Topic                           The Atmosphere and its Parameters


Learning Objectives             The cadet shall be to:-

  1. Explain the composition of the earth’s atmosphere.
  2. Explain the sequence and the extent of each layer.
  3. Know significance of troposphere and stratosphere.
  4. Explain atmospheric pressure with special reference to mb, hpa & Kg/cm^2.
  5. Explain atmospheric temperature.


Critical Knowledge

  1. Sequence and extent of each layer.
  2. Concept of DALR & SALR
  3. Relative Humidity & Dew Point


Atmosphere and its composition

  • It is a 800-Kms thick air envelope surrounding and bound to it because of earth’s gravitational attraction.
  • Compared to the size of the earth, this air envelope is indeed small. It is akin to a tissue paper enveloping an orange.
  • It not only supports but also protects all life on the earth from harmful solar radiation either by reflecting them back or by absorbing them.
  • Atmosphere is a mixture of oxygen (21%), nitrogen (78%), carbon dioxide (0.037%), other gases like hydrogen, helium, argon, neon, krypton, xenon and ozone and their compounds like halocarbons and fluorocarbons and a miniscule portion of water vapour and dust particles and impurities.
  • In the school in chemistry class, little importance was given to water vapour adn dust particles.
  • But in meteorology, water vapor is a critical constituent of atmosphere because water vapor is the powerhouse of weather.
  • Dust particles is an important constituent which provides essential nucleus for formation of rains, fog and other condensates.


Geometry of Atmosphere

  • Consult Diagram 1 in the accompanying document 04 Atmosphere Diagrams.docx
  • Troposphere (upto 8 at poles, upto 12km over UK & 16 km at equator). (Students find it difficult to remember the extent of troposphere at the poles and the equator. Use this memory aid. When water is churning, there is a dip on the axis of rotation. Here imagine the earth churning around it axis with a dip at the poles)
  • The height of the troposphere decreases in winter and increases in the summer.
  • Tropopause separates Troposphere and Stratospheres.
  • Stratosphere (16 to 50Km). Ozonosphere between 20 to 35 km height.
  • Stratopause, at 47km separates Stratosphere and Mesosphere.
  • Mesosphere (50 to 80 km)
  • Mesopause at 80km separates Mesosphere and Ionosphere.
  • Ionosphere (80 to >200 km)‏



Importance of Troposphere & Ionosphere

For mariners, the troposphere and the ionosphere are more relevant because

  • Troposphere
    • Around 90% of the atmosphere by weight lies in the troposphere
    • Clear skies and weather above 16km.
    • Nearly all water vapors is confined to the troposphere and hence majority of weather phenomena are confined to the Troposphere.
    • To be more exact, many of the world’s weather systems first begin developing. between about 02 and 08 km above the Earth’s surface.
  • Ionosphere
    • In the Ionosphere, ionized gases affect EM wave propagation used in Radio and Radars.
    • During solar flares magnetic compass and radio communications are badly affected.


There are a number of atmospheric parameters, but the for mariners, the important ones are Atmospheric Pressure, the Atmospheric temperature and the humidity.

Atmospheric Pressure

  • Consult Diagram 2 in the accompanying document 04 Atmosphere Diagrams.docx
  • Atmospheric pressure is the weight per unit area of the entire 800 km high air column above the surface of the earth.
  • Weight of this column is a Force which is = m * a

Mass of the air in the column (m) *   Earths Gravity of 9.81m/sec2 (a)‏


Concept of Mercury barometer

  • Consult Diagram 3 in the accompanying document 04 Atmosphere Diagrams.docx
  • The pressure exerted by the Mercury column in the tube on the surface of the Mercury in the trough will balance the pressure exerted by the atmosphere on the same surface of mercury in the trough.
  • If the atmospheric pressure decreases, the mercury column will fall.
  • If the atmospheric pressure increases, the mercury column will rise.


Measurement of Atmospheric Pressure

  • Pressure is defined as Force per Unit area and SI measure in physics is Newton per square meter
  • In the Air Conditioning and Pump Industry, the unit of measurement for pressure is 1 Bar.
  • But this unit is too big to measure atmospheric pressure – It is like measuring your height in kilometres or your weight in tons.
  • Meteorologists use a smaller unit of Measurements called millibar (mb), where 1Bar = 1000 mb.
  • Now “mb” has been replaced by a hectoPascal (hPa).
  • And therefore 1 Bar = 1000 mb = 1000 hPa .


Comparison between various units is as follows:-

  • One bar = 100,00 Newton/meter^2 .
  • So 1 mb = 1hPa = 100 Newtons / meter^2
  • 1000 hPa = 750.1 mm of Mercury column
  • 1000 hPa is also = 1.02kg per cm^2 = 10.2 t /m^2


Standard Atmospheric Pressure

  • Atmospheric pressure changes all the time.
  • It could be more than 1000hPa or less than 1000hPa.
  • After statistical analysis, it has been established that Standard Atmospheric pressure on earth’s surface (which is called Main Sea Level) is
    • = 760mm of Hg column
    • = 1013.25 hPa =
    • = 1.033 kg per cm^2
  • One atmosphere (101.325 kPa or 14.7 psi) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.


Concept of HP and LP

  • LP and HP are relative terms and not absolute ones.
  • Atmospheric pressure will change with space and time.
  • For example, At MSL, mean air pressure is usually 1013.2 hPa.
  • But In space, a few meters above MSL, it will be less (LP)
  • Or at night time it will be more (HP) than day.
  • Equator is normally considered as a Low Pressure Belt called Equatorial Low. .
  • But the Equatorial low (1012hPa) becomes a High Pressure (HP) area in relation to the Thar desert and causes SW monsoons.
  • A Factoid – The highest barometric pressure ever recorded on Earth was 32.31 inches (109.4kPa), measured in Agata, U.S.S.R., on December 31, 1968. Agata is located in northern Siberia. The temperatures was between -40° and -58°.
  • The lowest pressure ever measured was 25.69 inches (87kPa), set on Oct. 12, 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.


Magnitude of Pressure Change

  • Atmospheric pressure does not change much at sea level. A severe storm will seldom have a pressure below 980 mb and high pressure seldom go over 1036hPa.
  • But comparatively small change can bring about hugely different types of weather.
  • For example, If expected atmospheric pressure is 1000hPa and the actual pressure reads 997, it indicates an approaching storm.
  • You will understand later, how easy it is to make an error of 3hPa. An error of parallax and error of overlooking the semidiurnal pressure variation can be cause an error of 3hPa.
  • An error of 5 hPa, can mislead you about a typhoon.


Units of measurement of Temperature

  • The other parameter of weather, the temperature is easy to understand and the Scales of temperature are
    • Kelvin (K),
    • Celesius (C )
    • and Farenheit (F).
  • Conversion Factor
  • F° –> C°   =  (F° -32)*5/9
  • C° –> F°   =  (C° *9/5) + 32
  • C° –> K°   =  C° +273° (so 0°  = 273K)‏



Topic                                                               Humidity


Learning Objectives

The cadet shall be to:-

  1. Know importance of water vapor in meteorology.
  2. Define and understand Absolute, Specific and Relative Humidity & Dew Point
  3. Understand relation between Dew Point and RH.
  4. Calculate dew point using tables.
  5. Know relationship between humidity and hold ventilation.


Duration                                             2 Lectures


Critical Knowledge

  1. Be able to interpret RH Equation.
  2. Dew point is not rain.
  3. Dry and wet air as per meteorology.



  • Humidity is the third factor of Atmosphere and it is the quantity of water vapor present in the atmosphere.
  • There are various ways of expressing the quantum of water vapor in the air – Vapor pressure, Absolute Humidity, Specific Humidity and Relative Humidity and Dew Point.
  • Of these the last two, Relative Humidity and Dew Point, concern us the most.



  • Consult Diagram 4 in the accompanying document 04 Atmosphere Diagrams.docx
  • Evaporation occurs at all temperatures.
  • Evaporation occurs some molecules overcome the surface tension and escape. .
  • In a closed container, the process stabalises when same number of molecules return the liquid as those which have escaped.
  • At this point the vapor is said to be saturated.
  • As temperature increases, there is a greater to and fro movement of molecules.
  • In a closed container, the process stabalises when same number of molecules return the liquid as those which have escaped.
  • At this point the vapor is said to be saturated.


Relative Humidity

  • RH is the percentage ratio of the actual water vapor contained in a given sample of air, to the maximum quantity of water vapor that the sample can hold at that temperature.

Present Quantity of water vapor X 100

  • H. =  ———————————————–

Max  quantity of water vapor possible at that temp.

  • if no vapor is allowed to come in or go out of that sample of air
    • And If the temperature in a given sample of air is raised, Its capacity to hold water increases (denominator will Increase)‏. And, RH decreases.
    • Which means that the air becomes relatively drier.
    • Or If the sample or air is cooled, the denominator will decrease and the RH will increase.


To understand the concept of RH, let us take case of Delhi and Mumbai in the month of May. It is known that though Delhi is much hotter than Mumbai, it is Mumbai which is more humid than Delhi. Let us examine this situation using the formula above.

  • The Numerator In case of Mumbai, the numerator is much higher than Delhi because of proximity of the sea.
  • The Denominator In case of Mumbai the denominator is lower than Delhi because Mumbai ambient temperature is lower than Delhi and therefore the capacity of air to hold moisture is also lower.
  • This Mumbai has high numerator and lower denominator and therefore higher RH.


Dew Point

  • In Meteorology, Dry Air means its RH <100%.
  • When RH of a given sample of air becomes 100%, the air is called either Wet or Saturated air.
  • In the Atmosphere, when air is progressively cooled, it’s RH increases steadily .
  • At some temperature, the RH becomes 100% (The air is now wet or saturated.)
  • The temperature at which a given sample of air becomes saturated, is called Dew Point temperature of that sample of air.
  • It is popularly known as just the ‘Dew Point’.
  • This is a very very important concept and its definition should known by heart. Classic definition of Dew Point (DP) is
    • The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water.
  • Dew point of a sample of air would therefore depends upon it temperature and its RH.
  • Both RH and Dew Point are found by using a hygrometer or a psychrometer and then consulting a meteorological table.
  • When the dew point temperature falls below freezing it is often called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.


Sweat in the Cargo Hold

  • Sweat in the cargo hold is the condensation of water vapour present in the hold into water droplets.
  • Sweat causes damage to cargo.
  • Sweat happens when there is adequate water vapor in the hold and any part of the hold is cooled to below dew point.


Atmospheric Sweat

  • Imagine ship loaded in Europe in winter where the ambient temperature is cold and dry.
  • The temperature in the hold will also be cold and dry and much above the dew point.
  • Within a few days the ship enters the warmer and moister Mediterranean.
  • The atmosphere in the hold continues cold and dry.
  • The ambient Mediterranean temperature will be warmer and more humid.
  • If ventilation is allowed in this situation, the incoming warm and moist air will be cooled inside the hold and its temperature will soon reach dew point.
  • The water vapor in the incoming air will condense and dew will be formed in the hold and on the cargo. This is not at all desirable.
  • Remedy When ventilation is mandatory as in the case of perishable cargoes, the outside air should be  dehumidified before being pumped into the hold.


Cargo Sweat

  • Some cargo like hides, skins, ores have lots of internal moisture.
  • In the hold, such cargoes give out vapour and increase the moisture content of the hold.
  • If hold temperature decreases, the atmosphere gets saturated.
  • Vapor condenses and sweat is formed on the steel part of the hold and water drips on the cargo.
  • Solution
    • Use large capacity exhaust fans.
    • Use dunnage to keep cargo off the steel parts of hold.
    • Cover top of cargo with cardboard or bamboo mats.


Ships Sweat

  • When temperature of the sea is much lower than the atmosphere, the underwater part of the ship gets cooled by contact with the sea.
  • Sweat forms on the steel part of the ship and contaminates cargo by dripping onto it or wet the bottom of cargo touching the deck.
  • Solution
    • Use dunnage to keep the cargo away from the steel parts of the hold.
    • Use ventilation to reduce the humidity of the hold.



Topic                                                   Changes in Atmosphere

Learning Objectives

The cadet shall be to:-

  1. Explain how diurnal, seasonal, geographical and altitudinal variations in
  2. temperature, pressure and humidity occur.
  3. Explain adiabatic changes, DALR, SALR, Isothermal and inversion of temperature.
  4. Explain unsaturated and saturated states.


Duration                                             2 Lectures


Critical Knowledge

  1. Why DALR is always more than SALR


Variations in Atmospheric Parameters

Atmospheric Parameters of temperature, Pressure and Humidity are never constant and vary both in Time and Space.

  • Variations of Temperature
    1. Diurnal change of temperature
    2. Vertical Variation of Temperature
  • Variations of Pressure
    1. Diurnal change of temperature
    2. Vertical Variation of Temperature
  • Humidity


Diurnal change of temperature

  • The word diurnal means twice a day.
  • Thus there are two occasions everyday when there is a change.
  • The maxima generally occurs at 1400 hrs.
  • Minima generally occurs at half an hour after the sunrise.
  • There is a tendency to mix up between diurnal change of temperature and semi-diurnal change of atmospheric pressure.


Attitudinal variation of Temperature

To understand the altitudinal variations of Atmospheric Temperature, it is first necessary to understand:

  • The concept of parcel of air
  • & The concept of lapse rate


The concept  Parcel of air.

  • The concept of a parcel of air is best illustrated by a balloon. It is separated from its surroundings and its rise or movement is independent of the surroundings.
  • Parcel or air means a homogeneous mass of air which could be as small as 1nm radius or an airmass spread over 1000 sq. miles.


Concept of Lapse

  • A phrase called Lapse Rate is used in relation to altitudinal variation of temperature.
  • Lapse rate is defined as the decrease of an atmospheric temperature with height.
  • This phrase can be used in relation to atmospheric pressure or humidity as well.
  • When no variable is clearly mentioned, the lapse rate pertains to the temperature.
  • Negative Lapse rate means Rate of


There are two types of lapse rates :-

  • Environmental lapse rate (ELR) pertains to air which is termed as a “Standard Atmosphere (as defined by International Organisation of Standardisation) where:-
    • The air is not rising.
    • & it contains no moisture (Zero Moisture)
  • Adiabatic Lapse Rate pertains to a parcel of air which :-
    • Rises or sinks adiabatically (no exchange of heat with
    • Has moisture in it either dry {RH<1005} or Saturated {RH=+100%} .


There are three types of Environmental lapse rate

  • Consult Diagram 5(a), 5(b) & 5(c) in the document file 04 Atmosphere Diagrams.docx
    • Temperature Lapse Rate where temperature decreases with height and which is the most common lapse rate in the troposphere. The lapse rate of Standard Atmosphere is 49 °C/1000 m. up to 20 km. ‏(a5)
    • Negative Lapse Rate (Temperature Inversion) where ttemperature increases with altitude (5b).
    • Zero Lapse rate (Isothermal) where temperature is constant in relation with altitude (5c).


Temperature profile of atmosphere upto 130km:-

  • Consult Diagram 6 in the accompanying document 04 Atmosphere Diagrams.docx
    • In the troposphere, upto 20km altitude, the profile is of lapse rate.
    • Thereafter there is an isothemral or zero lapse rate
    • Beyond that, it is negative lapse rate (inversion of temperature) followed by a lapse rate, isothermal and then till the top of atmosphere it is inversion of temperature.
    • Our main concern is the lapse rate in the troposphere (upto 20km altitude)


What is Adiabatic Lapse Rate

Due to convection, parcel of air will either rise from the surface of the earth or sinks to the surface of the earth.

  • When it rises, the parcel of air expands and it’s temperature falls.
  • If the parcel of air sinks to the surface of the earth, it will contract and it’s temperature will rise.
  • When this change of temperature inside the parcel of air occurs without any exchange of heat with the surrounding, it is known as Adiabatic change.
  • Such adiabatic change of temperature is possible in a parcel of air because air is a poor conductor of heat.
  • There are two types of Adiabatic Lapse Rates:-
    • Dry adiabatic lapse rate (DALR)‏
    • Saturated Adiabatic Lapse Rate (SALR) – Also called Moist Adiabatic Lapse Rate)‏


Dry Adiabatic Lapse Rate

  • Dry Adiabatic Lapse Rate is the rate at which the temperature in a rising parcel of dry air (which means an unsaturated air and does not mean Zero Moisture) decreases with increasing height, under adiabatic conditions.
  • The DALR is 10° C per kilometre.
  • The reduction of temperature occurs because:-
    • The layer of air closes to the earth is heated up most and begins to rise.
    • As the parcel of air rises, it begins to expand Adiabatically.
    • To expand, the airmass uses its internal energy.
    • As the air a poor conductor, there is no heat exchange with the atmosphere outside the parcel of air and internal energy in the parcel of air is used up.
    • Loss of internal energy = Fall in temperature =


Concept of Latent Heat of Evaporation

Vaporization is a surface process whereas the evaporation is a bulk process.


Vaporization is a spontaneous process which occurs at the surface of a liquid for conversion of a liquid to its vapor; it is the reverse of the process of condensation.

The rate of vaporization increases as the temperature increases.

Vaporization depends on the external factors such as wind speed, humidity, temperature and the surface area of the liquid.

When the liquid has strong inter-molecular forces, the evaporation rate becomes slow. Vaporization takes place at a faster rate when the external pressure is low.

When evaporation takes place, the external environment gets cool down, because heat is absorbed for this process to occur. This is an endothermic reaction.

For 1 mol (18 g) of water to be completely vaporized, it needs 44.10 kJ (10.54 kcal) 585.6 cal/gm.



Evaporation is where liquid changes to gas at its boiling point.

Evaporation is independent of the temperature, but depends on atm. pressure. At high altitudes, the atmospheric pressure is less and so the boiling point is low.



What is the difference between Evaporation and Vaporization?

  • Vaporization is the process of changing a liquid to its vapors at a temperature below its boiling point. Evaporation is the process of changing a liquid to its vapors at its boiling point.
  • Vaporization can take place at any temperature. Evaporation takes place at the boiling point.
  • During the vaporization, temperature of the liquid changes. During the evaporation, temperature is a constant (=boiling point).
  • Vaporization is a surface process. It takes place only at the surface of a liquid. Evaporation takes place over the entire mass of the liquid.
  • Vaporization is a slow and silent process. Evaporation is a speed and violent process.
  • The rate of vaporization depends on the surface area of the liquid, speed of the wind, humidity and the temperature. The rate of evaporation is independent of the surface area of the liquid, wind speed, humidity and the temperature.


Steam vs Vapor 

Steam is a Vapor when liquid boils.

Vaporization also takes place when we hang washed clothes in air. It is then called evaporation.





Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process. An example is a state of matter change, meaning a phase transition, such as ice melting or water boiling.


In meteorology, latent heat is an important component of Earth’s surface energy budget which consists of evaporation  of water at the surface and subsequent condensation of water vapor in the troposphere.




Examples of Latent Heat

22.6 x 10^5 J of heat energy must be added to turn one kilogram of water from liquid to gas at 100 degrees Celsius.




Useful Constants:

1 calorie = 4.186 J

Latent heat of vaporization of water = 539 cal/g = 2256 kJ/kg

Latent heat of fusion of water = 79.5 cal/g = 333 kJ/kg



Saturated Adiabatic Lapse Rate

  • SALR is the rate at which the temperature in a rising parcel of moist air decreases with increasing height, under adiabatic conditions.
  • Initially the parcel of air cools at the DALR and RH rises.
  • At some stage, the parcel of air gets saturated (RH = 100%)‏ and the rate of fall in temperature decreases because:-
    • At Dew Point, condensation of water vapor begins.
    • The latent heat of evaporation is released.
    • This internally released heat reduces (note that it will not reverse but only reduce) the lapse rate to SALR.
    • SALR is always lesser than DALR.
    • At temperatures above freezing it is +4.9 °C/km.
    • When the moisture contents of a rising parcel of air are not known, SALR is presumed to be 6.5°C/km.
    • Further fall in temperature in the rising parcel of air now occurs at the SALR.


Semi-diurnal change of atmospheric pressure.

  • The word semi-diurnal means twice two times a day. Thus there are four occasions everyday when there is a change.
  • The maxima generally occurs at 1000 & 2200 hrs & Minima at 0400 & 1600 hrs.
  • The semidiurnal range means the difference between the maxima and the minima.
  • If pressure at 0400hrs is 997hPa and at 1000hrs it is 999 hPa, the range will be 2.0 hPa.
  • The range is higher in the tropics (3 hPa) than in the middle latitudes (0.8hPa )
  • Overlooking this semi-diurnal range can result in wrong judgement about a Tropical Revolving Storm (Hurricane), where a 3 hPa variation may confirm or rule out a storm. .


Why does this diurnal pressure change occur?

This change occurs due to the westward movement of a wave in the upper atmosphere, This movement occurs in relation to the sun’s apparent daily journey round the earth. (mark the word apparent) which was discovered in 1956 by Bernhard Haurwitz. He theorized that as the sun moves it causes warming of the upper atmosphere (mainly the thermosphere) and the amplitudes of both cycles depend on latitude, season and altitude.


Pressure – Altitude Relationship

  • Consult Diagram 7 in the accompanying document 04 Atmosphere Diagrams.docx
  • Atmospheric Pressure decreases with height.
  • This is attributable to decrease in the length of air column above you as altitude increases.
  • As shown in the diagram, the drop is linear in the beginning and after an altitude of about 10km, it begins to decrease exponentially.



Humidity – Altitude Relationship

Humidity almost invariably decreases with altitude and is not of great concern to a mariner.