Bài giảng khí hậu học chương 5

• The opposite process is condensation, wherein water vapor molecules collide with the water surface and bond with adjacent molecules.. • The vapor pressure of a volume of air depends on

Chapter 5

Atmospheric moisture

By Vu Thanh Hang, Department of Meteorology, HUS

G304 – Physical Meteorology and Climatology

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

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 watervapor 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.)

• 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/m 3 )

• 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 Ærelative 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.

5.2 Water vapor and liquid water (cont.)

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.

• 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 abimetallic strip and rotating drum to give a continuous record of temperature and humidity

5.4 Measuring humidity (cont.)

• 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

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

contraction); c v 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

• c v.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 + c v.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.

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.

5.6 Cooling the air to the dew or frost point

(cont.)

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.

5.6 Cooling the air to the dew or frost point

(cont.)

5.7 Forms of condensation

the early morning after a clear, windless night.

• At night the loss of longwave radiation can cause the surface

to cool diabatically Æ air in contact with cold surface cools by conduction Æ temperature decreases to the dew point Æ condensation.

5.7 Forms of condensation (cont.)

• 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.

5.7 Forms of condensation (cont.)

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.

5.7 Forms of condensation (cont.)

• Fog can form by the lowering of the air temperature to the dew point, an increase in the water vapor content, or the mixing of cold air with warm, moist air

raindrops

5.7 Forms of condensation (cont.)

air above a water surface.

5.7 Forms of condensation (cont.)

radiation causes cooling to the dew point

5.7 Forms of condensation (cont.)

horizontally over a cooler surface.

5.7 Forms of condensation (cont.)

upward along a sloping surface, expanding and cooling.

5.8 Formation and dissipation of cloud

droplets

associated with rising air parcels.

• The dew point decreases as the air rises, at the rate

of about 0.2°C/100m Æ the dew point lapse rate

• If the air temperature and dew point start out at 18°C and 10°C, respectively, an ascent of 1000m is necessary to cause saturation.

5.8 Formation and dissipation of cloud

• The processes that lead to the formation of a cloud do not continue forever Æ lifting will cease, no further condensation Æ cloud development ends

5.8 Formation and dissipation of cloud

droplets (cont.)

• Consider a rising air parcel begins to subside Æ warms

at the SALR Æ evaporation of droplets

• The evaporation continues until the parcel has descended back to LCL

• Below the LCL, air parcel wamrs at the DALR Æ all droplets will have evaporated

• At the initial level, the air will have its original temperature and dew point

• Æ reversible processes