Download Water Vapor in the Air

Chapter 12 Lecture Outline
Moisture, Clouds,
and Precipitation
Focus Question 12.1
• Why is latent heat important to our weather?
Water’s Changes of State
• Heat energy
• 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
–Heat exchanged between water and
surroundings during phase change
–Important in atmospheric processes
Water’s Changes of State
• Three states of matter
1. Solid — ice
2. Liquid — water
3. Gas — water vapor
• To change state, heat must be either
absorbed or released
Water’s Changes of State
• Processes
• Evaporation
–Liquid is changed to gas
–540-600 calories per gram added
–Latent heat of vaporization
• Condensation
–Water vapor (gas) is changed to a liquid
– 540-600c of heat energy/gram is released
–Latent heat of condensation
Water’s Changes of State
Water’s Changes of State
• Processes
• Melting
–Solid is changed to a liquid
–80 calories per gram added
–Latent heat of melting
• Freezing
–Liquid is changed to a solid
–80c of heat energy/gram is released
–Latent heat of fusion
Water’s Changes of State
• Processes
• Sublimation
–Solid is changed directly to a gas
–680 calories per gram of water are added
• Deposition
–Water vapor (gas) changed to a solid
– 680c of heat energy is released
Water’s Changes of State
Water’s Changes of State
Water Phase Changes
Focus Question 12.2
• If the temperature remains unchanged and
the amount of water vapor in the air
decreases, how does relative humidity
change?
Humidity: Water Vapor in the Air
• Amount of water vapor in the air
• Saturated air
–Air filled to capacity with water vapor
–Capacity is temperature dependent
–Warm air has a much greater capacity
• Water vapor adds pressure
–Vapor pressure
Humidity: Water Vapor in the Air
• Measuring humidity
• Mixing ratio
–Mass of water vapor in a unit of air
compared to the remaining mass of dry air
–Measured in g/kg
• 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)
Humidity: Water Vapor in the Air
Humidity: Water Vapor in the Air
• Measuring humidity
• Relative humidity
–Expressed as a percent
–Saturated air
• Content equals capacity
• Has 100% relative humidity
• Dew point temperature
• Temperature to which a parcel of air
would need to be cooled to reach
saturation
-Relative humidity can be
changed in two ways:
Relativemoisture
humidity
canthe
be
1. Adding
raises
changed
in two ways
relative humidity
Removing moisture lowers the
relative humidity
2. Changing the air temperature
 Lowering the temperature raises the
relative humidity
 Raising the temperature lowers the
relative humidity
Humidity: Water Vapor in the Air
Humidity: Water Vapor in the Air
Relative Humidity
change with
Temperature
Change
- It is “related” to T!
Dew point temperature
Cooling the air below
the dew point
causes
condensation
 e.g., dew, fog, or cloud
formation
 Water vapor requires
a surface to condense
on
 Dew Point Temp does
not change when the
air temp changes
Humidity: Water Vapor in the Air
• Two types of hygrometers are used to
measure humidity:
1. Psychrometer
• Compares temperatures of wet-bulb
thermometer and dry-bulb thermometer
• Greater difference = lower relative
humidity
• If air is saturated, both thermometers
read the same temperature
2. Hair hygrometer
• Reads the humidity directly
Humidity: Water Vapor in the Air
Humidity: H2O Vapor
in the Air
 Hair hygrometer—Reads the
humidity directly
Focus Question 12.3
• Describe the importance of adiabatic cooling
in cloud formation.
The Basis of Cloud Formation: Adiabatic
Cooling
• Adiabatic temperature changes
• Air is compressed
–Motion of air molecules increases
–Air warms
–Descending air is compressed
• Air expands
–Air parcel does work on the surrounding air
–Air cools
–Rising air expands
The Basis of Cloud Formation: Adiabatic
• Cooling
Adiabatic rates
• Dry adiabatic rate
– Unsaturated air
– Rising air expands & cools at :
10°C/1000 m (5.5F/1000 ft.)
– Descending air compresses and warms at:
10°C/1000 m (5.5F/1000 ft.)
• Wet adiabatic rate
– Begins at condensation level
– Air has reached the dew point
– Condensation is occurring and latent heat is being
liberated
– Heat released by condensing water reduces cooling
rate
– Rate varies from 5°C to 9°C/1000 m
The Basis of Cloud Formation: Adiabatic
Cooling
Focus Question 12.4
• How do orographic lifting and frontal wedging
force air to rise?
Processes That Lift Air
• Orographic lifting
• Elevated terrains act as barriers
• Result can be a rainshadow desert
Processes That Lift Air
• Frontal wedging
• Cool air acts as a barrier to warm air
• Fronts are part of middle-latitude cyclones
Processes That Lift Air
• Convergence
• Air flows together
and rises
Processes That Lift Air
• Localized convective lifting
• Unequal surface heating causes pockets of
air to rise because of their buoyancy
Focus Question 12.5
• How does atmospheric stability affect our
weather?
The Weathermaker: Atmospheric Stability
• Stability of air determines:
• Type of clouds that develop
• Intensity of the precipitation
The Weathermaker: Atmospheric Stability
• Types of stability
• Stable air
– Resists vertical displacement
• Cooler and denser than surrounding air
• Wants to sink
– No adiabatic cooling
– Widespread clouds with little vertical thickness
– Precipitation is light to moderate
– Absolute stability
• Environmental lapse rate less than wet adiabatic rate
The Weathermaker: Atmospheric Stability
The Weathermaker: Atmospheric Stability
• Absolute instability
• Acts like a hot air balloon
• Rising air
– Warmer and less dense than surrounding air
– Rises until it reaches altitude with same temperature
• Adiabatic cooling
• Environmental lapse rate greater than dry adiabatic rate
• Clouds are often towering
• Conditional instability
– Atmosphere is stable for an unsaturated parcel of air
but unstable for a saturated parcel
The Weathermaker: Atmospheric Stability
The Weathermaker: Atmospheric Stability
Focus Question 12.6
• What is the function of condensation nuclei in
cloud formation?
Condensation and Cloud Formation
• Condensation
• Water vapor changes to a liquid and forms
dew, fog, or clouds
• Water vapor requires a condensation
surface
–On the ground
• Grass, a car window, etc.
–In the air are tiny bits of particulate matter
called condensation nuclei
• Dust, smoke, ocean salt crystals, etc.
Condensation and Cloud Formation
• Clouds
• Made of millions and millions of
– Minute water droplets, or
– Tiny crystals of ice
• Classification based on form
–Cirrus
• High, white, thin
–Cumulus
• Globular, puffy, cloud masses
• Vertically developed
–Stratus
• Sheets or layers that cover much of the sky
• Horizontally developed
Condensation and Cloud Formation
Condensation and Cloud Formation
Condensation and Cloud Formation
• Clouds classified based on height
• High clouds
–Above 6000 m
• Cirrus, cirrostratus, cirrocumulus
• Middle clouds
–2000 to 6000 m
• Altostratus and altocumulus
• Low clouds
–Below 2000 m
• Stratus, stratocumulus, and
nimbostratus (nimbus means “rainy”)
Condensation and Cloud Formation
Condensation and Cloud Formation
• Clouds of vertical development
–From low to high altitudes
–Called cumulonimbus
–Often produce rain showers and
thunderstorms
Focus Question 12.7
• Define fog.
Fog
• Fog is a cloud with its base at or
near the ground
• Considered an atmospheric
hazard
• Most fogs form because of
–Radiation cooling, or
–Movement of air over a cold surface
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
Fog
Fog
Fog
• Evaporation fogs
• Steam fog
–Cool air moves over warm water
–Water has a steaming appearance
• Frontal fog, or precipitation fog
–Forms during frontal wedging when
warm air
lifted over colder air
–Rain evaporates to form fog
Fog
Fog
Focus Question 12.8
• Describe the 2 mechanisms that produce
precipitation.
How Precipitation Forms
• Cloud droplets
• < 20 micrometers (0.02 millimeter) in
diameter
• Fall incredibly slowly
• Formation of precipitation – 2 ways
• Bergeron process
–Temperature in the cloud is supercooled
–Ice crystals collect water vapor
–Large snowflakes form and fall to the
ground or melt and turn to rain
How Precipitation Forms
How Precipitation Forms
• Collision-coalescence process
–Warm clouds
–Large hygroscopic condensation
nuclei
–Large droplets form
–Droplets collide with other droplets
during their descent
How Precipitation Forms
Focus Question 12.9
• Explain why snow can sometimes reach the
ground as rain, but the reverse does not
occur.
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
Forms of Precipitation
• Sleet and glaze
– Sleet
• Small particles of ice in winter
• Occurs when warmer air overlies
colder air
• Rain freezes as it falls
– Glaze, or freezing rain
• Impact with a solid causes freezing
Forms of Precipitation
Forms of Precipitation
• Hail
–Hard rounded pellets
• Concentric shells
• Most diameters range from 1 to 5 cm
–Formation
• In large cumulonimbus clouds
• Layers of freezing rain are caught in
violent up- and down-drafts
• Pellets fall when they become too
heavy
Forms of Precipitation
Forms of Precipitation
• Rime
– Forms on cold surfaces
• Freezing of supercooled
fog
• Freezing of cloud droplets
Focus Question 12.10
• How is precipitation measured?
Measuring Precipitation
• Rain
– Easiest form to measure
– Measuring instruments
• Standard rain gauge
–Uses a funnel to collect rain
–Cylindrical tube measures in cm or
inches
• Radar is also used to measure
the rate of rainfall
Measuring Precipitation
Measuring Precipitation
Measuring Precipitation
• Snow has two measurements:
1. Depth
2. Water equivalent
• General ratio is 10 snow units to
1 water unit
• Varies widely