Meteorology, ch. 4 Moisture, Clouds, & Precipitation

2 Question of the Day  What are the three phases of matter?

Changes of state of water vHeat energy –Latent heat Stored or hidden heat Important in atmospheric processes –States of Matter Solid, liquid, gas Heat must be absorbed or released vHeat energy –Latent heat Stored or hidden heat Important in atmospheric processes –States of Matter Solid, liquid, gas Heat must be absorbed or released

Changes of state of water Figure 17.2

Humidity vAmount of water vapor in the air –Saturated air  Filled to capacity §Temperature dependent  Water vapor adds pressure (called vapor pressure) to the air vAmount of water vapor in the air –Saturated air  Filled to capacity §Temperature dependent  Water vapor adds pressure (called vapor pressure) to the air

Measuring Humidity vAbsolute Humidity –mass of water vapor/ volume of air vMixing ratio (used by meteorologists) –Often measured in grams per kilogram vRelative humidity –Actual air water vapor content compared with Amount of water vapor needed for saturation at that temperature (and pressure) –Saturated air is 100% RH –Altered by the +/- of water vapor, change in temperature –dew point (temp. to be cooled for saturation) vAbsolute Humidity –mass of water vapor/ volume of air vMixing ratio (used by meteorologists) –Often measured in grams per kilogram vRelative humidity –Actual air water vapor content compared with Amount of water vapor needed for saturation at that temperature (and pressure) –Saturated air is 100% RH –Altered by the +/- of water vapor, change in temperature –dew point (temp. to be cooled for saturation)

Relative humidity changes at constant temperature Figure 17.4

Relative humidity changes at constant water-vapor content Figure 17.5

Typical daily variations in temperature and relative humidity Figure 17.6

Instruments used for Humidity vTwo types of Hydrometer –Psychrometer - compares temperatures of wet- bulb thermometer and dry-bulb thermometer –100% RH, 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 vTwo types of Hydrometer –Psychrometer - compares temperatures of wet- bulb thermometer and dry-bulb thermometer –100% RH, 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

Adiabatic Temperature Change vHappens when –Air is compressed (warms) –Air expands (cools) vRates –Dry Adiabatic (DAR) –unsaturated; 1C/100 m (5.5F/1000’) –Wet Adiabatic (WAR) –starts at condensation level (reached dew pt.) –.5-.9C/100m vHappens when –Air is compressed (warms) –Air expands (cools) vRates –Dry Adiabatic (DAR) –unsaturated; 1C/100 m (5.5F/1000’) –Wet Adiabatic (WAR) –starts at condensation level (reached dew pt.) –.5-.9C/100m

Questions of the Day  What does it mean when air is saturated?  What is the RH reading for saturated air?  What does it mean when air is saturated?  What is the RH reading for saturated air? 12

Adiabatic cooling of rising air Figure 17.9

Processes that lift air vOrographic lifting –Result can be a rain shadow desert vFrontal wedging –Cool air acts as a barrier to warm air –Fronts are part of the storm systems vConvergence vLocal convection lift (thermals) vOrographic lifting –Result can be a rain shadow desert vFrontal wedging –Cool air acts as a barrier to warm air –Fronts are part of the storm systems vConvergence vLocal convection lift (thermals)

Processes that lift air Figures 17.10, 17.12, 17.13, &

Stability of air vTypes 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 vTypes 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

Absolute stability Figure 17.17

Stability of air vStable Air –resists vertical movement –Widespread clouds; precipitation light to moderate vUnstable Air –vertical movement –Warmer, less dense than surrounding air –Continues to rise until it reaches an altitude with the same temperature vStable Air –resists vertical movement –Widespread clouds; precipitation light to moderate vUnstable Air –vertical movement –Warmer, less dense than surrounding air –Continues to rise until it reaches an altitude with the same temperature

Types of Stability vAbsolute Stability –ELR is less than the WAR –Temperature Inversions vAbsolute instability –ELR > DAR vConditional instability –ELR between DAR and WAR –Towering cumulus clouds vAbsolute Stability –ELR is less than the WAR –Temperature Inversions vAbsolute instability –ELR > DAR vConditional instability –ELR between DAR and WAR –Towering cumulus clouds

Absolute instability Figure 17.18

Conditional instability Figure 17.19

How Stability Changes vInstability –solar radiation at lower layer –heating air from below as it moves over a warm surface –upward movement of air –radiation cooling from cloud tops vStability –radiation cooling of surface after sunset –cooling air from below as it moves over a cold surface –subsidence vInstability –solar radiation at lower layer –heating air from below as it moves over a warm surface –upward movement of air –radiation cooling from cloud tops vStability –radiation cooling of surface after sunset –cooling air from below as it moves over a cold surface –subsidence

Cloud Formation vCondensation –visible aggregate of minute droplets of water, ice crystals or both –Adiabatic cooling –Two things needed for clouds to form –air to be saturated or close to it –condensation nuclei –ground or in atmosphere (dust, smoke, salt) –hydroscopic nuclei are most effective sites vCondensation –visible aggregate of minute droplets of water, ice crystals or both –Adiabatic cooling –Two things needed for clouds to form –air to be saturated or close to it –condensation nuclei –ground or in atmosphere (dust, smoke, salt) –hydroscopic nuclei are most effective sites

Cloud Classification vBased on Form –Cirrus – high, white, thin –Cumulus- globular cloud masses often associated with fair weather –Stratus – sheets or layers that cover much of the sky vHeight –High: base at 6000m –Cirrus, Cirrostratus, Cirrocumulus –Middle: base between m –Altostratus, Altocumulus –Low: base below 2000m –Stratus, Stratocumulus, Nimbostratus –Clouds of Vertical development vBased on Form –Cirrus – high, white, thin –Cumulus- globular cloud masses often associated with fair weather –Stratus – sheets or layers that cover much of the sky vHeight –High: base at 6000m –Cirrus, Cirrostratus, Cirrocumulus –Middle: base between m –Altostratus, Altocumulus –Low: base below 2000m –Stratus, Stratocumulus, Nimbostratus –Clouds of Vertical development

Cirrus clouds Figure A

Altostratus clouds Figure E

Cumulus clouds Figure G

Classification of clouds according to height and form (continued) Figure 17.20

Fog vConsidered an atmospheric hazard –Cloud with its base at or near the ground vMost fogs form because of –Radiation cooling or mvem’t over cold surface  Radiation Fog  Advection Fog  Upslope Fog –Evaporation Fog  Steam fog  Frontal/precipitation fog vConsidered an atmospheric hazard –Cloud with its base at or near the ground vMost fogs form because of –Radiation cooling or mvem’t over cold surface  Radiation Fog  Advection Fog  Upslope Fog –Evaporation Fog  Steam fog  Frontal/precipitation fog

Precipitation vCloud droplets –Less than 20 micrometers (0.02 millimeter) in diameter –Fall incredibly slow vFormation 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 vCloud droplets –Less than 20 micrometers (0.02 millimeter) in diameter –Fall incredibly slow vFormation 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

Particle sizes involved in condensation and precipitation

The Bergeron process Figure 17.25

Precipitation vFormation 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 vFormation 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

Figure The collision- coalescence process

Precipitation vForms of 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 –Sleet and glaze  Sleet  Glaze –Hail  Occurs in large cumulonimbus clouds –Rime  Freezing of supercooled fog or cloud dropplets vForms of 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 –Sleet and glaze  Sleet  Glaze –Hail  Occurs in large cumulonimbus clouds –Rime  Freezing of supercooled fog or cloud dropplets

The standard rain gauge Figure 17.31