Meteorology

What is Meteorology?

Meteorology is the science of the atmosphere. The atmosphere is the media into which all air pollutants are emitted.

Atmospheric processes such as the movement of air (wind) and the exchange of heat dictate the fate of pollutants as they go through the stages of transport, dispersion, transformation and removal.

Air pollution meteorology is the study of how these atmospheric processes affect the fate of air pollutants.

Knowledge of air pollution meteorology is used to manage and control the release of pollutants into the ambient air.

Managing the release of air pollutants helps to ensure thatambiant polluant concentrations comply with national ambient air quality standards.

Knowledge of air pollution meteorology is essential in order to understand the fate and transport of air pollutants.

Importance of meteorology

Understanding air pollution meteorology and its influence in

pollutant dispersion is essential in air quality planning activities.

Engineers use this knowledge to help locate air pollution

monitoring stations and to develop implementation plans to

bring ambient air quality into compliance with standards.

Meteorology is used in predicting the ambient impact of a new

source of air pollution and to determine the effect on air quality

from modifications to existing sources.

When meteorological conditions develop that are not

conducive to pollutant dispersion, governmental air pollutionagencies must act fast to ensure that air pollutants don’t build

up to unacceptable levels in the air we breathe.

When pollutant levels become excessively high, an air pollution

episode results and emissions into the atmosphere must be

curtailed.

Donora SMOG, 1948

The towns people were accustomed to receiving some emissions

from the local steel mill, zinc smelter, and sulphuric acid plant.

But, they were not prepared for the dangerously high

concentrations of pollutants that built up and became trapped

over the town.

The meteorological conditions in Donora during this five-day

period (high pressure system and strong temperature inversion)

produced light winds and dense fog.

The air was not able to move horizontally or vertically and just

lingered over the town.

Solar Radiation

At upper boundary of atmosphere, incoming solar

radiation=340W/m2

Maximum intensity at λ = 0.4 to 0.8 μm = visible portion of

electromagnetic spectrum

42% of energy

Absorbed by higher atmosphere

Reflected by clouds

Back-scattered by atmosphere

Reflected by earth’s surface

Absorbed by water vapor & clouds

48% adsorbed by land and water

Insolation

Quantity of solar

radiation reaching a

unit area of the earth’s

surface

Angle of incidence

Thickness of the

atmosphere

Characteristics of surface

Albedo

fraction of incident

radiation that is

reflected by a surface

Earth’s albedo is

about 30%

Solar Incidence Angle

Angle between sun’s rays and

an imaginary line

perpendicular to the surface

Maximum solar gain is

achieved when incidence

angle is 0º

Tangent in morning and

approximately perpendicular

angle depends on surface

BASICS OF AIR MOVEMENT

Air generally moves from the poles to the equator, this

is because air flows from high pressure to low pressure

High pressure forms when cold air sinks (at the poles)

Low pressure forms when warm air rises (at the

equator)

But the air doesn’t flow in a straight line

Coriolis Effect

Causes air to move in a curved

path

It is caused by the Earth spinning

on its axis

The Earth spins fastest at the

equator, and slowest near the

poles

As air moves from the equator to the pole, it will travel faster

than the land beneath it

causing the air to follow a

curved path

LAND AND SEA BREEZE

Land surface heats and cools quicker than the sea or other

water bodies, temperature gradients develops, that can result in

the generation of localized wind flows.

A sea breeze develops during the day as the air over the land

warms more quickly than the air over the sea.

It rises, bringing in an onshore breeze, with a return flow aloft.

At night the opposite occurs and a land breeze develops, flowing towards the sea under an area of subsidence.

LAND AND SEA BREEZE

Sea breezes are generally strongest during the day in

summer and land breezes strongest during winter nights.

They can both have significant effects on air quality over

urban areas, as they are recirculating air currents that canreturn pollutants (instead of removing them) to an area from

which they were released earlier in the day .

SEA BREEZE- at day time

LAND BREEZE- at night time

Valley breeze

Mountain and valley winds are generated due to similar

heating and cooling mechanisms to sea−land breezes.

During the day the air above a slope is heated and becomes

warmer than neighboring air at the same height above sea

level and It rises due to convection, and up slope mountain

winds occur.

At night the mountain slopes cool more quickly than the

surrounding air, and the cool air drains down the slope,

generating valley winds.

This heating and cooling often results in closed circulation patterns, which can trap and/or recirculate air pollution in the

mountain −valley system .

Valley breeze

Important meteorological parameters that influence air pollution

classified into

Primary Parameters

Wind direction and speed

Temperature

Atmospheric Stability

Mixing Height

Secondary Parameters

Precipitation

Humidity

Solar Radiation

Visibility

Wind Direction and Speed

Higher wind speed at or near the discharge of pollutant, the

more rapidly the pollutants carried away.

Concentration of pollutants reduces rapidly due to dilution.

Greater the speed greater the dilution and carried away

At lower speed of wind, pollutants tend to be concentrated

near the area of discharge.

And longer the period of such wind greater will be the

concentration of pollutant

In rough terrains, it cannot be assumed that speed & direction

will subsequently govern the motion of contaminates

Turbulence

Not always completely

understood

Two types:

Atmospheric heating

Causes natural convection

currents

Thermal eddies

Mechanical turbulence

Results from shear wind effects

Result from air movement over

the earth’s surface, influenced

by location of buildings and

relative roughness of terrain.

Temperature

With increase in the height, temp of the normal air decreases above earth

surface (in Troposphere)

The rate at which temp decreases with increase in height is termed as

Lapse Rate or Environment Lapse Rate [ELR] {6.50C/KM}

The rate at which temp of a parcel of air pollutant decreases with increase

in height above earth surface is termed as Adiabatic Lapse Rate [ALR]

(as this change of temp takes place without exchange of any heat in the system)

It is taken as

6.00C/KM for Wet process

9.80C/KM for Dry process

Lapse Rate

Lapse rate is the negative of temperature gradient

( dT/dz) Dry adiabatic lapse rate Γ = - 1°C/100m

Change in Temp. with change in height

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

What is adiabatic cooling?Why does air cool when it rises?

When air rises it encounters lower pressure Momentarily, as the air

parcel rises it has a higher pressure than the surrounding

molecules.

This mean that there is more force exerted by the moleculesinside the parcel that is being exerted by the molecules outside

the parcel and the parcel expands.

This means there is more force exerted

from inside the parcel than there is from the outside and the parcel expands.

In order for the parcel to expand it has to push

away (displace) the surrounding molecules. Thus,

the molecules inside the parcel must use some of

their internal energy in order to do this work.

Since the temperature is a function of the internal

energy, when the internal energy decreases, then

the temperature decreases.

Thus, the rising parcel expands and cools.

This process is called adiabatic cooling.

Vertical Temperature Profiles

Environmental lapse rate (ELR)

Adiabatic lapse rate (ALR)

If,

ELR < ALR : sub adiabatic

condition, atmosphere is stable.

ELR << ALR : Inversion conditions.

Very stable atmosphere.ELR= ALR : atmosphere is neutral.

ELR> ALR : super adiabatic

condition, atmosphere is unstable.

Shapes of plumes depends upon atmospheric stability conditions.

Super Adiabatic Environment /

Unstable Environment (ELR>ALR)

ELR> ALR = super adiabatic condition, atmosphere

is unstable

In this type of system in which pollutant always

remains warmer than the air surrounding it.

Hence rising air pollutant keeps on rising into the

atmosphere (which may cause destruction to

stratosphere)

ELR in this type of system is termed as Super

Adiabatic Lapse Rate

Sub-Adiabatic Environment /

Stable Environment (ELR<ALR)

ELR< ALR = sub adiabatic condition, atmosphere is

stable

In this type of system pollutant doesn’t reach upto

stratosphere because it is stable system.

ELR in this type of system is termed as Sub-

Adiabatic Lapse Rate

Neutral Environment (ELR=ALR)

Here ELR=ALR

Inversion

If temp above surface of the earth increases with

increase in height, this condition is termed as Inversion

condition

The rate at which temp increases with increase in height

is called Negative lapse rate

Inversion is generally found when earth cools rapidly

compared to air surrounding it.

Inverse also exist if high pressure region is surrounded by

low pressure region

ELR << ALR= Inversion conditions. Very stable

atmosphere.

Adiabatic lapse rateEnvironmental lapse rate

Problem 1:

Determine whether the atmosphere is

unstable, neutral or stable for the following

case.

• Initial Temperature = 125 0C

• Final Temperature = - 65 0C

• Initial Height = 200 m

• Final Height = 21000 m

Solution:

Since no wind direction information is given, the stability is determined

from the environmental temperature gradient dT/dz.

• dT/dZ = (125-(-65))/(200 – 21000)

= 190/(-20800)

= -0.9140C/100m

=-9.14 0C/km

The atmosphere is slightly stable – near neutral condition .

Problem 2:

Z(m) T(ºC)

2 -3.05

318 -6.21

Solution:

Problem 3:

Z(m) T(ºC)

10 5.11

202 1.09

Solution:

Problem 4:

Z(m) T(ºC)

18 14.03

286 12.56

Solution:

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 affect plume dispersion differently

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

Super adiabatic 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 environment

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 weakly stable

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

High stacks are needed

Lofting Plume

Favorable in the sense that fewer

impacts at ground level.

Light winds & light turbulence

Pollutants go up into environment.

They are created when

atmospheric conditions are

unstable above the plume and

stable below.

Fumigation Plume

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.

Neutral Plume

It tend to rise vertically until it

reaches air density similar to that

of plume itself.

It is often converted to coning if

wind velocity is grater than 10

m/sec and when cloud cover

blocks solar radiation.

Trapping Plume

This type of plume is observed

when inversion layer exist over and

under the super adiabatic

environment

In this case, pollutant are trapped

in between the two inversion

layers. Hence, it is termed as

trapped plume.

Dispersion from Point Sources

Pollutants emitted in plume form

Impact on air quality depends on dispersion, which depends on

the height of plume

Plume rise affects transport

Effects maximum ground level concentrations (MGLCs)

Effects distance of MGLCs

Effects of Terrain on the Plume Pattern

Impact of Building and Stack Location

Impact of Stack Height: Stack Upwind of Building

Impact of Stack Height: Building Supported Stack

Impact of Stack Height: Stack Downwind of

Building

Plume Affected by Natural Terrain Irregularity

Plume Near Very Large Obstacle

Wind Rose Diagram

The wind rose is the time honored method of graphically

presenting the wind conditions, direction and speed, over a

period of time at a specific location.

Accurate estimation of the dispersion of pollutants in the

atmosphere require a knowledge of the frequency distribution

of wind directions as well as wind speed.

To create a wind rose, average wind direction and wind speed

values are collected at a site, at short intervals, over a period

of time, e.g. 1 week, 1 month, 1 year or longer.

The collected wind data is then sorted by wind direction so

that the percentage of time that the wind was blowing from

each direction can be determined.

Wind Rose Diagram

Presented in a circular format, the wind rose shows the

frequency of winds blowing from particular directions over a

particular period.

The length of each "spoke or segment" around the circle is

related to the frequency that the wind blows from a particular

direction per unit time.

Each concentric circle represents a different frequency, starting

from zero at the center to increasing frequencies at the outer

circles.

A wind rose plot may contain additional information, in that

each spoke/segment is broken down into color-coded bands

that show wind speed ranges.

Wind Rose Diagram

Wind roses typically use 16 cardinal directions, such as north (N),

NE, etc., although they may be subdivided into as many as 32

directions.

In terms of angle measurement in degrees, North corresponds to

0°/360°, East to 90°, South to 180° and West to 270°.

Uses of wind rose

1. Sailors use wind rose.

2. Architects do, or should, use wind

rose information for the siting of

buildings and stadiums.

3. Wind-power "farms" (wind mills) do

extensive wind rose type studies

prior to erecting their wind

turbines.

4. Also used for siting of industries in

order to minimize impact of air

pollution on neighboring cities

5. Runway design

Wind Velocity Profile

Friction retards wind movement

Friction is proportional to surface roughness

Location and size of surface objects produce different

wind velocity gradients in the vertical direction

Area of atmosphere influenced by friction- few hundred m

to several km above earth’s surface

Wind speed varies by height

International standard height for wind-speed

measurements is 10 m

Dispersion of pollutant is a function of wind speed at the

height where pollution is emitted

But difficult to develop relationship between height and

wind speed

Wind Velocity Profile

Maximum height

of wind profiles

indicate where

effects of

surface

roughness end

and where

gradient windbegins

Calculating Effective Stack Height Wind Speed: Deacons’ power law

The surface wind speed is different at the effective stack height,

which is used in the Gaussian model. At elevation z1, wind speed u1 is given by the following equation and table.

Where:

u = wind speed at altitude z (m/s)

z1 = effective stack height or altitude at which wind speed is

needed, m

z= height above surface at which wind speed is measured, m

(usually 10 meters)

p = exponent is a function of atmospheric stability class

Value of p for Deacons power law irrespective of type of terrain

Stability Classes

Developed for use in dispersion models

Stability classified into 6 classes (A – F)

A: strongly unstable – Large lapse rates

B: moderately unstable

C: slightly unstable

D: neutral- Less or zero lapse rate

E: slightly stable – mild inversion

F: moderately stable – moderate to severe

inversion

Problems

1. The wind speed at 10 m altitude is 5m/s. Find the wind speed at 250 m

altitude for the following stability conditions.

A. Large lapse rate

B. Zero or small lapse rate

C. Moderate inversion

D. Large inversion

2. Wind speed at 10 m altitude is 2m/s. Find the wind speed at 200 m

altitude for following cases

A. Large lapse rate

B. Zero or small lapse rate

C. Moderate inversion

D. Large inversion

Maximum Mixing Depth (MMD)

The dispersion of pollutants in the lower atmosphere is greatly

aided by the convective and turbulent mixing that takes

place.

The vertical extent to which this mixing takes place depends

on the environmental lapse rate which varies diurnally, from

season to season and is also affected by topographical

features.

The depth of the convective mixing layer in which vertical movement of pollutants is possible, is called the maximum

mixing depth (MMD).

The maximum mixing depth (sometimes called the mixing

height) is obtained by projecting the dry adiabatic lapse rate

line to the point of intersection with the atmospheric

temperature profile

These profiles are usually measured at night or early in the

morning.

An air parcel at a temperature warmer than the existing

ground level temperature rises and cools according to

adiabatic lapse rate.

The level where its temperature becomes equal to the

surrounding air gives the MMD value.

Urban air pollution episodes are known to occur when MMD is

1500 m or less.

Summary….of MMD

Parcel heated by solar radiation at earth’s surface

Rises until temperature T’ = T

Where, T’ = particle’s temp

T = atmospheric temp

Achieves neutral equilibrium, no tendency for

further upward motion

Determination of Maximum Mixing Height

The following steps can be used to determine the

maximum mixing height for a day from a temperature

profile:

1. Plot the temperature profile (ELR), if needed.

2. Plot the maximum surface temperature for the day on the

graph for morning temperature profile.

3. Draw a dry adiabatic line (-10C/100m) from the point of

maximum surface temperature to the point where it interests

the morning temperature profile.

4. Read the corresponding height above ground at the point of

intersection obtained in step 2. This is the maximum mixing

height for the day.

Practice problem (MMD)

Calculate the maximum mixing height from the

following early morning temperature data given

below:

The maximum surface temperature for the day was

15oC.

(Hint:- Follow steps given in previous slide. From tabular

values plot ELR and 150 C is air parcel temp in contact

with surface apply DALR to it)

1.

Height (m) 0 250 350 450 550 650

Temperature (0C) 9.5 12.2 15.1 15.6 16.2 16.5

Radiosonde

A radiosonde (Sonde is French for probe) is a

unit for use in weather balloons that measures

various atmospheric parameters and transmits

them to a fixed receiver.

Radiosondes may operate at a radio

frequency of 403 MHz or 1680 MHz and both

types may be adjusted slightly higher or lower

as required.

A rawinsonde is a radiosonde that is designed to only measure wind speed and direction.

Rawinsondes are usually referred to as

Radiosondes.