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Chapter 4
Water in the Atmosphere
Figure CO: Chapter 4, Water in the Atmosphere - Spring storm over field
© TobagoCays/ShutterStock, Inc.
Humidity measures how much water
vapor is in the air
• Water vapor gets into the atmosphere by
evaporation (and sublimation)
• Water vapor leaves the atmosphere by
condensation (and deposition)
• Evaporation increases humidity and
condensation decreases humidity
• Saturation occurs when rate of condensation =
rate of evaporation
Figure 01ab: The sequence of events that leads to saturation of air.
Figure 01cd: The sequence of events that leads to saturation of air.
Why measure water vapor content
(humidity) in the air?
• Water changing phase is a source of energy for
storms
• Water vapor is the source of clouds and
precipitation
• The amount of water vapor (humidity)
determines the rate of evaporation
• Water vapor is the most abundant greenhouse
gas
One way to measure humidity: mixing
ratio
• Mixing ratio is the ratio of the mass of water
vapor in a given volume of air to the mass of the
other molecules in that volume of air
• Mixing ratio does not change if the temperature
changes
• Evaporation and condensation change the
mixing ratio
Figure 02: the vertical distribution of the mixing ratio for three different
atmospheric conditions
Another way to measure humidity:
water vapor pressure
• Water vapor pressure is the amount of the total
atmospheric pressure that is due to the water
vapor molecules
• Water vapor pressure is often called just vapor
pressure
• Vapor pressure is measured in mb, just like total
atmospheric pressure
• Saturation vapor pressure depends on
temperature
Figure 03: Vapor pressure as a function of temperature
Source: Black Rock Forest Consortium.
Another way to measure humidity: the
relative humidity
• Relative humidity is expressed as a percentage,
where 100% is saturation
• Relative humidity can be defined in terms of the
vapor pressure
• Relative humidity = 100% x vapor pressure ÷
saturation vapor pressure
• A low relative humidity allows a higher
evaporation rate
Figure B01: Heat index values
Relative humidity has disadvantages
• Relative humidity tells how the air is from
saturation
• 0% relative humidity: No water vapor
• 100% relative humidity is saturated
• But air at a high temperature with relative
humidity of 50% may have more water vapor
than air at a lower temperature with a relative
humidity of 90%
Figure 04: A climatology of hourly temperature and relative humidity
Data from: Diurnal Cycle. Retrieved December 10, 2010, from
http://vista.cira.colostate.edu/improve/data/graphicviewer/diurnal.htm
Condensation in air (not on a surface
like frost or dew)
• In a lab with perfectly clean air, saturation
requires a relative humidity of more than 200%.
RH > 100% is supersaturation
• Condensation is inhibited by the curvature effect
• Small, very curved droplets have molecules with
few neighbors, and are quick to evaporate
Figure 07: The smaller the drop, the more curved the surface
Condensation in the atmosphere
(continued)
• Supersaturation (RH > 100%) does not occur in
the atmosphere. The solute effect cancels the
curvature effect.
• When the relative humidity reaches 100%, cloud
particles form.
• Cloud at the surface is called fog.
• Fog reduces visibility to less than 1 km or 0.6
miles
• Heavy fog is a travel hazard
Another way to measure humidity: dew
point
• Dew point is also called the dewpoint
temperature, abbreviated as Td or TD
• Dew point is defined as the temperature to which
air must be cooled (without changing the
pressure) to become saturated
• Dew point does not exceed the temperature
• The dew point depression is the difference
between the temperature and the dew point
Figure 05: Dew in spider web
Courtesy of Steven Ackerman
Figure T01: Various humidity quantities for two air temperatures and two
relative humidities for an atmospheric pressure of 1000 mb
More on dewpoint
• When the dewpoint is below 0°C (32°F), it is
called the frost point, because deposition (water
vapor to ice) in the form of frost will occur when
the air becomes saturated
• When air cools to the dewpoint, condensation
occurs
• On surfaces, this condensation is called dew (or
frost)
Figure 06: Frost
Courtesy of Steven Ackerman
Frozen Dew
• Occurs in two steps
• First, condensation occurs and the temperature
is above freezing (32ºF); that is, the dew point is
above freezing
• The condensation is dew
• Second, the temperature falls below freezing.
The dew freezes to frozen drops
• This ice is called frozen dew or black ice
Frozen Dew (continued)
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•
•
•
•
Frozen dew is also called “black ice”
It is a major traffic hazard
It also causes slips and falls for people on foot
Frozen dew is hard to see
Frozen dew frequently forms on roads where
there is a significant slope, as well as bridges
and overpasses
Condensation
• In the lab with perfectly clean air (no aerosol)
takes a relative humidity of more than 200%
– Relative humidity > 100% is supersaturation
• Condensation is inhibited by the curvature effect
• Small, very curved droplets have molecules with
few neighbors, and are quick to evaporate
Condensation in the atmosphere
• Is inhibited by the curvature effect
• Is enhanced by the solute effect
• Some aerosol, salt particles for example,
dissolve and have the ability to hold on to water
molecules and suppress evaporation
• Other aerosol particles form nuclei, or small
surfaces for condensation
Condensation in the atmosphere
(continued)
• A cloud nucleus gives water molecules more
neighbors, by acting like a small flat surface
• There are always abundant cloud condensation
nuclei in the atmosphere—dust, salt, pollen,
pollutants
• The solute effect permits condensation at RH <
100%. This is called haze
Ice in clouds
• Deposits (deposition) on small particles called
ice nuclei (clay minerals, tiny ice crystals)
• There is a scarcity of ice nuclei at high
subfreezing temperatures near but < 32ºF
• Many water droplets do not freeze at
subfreezing temperatures, called supercooling.
• Below -40ºC (or F), all water drops freeze
Ice in clouds (continued)
• Ice takes on different crystal shapes in clouds,
depending on temperature and supersaturation
• Clouds are saturated with respect to water,
supersaturated with respect to ice
• The saturation vapor pressure over ice is less
than that over water
• There can be ice fog (inland Alaska)
Nucleation
• Is the initial formation of a cloud droplet around
any type of particle
• Homogeneous nucleation
– Droplet is formed only by water molecules
– Only occurs at temperatures below -40°C
• Heterogeneous nucleation
– Uses cloud condensation nuclei (CCN)
• Hygroscopic nuclei dissolve in water
• Hydrophobic nuclei don’t dissolve in water
Ice Nucleation
• Ice nuclei are particles around which ice crystals
form
– Deposition nucleation—Ice forms from vapor by
deposition onto the ice nucleus when the air is
supersaturated with respect to ice
– Freezing nucleation—A supercooled drop freezes
without the air of a nonwater particle
– Immersion nucleation—The nucleus is submerged in
a liquid drop, causing the drop to freeze
– Contact nucleation—Ice nuclei collide with
supercooled drops and the drop freezes immediately
Types of Fog
• Radiation fog—cooling on clear nights
– Light winds required
– Common in valleys
• Advection fog—warm air advected over a cold
surface cools
• Evaporation fog (frontal fog)—form when water
evaporates from rain and saturates air beneath
– Associated with inversions and warm fronts
– Also when cold air flows over a warm lake (steam fog)
• Upslope fog—rising air cools to saturation
Figure 08: Fog obscures the view of the Sears Tower
Courtesy of Steven Ackerman
Figure 09: Number of days with fog across US
Figure 10: Fog in satellite image
Courtesy of SSEC and CIMSS, University of Wisconsin-Madison
Figure 11: Advection fog
© Manamana/ShutterStock, Inc.
Figure 12: Steam fog
© James Robertson, www.flickr.com/photos/shingen_au
Lifting mechanisms that form clouds
• Most clouds form when air cools to the dew point
as a parcel of air rises vertically as an updraft
• Orographic lifting—air flows up over a mountain
• Frontal lifting—when less dense warm air is
forced to rise over cooler, denser air
• Convection—air near the surface warms and
rises
• Convergence—when air near the ground
converges, or is squeezed together, and rises
Figure 13A: Orographic Lifting
Figure 13B: Frontal Lifting
Figure 13C: Convection
Figure 13D: Convergence of air at surface
The saturated adiabatic lapse rate
• Saturated air parcels have relative humidity
100%
• Rising saturated air parcels expand and cool,
but not at the dry adiabatic lapse rate of 10°/km
• Condensation in the saturated air parcel
releases latent heat that acts to warm the air
parcel
• Cooling dominates, but at a lesser rate
• The saturated adiabatic lapse rate is about
6°/km
Figure 14: Saturated ascent of a parcel
Static stability and saturated air parcel
• Saturated air parcels can rise freely if their
temperature is higher than that of their
environment
• Saturated air parcels can rise freely if their
environment’s lapse rate is greater than the
saturated adiabatic lapse rate
– Thus the parcel will be warmer than its environment
– An environmental lapse rate > the saturated adiabatic
lapse rate is called conditionally unstable
– The level at which rising air becomes saturated is the
lifting condensation level
Figure 15: Stability diagram
Figure T02: Atmospheric Stability Summary
Cloud Classification
• Layered clouds are much wider than tall
– Stratus describes layered clouds
• Convective clouds are as tall or taller than wide
– Cumulo describes convective clouds
• High clouds of ice crystals
– Cirro describes a high cloud
• Middle clouds form below high clouds
– Alto describes a middle cloud
• Nimbus describes a cloud causing precipitation
Figure T03: Common Cloud Types
Figure 16: Schematic of cloud types
Ten Cloud Types
• Stratus—like fog hovering above the ground
• Stratocumulus—low-lying cloud combining
layered and convective cloud types
• Cumulus—flat bases and intricately contoured
domed tops
– Fair-weather cumulus
– Cumulus congestus—tall relative to their width
• Can produce brief heavy rain for a short time
Figure 17: Stratus
Courtesy of Lil Ackerman
Figure 18: Stratocumulus
© Olga Miltsova/ShutterStock, Inc.
Figure 19: Cumulus
Courtesy of Steven Ackerman
Ten Cloud Types (continued)
• Precipitating clouds
• Nimbostratus—deep precipitating cloud
• Cumulonimbus—thunderstorm clouds
– Extend to high altitudes
– Produce large amounts of precipitation, severe
weather, and even tornadoes
– Flattened anvil shape of the top of the cloud
– Under the anvil, sinking air may create pouches
called mammatus
Figure 20: Nimbostratus.
© Demydenko Mykhailo/Fotolia.com
Figure 21: Cumulonimbus
Courtesy of David W. |Martin, SSEC, University of Wisconsin-Madison
Figure 22: Mammatus
Courtesy of Anne Pryor
Ten Cloud Types (continued)
• Middle clouds
– Altostratus—layered clouds made up mostly of water
droplets
– Altocumulus—similar to stratocumulus with a higher
base
• High clouds
– Cirrocumulus—similar to altocumulus but made of ice
and have smaller elements
– Cirrostratus—layerlike, uniform, made of ice
– Cirrus—wispy, fibrous clouds made of ice
Figure 23: Altostratus
Courtesy of Ralph F. Kresge/NOAA
Figure 24: Altocumulus
Courtesy of Lil Ackerman
Figure 25: Cirrocumulus
Courtesy of Steven Ackerman
Figure 26: Cirrostratus
© pzAxe/ShutterStock, Inc.
Figure 27: Cirrus
© Megmomster/Dreamstime.com
Clouds and the Greenhouse Effect
• Clouds play a critical role in the global warming
debate
• Clouds reflect solar radiation—a cooling effect
• Clouds are good emitters and absorbers of
longwave radiation—a warming effect
• Which dominates?
– Cirrus have a net warming effect
– Stratus have a net cooling effect
– Small particles, net cooling; large particles, warming
• Today’s distribution of clouds—net cooling
Figure 28: In the solar spectrum, clouds tend to cool Earth. In the longwave
spectrum, they tend to warm the planet.
Cloud Composition
• Includes the phase(s) of water, size and number
of particles, habit (shape) of ice crystals, if any
– Continental clouds have more and smaller drops than
marine clouds
– Ice crystal habit, hexagonal plate, needle, column,
dendrite
– Warm clouds have temperatures above freezing
throughout; cold clouds have temperatures below
freezing
• Precipitation is any liquid or solid water particle
that falls from the atmosphere and reaches the
ground
Figure 29: Ice crystal shapes
Precipitation
• Does not form by condensation like the process
cloud particles form
• Condensation and deposition are too slow to
produce precipitation
• It would take more than 2 days to grow raindrops
by condensation
• A single average raindrop has the mass of
1,000,000 cloud droplets
Precipitation processes in the
atmosphere: there are two
• In warm clouds, with no ice crystals, raindrops
form by the collision—coalescence process
• A few cloud droplets are bigger than the others
• The few large droplets fall faster and collide with
the slower, smaller ones, continuing to grow
• Collision helps create precipitation in cold clouds
– Accretion when crystals sweep up water drops
• Graupel when crystal disappears in a large ice particle
– Aggregation when ice crystals collide and stick
– Snowflake is an individual ice crystal or aggregate
Figure 30: Very small droplets may flow around the larger drops and avoid
colliding with them.
Figure 31: Falling ice crystals
The Ice Crystal Process
• Occurs in cold clouds with tops colder than 0ºC
• Also called the Bergeron-Wegener Process
• These clouds contain mostly cloud droplets and
a few ice crystals
• The cloud is saturated for water and
supersaturated for ice
• The few ice crystals grow and the drops
evaporate to feed the ice crystals
Figure 32: Beaker experiment for saturation
Figure 33: Ice crystals growing over time
Figure 34: The differences between the saturation vapor pressure over ice
and over water
Figure 35: Fallstreaks are wisps of ice particles that fall out of a cloud
Courtesy of Steven Ackerman
Precipitation Types
• Virga—rain that evaporates in the air
– Fallstreaks are the ice equivalent of virga
• Rain—drops at least 0.5 mm in diameter
– Smallest drops are called drizzle
• Snow—snowflakes and temperature from cloud
base to ground is less than 0°C
• Freezing rain—cold air near the surface freezes
melted precipitation on contact with surfaces
• Sleet—frozen raindrops
• Hail—lumps of ice that form in cumulonimbus
Figure 36A: Rain
Figure 36B: Snow
Figure 36C: Freezing rain
Figure UN01: Results of an ice storm
© Baudy/Dreamstime.com
Figure UN02: Inches of freezing rain
Source: NOAA/NWS
Figure 36D: Sleet
Figure 37A: Climatology of annual precipitation over the land regions of the
world.
Courtesy of University of Washington, Joint Institute for the Study of the
Atmosphere and Ocean (JISAO)
Figure 37B: Climatology of annual rainfall across the world, including the
oceans, based on satellite observations.
Courtesy of University of Washington, Joint Institute for the Study of the
Atmosphere and Ocean (JISAO)
Figure 38: Snowfall map
Prepared by Colorado Climate Center, Colorado Sate University,
copyright © 1997.
Figure 39: 30-Year Climatology of Freezing Precipitation
Adapted from Cortinas, V. J., et al., Monthly Weather Review, April 2004
Figure 40: Precipitation Ladder
Figure 41: Air flow over a mountain
Figure T04: Difference in Temperature, Cloud Cover, and Precipitation on the
Windward and Leeward Sides of the Cascade Mountains in Washington State
Data from the Western Regional Climate Center,
http://www.wrcc.dri.edu/summary/lcd.html.