Jacobson M.Z. Atmospheric Pollution

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6.2.1. Geostrophic Wind

The type of wind involving the least number of forces is the geostrophic (“Earth-

turning”) wind. This wind involves only the PGF and ACoF. It arises above the

boundary layer, where surface friction is negligible, and along straight isobars. An

isobar is a line of constant pressure. Suppose a horizontal pressure gradient, repre-

sented by two parallel isobars, exists, such as in the top diagram of Fig. 6.2. The

PGF causes still air to move from high to low pressure. As the air moves, the ACoF

deflects the air to the right. The ACoF continues deflecting the air until the ACoF

exactly balances the magnitude and direction of the PGF (geostrophic balance).

Figure 6.2 shows that the resulting geostrophic wind flows parallel to isobars. The

closer the isobars are together

, the faster the geostrophic wind. In reality, geostroph-

ic balance occurs following a process called geostrophic adjustment, during which

the wind overshoots then undershoots its ultimate path in an oscillatory fashion. In

the Southern Hemisphere, the geostrophic wind flows in the opposite direction from

that shown in the top of Fig. 6.2.

6.2.2. Surface Winds along Straight Isobars

When isobars are straight near the surface, the FF affects the equilibrium wind speed

and direction. The bottom of Fig. 6.2 shows the wind direction that results from a

balance among the PGF, ACoF, and FF at the surface in the Northern Hemisphere.

Friction, which acts in the opposite direction from the wind, slows the wind.

Because the magnitude of the ACoF is proportional to the wind speed, a reduction in

wind speed reduces the magnitude of the ACoF. Because the sum of the FF and the

ACoF must balance the PGF, the equilibrium wind direction shifts toward low pres-

sure. On average, surface friction turns winds 15 to 45 toward low pressure, with

lower values corresponding to smooth surfaces and higher values corresponding to

rough surfaces.

148 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Surface

PGF

Aloft

ACoF

FF+ACoF

Geostrophic wind

Surface wind

PGF

ACoF

Figure 6.2. Forces acting to give winds aloft and at the surface in the Northern Hemisphere.

6.2.3. Gradient Wind

When centers of low and high pressure exist aloft, the wind is controlled by the PGF,

ACoF, and ACfF. The resulting wind is the gradient wind. For a low-pressure center,

the PGF acts toward and the ACfF acts away from the center of the low. Balance

requires that the ACoF also oppose the PGF. Figure 6.3(a) shows the resulting

counterclockwise (cyclonic) gradient wind in the Northern Hemisphere. Around a high-

pressure center aloft in the Northern Hemisphere, the PGF and ACfF act away from and

the ACoF acts toward the center of the high. Figure 6.3(b) shows the resulting clock-

wise (anticyclonic) wind. In the Southern Hemisphere, gradient winds ﬂow clockwise

around low-pressure centers and counterclockwise around high-pressure centers.

6.2.4. Surface Winds along Curved Isobars

Near the surface, the FF affects the ﬂow around centers of low and high pressure.

A surface low-pressure center is a cyclone, and a surface high-pressure center is an

anticyclone. Figure 6.4 shows the force balances and resulting winds in the presence

of a surface (a) low-pressure system and (b) high-pressure system in the Northern

Hemisphere. In the low-pressure case, surface winds rotate counterclockwise

EFFECTS OF METEOROLOGY ON AIR POLLUTION 149

L ACoF +

ACfFPGF

Gradient

wind

834 mb

830 mb

PGF +

ACfF

ACoF

834 mb

830 mb

Gradient

wind

(a) (b)

Figure 6.3. Gradient winds around a center of (a) low and (b) high pressure in the Northern

Hemisphere, and the forces affecting them.

Surface

wind

1,000 mb

996 mb

ACoF

ACfF

PGF

PGF +

ACfF

ACoF

1,016 mb

1,012 mb

Surface

wind

(a) (b)

Figure 6.4. Surface winds around centers of (a) low and (b) high pressure in the Northern

Hemisphere and the forces affecting them.

(cyclonically), but the FF causes them to converge toward the center of the low. In the

high-pressure case, surface winds rotate clockwise (anticylonically), but the FF caus-

es them to diverge away from the center of the high. In the Southern Hemisphere,

winds converge clockwise around a surface low-pressure center and diverge counter-

clockwise around a surface high-pressure center.

6.3. GLOBAL CIRCULATION OF THE ATMOSPHERE

Air pollution is affected by winds, winds are affected by large-scale pressure systems,

and large-scale pressure systems are affected by the global circulation of the atmosphere.

Figure 6.5 shows features of the global circulation, including the major circulation cells,

the belts of low and high pressure, and the predominant wind directions.

Winds have a west-east (zonal), south-north (meridianal), and vertical compo-

nent. The three circulation cells in each hemisphere shown in Fig. 6.5 represent the

meridianal and vertical components of the Earth’s winds, averaged zonally (over all

longitudes) and over a long time period. The cells are symmetric about the equator and

extend up to the tropopause (Section 3.3.1.2), which is near 18 km altitude over the

equator and near 8 km altitude over the poles.

Two cells, called Hadley cells, extend from 0 to 30N and S latitude, respectively.

These cells were named after George Hadley (1685–1768), an English physicist and

150 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 6.5. Diagram of the three major circulation cells in the atmosphere, the predominant surface pressure

systems, and the predominant surface wind systems on the Earth; H, high pressure; L, low pressure.

Elevated high and low pressures are relative to pressures at the same altitude. The circled X’s denote winds

going into the page (west to east). The circled dots denote winds coming out of the page (east to west).

Polar cell

Ferrel cell

Hadley

cell

Equatorial low - 0 °N

(Doldrums), ITCZ

Subtropical high - 30

°N

(Horse latitudes)

Subpolar low - 60

°N

Polar high - 90

°N

Polar high - 90

°S

Subtropical high - 30

°S

Subpolar low 60

°S

Northeast trade winds

Southwesterlies

Southeast trade winds

Northwesterlies

Polar easterlie

Polar easterlies

Polar cell

Ferrel cell

Hadley

cell

West

East

meteorologist who, in 1735, ﬁrst proposed the cells in a paper called, “Concerning the

Cause of the General Trade Winds,” presented to the Royal Society of London.

Hadley’s original cells, however, extended between the equator and poles.

In 1855, William Ferrel (1817–1891; Fig. 6.6)

an American school teacher, meteorologist, and

oceanographer, published an article in the Nashville

Journal of Medicine pointing out that Hadley’s one-

cell model did not ﬁt observations so well as did the

three-cell model shown in Fig. 6.5. In 1860, Ferrel

went on to publish a collection of papers showing

the ﬁrst application of mathema-tical theory to ﬂuid

motions on a rotating Earth. Today, the middle cell

in the three-cell model is named the Ferrel cell.

Ferrel cells extend from 30 to 60 in the Northern

and Southern Hemispheres. Two Polar cells extend

from 60 to 90N and S latitude, respecti

vely.

6.3.1. Equatorial Low-Pressure Belt

Circulation in the three cells is controlled by heating

at the equator, cooling at the poles, and the rotation

of the Earth. Air rises over the equator because

the sun heats this region intensely. Much of the

heating occurs over water, some of which evapo-

rates. As air containing water vapor rises, the air expands and cools, and the water

vapor recondenses to form clouds of great v

ertical extent. Condensation of water

vapor releases latent heat, providing the air with more buoyancy. Over the equator,

the air can rise up to about 18 km before it is decelerated by the stratospheric inver-

sion. Once the air reaches the tropopause, it cannot rise much farther, so it diverges to

the north and south. At the surface, air is drawn in horizontally to replace the rising

air. So long as divergence aloft exceeds convergence at the surface, surface air pres-

sure decreases and air pressure aloft increases (relative to pressures at the same

altitude but other latitudes). The surface low-pressure belt at the equator is called the

equatorial low-pressure belt. Because pressure gradients are weak, winds are light,

and the weather is often rainy over equatorial wa

ters, this region is also called the

doldrums.

6.3.2. Winds Aloft in the Hadley Cells

As air diverges toward the north in the elevated part of the Northern Hemisphere

Hadley cell, the ACoF deﬂects much of it to the right (to the east), giving rise to

westerly winds aloft (winds are generally named after the direction that they originate

from). Westerly winds aloft in the Northern Hemisphere Hadley cell increase in

magnitude with increasing distance from the equator until they meet equatorward-

moving air from the Ferrel cell at 30N, the subtropical front. The front is a region of

sharp temperature contrast. The winds at the front are strongest at the tropopause,

where they are called the subtropical jet stream. Winds aloft in the Southern

Hemisphere Hadley cell are also westerly and culminate in a tropopause subtropical

jet stream at 30S.

EFFECTS OF METEOROLOGY ON AIR POLLUTION 151

Figure 6.6. William Ferrel (1817–1891).

6.3.3. Subtropical High-Pressure Belts

As air converges at the subtropical fronts at 30N and S, much of it descends. Air is

drawn in horizontally aloft to replace the descending air. So long as inﬂow aloft

exceeds outﬂow at the surface, surface air pressure builds up. The surface high-

pressure belts at 30N and S are called subtropical high-pressure belts. Because

descending air compresses and warms, evaporating clouds, and because pressure gra-

dients are relatively weak around high-pressure centers, surface high-pressure systems

are characterized by sunny skies and light winds. Sunny skies and the lack of rainfall

at 30N and S are two reasons why most deserts of the world are located at these lati-

tudes. The light winds forced many ships sailing at 30N to lighten their cargo, the

heaviest and most dispensable component of which was often horses. Thus the 30N

latitude band is also called the horse latitudes.

6.3.4. The Trade Winds

At the surface at 30N and S, descending air diverges both equatorward and poleward.

Most of the air moving equatorward is deﬂected by the ACoF to the right (toward the

west) in the Northern Hemisphere and to the left (toward the west) in the Southern

Hemisphere, except that friction reduces the extent of ACoF turning. The resulting

winds in the Northern Hemisphere are called the northeast trade winds because they

originate from the northeast. Those in the Southern Hemisphere are called the south-

east trade winds because they originate from the southeast. Sailors from Europe have

used the northeast trades to speed their voyages westward since the ﬁfteenth century.

The trade winds are consistent winds. As seen in Fig. 6.5, the northeast and southeast

trade winds converge at the Intertropical Convergence Zone (ITCZ), which moves

north of the equator in the Northern Hemisphere summer and south of the equator in

the Southern Hemisphere summer, generally following the direction of the sun. At the

ITCZ, air convergence and surface heating lead to the rising arm of the Hadley cells.

6.3.5. Subpolar Low-Pressure Belts

As surface air moves poleward in the Ferrel cells, the ACoF turns it toward the east in

both hemispheres, but surface friction reduces the extent of turning, so that near-surface

winds at midlatitudes (30 to 60N and S) are generally westerly to southwesterly (from

the west or southwest) in the Northern Hemisphere and westerly to northwesterly in the

Southern Hemisphere. In both hemispheres, poleward-moving near-surface air in the

Ferrel cell meets equatorward-moving air from the Polar cell at the polar front, which

is a region of sharp temperature contrast between these two cells. Converging air at the

surface front rises and diverges aloft, reducing surface air pressure and increasing air

pressure aloft relative to pressures at other latitudes. The surface low-pressure regions at

60N and S are called subpolar low-pressure belts. Regions of rising air and surface

low pressure are associated with storms. Thus, the intersection of the Ferrell and Polar

cells is associated with stormy weather. Unlike at the equator, surface pressure gradients

and winds at the polar front are relatively strong. West–east wind speeds also increase

with increasing height at the polar fronts. At the tropopause in each hemisphere, they

culminate in the polar front jet streams. Whereas the subtropical jet streams do not

meander to the north or south over great distances, the polar front jet streams do. Their

predominant direction is still from west to east.

152 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

EFFECTS OF METEOROLOGY ON AIR POLLUTION 153

1018

1014

1006

1022

1018

1022

1018

1010

1006

1022

1026

-180 -170 -160 -150 -140 -130 -120 -110

Latitude (degrees)

Longitude (degrees)

= 17.67 m/s

(a)

5450

5600

5550

5800

5900

5550

5500

5450

5400

5600

5900

5850

-180 -170 -160 -150 -140 -130 -120 -110

Latitude (degrees)

Longitude (degrees)

= 32.15 m/s

(b)

Figure 6.7. Map of (a) sea-level pressures (mb) and near-surface winds (m/s) and (b) 500-mb

heights (m) and winds (m/s) obtained from NCEP (2000) for August 3, 1990, at 12 GMT for

the northern Paciﬁc Ocean. Height contours on a constant pressure chart are analogous to

isobars on a constant height chart; thus, high (low) heights in map (b) correspond to high

(low) pressures on a constant height chart. The surface low-pressure system at 148W,

53N in (a) is the Aleutian low. The sur face high-pressure system at 134W, 42N in (a) is

the Paciﬁc high.

6.3.6. Westerly Winds Aloft at Midlatitudes

One would expect the ACoF to deﬂect air moving equatorward aloft in the Ferrel cells

to the west, creating easterly winds (from east to west) aloft at midlatitudes in both

hemispheres. In fact, the Coriolis force does act on the winds in this way, but the

winds aloft in these regions are accelerated in the opposite direction (from west to

east) by the movement of air around pressure centers.

Descending air at 30N and S creates centers of surface high pressure, and rising

air at 60N and S creates bands or centers of surface low pressure. Figure 6.7(a)

shows an example of surface high- and low-pressure centers formed over the Paciﬁc

Ocean in the Northern Hemisphere at these respective latitudes. Surface winds mov-

ing around a Northern Hemisphere surface high-pressure center travel clockwise

(diverging away from the center of the high), and surface winds moving around a sur-

face low-pressure center travel counterclockwise (converging into the center of the

low). Indeed, these characteristics can be seen in Fig. 6.7(a),

which shows winds trav-

eling clockwise around the highs and counterclockwise around the lows. The reason

winds move the way they do around pressure centers was described in Sections 6.2.3

and 6.2.4. The positions of the highs and lows shown in Fig. 6.7(a) create a near-

surface west-to-east ﬂow that meanders sinusoidally around the globe. The ﬂow

created by these highs and lows is consistent with the e

xpectation that near-surface

winds in the Ferrel cell are predominantly westerly.

Figure 6.7(b) shows an elevated map corresponding to the surface map in

Fig. 6.7(a). The elevated map shows height contours on a surface of constant pressure

(500 mb) and winds traveling around centers of low and high heights. Height contours

on a constant pressure chart are analogous to isobars on a constant altitude chart; thus,

high (low) heights in Fig. 6.7(b) correspond to high (low) pressures on a constant

height chart. The lows aloft lie slightly to the west of the surface lows. The ﬁgure indi-

cates that winds traveling around the highs and lows aloft connect, as they do at the

surface, resulting in sinusoidal west-to-east ﬂow around the globe. Thus, high-and

low-pressure systems aloft are responsible for westerly winds aloft in the Ferrel cell.

6.3.7. Polar Easterlies

Air moving poleward aloft in the Polar cells is turned toward the east in both

hemispheres by the ACoF, causing elevated winds in the Polar cells to be westerly. At

the poles, air aloft descends, increasing surface air pressures. The surface high-

pressure regions are called Polar highs. Air at the polar surface diverges equatorward.

The ACoF turns this air toward the west. Friction is weak because polar surfaces are

either snow or ice, and the resulting surface winds in the Polar cells are easterly and

called polar easterlies.

6.4. SEMIPERMANENT PRESSURE SYSTEMS

The subtropical high-pressure belts in the Northern and Southern Hemispheres are

dominated by surface high-pressure centers over the oceans. These high-pressure cen-

ters are called semipermanent surface high-pressure centers because they are

usually visible on a sea level isobar map most of the year. These pressure systems tend

to move northward in the Northern Hemisphere summer and southward in the winter.

On average, they are centered near 30N or S. In the Northern Hemisphere, the two

154 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

semipermanent surface high-pressure systems are the Paciﬁc high (in the Paciﬁc

Ocean) and the Bermuda–Azores high (in the Atlantic Ocean). In Fig. 6.7(a), the sur-

face high-pressure center at 134W, 42N is the Paciﬁc high. In the Southern

Hemisphere, semipermanent high-pressure systems are located at 30S in the south

Paciﬁc, south Atlantic, and Indian Oceans.

The subpolar low-pressure belt in the Northern Hemisphere is dominated by semi-

permanent surface low-pressure centers. The subpolar low-pressure belt in the

Southern Hemisphere is dominated by a band of low pressure. In the Northern

Hemisphere, the two semipermanent low-pressure centers are the Aleutian low (in the

Paciﬁc Ocean) and the Icelandic low (in the Atlantic Ocean). These pressure systems

tend to move northward in the Northern Hemisphere summer and southw

ard in the

winter, but generally stay between 40 and 65N. In Fig. 6.7(a), the surface low-pressure

center at 148W, 53N is the Aleutian low.

6.5. THERMAL PRESSURE SYSTEMS

Whereas semipermanent surface high- and low-pressure centers exist over the oceans

all year, thermal surface high- and low-pressure systems form over land seasonally.

Thermal pressure systems form in response to surface heating and cooling, which

depend on properties of soil and water, such as speciﬁc heat. Speciﬁc heat (J kg

1

)

is the energy required to increase the temperature of 1 g of a substance 1 K. Soil has a

lower speciﬁc heat than does water, as shown in Table 6.1. During the day, the addition

of the same amount of sunlight increases the temperature of soil more than it does that

of water. During the night, the release of the same amount of thermal-IR energy

decreases soil temperature more than it does that of water. As such, land heats during

the day and cools during the night more than does water. Similarly, land heats during

the summer and cools during the winter more than does water.

Speciﬁc heats vary not only between land and water, but also between different

soil types, as shown in Table 6.1. Because sand has a lower speciﬁc heat than does

clay, sandy soil heats to a greater extent than does clayey soil during the day and sum-

mer. The preferential heating of sand in comparison with that of clay and the

preferential heating of land in comparison with that of water is an important factor that

results in thermal low-pressure centers.

When a region of soil heats, air above the soil warms, rises, and diverges aloft,

creating low pressure at the surface. Because the low-pressure system forms by heat-

ing, it is called a thermal low-pressure system. Thermal low-pressure centers form in

the summer over deserts and other sunny areas (e.g., the Mojave Desert in southern

California, the plateau of Iran, the north of India). These areas are all located near

30N, the same latitude as the semipermanent highs. Descending air at 30N due to the

highs helps to form the thermal lows by evaporating clouds, clearing the skies. The

descending air also keeps the thermal lows shallow (air rising in the thermal lows

EFFECTS OF METEOROLOGY ON AIR POLLUTION 155

Table 6.1. Speciﬁc Heats of Four Media at 298.15 K

Dry air at constant pressure 1004.67

Liquid water 4185.5

Clay 1360

Dry sand 827

Substance Speciﬁc Heat (J kg

1

)

diverges horizontally at a low altitude). In some cases, such as over the Mojave Desert,

the thermal lows do not produce clouds due to the lack of water vapor and shallowness

of the low. In other cases, such as over the Indian continent in the summer, rising air in

a low sucks warm, moist surface air in horizontally from the ocean, ultimately produc-

ing heavy rainfall. The strong sea breeze due to this thermal low is the summer

monsoon, where a monsoon is a seasonal wind caused by a strong seasonal variation

in temperature between land and water.

During the winter, when temperatures cool over land, air densities increase, caus-

ing air to descend. Aloft, air is drawn in horizontally to replace the descending air,

building up surface air pressure. The resulting surface high-pressure system is called a

thermal high-pressure system. In the Northern Hemisphere, two thermal high-

pressure systems are the Siberian high (over Siberia) and the Canadian high (over

the Rocky Mountains between Canada and the United States). As with thermal low-

pressure systems, thermal high-pressure systems are often shallow.

6.6. EFFECTS OF LARGE-SCALE PRESSURE SYSTEMS

ON AIR POLLUTION

Semipermanent and thermal pressure systems affect air pollution. Table 6.2 compares

characteristics of such pressure systems, including their effects on pollution. Semi-

permanent low-pressure systems are associated with cloudy skies, stormy weather, and

fast surface winds. Thermal low-pressure systems, which are often shallow, may or may

not produce clouds. Air rises in both types of low-pressure systems, dispersing near-

surface pollution upward. When clouds form in low-pressure systems, they block

sunlight that would otherwise dri

ve photochemical reactions, reducing pollution further.

Surface high-pressure systems are characterized by relatively slow surface winds,

sinking air, and cloud-free skies. In such pressure systems, air sinks, conﬁning near-

surface pollution. In addition, the slow near-surface winds associated with high-

pressure systems prevent horizontal dispersion of pollutants, and the cloud-free skies

caused by the pressure systems maximize the sunlight available to drive photochemi-

cal smog formation. In sum, the major effects of pressure systems on pollution are

through vertical pollutant transfer, horizontal pollutant transfer, and cloud cover. Each

of these effects is discussed in turn.

156 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Table 6.2. Summary of Characteristics of Northern Hemisphere Surface Low- and

High-Pressure Systems

Latitude range 45–65N25–45N25–45N45–65N

Surface pressure gradients Strong Varying Weak Varying

Surface wind speeds Fast Varying Slow Varying

Surface wind directions Converging, Converging, Diverging, Diverging,

counterclockwise counterclockwise clockwise clockwise

Vertical air motions Upward Upward Downward Downward

Cloud cover Cloudy Cloud-free or cloudy Cloud-free, sunny Cloud free

Storm formation? Yes Sometimes No No

Effect on air pollution Reduces Reduces Enhances Enhances

Surface Low-Pressure Systems Surface High-Pressure Systems

Characteristic Semipermanent Thermal Semipermanent Thermal

6.6.1. Vertical Pollutant Transport

Pressure systems affect vertical air motions and, therefore, pollutant dispersion by

forced and free convection (Section 3.2.2). In a semipermanent low-pressure system,

for example, near-surface winds converge and rise, dispersing near-surface pollutants

upward. In a semipermanent high-pressure system, winds aloft converge and sink, con-

ﬁning near-surface pollutants. Both cases illustrate forced convection. In thermal

low-pressure systems, surface warming causes near-surface air to become buoyant and

rise. In thermal high-pressure systems, surface cooling causes near-surface air to

become negatively buoyant and sink or stagnate. Both cases illustrate free convection.

To understand better how free convection in thermal pressure systems and forced con-

vection in semipermanent pressure systems af

fect pollutant dispersion, it is necessary

to discuss adiabatic processes and atmospheric stability.

6.6.1.1. Adiabatic and Environmental Lapse Rates

Whether air rises or sinks buoyantly in a thermal pressure system depends on

atmospheric stability, which depends on adiabatic and environmental lapse rates.

These terms are discussed next.

Imagine a balloon ﬁlled with air. The air pressure inside the balloon exactly equals

that outside the balloon. Imagine also that no energy (such as solar or thermal-IR ener-

gy or latent heat energy created by condensation of water vapor) can enter or leave the

balloon, but that the balloon’s membrane is ﬂexible enough for it to expand and con-

tract due to changes in air pressure outside the balloon. Suppose now that the balloon

rises. Because air pressure always decreases with increasing altitude, the balloon must

rise into decreasing air pressure. For the air pressure inside the balloon to decrease to

the air pressure outside the balloon, the balloon must expand in volume. This type of

expansion, caused by a change in air pressure alone, is called an adiabatic expansion.

Solar heating and latent heat release are diabatic heating processes and do not con-

tribute to an adiabatic expansion.

During an adiabatic expansion, kinetic energy of air molecules is converted to

work to expand the air. Because temperature is proportional to the kinetic energy of air

molecules (Equation 3.1), an adiabatic expansion cools the air. In sum, rising air

expands and expanding air cools; thus, rising air cools during an adiabatic expansion.

The rate of cooling during an adiabatic expansion near the surface of the Earth is

approximately 9.8 K or C per kilometer increase in altitude. This rate is called the

dry or unsaturated adiabatic lapse rate (

). Lapse rates are opposite in sign to

changes in temperature with height; thus, a positive lapse rate indicates that tempera-

ture decreases with increasing height.

If the balloon in our example rises in air saturated with water vapor, the resulting

adiabatic expansion cools the air and decreases the saturation vapor pressure of water

in the balloon, causing the relative humidity to increase to more than 100 percent and

water vapor to condense to form cloud drops, releasing latent heat. The rate of temper-

ature increase with increasing height due to this latent heat release is typically 4 K

1

, but it increases to 8 K km

1

in the tropics. Subtracting this latent-heat release

rate from the dry adiabatic lapse rate gives a net lapse rate during cloud formation of

between 6 and 2 K km

1

. This rate is called the wet-, saturated-, or pseudoadiabatic

lapse rate (

). It is the negative rate of change of temperature with increasing alti-

tude during an adiabatic expansion in which condensation also occurs. The wet

adiabatic lapse rate is applicable only in clouds.

EFFECTS OF METEOROLOGY ON AIR POLLUTION 157