Change of State

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Change of State

• Gases are highly compressible
• The density of a gas can be increased only up to
  a certain level
• After the threshold it changes state into liquid
  or solid

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Phases Phase changes involve energy gain or
loss.
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The process whereby molecules break free of
liquid water is known as evaporation. The
opposite process is condensation, wherein water
vapor molecules become a liquid. The change of
phase directly from ice to water vapor, without
passing into the liquid phase, is called
sublimation. The reverse process (from water
vapor to ice) is called deposition.
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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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Saturation

• When enough WV exists in the air that evaporation
  is equal to condensation

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Indices of Water Vapor Content

• There are several ways to calculate how much
  water vapor is in the atmosphere
• We will explore the most used ones, and their
  characteristics

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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. The vapor pressure of a volume
of air depends on both the temperature and the
density of water vapor molecules. The saturation
vapor pressure is an expression of the maximum
water vapor that can exist. The saturation vapor
pressure depends only on temperature.
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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.
Specific humidity expresses the mass of water
vapor existing in a given mass of
air. Saturation specific humidity is the maximum
specific humidity that can exist and is directly
analogous to the saturation vapor pressure. The
mixing ratio is a measure of the mass of water
vapor relative to the mass of the other gases of
the atmosphere. The maximum possible mixing
ratio is called the saturation mixing ratio.
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Vapor Pressure

• Fraction of the total atmospheric pressure due to
  water vapor
• It is affected by density for the most part, and
  temperature in smaller amount
• It is measured in mb, kPa, or hPa

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Absolute Humidity

• Density of water vapor contained in 1 m3 of air
• It is not widely used because it is highly
  dependent on temperature
• The density of 1 m3 of air changes as a function
  of the temperature

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Specific Humidity

• The mass of water vapor of exactly 1 kg of air
• Specific humidity is the ratio of water vapor to
  air (including water vapor and dry air) in a
  particular volume. Specific humidity ratio is
  expressed as a ratio of kilograms of water vapor,
  mw, per kilogram of mixture, mt .
• That ratio can be showed as

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Mixing Ratio

• Mixing or humidity ratio is expressed as a ratio
  of kilograms of water vapor, mw, per kilogram of
  dry air, md, at a given pressure

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Mixing Ratio 2

• Mixing ratio can also be expressed with the
  partial pressure of water vapor
• where
• d 0.62197 is the ratio of gas constants for
  dry and moist air
• pw partial pressure of water vapor in moist
  air
• pa atmospheric pressure of moist air

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Relative Hmidity

• The ratio of the partial pressure of water vapor
  to the saturated vapor pressure of water at a
  given temperature
• Relative humidity is expressed as a percentage
  and is calculated in the following manner
• p(H2O) is the partial pressure of water vapor in
  the gas mixture
• p(H2O) is the saturation vapor pressure of water
  at the temperature of the gas mixture
• .

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Relative humidity, RH, relates the amount of
water vapor in the air to the maximum possible at
the current temperature. RH (specific
humidity/saturation specific humidity) X
100 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. 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.
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In (a), the temperature of 14C 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.
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Dew Point

• Dew point is the temperature at which water vapor
  saturates from an air mass into liquid or solid
  usually forming rain, snow, frost, or dew
• Dew point normally occurs when a mass of air has
  a relative humidity of 100.
• This happens in the atmosphere as a result of
  cooling through a number of different processes.

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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.
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How to Achieve Saturation

• Adding Water Vapor
• Mixing Cold Air with Warm Moist Air
• Lowering the Temperature to the Dew Point

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The simplest and most widely used instrument for
measuring humidity is the sling psychrometer,
which has two thermometers called the wet bulb
and dry bulb. The difference between the two
temperatures, the wet bulb depression, depends on
the moisture content of the air and can be used
to determine dew point and relative humidity.
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The value corresponding to the row for the dry
bulb temperature and the column for the wet bulb
depression yields the dew point temperature.
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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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The effect of humidity and high temperatures can
be expressed in a heat index. The apparent
temperatures caused by the combination of heat
and humidity provide important guidelines for
people. At values between 41 C to 54 C muscle
cramps or heat exhaustion are likely for
high-risk people. Apparent temperatures above 54
C (129 F) are considered extremely
dangerous, and heat stroke is likely for at-risk
people.
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Diabatic Vs Adiabatic Processes

• Diabatic Energy is added or removed from a
  system
• Adiabatic Temperature changes, but no heat is
  added or removed

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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.
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First Law of Thermodynamics

• "Energy can be transformed, but it can neither be
  created nor destroyed.
• dH p . dV c .dT
• dH heat added to the system
• p pressure
• dV change in volume
• c specific heat of air
• dT change in temperature

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What does it mean?

• dH p . dV c .dT
• 0 p . dV c . dT
• -p . dV c . dT
• or
• p . dV -c . dT
• If no heat is added or removed
• Decrease in volume (dV) causes increase in
  temperature (dT)
• Increase in Temperature causes decreases in volume

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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).
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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).
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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.
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Lapse Rate

• Rate of change in an atmospheric variable,
  usually temperature, with height in an atmosphere
• Environmental lapse rate which refers to the
  actual change of temperature with altitude for
  the stationary atmosphere (i.e. the temperature
  gradient)
• The adiabatic lapse rates which refer to the
  change in temperature of a mass of air as it
  moves upwards
• There are two adiabatic rates
• Dry adiabatic lapse rate
• Moist adiabatic lapse rate

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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.
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Dew is liquid condensation on a surface that
occurs during the early morning after a clear,
windless night. The formation of frost is
similar to that of dew, except that saturation
occurs when the temperature is below 0 C
depositing small ice crystals. Frozen dew begins
when saturation forms liquid dew at temperatures
slightly above 0 C. When further cooling brings
its temperature below the freezing point, the
liquid solidifies into a thin, continuous layer
of ice.
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Fog is a cloud whose base is at or near ground
level. Radiation fog (ground fog) develops when
the nighttime loss of longwave radiation causes
cooling to the dew point. Advection fog forms
when relatively warm, moist air moves
horizontally over a cooler surface. Upslope fog
is formed by adiabatic cooling as air flows
upward along a sloping surface, expanding and
cooling. Precipitation fog forms from the
evaporation of falling raindrops. Steam fog
occurs when cold, dry air mixes with warm, moist
air above a water surface.
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The different types of fog commonly found
throughout North America.
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The dew point decreases as the air rises, at the
rate of about 0.2 C/100 m (1.1 F/1000 ft). This
decrease is called the dew point lapse rate. As
unsaturated air is lifted, its temperature
therefore approaches the dew point by 0.8 C for
every 100 m of ascent (i.e., 1.0 C minus 0.2
C). Thus, if the air temperature and dew point
start out at 18 C and 10 C, respectively, an
ascent of 1000 m is necessary to cause saturation.
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The next chapter examines cloud development and
forms.