Meteorology, ch. 4Meteorology, ch. 4Meteorology, ch. 4Meteorology, ch. 4

Moisture, Clouds, & Precipitation

Moisture, Clouds, & Precipitation

22

Question of the DayQuestion of the Day

What are the three phases of matter? What are the three phases of matter?

Changes of state of water Changes of state of water v Heat energy

– Latent heat• Stored or hidden heat • Important in atmospheric processes

– States of Matter• Solid, liquid, gas• Heat must be absorbed or released

v Heat 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 waterChanges of state of water

Figure 17.2

HumidityHumidity

v Amount of water vapor in the air – Saturated air

Filled to capacity§ Temperature dependent

Water vapor adds pressure (called vapor pressure) to the air

v Amount of water vapor in the air – Saturated air

Filled to capacity§ Temperature dependent

Water vapor adds pressure (called vapor pressure) to the air

Measuring HumidityMeasuring Humidityv Absolute Humidity

– mass of water vapor/ volume of air

v Mixing ratio (used by meteorologists)– Often measured in grams per kilogram

v Relative 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)

v Absolute Humidity– mass of water vapor/ volume of air

v Mixing ratio (used by meteorologists)– Often measured in grams per kilogram

v Relative 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

Relative humidity changes at constant temperature

Figure 17.4

Relative humidity changes at constant water-vapor

content

Relative humidity changes at constant water-vapor

content

Figure 17.5

Typical daily variations in temperature and relative

humidity

Typical daily variations in temperature and relative

humidity

Figure 17.6

Instruments used for Humidity

Instruments used for Humidity

v Two 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

v Two 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

Adiabatic Temperature Change

v Happens when– Air is compressed (warms)– Air expands (cools)

v Rates– Dry Adiabatic (DAR)

– unsaturated; 1C/100 m (5.5F/1000’)

– Wet Adiabatic (WAR)– starts at condensation level (reached dew pt.)– .5-.9C/100m

v Happens when– Air is compressed (warms)– Air expands (cools)

v Rates– 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 DayQuestions 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?

1212

Adiabatic cooling of rising air

Adiabatic cooling of rising air

Figure 17.9

Processes that lift airProcesses that lift air

v Orographic lifting– Result can be a rain shadow desert

v Frontal wedging– Cool air acts as a barrier to warm air – Fronts are part of the storm systems

v Convergencev Local convection lift (thermals)

v Orographic lifting– Result can be a rain shadow desert

v Frontal wedging– Cool air acts as a barrier to warm air – Fronts are part of the storm systems

v Convergencev Local convection lift (thermals)

Processes that lift airProcesses that lift air

Figures 17.10, 17.12, 17.13, & 17. 14

Stability of air Stability of air

v 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

v 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

Absolute stabilityAbsolute stability

Figure 17.17

Stability of air Stability of air

v Stable Air– resists vertical movement– Widespread clouds; precipitation light to

moderate v Unstable Air

– vertical movement– Warmer, less dense than surrounding air– Continues to rise until it reaches an altitude

with the same temperature

v Stable Air– resists vertical movement– Widespread clouds; precipitation light to

moderate v Unstable Air

– vertical movement– Warmer, less dense than surrounding air– Continues to rise until it reaches an altitude

with the same temperature

Types of StabilityTypes of Stability

v Absolute Stability– ELR is less than the WAR– Temperature Inversions

v Absolute instability – ELR > DAR

v Conditional instability – ELR between DAR and WAR– Towering cumulus clouds

v Absolute Stability– ELR is less than the WAR– Temperature Inversions

v Absolute instability – ELR > DAR

v Conditional instability – ELR between DAR and WAR– Towering cumulus clouds

Absolute instabilityAbsolute instability

Figure 17.18

Conditional instabilityConditional instability

Figure 17.19

How Stability ChangesHow Stability Changesv Instability

– 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

v Stability– radiation cooling of surface after sunset– cooling air from below as it moves over a

cold surface– subsidence

v Instability– 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

v Stability– radiation cooling of surface after sunset– cooling air from below as it moves over a

cold surface– subsidence

Cloud FormationCloud Formation

v Condensation – 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

v Condensation – 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 ClassificationCloud Classificationv Based 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

v Height– High: base at 6000m

– Cirrus, Cirrostratus, Cirrocumulus– Middle: base between 2000-6000m

– Altostratus, Altocumulus– Low: base below 2000m

– Stratus, Stratocumulus, Nimbostratus– Clouds of Vertical development

v Based 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

v Height– High: base at 6000m

– Cirrus, Cirrostratus, Cirrocumulus– Middle: base between 2000-6000m

– Altostratus, Altocumulus– Low: base below 2000m

– Stratus, Stratocumulus, Nimbostratus– Clouds of Vertical development

Cirrus cloudsCirrus clouds

Figure 17.21 A

Altostratus cloudsAltostratus clouds

Figure 17.21 E

Cumulus cloudsCumulus clouds

Figure 17.21 G

Classification of clouds according

to height and form (continued)

Classification of clouds according

to height and form (continued)

Figure 17.20

Fog Fog v Considered an atmospheric hazard

– Cloud with its base at or near the ground

v Most 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

v Considered an atmospheric hazard– Cloud with its base at or near the ground

v Most 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 Precipitation

v Cloud droplets – Less than 20 micrometers (0.02 millimeter)

in diameter – Fall incredibly slow

v 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

v Cloud droplets – Less than 20 micrometers (0.02 millimeter)

in diameter – Fall incredibly slow

v 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

Particle sizes involved in condensation and

precipitation

Particle sizes involved in condensation and

precipitation

The Bergeron processThe Bergeron process

Figure 17.25

Precipitation Precipitation

v 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

v 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

Figure 17.26

The collision-coalescence

process

Precipitation Precipitation

v Forms 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

v Forms 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 gaugeThe standard rain gauge

Figure 17.31