deals with temperature, density, pressure, winds and humidity parameters of the atmosphere; Prssure gradient force, coriolis force, gravity force and friction force and winds and currents, ; pressure lows and highs, atmospheric circulation, winds.

1. Atmospheric Pressure and
Winds
Dr. Akepati S. Reddy
Professor, School of Energy & Environment
Thapar University, Patiala – 147 001
Punjab (INDIA)

2. Air Density, Temperature and Pressure
Air or atmospheric density
• Mass per unit volume (kg/m3) and indicated by ‘ρ’
• Mass subjected to gravity results in weight (same mass has different
weights depending on varying gravity of different planets
• At MSL, air density is 1.2 kg/m3 and it decreases with altitude
• Air density is pressure and temperature dependent
• At the same temperature and pressure conditions, density of the
moist air is lower than that of dry air – Why?
Air or atmospheric temperature
• Temperature is a measure of average speed of moving molecules
(kinetic energy)
• Measured in Kelvin scale (K) – at 0 K, there is no kinetic energy
• Temperature increases with increasing air density
• Atmospheric temperature is affected by
– Short-wave radiation from above and long-wave from below - further,
sensible and latent heat received from below (earth surface)
– Varying radiation absorption/emission properties of the atmosphere

3. Air Density, Temperature and Pressure
Atmospheric temperature
• Atmosphere has a distinctive temperature profile
• Temperature profile of the lower atmosphere is very variable (due
to the variable heating and cooling by earth surface (sensible and
latent heat, and radiation)
• Average temperature of the Earth is 288 K – in troposphere it
decreases with altitude (average vertical lapse rate is 6.6°C/km)
Atmospheric pressure
• Pressure is force per unit area – force is mass multiplied by
acceleration (kg.m/sec.2) – acceleration is change in velocity
through time (m/sec./sec. or m/sce2)
• Pressure units: N/m2 or Pa; Bars/millibars (1 mb = 100 Pa) –at MSL
pressure is 1013 mb or 101.3 kPa or 10130 kg air over one m2 area
• Atmospheric pressure decreases with altitude (700 mb at 3 km, 500
at 5.5 and 300 at 10) – mass of the overlying air reduces with height

4. Density, Temperature and Pressure, and Winds
Atmospheric pressure
• Atmospheric pressure also varies horizontally on the earth surface
(results from unequal heating of the earth surface)
– Recorded highest & lowest sea level pressures: 1084 mb and 870 mb
(typical range is 950 mb to 1050 mb)
– Surface pressure tendency over the fast several hours is useful in local
short range weather predictions
• Pressure (scalar quantity) is exerted in all directions – still an air
parcel is in equilibrium (pressure exerted by it is balanced by the
force -gravity pull- exerted by overlying air: hydrostatic equilibrium)
• Air parcels with pressures different from the surroundings, have
disturbed hydrostatic equilibria, and move in the atmosphere
• Pressure gradient force is responsible winds
– Despite very large vertical pressure gradients, because of the
hydrostatic equilibrium, vertical movement of air is very limited –
Vertical pressure gradient force operates opposite to gravity force
– Hydrostatic equilibrium or balance is disturbed in case of convection
currents and thunderstorms
– Horizontal pressure gradient on the other hand causes winds, though
it is many times lower than the vertical pressure gradient

5. Density, Temperature and Pressure, and Winds
Winds
• Movement of wind is due to the pressure gradient force from high
pressure region to low pressure region
• Divisible into surface winds and aloft or upper atmosphere winds
and also into vertical currents
• Winds carry and transport heat, moisture and pollutants, and wind
create conditions for clouds formation/dissipation and precipitation
• Wind is a vector quantity and has both speed and direction
components
– Increasing PGF (closer spacing of isobars) increases wind speed
– Wind speed is influenced by friction force (slows down the wind)
– Wind is named after from where it is blowing (west wind: wind coming
from the west)
– Wind direction (indicated in degrees in the clockwise direction from
the north) is influenced by both friction and coriolis effect

6. Density, Temperature and Pressure, and Winds
Winds
• General wind pattern of the Earth ( also called general circulation,
global circulation, or primary circulation) includes three circulation
cells (Hadley cells, Ferrel cells and Polar cells)
– Trade winds, Westerlies, and Polar Easterlies
– Geostrophic winds, and Jet streams
• Secondary circulation winds: Regional scale winds
– Monsoon winds ?
• Tertiary circulation winds: local winds (upto 100 km distance)
– Sea level breezes and mountain and valley breezes
• Wind is an important renewable energy source (indirect solar
energy)
– Wind speed matters a for wind energy (energy potential is
proportional to the cube of wind speed)
– Consistent winds are preferred (variability is not desirable)

7. Due to compressibility of air, atmospheric pressure decreases faster
near the surface but less so aloft

8. International Standard atmosphere
Pressure: 1013.25 hPa
Density: 1.225 kg/m3
Temperature: 288.15 K
Acceleration due to gravity: 9.80665m/sec.2
Speed of sound: 340.294 m/sec.

9. Measurement/Monitoring and Analysis
• Temperature
– Bimetallic thermometers (differential expansion of two different metals
by temperature is the basis of measurement)
– Electronic thermometers (elec. resistance changes with temp.)
• Pressure
– Mercury barometer (Evangelista Torricelli, 1643)
– Aneroid barometer – no liquid is used – air pressure deforms an
evacuated chamber and this is the basis of measurement
• Winds
– Wind velocity is measured by Anemometer
– Wind direction by Wind Vane (aerovanes for both speed and direction)
• Humidity (dew point monitoring!)
– Dew point: Temp. to which air needs cooling for saturation moisture
– Humidity is measured by hygrometer (Filamentous hygrometers – hair
expands/contracts with humidity variation; Electrical hygrometers –
chemicals absorb moisture and change resistance)
– Sling psychrometer (measures cooling by evaporation) and Wet bulb
and dry bulb thermometers

11. Measurement/Monitoring and Analysis
• Monitored at the surface and at higher altitudes and use
– Surface weather stations
– Doppler radar (detects precipitation types and amounts , and
measures wind velocities)
– Radiosondes
• Package of instruments (thermometer, barometer, hygrometer and
transmitter)
• Launched twice daily (at 0000 and 1200 Universal Time Coordinate
on balloons from earth stations
• As the balloon ascends, temperature, dew point and wind are
measured and reported as a function of pressure (radiosonde
telemetry)
– Geostationary satellites and aeroplanes
• Data analysis and calculations for the parameters through indirect
measurements

12. Measurement/Monitoring and Analysis
• Wind roses from wind data analysis
– Wind speed, direction and frequency are pictorially presented
– How to construct wind roses and how to read them?
– Wind energy potential assessment
• Theoretically available power of a wind is expressed as
– Density of air decreases with temperature and altitude
– Wind velocity is the major factor in power generation (20% increase in
velocity increases power output by 73%)
• Potential temperature: Temperature that a parcel of air (at pressure
P) would acquire if adiabatically brought to a standard pressure P0
– It is denoted by θ and given by
P = 1/2 ρ A v3
P = power (W)
ρ = density of air (kg/m3)
A = area perpendicular to the wind (m2)
v = wind velocity (m/s)
T is current temperature (in K) of air parcel
R is the gas constant of air
Cp is specific heat capacity at constant pressure
R/ Cp for air is 0.286

14. Atmospheric Pressure Gradient Force
• Horizontal pressure differences are mapped in the Average Sea
Level Pressure Charts (constant height charts) using isobars
– Pressure differences in the upper atmosphere are mapped in the
Constant Pressure Charts using iso-hypse (iso-heights)
• Iso-hypse gradient and horizontal pressure gradient influence the
speed of surface winds and upper atmospheric winds respectively
• Atmospheric pressure patterns are controlled by
– Temperature changes (thermal air pressure lows and highs)
– Earth rotation also creates air pressure systems (dynamic air pressure
lows and highs)
• Moving air masses (winds) affect changes in atmospheric
pressures
• Temperature changes can be
– Latitudinal (high temperature at equator and decreasing temperature
with higher latitudes
– Land and ocean surfaces (land surfaces rapidly heated and rapidly
cooled when compared to oceans)
– Elevation/Topography of the surface

15. January July
Isobars and Mean Sea Level Pressure Maps
Depicts how pressure changes while holding the height
constant
Good weather analysis tool
Surface/station pressure is reduced to sea level and
depicted by isobars (lines of equal MSLP) – station
pressure is adjusted for elevation to obtain the SLP
Wind speed is proportional to distance between isobars
These maps show low and high pressure centres, and troughs and ridges
Troughs: curved isobars forming elongated regions of low pressure
Ridges: curved isobars forming elongated regions of high pressure
 
ratiopressuretotaltopressureourwater vapis
61.01Texp v
10
10
rhere
rThere
TR
zg
PP
vd

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

17. Isohypse (Isoheights) Constant Pressure Charts
Pressure is held constant.
Used to describe upper air
conditions.
Prepared twice a day at
0000 and 1200 UTC for
several mandatory
pressure levels (925, 850,
700, 500 mb, etc.)
Temperature, humidity and
wind data required is
provided by radiosonde -
data is supplemented from
other sources (aircraft and
satellites)
Forecast data is also
depicted on constant
pressure charts

19. 2211PP
ConstantPVor
1
VPV
V
P



Pressure of a given mass of an ideal gas
is inversely proportional to its volume at
a constant temperature
Boyle’s law
Avogadro number and molar volume
The number of elementary particles (molecules and/or atoms) per mole
of a substance (6.022×10 23 mol -1) – it is expressed by the symbol NA
Molar volume: volume occupied by one mole of ideal gas – Its value at
STP is 22.414 L/mol and at NTP is 22.414 L/mol - it is same for all the
gases or mixture of gases
Volume of an ideal gas at constant
pressure is directly proportional to the
absolute temperature.
V1 = original volume and V2 = new volume
T1 = original temperature and T2 = new temp.
Charle’s law
2
2
1
1
T
V
T
1
Constant
T
V
or
T
V
TV




2
2
1
1
T
P
Constant
T
P
or
T
P
TP Gay-Lussac’s law

20. Ideal gas law for dry air
Ideal Gas Law expresses the relation between pressure, temperature
and volume or density in an ideal or perfect gas – expressed as
P V = n Ru T or p V = m R T or P = (m / V) R T or P = ρ R T
Ru (universal gas constant) = 8314.47 (J/kmol/K) n is number of moles
R (individual gas constant)(R = Ru / Mgas) = 8314/29 = 287 J/Kg/K
V = volume of gas (m3) p = absolute pressure (N/m2, Pa)
m = mass of gas (kg) T = absolute temperature (K)
ρ = density (kg/m3) (ρ = m / V) Mgas = molecular weight of the gas
Ideal gas law for moist air
• Daltons Law states that the total pressure exerted by a mixture of
gases is the sum of the partial pressures of the individual gases
pt = pa + pw pt is total pressure
pa is dry air partial pressure pw is water vapour partial pressure
• Dry Air Partial Pressure is pa = ρa (286.9 J/kg K) T
• Water Vapor Partial Pressure is pw = ρw (461.5 J/kg K) T
• For moist the ideal gas law can be written as P = ρ R Tv
Here R should be for moist air rather than for dry air – Instead correction is
made to temperature, T (as virtual temperature,Tv) Tv = T (1 + 0.61 r)
Here r is volume ratio of water vapour in the moist air

21. Pressure Gradient Force and Wind Systems
• Consider a warm column of air and a cold column of air separated
by 3000 kM distance, and assume
– 1005 mb pressure at near sea level and 600 mb pressure at 5500 m
altitude for the warm air column
– 1020 mb pressure at near sea level and 400 mb pressure at 5500 m
altitude for the cold air column
– 500 mb pressure is measured at 5880 m altitude for worm column and
at 4800 m altitude for cold column
• Difference in pressure at 5500 m altitude between the two columns
will initiate horizontal flow of air from warm column to cold column
– Height gradient indicates the magnitude of force causing the air
movement aloft
– Iso-heights for the warm column and the cold column can be used in
estimating the height gradient
• Similarly at the sea level, horizontal flow of air from the cold
column to the warm column is initiated
– Iso-bars between the warm air column and the cold air column can be
used in finding the pressure gradient
– Sea level pressure maps can be used for computing the HPGF

22. Horizontal pressure gradient between the two
columns is
The height gradient (difference in height of a
particular pressure value, 500 mb, for upper
altitudes) between the two columns is
mb/kM005.0
kM3000
)10051020(



mb
HPGF
m/kM36.0
kM3000
)48005880(
gradientHeight 


m
Higher pressure is usually associated with fair
weather and clear skies, and lower pressure
with storms (tornadoes and hurricanes)
Pressure Gradient Force and Wind Systems
Vertical air movement of the air is the result of
net force of vertical pressure gradient force
and the vertical gravity force (acts opposite to
the VPGF – disturbed hydrostatic equilibrium)
Vertical motion of air masses produce winds
ZgP  Hydrostatic equation:

24. Low Pressure & High Pressure Wind Systems
• Atmospheric pressure patterns are controlled by
– Temperature changes created by differential heating (thermal air
pressure lows and highs)
– Dynamic air pressure lows and highs created by upper level winds and
earth rotation
• Temperature changes can be
– Latitudinal (high temperature at equator and low at higher latitudes
– Land and ocean surfaces (land surfaces are rapidly heated and rapidly
cooled when compared with ocean surfaces)
• The horizontal pressure gradients on the earth surface and the
height gradients of the upper atmosphere are highly dynamic
– Average Sea Level Pressure charts and Constant Pressure Charts are
used to show the dynamism
• Wind movement on the surface, in the upper atmosphere and
vertical movement of winds influence the pressure gradients
– Surface low pressure wind systems are maintained or intensified by
the divergence aloft of air

25. Colder earth surfaces on the other
hand develop cold air columns –
Characterized by surface thermal
highs and by upper altitude lows, and
by surface divergence and upper
altitude convergence of air
Warmer earth surfaces through
heating develop warm air columns
– Characterized by surface thermal
lows and by upper altitude highs
and by surface convergence and
upper altitude divergence of air
Surface high pressure
Cool sinking air
Surface low pressure
Warm rising air

26. Coriolis Force and Friction Force,
and Wind Systems
Winds created by the pressure gradient force are modified by
Coriolis force and friction force
Coriolis force
• Proportional to the speed of wind and varies with altitude (because
of friction force) and latitude
– Coriolis force changes only the wind direction but not speed (the level
of coriolis deflection is influenced by the wind speed)
– The amount of Coriolis deflection increases with latitude (zero at
equator and the maximum at poles)
– It acts at right angles (towards right in northern hemisphere and
towards left in southern hemisphere) to the direction of wind
• Moves air counter-clock-wise around a low pressure system and
clock-wise around a high pressure system in northern hemisphere

28. Wind Systems of Atmospheric
Lows and Highs
In the northern hemisphere
– Air moves out (diverges) from a
surface high in the clockwise
direction and it moves in (converges)
into the surface low in the counter-
clockwise direction due to Coriolis
effect and friction
– In the higher atmosphere, with no
friction and with only pressure
gradient force and Coriolis effect in
action, air masses (winds or
geostrophic winds) run parallel to
isotherms (clockwise direction
around the highs, and counter-
clockwise direction around the lows)
In the southern hemisphere the wind
direction of convergence and
divergence is opposite to that of the
northern hemisphere

29. Coriolis Force and Friction Force,
and Wind Systems
Friction force
• It is Earth’s surface drag and it is limited to planetary boundary layer
(1 to 3 kM) – reduces with altitude – surface roughness increases
• Experienced by surface winds (through slowing down the wind
speeds it is reduces the Coriolis deflection)
In the upper atmosphere beyond the boundary layer, friction force is
negligible and Coriolis force is exactly equal and opposite to PGF
• Wind moves parallel to the isobars at constant speed (these winds
are called geostrophic winds)
– The air masses run parallel to the isobars (clockwise direction around
the highs, and counter-clockwise direction around the lows)
– Sub-geostrophic flow occurs around low pressure centres and super-
geostrophic flow occurs around high pressure centres
• Gradient winds: winds that blow at constant speed parallel to the
curved isobars (around highs/lows of the upper atmosphere)

30. Global Pressure Patterns and General Circulation
• Temperature difference between the equator and the poles
generates global general circulation
– Thermal lows at the equator and thermal highs at the poles are
generated
– Large scale vertical air movement generates pressure differences
across the Earth and assist in the development of surface winds
– 60% of the heat energy redistribution is by the atmospheric circulation)
• For an ideal earth (non-rotating and all oceanic earth) a single
circulation cell is expected
– Air raises near the equator, moves towards the poles in the upper
atmosphere, descends near the poles and moves towards the equator
on the earth surface
• What makes global air circulation very complicated?
– Latitude (radiation received varies)
– Earth’s rotation and tilt of the earth’s rotational axis (seasonality)
– Positions of continents and oceans - northern hemisphere has more
land than the southern hemisphere
– Altitude and roughness of the earth surface
– Cloud cover (during daytime clouds reflect radiation and during nights
clouds prevent escape of radiation

31. Global Pressure Patterns and General Circulation
• Due to the rotation, instead of a single cell, three cells of large-scale
circulation (of rising and descending air) are existing
– Hadley cells, Ferrel cells and Polar cells
• Continents gain and loose heat much more quickly than oceans
– Become warmer during day time and colder during nights (land-sea
breezes)
– Become much warmer in summers and much colder in winters - low
pressure cyclones are developed during summer and high pressure
anticyclones during winters - (monsoons!)
– Coastal land areas stay warmer in winter and cooler in summer
– Because of the land-sea distribution differences, mid-latitude cyclonic
depressions are rapidly developed in the northern hemisphere oceans

32. Atmospheric pressure patterns
Thermal highs and lows
• Equatorial thermal lows at 5 N to 5 S (intertropical convergence
zone –ITCC)
• Polar thermal highs at 90 N and S: Characterized by descending air
• Monsoon lows
• Highs and lows associated with land-sea breezes and Mountain-
valley breezes
Rotation induced or dynamic air lows and highs
• Earth rotation causes accumulation of air at certain latitudes (highs)
and air divergence at other latitudes (lows)
• Sub-tropical highs (at 25 to 40 N and S) characterized by descending
dry air and clear skies
• Sub-polar lows (at 55 to 70 N and S) characterized by ascending air
and storm centers (warm air from low latitudes is lifted up by the
cold polar air)

33. Global Pressure Patterns & Climate Zones
Tropical climate:
• Intertropical convergence zone (ITCC)
• Equatorial thermal lows near the equator (between 5 N to 5 S)
• Characterized by high sun angles, long days, high surface
temperatures, ascending air, heavy precipitation, cloud cover and
thunder storms
• This zone shifts to the north of the equator in summer and to the
south in winter
Subtropical climate:
• This zone is between 25-40° N and between 25-40 S latitudes
• Air rising at the equator spreads out, cools, and descends here
• This zone is associated with clear skies, low rainfall, and high day
time temperatures (>40°C)
• Many of the hot deserts are found here
• This zone expands towards higher latitudes during summer

34. Global Pressure Patterns & Climate Zones
Temporate climate:
• Between 50-60 N and between 50-60 S latitudes
• Some of the descending air of subtropical zone (warm surface air)
and the cold surface air from the poles moves towards this zone
• The warm surface air and the cold polar collide in this zone, and rises
up, developing a low pressure zone
• This zone is cyclonic in nature, and is associated with the
development of frontal depressions (more dominant in winters)
• Sometimes, during summers, the subtropical highs expand into, and
the temperate zone experience calming influence on weather
Polar zone of climate:
• Polar thermal highs at 90 N and S
• Characterized by very low temperatures, descending and heavy air,
and creation of highs
• This zone has permanent, thick snow and ice cover
• This zone can be as dry as hot deserts of subtropical climate zone

35. Winds and Currents
• Horizontal motion of air is considered as wind and vertical motion
as current
• Wind is a constituent of weather and wind is also a determinant of
other elements of the weather (temperature and precipitation)
• Air movement is because of the interacting forces imbalances
– Interacting forces include real forces (horizontal & vertical PGFs,
gravity force and friction force) and apparent forces (Coriolis force)
– Air movement is meant for the balancing or equilibriating the forces
– Since the factors causing the force imbalances are constantly
changing, the equilibriating process is an unending process
• Vertical movement of air occurs only when gravity is not balanced
by VPGF (at lows and highs)
– Air moves up when the vertical pressure gradient force is stronger
then the gravity force, and moves down when it is weaker
• Moving winds experience no Coriolis deflection at the equator and
the deflection increases with latitude (the maximum at poles)
• Winds in the upper atmosphere experience no surface drag
(frictional force) and hence have higher wind speeds (geostrophic)

36. Winds and Currents
Surface winds
• Intertropical convergence zone (ITCZ): the equatorial (between 5 N
and 5 S) belt of variable winds and calm (also known as doldrums)
• Trade winds: North-East and South-East winds seen at 5 to 25 N and
S latitudes; the winds are steady and persistant
• Horse latitudes: subtropical belt of variable winds and calm; seen in
30 to 35 N and S latitudes
• Weterlies: seen at 35 to 60 N and S latitudes; these winds are
neither persistent nor steady.
• Polar front: seen in 60 to 65 N and S latitudes; it is a zone ofconflict
among different air masses – the boundary between the polar
easterlies and the westerly winds of the mid latitudes – it separates
the cold polar air from the warm temperate air
• Polar easterlies: seen at 65 to 80 N and S latitudes; more prevalent
in the southern hemisphere than in the northern hemisphere
• Polar zone: seen in 80 to 90 N and S latitudes; variable winds and
calm are characteristics

37. Winds and Currents
Winds aloft
• Surface lows/highs have matching upper atmosphere highs/lows
• In the upper atmosphere air moves under the influence of PGF and
Coriolis force and parallel to the isobars
• Upper atmosphere has westerly jet streams - Rivers of extremely
high speed winds - occurs in the zones of strong temperature
contrasts – polar and sub-tropical jet streams
Winds of the upper atmosphere
• Upper level westerlies: seen between 25-90 latitudes
• Tropical high pressure belt (5 to 20 N and S latitudes)
• Equatorial easterlies
Jet streams:
• Polar jet: moves from west to east at ~10 kM altitude – when the
cold air from poles meets the warm air from the other side, strong
temperature and pressure gradient is developed - seasonally shifts
• Subtropical jet (westerly jet) – located at 13 kM altitude above the
subtropical high zone – this jet is relatively weaker because of the
weeker latitudinal temperature and pressure gradients

38. Regional Winds and Local winds
Regional winds
• Monsoons
Local winds are caused by contrasts in heating of the
atmosphere at the surface
• Water and land, at coastlines of seas and lakes
• Urban and rural areas
• Vegetated and unvegetated areas
• Wet and dry areas
• Snow-covered ground and bare ground
Day time sea breeze Night time land breeze

39. Summer monsoon