Climatology/Air pressure

Introduction
Atmosphere is  an  envelope  of  gases  circling  the  earth. It is  mainly  made  up  colourless  and odorless  gases  of  Nitrogen,  Oxygen,  Carbon-dioxide  and  many  more. Pressure can be defined as the force exerted by the atmosphere at a given place and time. Generally it is expressed in millibar (mb). The atmospheric pressure is determined by the mass of a column of air lying above a certain level or surface. The increase or decrease in pressure of the air is determined by various factors. Atmospheric pressure is a force of dry air exerted on a particular place or surface. The column of dry air causes the pressurization due to its own mass. The air above is exerting pressure on the lower level, and is also known as barometric pressure. Atmospheric pressure declines very rapidly with increasing height above the sea/earth surface.

The first atmospheric pressure measuring instrument was invented by  E.  Torricelliin in 1643. In Torricelli’s barometer, mercury was used. One inch of mercury pressure is equivalent to about 33.8639 mb at sea level. The air pressure is shown on maps using isobars -- lines that join sites with the same air pressure.

Pressure Gradient
The atmospheric pressure gradient refers to the change in the pressure per unit distance between two places  along  a  line  on  an  isobar  map  of  anyarea. The maximum  pressure  gradient  is observed along a line perpendicular to the isobar. It is more when the isobars are closely spaced, but when it is widely spaced, the pressure gradient is lesser.

Variation in the Atmospheric Pressure
The atmospheric pressure varies from place to place and from time to time. On the basis of time, diurnal and seasonal variations can be observed.

Diurnal Variation
The periodic pressure change is observed on daily basis. Two highs and two lows of air  pressure  is  seen  each  day. The high  point  occurs  at  10am  and 10pm  while the  low pressure  occurs  at  4pm  and  4am. This is  also  called  as semidiurnal  observation  in pressure variation. It is because of difference of 12 hours between them. The mean daily changes in the  air  pressure  can  be  found  out  by  calculating  the  average  hourly  observed  pressure  for  a long  time. Insolation, heating, cooling and radiation are the factors for the diurnal changes in the air pressure.

Seasonal or Annual Variation
The amount of insolation received in a particular region varies from one season to another season. Because of this variation, seasonal or annual variation in the atmospheric pressure  is  found. The amount of insolation received in a particular region varies from one season to another season. Because of this variation, seasonal or annual variation in the atmospheric pressure  is  found.

Factors Affecting Air Pressure
There are  three  important  factors  affecting  the  air  pressure. They are  temperature  of  the  air, altitude  and  moisture. Briefly known  as  TAM.
 * Temperature(T)
 * Altitude  (A)
 * Moisture  (M)

Distribution of Atmospheric Pressure
Atmospheric pressure  varies  from place  to  place  and  season  to  season. Its distribution over the globe is not uniform. Variation is seen in both perspectives :--
 * vertical
 * horizontal

Vertical Distribution of Atmospheric Pressure
The air is compressible.Its density is greater in lower layers as compared to the upper layers of the atmosphere.The atmospheric pressure decreases with increasing height. The vertical  distribution  of  atmosphere  is  influenced  by  temperature,  water vapors and altitude. In  the   higher  altitude,  atmosphere  becomes  thin  and intermolecular  space  is  more.

Horizontal Distribution of Atmospheric Pressure
The distribution  of  atmospheric  pressure on  the  earth  is  explained across  the  latitude. Distribution of air pressure on the earth's surface is shown by means of isobars.Isobars are line drawn through points of equal pressure. This is considered  as horizontal distribution  of  atmospheric  pressure. distribution of  pressure  belts is  very  distinct  and  classifiable.Based  on  the  characteristics  of  different  belts,  they  are  grouped into four: They are:- Last three belt have  two cases of each –northern and southern hemisphere. In fact the  first belt has also two cases but both north and south cases of equatorial low forms a single belt. That is why, it is call as one.
 * Equatorial low pressure belt
 * Sub-tropical high pressure belt
 * Sub-polar low pressure belt
 * Polar high pressure belt

Equatorial Low Pressure  Belt
The Equatorial belt extends  from  10°  north  to  10°  south  latitude. It is  a thermally induced belt because here the temperature remains very high throughout the year due to the vertical sun’s rays.The average pressure in this belt is less rhan 1013millibars,but in the eastern hemisphere it is generally less than 1009milibars.there is almost an absence of horizontal movement of wind, the calm condition is termed as doldrum. The winds converging from both hemisphere’s high pressure belts results into a zone of convergence. It is known as inter tropical convergence zone  (ITCZ). The risen  air  from  the  equatorial  low  reaches  to  the  upper  troposphere  and  dragged  towards poles. By reaching in the tropics, air descends from 200to 350 latitudes in both the hemispheres. It is caused by cooling of the air. Cool air is heavier.

Sub-Tropical High Pressure Belts
These belts are extended from 20°to 35°latitudes in both hemisphere. These belts are situated over tropic of Cancer and tropic of Capricorn. There is subsidence of air from the upper troposphere in this zone. The wind moves equatorward to fill the temporarily created vacuum/ gap produced  by  rising  air  at  the  low  pressure  zone. Hence, an  atmospheric  cell  is  created  by rising air at equator –moving up –getting drifted towards pole –getting subsided due to cooling –becoming heavier –climbing down at sub-tropical high and finally moving towards equator to fill  the  gap  created  by  rising  air,This  cell  (circular  motion)  is  known  asHedley  cell .A  calm  and  feeble  wind  is  created  in  this  region  which  is known as horse latitude. In early day sailing vessel  with cargo  of  horses  was  very  difficult under such calm conditions. The horses were thrown into thesea to reduce the load of the ship.

Sub-Polar Low Pressure Belts
These belts are found between 50° to 70° latitudes in both the hemispheres. These belts  are  induced  due  to  ascend  of  air  as  a  result  of  convergence  of  wind coming   from sub-tropical   high   pressure   belts   (westerlies)   and   polar   high   pressure   belts (easterlies).The air moving fromsub-tropical high to sub-polar low – rises above – gets cooled – diverted  towards  equatorward  and  descends  at  sub-tropical  high – makes  a  cell  (circulation motion) known as Ferrel’s cell.During  winter  season,  because  of  high  contrast  of temperature between  land  and  sea,  this  belt is  broken  into  two  low  pressure  centers  in  northern hemisphere one  in  the  vicinity  of  the  Aleutian  Island,  and  other  between  Iceland  and  Green Land. During the summer season, the variation is less.

Polar High Pressure Belts
High pressure prevails over both the polar regions due to excessive cold condition. The cold climatic condition itself is caused by slanting sun’s ray at the poles.These pressure zones, thermal factor is more important than dynamic factor. The air coming from polar region –rises  up  at  the  sub-polar  low –finally  pushed  towards  pole  and  descends  at  the polar high. This also makes a cell known as Polar cell.

Factors Controlling Pressure Belt System
There are  two  main  factors  controlling  the  pressure  belt  system  over  the  globe are following:--
 * Thermal factor
 * Dynamic factor

Thermal factor
Equatorial  region   receive   intense   solar   energy throughout  the  year. Polar regions  are  extremely  cold  throughout  the  year  due  to  less  effective and inclined solar rays. More solar energy in the equatorial region causes the air to expand and thus low air pressure is created.At poles, due to extreme cold conditions the air is very  cold  and  hence,  high  air  pressure  is  observed. Therefore, the  creation  of  equatorial  low and  polar  high  are  caused  by  thermal  factor. That is  why,  they  are  called  asthermally  induced pressure belts.

Dynamic Factor
The equatorial low as well as polar highs are explained by the thermal factors. But sub-tropical highs and sub-polar lows are not coming  into  the  line  of  thermal explanation. The warm  air  uplifted  from  the equatorial low is pushed towards pole and due to cooling it subsides over tropics. It is considered as a dynamic factor of the development of subtropical highs. wind blows from the subtropical highs to equatorial low as  well  as  towards  pole. Since pole  has  thermally  induced  high  pressure. Both winds meet in the subpolar low around 50 to 70 degree latitudes in both the hemispheres. Therefore, subpolar lows also have dynamic origin.

Changing Seasons and Pressure Belts
The earth’s  axis  is  inclined  by  66030’from  the  horizontal  plane. On this  inclined  axis,  it  is revolving around the sun. Due to this reason, the orbit around the sun is elliptical in shape The sun’s rays are vertical on equator on 21 March and 22 September. This situation is known as equinox. Equinox means the day and night areequal throughout the globe. The  wind   coming   from   both   the   hemispheres   converges   at   equator. It  is   inter   tropical convergence  zone  (ITCZ). After 21  March,  the  northern  hemisphere  starts  tilting  towards  the sun.January being on of the summer months in the southern hemisphere,the continent of Australia, Africa,and South America are warmer. there are distinct low pressure cell over them.Vertical sun rays in the northern hemisphere is marked by increasing energy. This energy heats the areas/ air. Therefore, ITCZ migrates towards north along the thermal equator. Since the ITCZ is temperature induced low pressure (already discussed before) the migration of ITCZ is marked by the shifting of all pressure belts northward.The maximum northward departure of sun is seen by 22 June, the effective maximum average energy received in the northern hemisphere is observed in  July. The northward  maximum  departure  of  ITCZ  is  seen  upto  25 degree from  equator .Land  is  getting  heated  much  quickly  as  well  as  greater  temperature  is  recorded  there,  but  the same  is  not  with  water  body/  oceans. Greater effective  energy  is  accumulated  over  the  land, ITCZ  is  also  havethe  tendency  to  depart  more  over  the  land. After 22  June,  the  retreat  is  seen  and  sun  again  shines  vertically  over  the  equator  by  22 September. Further onward the southern hemisphere is inclined towards the sun. The same case is repeated  in  the  case  of  southern  hemisphere  what  it  has  been  discussed  above. By 21 December,  sun  shines  vertically  over  the  tropics  of  Capricorn. After this  it  starts  northward migration and reached over the equator again by 21 March.

Seasonal Variations during July
End of  third  week  of  June,  the  sun  shines  directly  over  the  tropic  of  Cancer. After that sun’s  rays  starts  turning  towards  south. July is  the  hottest  month  recorded  in  the  northern hemisphere. Northward departures  of  the  inclination  of  sun’s  rays  from  last  week  of  March increases  the  temperature  in  northern  hemisphere. the ITCZ  keeps  on  moving northwards. Its maximum departure is seen upto 25 degree north latitude in the large chunk of land of Asia. the ITCZ shifting is northwards, its shift is minimum on the large water bodies of Pacific and Atlantic.The effectiveness  of  the  sun’s  departure  is  observed  till  July  as  it  is  the  hottest  month  for northern  hemisphere,  while  the  southern  hemisphere  witness  the  coldest  conditions  in  July.

Seasonal Variations during January
End of  third  week  of  September  the  sun’s  rays  become  vertical  towards  southern hemisphere. The effectiveness  of  solar  radiation  increases  in  southern  hemisphere  with  the passage  of  time,  but  simultaneously  it  keeps  on  decreasing  in  the  northern  hemisphere. It all happens due to inclined rays of the sun in the northern hemisphere.Increasing temperature  causes  the  high  atmospheric  pressure  region  turnsinto a  low  air  pressure  zone. The continuous  widespread  high  pressure  belt  is  confined  only  to the southern parts of Pacific, Atlantic and Indian oceans. The subpolar low pressure belt is also reduces to a narrower belt in comparison to July. So the  case  of  northern  hemisphere  is  reversed  in  January  in  comparison  to  July. Everything is due  to  seasonal  changes due to  earth’s  revolution  round  the  sun. A huge  area  is  under  the influence  of  high  atmospheric  pressure.