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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.