Atmospheric Stability and

Air Pollution Meteorology

Prof Shirley Brooks and V Zungu (PhD…)

Aim of the Learning Guide

• Understand general atmospheric or climatic

patterns;

• Introduce key concepts of atmospheric

dynamics with reference to stability and

general air dispersal patterns;

• To apply atmospheric patterns in contexts

of air pollution, cloud formation, air

travelling, waste management etc…

Environmental Management 4

B Tech

CLIMATE, WEATHER

SYSTEMS and Diversity

INTRODUCTION

State weather the phrasing of each of the followingis technically (scientifically) correct or incorrect :

1. The climate of Cape Town changes several times in a day.

2. The weather of Cape Town is characterised bystrong winds and wet winters.

3. The climate on the plane’s route from CapeTown to Durban was not favourable for theflight.

4. The leafing pattern of savanna plants each yearis guided by the climate.

5. The climate of the Namib desert ischaracteristically dry.

INTRODUCTION

WEATHER:

= the state of the atmosphere, particularly with respect to its effect on humanactivities and in terms of its short-termvariability.

CLIMATE:

= the totality of weather over long periodsin a region, usually summarised by averages and statistical measures of variability

INTRODUCTION

Elements of weather:

- Temperature

- Humidity

- Rainfall

- Wind

- Cloud cover

- Sunshine

- Pressure

Elements of climate? = as for weather.

ENVI 1400 : Meteorology and Forecasting : lecture 2 7

TROPOSPHEREcumulusclouds

cirrostratusclouds

10km

20km

30km

40km

50km

70km

80km

90km

60km

110km

120km

130km

100km

MESOSPHERE

STRATOSPHERE

THERMOSPHERE

Mt Everest(8km)

ozone layer

meteorite

aurora

noctilucentclouds

tropopause

stratopause

mesopause

20

60

40

100

80

0-100 -80 -60 -40 -20 0 4020

100

10

1

0.1

0.01

0.001

0.0001

0.00001

TROPOSPHERE

MESOSPHERE

THERMOSPHERE

STRATOSPHEREA

ltit

ud

e (

km

)

Pe

rce

nta

ge

of

Atm

os

ph

eri

c M

as

s A

bo

ve

Temperature (C)

Tropopause

Stratopause

Mesopause

ozone

layer

Mt Everest

cumulonimbus

Vertical Structure:

Temperature

some of the risingair flows south

some of the risingair flows north

Rising air is now dry…

Dry air descendsat around 30º N

…and at around 30º S

Deserts DesertsThe descending air flows N and S

These are called circulationcells – the basic units of

Vertical atmospheric circulation

Hadley cells

Ferrell cells (30 - 60º)

Polar cells (60-90º)

Circulation patternsrepeat at 30-60º and

60-90º…

Dry

Dry

Wet

WetDry

Wet

Dry

Air rises and falls in Hadley, Ferrel, and Polar cells(vertical circulation)

Circulation cells explain global distribution of rainfall

Earth’s rotation determineswind direction(horizontal circulation,Coriolis force)

ITCZ and cell locations shift seasonallydepending on location of maximal heating of Earth’s surface

2.6

HIGH AND LOW PRESSURES

RELATIONSHIP BETWEEN AIR

PRESSURE AND WIND

DIVERGENCE

LOW PRESHIGH PRESHIGH PRES

CONVERGENCE

Wind always blows from a

HIGH PRES to a LOW PRES

ENVI1400 : Meteorology and Forecasting : lecture 3 13

Isobars at 4mb intervals

Recall: Saturation is when evaporation = condensation

Also remember that the

higher the air

temperature, the more

water vapor will be

present in the air at

saturation.

Relative humidity = actual water vapor in air/maximum water vapor

possible

Relative humidity depends on two factors:

the actual amount of moisture in the atmosphere

the temperature

remember that temperature determines how much water

vapor can be in the air at saturation

RH=[(vapor pressure)/(saturation vapor pressure)] X 100%

RH=[(mixing ratio)/(saturation mixing ratio)] X 100%

We can express/calculate Relative Humidity in a variety of ways:

Dew Point = the temperature to which air must be cooled in order to become

saturated

Dewpoint temperature is a better "absolute" measure of moisture in the air.

Why? Because it doesn't change when the air temperature changes; it only

changes when the moisture content changes. (Assuming constant pressure).

For example:

Temperature Dew Point Relative

Humidity

30 10 29%

20 10 53%

10 10 100%

High dew points mean high moisture content of the air, which often translates to

muggy and uncomfortable conditions.

In general, most people consider dew points above 20 degrees C very

uncomfortable (regardless of air temperature and relative humidity).

NOTE: Dew point temperature can NEVER be greater than the actual air

temperature

When air temperature = dew point temperature, RH = 100%One of the clues a

meteorologist uses for

forecasting tonight's low

temperature is to look at

today's dew point: if no fronts

are expected to come through,

tonight's low temperature will

not get much below today's

dew point. WHY?

Quick summary of conditions of saturation

• Air temp = dewpoint temp

• Relative humidity = 100%

• Mixing ratio = saturation mixing ratio

• Vapor pressure = saturation vapor pressure

Once saturation is reached:

1) If more water is added, then condensation will dominate

2) If temperature is decreased, then condensation will dominate

In other words, a cloud will form (given presence of CCN, etc.)

To make a cloud we need really only 3 things:

• Moisture

• Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN) (more

detail later on this)

• A method of cooling the air to saturation

So how, exactly, do convective clouds form in the atmosphere?

Pressure is essentially the

“weight” of the

atmosphere above you

As you go up, less

atmosphere is above you,

so pressure is less

This is why your ears

“pop” as you drive up a

mountain or go up in an

airplane

-- basically air inside your ears has

retained the pressure of the lower

elevation and starts to expand

Consider a “parcel” of air at

1000 millibars (that is, at the

ground)

Assume that the parcel is not

saturated

The parcel will exert 1000 mb

of pressure to counteract the

atmospheric pressure acting on

the parcel. i.e., parcel pressure

is in equilibrium

If we take this parcel of air at Earth’s

surface and somehow lift it up to 500

mb:

• as we go higher in the atmosphere,

there is less atmosphere above us

• with fewer molecules pressing on

our parcel of air, the molecules in the

parcel can move more and the parcel

can expand

• when a parcel expands, the

molecules are “doing work”

• when molecules do work, they lose

energy so the parcel cools

That is – RISING AIR ALWAYS

COOLS

ΔU = Q – W

Short aside: first law of thermodynamics

ΔU = Q – WChange in

internal

energy

Heat added to

the system

Work done by

the system

Recall: temperature is a measure of kinetic energy. As kinetic

energy increases, temp will increase. As kinetic energy

decreases, temp decreases.

So – we know that rising/sinking parcels are “doing work” – thus we know they are

change their internal energy.

When air parcels rise (or sink), the

process is labeled adiabatic.

Physically, this simply means the

parcel keeps its heat constant

(remember, heat and temperature are

not the same!!) (Q in the equation

above does not change)

Parcel Temperature Internal energy Work

Rises cools decreases done by

Sinks warms increases done to

PARCEL lapse rates

Rising air parcels will COOL.

IF UNSATURATED, their rate of

cooling is fixed: 10°C / km (ten

degrees celsius per kilometer).

A parcel that rises 500 m (1/2

km) will cool 5C, one that rises

1267 m will cool 12.67C. The

math is simple

This lapse rate is called the “dry

adiabatic lapse rate”.

Sinking parcels are – by

definition – unsaturated. WHY?

Their rate of warming is fixed at

the dry adiabatic lapse rate.

Don’t confuse “parcel” lapse

rates with “environmental lapse

rates” – the two are different!

I.e., sinking parcels always warm at 10°C / km

What happens when the rising parcel becomes saturated?

As an air parcel rises and cools, the relative humidity increases

When the parcel cools to the point when the parcel temperature and the dew point

temperature are equal, RH will be 100%

If lifting continues, the parcel will continue to cool – BUT the parcel would be

“supersaturated” (not good)

Thus, it MUST “expel” water vapor – & condensation occurs

The difference between

wet adiabatic lapse rate

(6 C / km) and dry

adiabatic lapse rate (10

C / km) is due to latent

heat release

Notice also that MOST

rising parcels first cool

at the dry rate, then

reach saturation & cool

at the wet rate.

Parcel air temp & dewpoint temp example

Cloud Formation

When we lift the air, where will condensation occur?

Depends on the moisture content of the air that is being lifted.

The lifting condensation level (LCL) is the altitude (usually

expressed as a pressure) at which the lifted air is cooled dry

adiabatically to saturation. Clouds will form at this level.

Typo: 6C/km

Now all we have to do is get the parcel of air lifted. We

can do that in four ways:

Orographic Lifting

Frontal Uplift (also known as frontal wedging)

Convergence

Convection

Orographic LiftingAir is forced upward by topography

As air is forced up the mountains (windward side) it cools, forms clouds, and maybe

precipitation

As air goes down the mountain on the leeward side, it is compressed and warms

Therefore it is usually wetter on the windward side than on the leeward side.

What is Stability?

Vertical motion of air parcels.

What is Atmospheric Stability

• The vertical movement of air molecules characterized by

certain basic conditions (Temperature) that determine the

general stability of the atmosphere.

• Downward motion – Adiabatic warming

• Upward motion – Adiabatic cooling

Stability & Movement

•A rock, like a parcel of air, that is in stable equilibrium will return to its

original position when pushed.

•If the rock instead departs in the direction of the push, it was in

unstable equilibrium.

Behavior of Rising and Sinking Air•Rising air expands, using

energy to push out, which

slows and adiabatically cools

the air.

•A parcel of air may be forced

to rise or sink, and change

temperature relative to the

environmental air, which is

sampled using radiosonde

balloons.

•The radiosonde balloon

expands in size from

approximately 6 feet to a

diameter between 24 and 32 ft

before it bursts. The balloon

carries the instrument package

to an altitude of

approximately 25 mi (27-37

km) where the balloon bursts

(at a pressure of

approximately 10 mb).

= Exerted Pressure

Key Terms When Discussing Stability

• Adiabatic Process - is when an air parcel cools by expansion or warms by compression with no exchange of heat from the the outside environment.

• Dry Adiabatic Rate - the rate at which an “unsaturated” parcel is cooled or warmed adiabatically (adiabatic process). The dry adiabatic rate is 10°C per 1000 m or 5.5°F per 1000 ft and it remains constant.

• Moist Adiabatic Rate - the rate at which a “saturated” parcel is cooled and warms with ascending or descending motion. This rate varies but it is less than the dry adiabatic rate due to latent heating from condensation offsetting the cooling. A commonly used value for the moist adiabatic rate is 6°C per 1000 m or 3.3°F per 1000 ft. This rate is not an adiabatic process due to latent heating.

• Environmental Lapse Rate - the rate at which ambient air temperature decreases with height. This rate can vary as well and must be measured by a radiosonde.

Lapse Rate

• Important characteristic of atmosphere is

ability to resist vertical motion: stability

• Affects ability to disperse pollutants

• When small volume of air is displaced upward

– Encounters lower pressure

– Expands to lower temperature

– Assume no heat transfers to surrounding

atmosphere

– Called adiabatic expansion

Adiabatic Expansion

To determine the change in temp. w/ elevation due to adiabatic expansion– Atmosphere considered a stationary column of air in a

gravitational field

– Gas is a dry ideal gas

– Ignoring friction and inertial effects

( dT/dz)adiabatic perfect gas = - (g M/ Cp)

• T = temperature

• z = vertical distance

• g = acceleration due to gravity

• M = molecular weight of air

• Cp = heat capacity of the gas at constant pressure

Adiabatic Expansion

( dT/dz)adiabatic perfect gas = -0.0098°C/m

or

( dT/dz)adiabatic perfect gas = -5.4°F/ft

Change in Temp. with change in height

Lapse rate

• Lapse rate is the negative of temperature

gradient

• Dry adiabatic lapse rate =

Metric:

Γ = - 1°C/100m or

SI:

Γ = - 5.4°F/1000ft

Conti….

• Important is ability to resist vertical motion:

stability

• Comparison of Γ to actual environment lapse rate

indicates stability of atmosphere

• Degree of stability is a measure of the ability of

the atmosphere to disperse pollutants

Atmospheric Stability

• Affects dispersion of pollutants

• Temperature/elevation relationship principal determinant of atmospheric stability

• Stable– Little vertical mixing

– Pollutants emitted near surface tend to stay there

– Environmental lapse rate is same as the dry adiabatic lapse rate

Stability Classes

• Developed for use in dispersion models

• Stability classified into 6 classes (A – F)

• A: strongly unstable

• B: moderately unstable

• C: slightly unstable

• D: neutral

• E: slightly stable

• F: moderately stable

Vertical Temperature Profiles

Environmental lapse rate (ELR)

Dry adiabatic lapse rate (DALR)

If:

ELR > DALR =sub adiabatic condition, atmosphere is stable.

ELR >> DALR= Inversion conditions. Very stable atmosphere.

ELR= DALR= atmosphere is neutral.

ELR< DALR = super adiabatic condition, atmosphere is

unstable.

Shapes of plumes depends upon atmospheric stability conditions.

Copyright R. R. Dickerson 2011 49

Stability and Thermodynamic Model

(Air movement behavior)

thermodynamic property – measurements or soundings

day

γa > γ₀ unstable

γb = γ₀ neutrally stable

γc < γ₀ stable

On day a a parcel will cool more slowly than surroundings – air will be

warmer and rise.

On day b a parcel will always have same temperature as surroundings –

no force of buoyancy.

On day c a parcel will cool more quickly than surroundings – air will be

cooler and return to original altitude.

Central Concepts

• The dry adiabatic lapse rate is one degree Celsius of cooling for every

100 meters (-1°C/100m, -10°C/kilometer). This is the parcel of Dry air in

the atmosphere!!!

• As the parcel of air rises and it cools, it will eventually cool to the dew

point when condensation can begin and clouds will form

• However- Air that is saturated with water has reached the dew point

temperature and is carrying as much moisture as that parcel of air is

capable of holding at that temperature.

• This saturated parcel of air has a saturated adiabatic lapse rate (also known

as wet adiabatic lapse rate) of 0.5°C/100 m (5°C/kilometer).

Creating Adiabatic Graph

• First must plot Environmental Lapse Rate Data.

• Create graph of the adiabatic temperature change

for the air parcels.

• Label where the air is cooling at the DAR and

SAR, identify the level of condensation where

clouds start to form, and where the air is stable

and unstable.

Graphing ELR and DALR

• Lapse rate = -DT/DZ = (T2-T1)/(Z2-Z1)

– DALR = 1°C/100m and WALR = 0.60C/100m

• Ground level Temp = 150C

• Dew Point Temp = ?

• Need to graph the adiabatic temperature change.

To do this I need three points:

– Point 1 – Ground level air temperature

– Point 2 – The condensation level

– Point 3 – The end point (highest elevation for the

problem)

Condensation Level

• Ground Level: 150C

• Mixing Ratio: 8 g/kg ~ Mixing ratio (w) is the amount of water vapor that is

in the air i.e. w is the grams of vapor per kg of dry air. w is an absolute

measure of the amount of water vapor in the air.

• Mass of Water Vapour to Mass of Dry Air:

• Cond… Level: 100c

• Need an ELR Graph10

ELR Graph

Temp C0

Hei

gh

t in

(M

)

DAR and Condensation

Level

(a)15o C – 10o = 50C.

(a)But need to know the

actual level in meters:

(a)50C * 100m = 500m

500m

There are 3 steps

calculate this • Determine T0 at the highest level

(e.g. 3000 m)1. Calculate the change in elevationbetween point 2 and point 3.

: Change in elevation

= 500m – 3,000m = -2,500 m

2. Calculate the amount oftemperature change between points 2and 3.

Amount of temperature change

= -2,500 m X .6oC/100m = -15o C

What does the -15oC mean? It means that the air will rise and cool by 15oC going from 500m to 3,000m.

3. The last step is to calculate the new

temperature

= 10o C + (-15o C) = – 5o C

i.e.

Step 3: Plot Ending Temperature

• Calculate change (∆) in elevation:

= starting elevation (h) – ending elevation (h)

= 500m – 3000m

= -2500

Calculate change (∆) in Temperature:

= ∆ elevation x Lapse Rate (dt/dz)

= -2500 x 0.60C/100m

= -15

Calculate ending Temperature:

= starting temperature + ∆ temperature

=100C +-150C

= -50C

Plot Change in Temperature from CL

Stable and Unstable

Mixing Height of atmosphere

The height of the base of the inversion layer from ground

surface.

General Characteristics of Stack

Plumes

• Dispersion of pollutants

• Wind – carries pollution downstream from source

• Atmospheric turbulence -- causes pollutants to

fluctuate from mainstream in vertical and crosswind

directions

• Mechanical & atmospheric heating both present at

same time but in varying ratios

• Affected plume dispersion is differently- location &

time

Plume Types

• Plume types are important because they help

us understand under what conditions there

will be higher concentrations of contaminants

at ground level.

Looping Plume

• High degree of convective

turbulence

• Superadiabatic lapse rate -- strong

instabilities

• Associated with clear daytime

conditions accompanied by strong

solar heating & light winds

• High probability of high

concentrations sporadically at

ground level close to stack.

• Occurs in unstable atmospheric

conditions.

Coning Plume

• Stable with small-scale turbulence

• Associated with overcast moderate to strong winds

• Roughly 10° cone

• Pollutants travel fairly long distances before reaching ground level in significant amounts

• Occurs in neutral atmospheric conditions

Fanning Plume

• Occurs under large negative

lapse rate

• Strong inversion at a

considerable distance above

the stack

• Extremely stable atmosphere

• Little turbulence

• If plume density is similar to

air, travels downwind at

approximately same elevation

Lofting Plume

• Favorable in the sense that fewer impacts at ground level.

• Pollutants go up into environment.

• They are created when atmospheric conditions are unstable above the plume and stable below.

Fumigation

• Most dangerous plume: contaminants are all coming down to ground level.

• They are created when atmospheric conditions are stable above the plume and unstable below.

• This happens most often after the daylight sun has warmed the atmosphere, which turns a night time fanning plume into fumigation for about a half an hour.

Vertical Temperature

Profiles

Environmental lapse rate (ELR)

Dry adiabatic lapse rate (DALR)

If,

ELR > DALR =sub adiabatic

condition, atmosphere is stable.

ELR >> DALR= Inversion

conditions. Very stable atmosphere.

ELR= DALR= atmosphere is

neutral.

ELR< DALR = super adiabatic

condition, atmosphere is unstable.

Shapes of plumes depends upon

atmospheric stability conditions.

THANK YOU