Download Humidity

Humidity
Humidity
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Humidity is the amount of water vapor in the atmosphere
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Water is found in all three phases in the atmosphere: gas
(water vapor), water (liquid), ice crystal (solid)
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Highest heat capacity of all solids and liquids except liquid
ammonia
Highest heat conduction of all liquids
Highest surface tension and dielectric constant of any liquid
Highest variable in all of atmospheric constituents (0-4%)
Well know Greenhouse Gas
Water expands 9% when freezing  ice flosts
Essential for life and climate
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Water molecule
The partly exposed hydrogen atom of one
molecule is attracted to the negative
oxygen atom of another molecule.
Constant Gas
Variable Gas
%
Name
Name
%
Nitrogen
N2
78.08
Water vapor
H2O
0–4
Oxygen
O2
20.95
Carbon dioxide
CO2
0.034
Argon
Ar
0.93
Ozone
O3
0.000004*
Neon
Ne
0.0018
Carbon
monoxide
CO
0.00002*
Helium
He
0.0005
Sulfur dioxide
SO2
0.000001*
Methane
CH4
0.0001
Nitrogen dioxide
NO2
0.000001*
Hydrogen
H2
0.00005
Aerosol
Xenon
Xe
0.000009
0.00001*
deposition
evaporation
condensation
Phase changes of water
sublimation
Hydrosphere
component/reservoirs
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Ocean (97.25%), cover 71% of Earth’s surface
Cryosphere (e.g. ice cap, glaciers (2.05%)
Groundwater and soils (0.7%)
Atmosphere (0.001%)
River (0.0001%)
Biosphere (0.00004%)
Hydrological Cycle
Consider a hypothetical jar containing pure water with a flat
surface and an overlying volume that initially contains no water
vapor (a).
As evaporation begins, water vapor starts to accumulate above
the surface of the liquid. With increasing water vapor content,
the condensation rate likewise increases (b).
Eventually, the amount of water vapor above the surface is
enough for the rates of condensation and evaporation to become
equal. The resulting equilibrium state is called saturation (c).
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Humidity refers to the amount of water vapor in the air.
The part of the total atmospheric pressure due to water
vapor is referred to as the vapor pressure or vapor
pressure (e).
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The vapor pressure of a volume of air depends on both
the temperature and the density of water vapor
molecules.
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The saturation vapor pressure (eS) is an expression of
the maximum water vapor that can exist. On the other
hand, saturation is the air that contain amount of water
greater than a threshold, water vapor tend to condense
into liquids faster than its evaporates.
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The air that contain amount of water less than threshold
amount is unsaturated (e<eS).
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Supersaturated is the temporary situation when threshold
value drops so quickly that condensation does not
remove water vapor fast enough.
Saturation
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an air parcel is saturated
when it holds the maximum
amount of water vapour
possible; addition of any
extra water vapour would lead
to condensation
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the saturation vapour
pressure is the vapour
pressure at saturation (big
surprise); it depends on
temperature: "warmer air can
hold more moisture"
Williams p62
dry
E>C
saturate
dE=C
warmed
E>C
cooled
E<C
SVP depends on temperature. As temperature increases, more
molecules are energetic enough to escape into the air.
Concept applies to an ice surface. SVP over ice is lower because
water molecules are bonded more tightly to ice.
For the temperatures of interest, some water molecules are energetic
enough to escape into atmosphere and SVP>0.
Humidity measurement
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ไซโครมิเตอร (Psychrometer)
ประกอบดวยเทอรโมมิเตอรสองอัน
ปลายกระเปาะของเทอรโมมิเตอร
อันหนึ่งหุมดวยผามัสลินชุบน้ํา เรียกวา
กระเปาะเปยก (wet bulb) และ
เทอรโมมิเตอรอีกอันเรียกวากระเปาะ
แหง (dry bulb)
ถามีการระเหยมากขึ้นก็จะทําให
อุณหภูมิระหวางเทอรโมมิเตอร
กระเปาะเปยกและกระเปาะแหงตางกัน
มากขึ้น ความตางของอุณหภมิ
เทอรโมมิเตอรกระเปาะเปยกและแหง
สามารถนํามาคํานวนหาคาความชื้น
สัมพันธได
Psychrometer
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Absolute humidity is the density of water vapor,
expressed as the number of grams of water vapor
contained in a cubic meter of air.
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Specific humidity expresses the mass of water vapor
existing in a given mass of air.
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Saturation specific humidity is the maximum specific
humidity that can exist and is directly analogous to the
saturation vapor pressure.
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The mixing ratio is a measure of the mass of water
vapor relative to the mass of the other gases of the
atmosphere.
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The maximum possible mixing ratio is called the
saturation mixing ratio.
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Relative humidity, RH, relates the amount of water
vapor in the air to the maximum possible at the current
temperature.
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RH = (specific humidity/saturation specific humidity) X
100%
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More water vapor can exist in warm air than in cold air,
so relative humidity depends on both the actual moisture
content and the air temperature.
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If the air temperature increases, more water vapor can
exist, and the ratio of the amount of water vapor in the air
relative to saturation decreases.
In (a), the temperature of 14?C has a saturation specific humidity of
10 grams of water vapor per kilogram of air. If the actual specific humidity
is 6 grams per kilogram, the relative humidity is 60 percent. In (b), the
specific humidity is still 6 grams per kilogram, but the higher temperature
results in a greater saturation specific humidity. The relative humidity is
less than in (a), even though the density of water vapor is the same.
The dew point is the temperature to which the air must be cooled to become
saturated and is an expression of water vapor content. In (a), the temperature
exceeds the dew point and the air is unsaturated. When the air temperature is
lowered so that the saturation specific humidity is the same as the actual
specific humidity (b), the air temperature and dew point are equal. Further
cooling (c) leads to an equal reduction in the air temperature and dew point
so that they remain equal to each other. When the temperature at which
saturation would occur is below 0 ?C, we use the term frost point.
The value corresponding to the row for the dry bulb temperature and the
column for the wet bulb depression yields the dew point temperature.
The value corresponding to the row for the dry bulb temperature and the
column for the wet bulb depression yields the relative humidity.
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Vapor pressure or vapor pressure
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Absolute humidity
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Specific humidity
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Mixing ratio
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Relative humidity
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Dewpoint temperature
Adiabatic processes
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basic idea....
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remember pressure
decreases with height
in the atmosphere
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and air parcel cools by
expansion as it
ascends, and warms
by compression as it
descends
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this is similar to air
feeling cold when
escaping from an
inflated tire
Adiabatic processes
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now for the details....
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an adiabatic process is a process which
takes place without the addition or removal
of heat from external sources
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as long as the air parcel remains unsaturated
and no condensation occurs, it cools at a
rate of 10oC for every km of ascent; this rate
of temperature change is the dry adiabatic
lapse rate
Adiabatic processes
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should condensation occur as the air parcel is cooled, latent heat
is released which partially counteracts the adiabatic cooling; the
reduced cooling rate is the moist adiabatic lapse rate, which
depends on temperature; an average value is 6oC per km of ascent
!!! Almost all clouds result from the rapid expansional cooling
of air when it ascends !!!
Mechanisms That Lift Air
1. Orographic Lifting: Forcing of Air Above a Mountain
(Land) Barrier.
2. Frontal Lifting: Displacement of One Air Mass
(Warmer) Over Another Air Mass (Cooler).
3. Convergence: Horizontal Movement of Air Into an
Area at Low Levels.
4. Localized Convective Lifting: Buoyancy (Heating).
Rainshadow Effect
Windward Side (Upwind): Precipitation Greatly
Enhanced
Leeward(Downwind): Low Precipitation
Frontal Lifting
Displacement of One Air Mass (Warmer) Over
Another Air Mass (Cooler)
Warmer Air Approaches Colder Air
Warmer Air Wedges Over the Colder Air
Warm Front
Smooth Slope
Colder Air Approaches Warmer Air
Colder Air Wedges Under the Warmer Air
Cold Front
Blunt Slope
Horizontal Convergence
Horizontal Movement of Air Into an Area at Low Levels
Mass of Air Not Evenly Distributed
Causes Areas of Higher and Lower Pressure
Pressure Difference Cause Wind
Horizontal Movement of Air Into a Low Pressure Zone
Causes Convergence Lifting
Localized Convection
Localized as Opposed to Global
Free Convection
Heating of Earth’s Surface in Localized Areas
Buoyancy: Lighter, Warmer Air Rises
Can Speed Up or Slow Down Other Lifting Mechanisms
•A diabatic process is one in which energy is added
to or removed from a system, such as air that is
warmed by conduction when in contact with a warm
surface or air that passes over a cool surface and
loses energy by conduction.
•The direction of heat transfer is in accordance with
the second law of thermodynamics, which dictates
that energy moves from regions of higher to lower
temperatures.
Processes in which temperature changes but no heat is added
to or removed from a substance are said to be adiabatic.
The rate at which a rising parcel of unsaturated air cools, called
the dry adiabatic lapse rate (DALR), is very nearly 1.0 ?C/100
m (5.5 ?F/1000 ft).
If a parcel of air rises high enough and cools
sufficiently, expansion lowers its temperature to
the dew or frost point, and condensation or
deposition commences.
The altitude at which this occurs is known as the
lifting condensation level (LCL).
The rate at which saturated air cools is the
saturated adiabatic lapse rate (SALR), which is
about 0.5 ?C/100 m (3.3 ?F/1000 ft).
Unlike the DALR, the SALR is not a
constant value. If saturated air cools
from 30 ?C to 25 ?C (a 5? decrease),
the specific humidity decreases from
27.7 grams of water vapor per
kilogram of air to 20.4. A 5 ?C drop in
temperature from 5 ?C to 0 ?C lowers
the specific humidity only 1.7 grams
for each kilogram of air. This brings
about less warming to offset the
cooling by expansion, as well as a
greater saturated adiabatic lapse rate.
The environmental lapse rate (ELR),
applies to the vertical change in
temperature through still air.
A balloon rising through air with an ELR
of 0.5 ?C/100 m passes through air
whose temperature decreases from
10 ?C at the surface, to 9.5 ?C at 100 m,
and 9.0 ?C at 200 m. The air within the
balloon cools at the dry adiabatic lapse
rate of 1.0 ?C/100 m, faster than the
ELR, and therefore attains a temperature
of 8 ?C at the 200-m level.