Download Ch. 23 Water in the Atmosphere

Ch. 23 Water in the Atmosphere
• Three states or phases of water in the atmosphere.
– ice (solid), water (liquid), water vapor (gas)
• Temperature is a measurement of the
movement of molecules.
- As you heat a substance, the molecules move faster.
- As you cool a substance, the molecules move slower.
Phase changes (Changes in State)
EVAPORATION – changes from liquid to gas
CONDENSATION – changes from gas to liquid
MELTING – changes from solid to liquid
FREEZING – change from liquid to solid
SUBLIMATION – changes from solid to gas
DEPOSITION – changes from gas to solid
Temperature
• Water freezes at 32 oF or 0 oC
• Water boils at 212 oF or 100 oC
• Lines on a weather map that connect points of
equal temperature are called
Isotherms
Humidity - water vapor in the atmosphere
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•
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“moisture” in the air.
controlled by rates of condensation and evaporation
When rate of evaporation equals rate of condensation,
then the air is saturated (equilibrium reached).
Saturated air holds 100 % of the water vapor that it
could hold at that temperature.
The temperature at which equilibrium occurs is called
the dew point.
• At temperatures below the dew point, condensation
occurs and liquid water droplets form.
• At this point, clouds form.
Condensation Nuclei
• Particles of dust, ash, salt, or other solids in
the air on which water vapor can form.
– At the poles there is very little precipitation because of the
cold temps and lack of condensation nuclei.
Coalescense
Absolute Humidity Relative Humidity
• A measure of the
actual amount of
water vapor in the
air.
• A way to express
the amount of
moisture in the air.
Absolute = mass of water vapor (g)
Humidity
volume of air (m3)
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Compares the amount of water vapor in the
air to the amount the air could hold at a
certain temperature (%),
A ratio of the actual water vapor content of
the air to the amount of water vapor needed
to reach saturation.
It is a measure of how close the air is to
reaching the dew point.
A common way to express the amount of
water vapor in the air.
Example: at 25oC, air is saturated when it
contains 20g of water vapor (per 1 kg of air).
If air at 25oC contains 5 g of water vapor, the
relative humidity is 5/20, or 25%.
Relative
Humidity
=
amount of water vapor in air
amount of water vapor needed
to reach condensation
X 100
The Relative Humidity – Saturation
curve
1.
2.
3.
4.
5.
6.
If T = 50
If T = 30
If T = 0
If H2O = 10
If H2O = 40
If H2O = 70
Dew Point and Dew
• When the air is nearly saturated with a relative humidity
of almost 100%, a small temperature drop will allow it
to reach its dew point.
• At the dew point, condensation occurs.
• Dew occurs when air cools by conduction
(contact with cooler surface features)
– Dew forms on these cooler surfaces (condensation)
– Most likely to form on cool, clear nights with little wind.
– If the dew point falls below the freezing temperature of
water, deposition can occur.
– Frost is not frozen dew
(frozen dew is rare and forms as clear beads of ice)
The heat index gives the temperature according to
how it feels compared to the actual temperature.
• The higher the humidity, the higher the index.
Measuring Relative Humidity
Psychrometer
(sling psychrometer)
Used with a table that lists relative
humidity based on differences
between wet-bulb and dry-bulb
thermometer readings
• No difference  100% humidity
(air is saturated)
Hygrometer
(hair hygrometer)
Works on the principle that
hair becomes longer as relative
humidity increases and shorter
has relative humidity
decreases.
Comparing wet-bulb and dry-bulb to determine Relative Humidity
• Dry = 80
differ = 10
• Dry =100
differ= 13
• Dry = 60
differ = 4
Measuring Humidity at High Altitudes
• Uses an electronic hygrometer
• Carried into the atmosphere by a radiosonde
• The radiosonde is attached to a weather balloon
Isotherms
• Collections of water droplets or ice
crystals suspended in air which form
when air is cooled and condensation
occurs
Four major processes can cause the cooling that
is necessary for clouds to form.
• Adiabatic Cooling
– The process by which the
temperature of an air mass
decreases as the air mass rises
and expands.
– The altitude at which this
condensation occurs is called
the condensation level, and
marks the base (bottom) of the
clouds.
• Mixing
- Occurs when two moist bodies
of air mix, cooling the
combined air below its dew
point.
• Lifting
- occurs when air is forced
upward and cools;
common as air is forced
upward to move over
mountains
• Advective cooling
– the process by which
temperature of an air
mass decreases as the air
mass moves over a cold
surface
Clouds need these factors to form:
• Water vapor must be able to condense.
• The dew point temperature
must be reached (saturation).
• There must be condensation nuclei
in the atmosphere for the water to form on.
Formation of a Water Droplet
Classification of Clouds
(classified by altitude)
Three
altitudes
High:
(above 6,000 m)
“cirro”
Middle:
(2000 m – 6000 m)
“alto”
Low:
(0 m – 2000 m)
“strato”
Classification of Clouds
(Classified by shape/form)
• Stratus
• Stratus means sheetlike or layered
• Flat, uniform, low
altitude
• Cover large areas of
the sky
• Produce little
precipitation
• Nimbostratus - dark
rain makers
• Altostratus – thinner
at middle latitudes
• Stratocumulus
Classification of Clouds
(Classified by shape/form)
• Cumulus
• Low altitude,
billowy, puffy
• Cumulus means
piled or heaped
• Hot humid days
• Cumulonimbus –
high, dark, storm
clouds,
thunderheads
• Altocumulus –
middle altitudes
• Stratocumulus –
low forming
combinations of
stratus and
cumulus
Classification of Clouds
(Classified by shape/form)
• Cirrus
• Feathery clouds
composed of ice
crystals
• Highest altitude of
any clouds; above
6,000 m
• Cirro and cirrus
means curly
• Cirrocumulus –
rare, high altitude,
billowy
• Cirrostratus – high,
transparent veil.
• Halo around
moon or sun
Fog
(a cloud that forms near the surface)
Radiation fog
• Forms from radiation
• The layer of air in contact
with the ground becomes
chilled to below dew point.
• Thickest in valleys and low
places.
• Also common around cities,
where pollutants act as
condensation nuclei.
Other types of fog
• Advection fog
– Forms when warm, moist air
moves across a cold surface.
– Common along coasts
• Upslope fog
– Forms by the lifting and cooling
of air as air rises along land
slopes.
• Steam fog
– A shallow layer of fog that
forms when cool air moves
over an inland warm body
water such as a river.
STEAM FOG
Weather Modification
• Cloud seeding:
– In areas of drought, scientists may attempt to induce
precipitation.
– Freezing nuclei or condensation nuclei are introduced into
a cloud.
– Silver iodide crystals are released from burners on the
ground or from flares dropped by aircrafts.
– Powdered dry ice is dropped from aircraft to cool cloud
droplets and cause ice crystals to form which melt to form
raindrops.
– Works too well sometimes, not at all sometimes.
– Could eventually be a way to reduce precipitation from
clouds before a storm gets too large.
Cloud Cover Symbols
Cloud Type Symbols
- any moisture that falls from
the air to Earth’s surface
• Forms in troposphere
• cumulonimbus and nimbostratus clouds
Forms of Precipitation
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Rain
- Liquid, 0.5 – 5 mm in diameter (smaller  drizzle)
Snow
– Solid, ice particles
Sleet
– Ice pellets, occur when rain falls through a layer of freezing air near the
ground. Sometimes it may not freeze until it hits the ground  glaze ice
(ice storm).
Freezing rain
- rain falls and freezes when it touches cold surfaces such as highways
and trees
Hail
– Solid, lumps of ice
– Forms in cumulonimbus clouds that keep the particles aloft until they are
too heavy and then fall as hail
Precipitation Formation
Hail
Causes of Precipitation
(Most cloud droplets are 20 micrometers in diameter)
Coalesence
• The formation of large
droplets by the combination
of smaller droplets.
Supercooling
• A condition in which a
substance is cooled to
below its freezing point,
condensation point, or
sublimation point without
changing state.
• Ice crystals do not form
because there are to few
freezing nuclei.
• Water vapor condenses on
available ice crystals.
Measuring Precipitation
• Rain gauge –
measures rainfall
• Measuring stick –
snow depth
• Water content of
snow – melt a
measured volume of
snow
• 10 cm of snow = 1 cm
of liquid water
Doppler Radar
• Measures the
intensity of rainfall.
• Works by bouncing
radio waves off
rain/snow.
• By timing the waves
return rate,
meteorologists can
detect the location,
direction of
movement, and
intensity of
precipitation.