Chapter 18
Moisture, Clouds, & Precipitation
18.1 WATER IN THE ATMOSPHERE
Changes of State
• Water vapor is the most important gas in the
atmosphere
• Changing states requires that energy is
transferred in the form of heat
• Solid
• Liquid
• Gas
Solid to Liquid
• Melting (heat is
absorbed)
• The heat used to melt ice
does not make a
temperature change.
(Latent Heat)
• The opposite is Freezing
(heat is released)
Liquid to Gas
• Evaporation (cooling
process)
• The energy absorbed by the
H2O molecules during
evaporation, gives them the
motion needed to escape
the surface of the liquid and
become a gas (Latent Heat
of Vaporization)
• The opposite is
Condensation (heat is
released)
Solid to Gas
• Sublimation – solid to gas without going to
the liquid state (heat absorbed)
• Opposite is Deposition – the conversion from
vapor (gas) straight to to a solid (heat
released)
Humidity
• Amt. of water vapor in the air
• Measure using a Hygrometer
• Relative Humidity – describes the moisture
content of air
– A ratio of the air’s actual water-vapor content
compared with the amt. of water vapor air can
hold at that temp. & pressure
– CAN BE CHANGED BY: + or – water vapor OR amt.
of moisture needed for saturation depends on
temp. relative humidity varies with temp.
Continued…
• Saturation – when saturated, warm air
contains more water vapor than cold air
• Dew Point – temp. to which a parcel of air
would need to be cooled to reach saturation
18.2 CLOUD FORMATION
Air Compression & Expansion
• When air is allowed to expand IT COOLS & when
its is compressed IT WARMS
• Adiabatic temperature changes – happen even
though heat isn’t + or – results in air
compressing or expanding
• Dry Adiabatic Rate – cooling or warming in
unsaturated air. (rate of change is 1oC per 100
meters)
• Wet Adiabatic Rate – rate of change in saturated
air; variable but less than the dry adiabatic rate
Processes That Lift Air
• 4 mechanisms that can cause air to rise are:
– Orographic lifting
– Frontal Wedging
– Convergence
– Localized Convective
Orographic Lifting
Frontal Wedging
Convergence
Localized Convective Lifting
Stability
• Stable air tends to remain in its original
position & unstable air tends to rise
• Air stability is determined by measuring
temp. of the atmosphere at different heights
• Most stable conditions = temperature
inversion
• Clouds won’t form when stable conditions
are present in the atmosphere
Condensation
• For any of these forms of condensation to
occur the air must be saturated
• Condensation nuclei – tiny bits of particulate
matter that serve as surfaces on which water
vapor condenses
18.3 CLOUDS TYPES &
PRECIPITATION
Types of Clouds
• Clouds are classified on the basis of their
form & height
• Cirrus (a curl of hair)
– High, white, thin, occur in patches
• Cumulus (a pile)
– Rounded individual masses, rising domes
• Stratus (a layer)
– Sheets or layers that cover much of the sky
Cirrus
Cumulus
Stratus
High Clouds
•
•
•
•
Cirrus
Cirrostratus
Cirrocumulus
Not precipitation makers, but may be a
warning of an approaching storm
Middle Clouds
• 2000 – 6000 meters high
• Alto as prefix
Low Clouds
•
•
•
•
Stratus
Stratocumulus
Nimbostratus
Occasionally produce light precipitation
Clouds of Vertical Development
• Do not fit into any 3 height categories
• Base is in the low height range but extends
upward into the middle or high ranges
Fog
• Fog is defined as a cloud with its base at or
very near the ground
• Fog can be caused by cooling or by
evaporation
How Precipitation Forms
• For precipitation to form cloud droplets must
grow in volume by roughly one million times
• Cold Cloud Precipitation
– Bergeron Process
• Supercooling
• Supersaturated
• Warm Cloud Precipitation
– Collision-Coalescence Process
Forms of Precipitation
• The type of precipitation that reaches Earth’s
surface depends on the temperature profile
in the lower few kilometers of the
atmosphere
• Rain
• Snow
• Sleet
• Hail