressure B 3 Which has higher rh A Photos R Grotjahn A raining in Patagonia B sunny La Campana Chile Common Cloud Categories 10 Common Cloud Types 10 combinations of these 5 names Cirrus wispy or hair like often high Alto middle elevation Stratus sheet like or layered Cumulus puffy heaped or vertical horizontal Nimbus precipitating 4 Common Cloud Groups Elevations of Common Cloud Groups Uncommon Cloud Categories Water versus Ice Cloud Florida Everglades R Grotjahn Photo R Grotjahn Cirrus Types R Grotjahn Desolation Pks CA R Grotjahn Altocumulus Mt Rainer R Grotjahn Wairikori Beach New Zealand R Grotjahn Stratus Big Sur CA R Grotjahn Altostratus stratus Nimbostratus Wilson s Prom Australial R Grotjahn Mt Cook NZ R Grotjahn Paria Canyon R Grotjahn Stratocumulus cumulus Photo R Grotjahn Cumulus congestus cumulonimbus Summary of major cloud types Odd clouds Part 1 Noctilucent clouds Nacreous clouds Odd clouds Forced uplift clouds pileus Gibraltor R Grotjahn Wasatch Front UT R Grotjahn Odd clouds severe weather Marshall CO R Grotjahn G Moore G Moore Most Common Altocumulus Catskill Creek T Cole End of lecture 3