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Bài giảng khí hậu học chương 5

G304 – Physical Meteorology and Climatology

Chapter 5
Atmospheric moisture

By Vu Thanh Hang, Department of Meteorology, HUS


5.1 The hydrologic cycle
• The total amount of precipitation for the entire globe is
relatively constant at about 104cm per year
• The movement of water between and within the
atmosphere and Earth is reffered to as the hydrological
cycle
• Atmospheric residence time for water vapor is only 10
days or so
• The hydrological cycle is a continuous series of
processes that occur simultaneously, has no real end
and beginning



5.1 The hydrologic cycle (cont.)


5.2 Water vapor and liquid water
• The process whereby molecules break free of liquid
water is known as evaporation.
• The opposite process is condensation, wherein water
vapor molecules collide with the water surface and bond
with adjacent molecules.
• 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.


5.2 Water vapor and liquid water (cont.)

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).


5.2 Water vapor and liquid water (cont.)
• 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 (mb).
• 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.



5.2 Water vapor and liquid water (cont.)


5.2 Water vapor and liquid water (cont.)
• Absolute humidity is the density of water vapor, expressed as
the number of grams of water vapor contained in a cubic meter
of air (g/m3).
• Specific humidity expresses the mass of water vapor existing
in a given mass of air (g/kg). q = mv/(mv + md)
• 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
(g/kg). r = mv/md
• The maximum possible mixing ratio is called the saturation
mixing ratio.


5.2 Water vapor and liquid water (cont.)
• 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 Æ
relative humidity depends on both the actual moisture
content and the air temperature.
• Air temperature increases Æ more water vapor can exist
Æ the ratio of the amount of water vapor in the air relative to
saturation decreases.


5.2 Water vapor and liquid water (cont.)

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.


5.2 Water vapor and liquid water (cont.)

The dew point is the temperature to which the air must be cooled to become saturated.
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.


5.3 Distribution of water vapor
• Water vapor gets into the atmosphere either from local
evaporation or from the horizontal transport of moisture from
other locations


5.4 Measuring humidity
• 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.


5.4 Measuring humidity (cont.)
Å Sling psychrometer

Hygrothermograph Æ


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.


5.4 Measuring humidity (cont.)
• Aspirated psychrometers are equipped with fans that
circulate air across the bulbs of the two thermometers.
• The hair hygrometer uses human hair that expands
and contracts in response to the relative humidity.
• A hygrothermograph is a hygrometer coupled with a
bimetallic strip and rotating drum to give a continuous
record of temperature and humidity.


5.5 High humidities and human discomfort
• The effect of humidity and high temperatures can be
expressed in a heat index (the apparent temperature).
• 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 are considered
extremely dangerous, and heat stroke is likely for at-risk
people.


5.5 High humidities and human discomfort
(cont.)


5.6 Cooling the air to the dew or frost point
• A diabatic process is one in which energy is added
to or removed from a system.
• 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.


5.6 Cooling the air to the dew or frost point
(cont.)
• The first law of thermodynamics states what happens when
heat is added to or removed from gases.
• If heat is added, there will be some combination of an
expansion of the gas and an increase in its temperature.
• The law is given in numerical form as:
ΔH = p.Δα + cv.ΔT
where ΔH = heat added to system; p is air pressure; Δα is
the change in volume (‘+’ for expansion and ‘-’ for
contraction); cv is the specific heat for air at a constant
volume; ΔT is the change in temperature.


5.6 Cooling the air to the dew or frost point
(cont.)
• p.Δα is the work performed by the gas as expansion occurs
• cv.ΔT refers to the change in internal energy
Æ Heat added to the air does not simply disappear but rather is
apportioned between temperature and volume changes.
• A process is adiabatic:
0 = p.Δα + cv.ΔT
Æ Work performed by the air (the expansion of the gas) causes
a decrease in internal energy (a decrease in temperature),
and work performed on the gas (compression) leads to
warming.


5.6 Cooling the air to the dew or frost point
(cont.)

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).


5.6 Cooling the air to the dew or frost point
(cont.)
• 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) Æ not a constant value


5.6 Cooling the air to the dew or frost point
(cont.)
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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