Metrology

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Atmosphere extends 200km, From ground in order they are

  • Troposphere (Weather changes) 8 above pole -16 above Equator
  • Stratosphere About 50 KM above SL
  • Mesosphere About 80 KM above SL
  • Ionosphere (Navigation and communication when using electromagnetic waves.) About 200 KM above SL

Zone of separation between Troposphere and Stratosphere is called Tropopause. Temp. of air normally falls as we move up. Sometimes Local influence cause temp. of air to

  • Increase with Height instead of falling (Temperature Inversion ).
  • Remain constant with height(Isothermal layer)

Adiabatic Lapse rate

Adiabatic change of temp of a parcel of air is the change in its temperature due to increase or decrease in its volume, Without Exchange of heat from the surrounding.

  • If volume increase , Temp Decrease and vice versa,
  • If a parcel of air is made to rise ,Its volume would increase.
  • So if a parcel of air is made to rise its volume would increase in accordance with the rarer air at that height. this expansion causes cooling.

Any parcel of air fully saturated is called wet air or saturated air. Not fully saturated is called dry air .

DALR (Dry adiabatic lapse rate) – Temp of dry parcel of air , which is made to rise or fall at a steady rate of 10 degree C per KM.

SALR (Saturated adiabatic lapse rate) – 5 Degree per KM.

As Saturated air is cooled , Its Capacity to hold water vapour decreases and the excess moisture condenses into water droplets. This condensation causes latent heat that warms up parcel of air.

Diurnal Variation Of atmospheric Temp. – Max at 1400 hours local time and reaches minimum at about half hour after sunrise. Diurnal range of air temp over land is 20 and over sea is 1 .

SI unit of pressure is Hectopascals or Millibars.

Lapse rate of atmospheric temp. is 115mb per km , At greater Height lapse rate is higher. Semi-Diurnal range 3 mb.

BAROMETRIC TENDENCY

Difference between the atmospheric pressure at the time of observation and the atmospheric pressure three hours earlier.

Due to this tilt, we have different seasons. The sun appears to be overhead on

  • The Equator around 21st March
  • Tropic of cancer around 22nd June
  • The equator again on around 23rd September
  • Tropic of Capricorn around 22nd December

ABSOLUTE HUMIDITY – Absolute Humidity is the mass of water vapour contained in a sample of air. It is usually expressed in grams per cubic meter

Relative Humidity = (present quantity of water vapour / Max possible at that temp. ) * 100

And we know Relative humidity is inversely proportional to temp.

If a sample of air was progressively cooled its relative humidity would increase that’s the air would become relatively more moist. At some temperature, the air would become wet that is relative humidity would become hundred per cent. The air is then said to be saturated and the temperature at which the surface is called dew point.


Dew point of air depends upon temperature and relative humidity.

Atmospheric sweat , Cargo sweat and Ship sweat.

Atmospheric Sweat – Air brought into hold is very small as compared to size of the hold. If the temp. inside the hold is lower then the dew point of air outside we will not do ventilation and the type of sweat that can form is called Atmospheric Sweat.

Cargo sweat – Some cargoes like Hides , Skins , Ores etc. can give off a Large quantity of water vapour making relative humidity of the air inside hold very high. Slightest cooling can cause sweat to form on the steel parts of the hold.(use dunnage to prevent)

Ship Sweat – When the temp of the water is much lower then that of the air above it ,The underwater part of the hold in contact with the sea would be cooled . Hence sweat would form on the steel parts below the waterline. (USE of dunnage and ventilation can also help.)