Moisture, Clouds, and Precipitation Lecture PDF

Chapter 17 Lecture Earth Science Fourteenth Edition, Global Edition Moisture, Clouds, and Precipitation Jennifer Mangan James Madison University © 2015 Pearson Education Ltd.  Changes of State of Water Heat energy – M...

Chapter 17 Lecture Earth Science Fourteenth Edition, Global Edition Moisture, Clouds, and Precipitation Jennifer Mangan James Madison University © 2015 Pearson Education Ltd.  Changes of State of Water Heat energy – Measured in calories – one calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius – Latent heat Stored or hidden heat Not derived from temperature change Important in atmospheric processes © 2015 Pearson Education Ltd.  Changes of State of Water Three states of matter – Solid – Liquid – Gas To change state, heat must be – Absorbed, or – Released © 2015 Pearson Education Ltd.  Changes of State of Water Processes – Evaporation Liquid is changed to gas 600 calories per gram of water are added – called latent heat of vaporization – Condensation Water vapor (gas) is changed to a liquid Heat energy is released – called latent heat of condensation © 2015 Pearson Education Ltd.  Changes of State of Water Processes – Melting Solid is changed to a liquid 80 calories per gram of water are added – called latent heat of melting – Freezing Liquid is changed to a solid Heat is released – called latent heat of fusion © 2015 Pearson Education Ltd.  Changes of State of Water Processes – Sublimation Solid is changed directly to a gas (e.g., ice cubes shrinking in a freezer) 680 calories per gram of water are added – Deposition Water vapor (gas) changed to a solid (e.g., frost in a freezer compartment) Heat is released © 2015 Pearson Education Ltd.  Changes of State of Water © 2015 Pearson Education Ltd.  Humidity Amount of water vapor in the air – Saturated air is air that is filled with water vapor to capacity – Capacity is temperature dependent – warm air has a much greater capacity – Water vapor adds pressure (called vapor pressure) to the air © 2015 Pearson Education Ltd.  Humidity Measuring humidity – Mixing ratio Mass of water vapor in a unit of air compared to the remaining mass of dry air Often measured in grams per kilogram – Relative humidity Ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure) © 2015 Pearson Education Ltd.  Humidity Measuring humidity – Relative humidity Expressed as a percent Saturated air – Content equals capacity – Has a 100 percent relative humidity Relative humidity can be changed in two ways – Add or subtract moisture to the air » Adding moisture raises the relative humidity » Removing moisture lowers the relative humidity © 2015 Pearson Education Ltd.  Humidity Relative humidity – Relative humidity can be changed in two ways Changing the air temperature – Lowering the temperature raises the relative humidity © 2015 Pearson Education Ltd.  Humidity Dew point temperature – Temperature to which a parcel of air would need to be cooled to reach saturation – Cooling the air below the dew point causes condensation e.g., dew, fog, or cloud formation Water vapor requires a surface on which to condense © 2015 Pearson Education Ltd.  Relative Humidity Changes at Constant Temperature © 2015 Pearson Education Ltd.  Relative Humidity Changes at Constant Water-Vapor Content © 2015 Pearson Education Ltd.  Daily Variations in Temperature and Relative Humidity © 2015 Pearson Education Ltd.  Humidity Relative humidity – Two types of hygrometers are used to measure humidity © 2015 Pearson Education Ltd.  Hygrometers Psychrometer – compares temperatures of wet-bulb thermometer and dry-bulb thermometer If the air is saturated (100 percent relative humidity) then both thermometers read the same temperature The greater the difference between the thermometer readings, the lower the relative humidity Hair hygrometer – reads the humidity directly © 2015 Pearson Education Ltd.  A Sling Psychrometer © 2015 Pearson Education Ltd.  Adiabatic Heating/Cooling Adiabatic temperature changes occur when – Air is compressed Motion of air molecules increases Air will warm Descending air is compressed due to increasing air pressure – Air expands Air will cool Rising air will expand due to decreasing air pressure © 2015 Pearson Education Ltd.  Adiabatic Heating/Cooling Adiabatic rates – Dry adiabatic rate Unsaturated air Rising air expands and cools at 1° C per 100 meters (5.5° F per 1000 feet) Descending air is compressed and warms at 1° C per 100 meters © 2015 Pearson Education Ltd.  Adiabatic Heating/Cooling Adiabatic rates – Wet adiabatic rate Commences at condensation level Air has reached the dew point Condensation is occurring and latent heat is being liberated Heat released by the condensing water reduces the rate of cooling Rate varies from 0.5° C to 0.9° C per 100 meters © 2015 Pearson Education Ltd.  Adiabatic Cooling of Rising Air © 2015 Pearson Education Ltd.  Processes that Lift Air Orographic lifting – Elevated terrains act as barriers – Result can be a rainshadow desert Frontal wedging – Cool air acts as a barrier to warm air – Fronts are part of the storm systems called middle- latitude cyclones © 2015 Pearson Education Ltd.  Orographic Lifting © 2015 Pearson Education Ltd.  Frontal Wedging © 2015 Pearson Education Ltd.  Processes that Lift Air Convergence where the air is flowing together and rising (low pressure) Localized convective lifting – Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy © 2015 Pearson Education Ltd.  Convergence © 2015 Pearson Education Ltd.  Localized Convective Lifting © 2015 Pearson Education Ltd.  Stability of Air Types of stability – Stable air Resists vertical displacement – Cooler than surrounding air – Denser than surrounding air – Wants to sink No adiabatic cooling Absolute stability occurs when the environmental lapse rate is less than the wet adiabatic rate © 2015 Pearson Education Ltd.  Absolute Stability © 2015 Pearson Education Ltd.  Stability of Air Stable air – Often results in widespread clouds with little vertical thickness – Precipitation, if any, is light to moderate © 2015 Pearson Education Ltd.  Stability of Air Absolute instability – Acts like a hot air balloon – Rising air Warmer than surrounding air Less dense than surrounding air Continues to rise until it reaches an altitude with the same temperature © 2015 Pearson Education Ltd.  Absolute Instability © 2015 Pearson Education Ltd.  Stability of Air Absolute instability Adiabatic cooling Environmental lapse rate is greater than the dry adiabatic rate Clouds are often towering Conditional instability occurs when the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parce © 2015 Pearson Education Ltd.  Conditional Instability © 2015 Pearson Education Ltd.  Stability of Air Determines to a large degree – Type of clouds that develop – Intensity of the precipitation © 2015 Pearson Education Ltd.  Condensation and Cloud Formation Condensation – Water vapor in the air changes to a liquid and forms dew, fog, or clouds – Water vapor requires a surface to condense on Possible condensation surfaces on the ground can be the grass, a car window, etc. Possible condensation surfaces in the atmosphere are called condensation nuclei – Dust, smoke, etc – Ocean salt crystals which serve as hygroscopic (“water- seeking”) nuclei © 2015 Pearson Education Ltd.  Condensation and Cloud Formation Clouds – Made of millions and millions of Minute water droplets, or Tiny crystals of ice – Classification based on Form (three basic forms) – Cirrus – high, white, thin – Cumulus – globular cloud masses often associated with fair weather – Stratus – sheets or layers that cover much of the sky © 2015 Pearson Education Ltd.  Condensation and Cloud Formation Classification based on – Height High clouds – above 6000 meters – Types include cirrus, cirrostratus, cirrocumulus Middle clouds – 2000 to 6000 meters – Types include altostratus and altocumulus Low clouds – below 2000 meters – Types include stratus, stratocumulus, and nimbostratus (nimbus means “rainy”) © 2015 Pearson Education Ltd.  Condensation and Cloud Formation Classification based on – Height Clouds of vertical development – From low to high altitudes – Called cumulonimbus – Often produce rain showers and thunderstorms © 2015 Pearson Education Ltd.  Classification of Clouds According to Height and Form © 2015 Pearson Education Ltd.  Fog Considered an atmospheric hazard Cloud with its base at or near the ground Most fogs form because of – Radiation cooling, or – Movement of air over a cold surface © 2015 Pearson Education Ltd.  Fog Types of fog – Fogs caused by cooling Advection fog – warm, moist air moves over a cool surface Radiation fog – Earth’s surface cools rapidly – Forms during cool, clear, calm nights Upslope fog – Humid air moves up a slope – Adiabatic cooling occurs © 2015 Pearson Education Ltd.  Advection Fog © 2015 Pearson Education Ltd.  Fog Types of fog – Evaporation fogs Steam fog – Cool air moves over warm water and moisture is added to the air – Water has a steaming appearance Frontal fog, or precipitation fog – Forms during frontal wedging when warm air is lifted over colder air – Rain evaporates to form fog © 2015 Pearson Education Ltd.  Steam Fog © 2015 Pearson Education Ltd.  Precipitation Cloud droplets – Less than 20 micrometers (0.02 millimeter) in diameter – Fall incredibly slow Formation of precipitation – Bergeron process Temperature in the cloud is below freezing Ice crystals collect water vapor Large snowflakes form and fall to the ground or melt during descent and fall as rain © 2015 Pearson Education Ltd.  Particle Sizes Involved in Condensation and Precipitation © 2015 Pearson Education Ltd.  The Bergeron Process © 2015 Pearson Education Ltd.  Precipitation Formation of precipitation – Collision-coalescence process Warm clouds Large hygroscopic condensation nuclei Large droplets form Droplets collide with other droplets during their descent Common in the tropics` © 2015 Pearson Education Ltd.  The Collision-Coalescence Process © 2015 Pearson Education Ltd.  Precipitation Rain and drizzle – Rain – droplets have at least a 0.5 mm diameter – Drizzle – droplets have less than a 0.5 mm diameter Snow – ice crystals, or aggregates of ice crystals © 2015 Pearson Education Ltd.  Precipitation Sleet – Wintertime phenomenon – Small particles of ice – Occurs when Warmer air overlies colder air Rain freezes as it falls © 2015 Pearson Education Ltd.  Precipitation Glaze, or freezing rain – impact with a solid causes freezing © 2015 Pearson Education Ltd.  Precipitation Hail – Hard rounded pellets Concentric shells Most diameters range from 1 to 5 cm – Formation Occurs in large cumulonimbus clouds with violent up- and downdrafts Layers of freezing rain are caught in up- and downdrafts in the cloud Pellets fall to the ground when they become too heavy © 2015 Pearson Education Ltd.  Hail © 2015 Pearson Education Ltd.  Precipitation Rime – Forms on cold surfaces – Freezing of Supercooled fog, or Cloud droplets © 2015 Pearson Education Ltd.  Precipitation Measuring precipitation – Rain Easiest form to measure Measuring instruments – Standard rain gauge Uses a funnel to collect and conduct rain Cylindrical measuring tube measures rainfall in centimeters or inches © 2015 Pearson Education Ltd.  The Standard Rain Gauge © 2015 Pearson Education Ltd.  Precipitation Measuring precipitation – Snow has two measurements Depth Water equivalent – General ratio is 10 snow units to 1 water unit – Varies widely – Radar is also used to measure the rate of rainfall © 2015 Pearson Education Ltd.