Transcript Moisture&Clouds

Moisture and Clouds
Weather Unit
When you see this megaphone,
Click it for audio information
Water in the Atmosphere
• Water vapor is the source
•
of all condensation
(clouds, fog, dew) and
precipitation (rain, snow,
sleet)
Most important
atmospheric gas to
understand weather
processes
Changes of State
• All changes require a
•
•
•
transfer in heat
Heat is absorbed to break
bonds
Heat is released when
bonds form
Latent heat: “hidden”;
major energy source for
thunderstorms, tornadoes
and hurricanes
Some general terms…
• Melting (solid to liquid): energy absorbed to
•
•
•
•
break bonds
Evaporation (liquid to gas): energy absorbed to
break bonds
Condensation (gas to liquid): energy released
when bonds formed
Sublimation (solid to gas): dry ice, for example
Deposition (gas to solid): dew, for example
Humidity
• General term for water vapor in the air
• Expressed as gm/kg
• Saturated air:
– Dependant on air temperature
– Warm air can “hold” more water vapor than cold air
– Considered saturated (full) when cannot hold any more water
vapor at that temperature
Relative Humidity (RH)
• A ratio of the air’s
•
actual water-vapor
content compared
with the amount of
water vapor air can
hold at that
temperature and
pressure
Indicates how near
the air is to saturation
More on Relative Humidity…
• Two ways it can change:
1) Adding or removing water
vapor
2) Lowering or raising air
temperature
• When water vapor
content remains
constant, a decrease in
temperature causes RH
to increase. Conversely,
an increase in
temperature causes RH
to decrease.
Dew Point
• Temperature to which air
•
•
would need to be cooled
to reach saturation
If cooled further, excess
water vapor would
condense (dew, fog,
clouds)
Cloud base indicates
elevation where dew
point is achieved (air is
saturated and
condensation occurs).
More on dew point…
• Dew points close to
•
current air temperature
indicate humid conditions
(air nearly saturated)
Dew points far from
current air temperature
indicate dry conditions
(air can “hold” much
more moisture before
saturated)
Measuring Humidity
• Psychrometer: measures
RH based on the amount
of evaporation that takes
place
• Dry Bulb (room temperature)
• Wet Bulb (wet wick)
• Water from wet bulb evaporates
•
•
as fanned, cooling the
temperature on thermometer.
Greater evaporation=dry air
Less evaporation=moist air
Measuring Relative Humidity
Cloud Formation
• Clouds form from condensation of water
vapor (vapor to liquid)
• Air becomes saturated when cools.
Remember that excess water vapor
condenses when saturation occurs.
• Air must be cooled to its dew point (100%
RH) for clouds to form.
Air Compression and Expansion
• When air expands, it
•
•
cools and when
compressed, it warms
(adiabatic changes).
Adiabatic changes don’t
involve adding or
subtracting heat, it simply
involves changes in
pressure.
Dry (unsaturated) air
cools and warms at a
different rate than wet
(saturated) air.
Forces That Lift Air
• In general, air resists vertical movement.
(It tends to remain at a constant elevation.)
• However, air that is heated at the surface
will become less dense and rise (localized
convective lifting).
• Mechanical processes, too, can force air
to rise (orographic lifting, frontal wedging
and convergence).
Processes That Lift Air
Processes That Lift Air
• Orographic Lifting: When
•
•
elevated terrains
(mountains) force air up
Air cools adiabatically on
windward side generating
much rain.
Air warms adiabatically
on leeward side forming
warm, dry conditions
(deserts).
• Frontal Wedging: When
•
•
warm and cold air
masses collide, producing
a front.
Cooler, denser air acts as
a barrier. Warmer, less
dense air is forced to rise
above.
Middle-latitude cyclones
produced (more on this
later).
Processes That Lift Air
• Convergence: Occurs
•
•
when air in the lower
atmosphere
convergences (comes
together) and lifting
results.
Converging air cannot go
down, so forced up.
Air cools adiabatically
when it rises, forming
clouds.
• Localized Convective
•
Lifting: Occurs when
unequal heating of the
Earth’s surface causes
pockets of air (thermals)
to be warmed more than
surrounding air. Air
becomes less dense and
rises, cooling
adiabatically.
Forms clouds and
precipitation.
Processes That Lift Air
• All processes that
•
force air to rise can
create clouds if
cooled to its
condensation level
(dew point)
Remember that
colder air cannot
“hold” as much water
vapor
Air Stability
• Air that rises cools
•
•
adiabatically. If rising
parcel remains colder
than surrounding air, it
sinks again (stable air).
Air continues to rise as
long as it is warmer (less
dense) than surrounding
air (unstable air).
Stable air tends to remain
in its original position,
while unstable air tends
to rise.
• Air stability is determined
•
•
•
by measuring the
temperature of the air at
various heights.
Aircraft and radiosondes
Temperature Inversions
occur when air becomes
warmer with height. (Air
on ground cools faster
than aloft. Warmer air
above creates a “cap” on
atmosphere below.)
Little vertical movement
Stability and Daily Weather
• When stable air is forced
•
•
above Earth’s surface,
the clouds that form are
widespread and have
little vertical thickness,
thus light precipitation.
When unstable air is
forced up, clouds are
often towering and can
generate thunderstorms
and tornadoes.
Picture from Mancelona,
MI
Condensation Nuclei
• Air must be saturated for
•
•
•
condensation to occur
Must be a surface for
water to condense on
Dew (grass, cars, etc)
Clouds (dust, smoke, salt
particles)
– Millions of particles
– So small that they remain
suspended until too “heavy”
(precipitation)