Atmospheric pressure

Also called: Air pressure · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

Atmospheric pressure (also called air pressure) is the weight of a column of air pressing on a unit area. The column runs from mean sea level to the top of the atmosphere.

It matters because differences in pressure from place to place make the winds blow. Pressure also falls with height, which affects breathing, aircraft and how weather maps are drawn.

Explanation

How pressure forms and how it is measured

• Why pressure is highest at the ground:
• Gravity pulls air towards the Earth.
• The lowest layers are squeezed by all the air above them.

• So air is densest near the ground, and pressure is highest there.

• Average sea-level pressure is 1,013.2 mb. The standard value used in science is 1,013.25 mb.

• The same pressure in other units:
• about 760 mm (30 inches) of mercury, or 101,320 pascals [3]

• 29.92 inches of mercury on US charts [2]

• Units:

• 1 millibar (mb) = 1 hectopascal (hPa) = 100 pascals (Pa).
• The pascal is the SI unit of pressure. It is named after Blaise Pascal (1623–1662) [2].

• Weather maps now usually show hPa. Because 1 hPa = 1 mb, the numbers stay the same.

• In weather work, pressure values usually range from about 100 to 1,050 mb [2].

• Why we do not feel this weight (Class 9):
• Air presses on our body from all sides, not just from above.

• The pressure inside our body pushes outwards with equal force, so the two cancel out.

• Barometer: the instrument that measures pressure. It has two main types [3]:

• Mercury barometer: air pressure holds up a column of mercury in a glass tube. The readings are corrected for local temperature and gravity. It is used in observatories and to check aneroid barometers [3].

• Aneroid barometer ("aneroid" means without liquid): a thin metal box with the air pumped out. Its walls bend in or out as pressure changes. It is small and easy to carry, so it is used in aircraft altimeters (instruments that show height) [3].

• Daily rise and fall:

• Pressure is highest around 10 a.m. and 10 p.m. and lowest around 4 a.m. and 4 p.m. [2].
• This daily change is largest near the equator and gets smaller towards the poles [2].

Vertical variation: pressure falls with height

• Near the ground, pressure falls fast, by about 1 mb for every 10 m you climb.
• The rate of fall is not constant:
• Higher up, the air is thinner (less dense).
• So each 10 m of height holds less air.

• So pressure falls by less for every 10 m as you rise.

• Standard atmosphere (NCERT Table 9.1):

Level	Pressure (mb)	Share of sea-level pressure	Temperature (°C)
Sea level	1,013.25	100%	15.2
1 km	898.76	~89%	8.7
5 km	540.48	~53%	–17.3
10 km	265.00	~26%	–49.7

• How fast the pressure falls in each layer:
• about 1.1 mb per 10 m from 0 to 1 km
• about 0.9 mb per 10 m from 1 to 5 km

• about 0.55 mb per 10 m from 5 to 10 km

• NCERT exercise: if surface pressure is 1,000 mb, it will be about 900 mb at 1 km.

• Temperature falls about 6.5 °C per km in the lower layers. This is the normal lapse rate (the rate at which temperature drops with height).
• Exponential fall: pressure and density roughly halve every 5.6 km. They drop to about 37% (1/e) every 7.64 km. This distance is called the scale height [5].
• About half of all the air lies below 5.5 km (NCERT). NOAA puts it at 5.6 km (18,000 feet) [2].
• The 500 mb level, a standard level on upper-air charts, lies on average at about 5,600 m. It sits higher in warm air and lower in cold air, because warm air is less dense [2].

Reducing pressure to sea level and hydrostatic balance

• The problem:
• Pressure falls with height.
• So a hill station would always show lower pressure than a coastal town, even in the same weather.

• This would hide the real pattern of highs and lows across the map.

• The fix: each station's reading is reduced to sea level. This means it is corrected to the value it would have at mean sea level. Sea-level pressure is the value found by this theoretical reduction of barometric pressure to sea level [3].

• Result: isobars (lines on a map joining places with equal pressure) then show only the sideways (horizontal) differences in pressure. These differences drive winds.
• Pressure gradient force: the push on air from higher pressure towards lower pressure.
• Hydrostatic balance:
• Pressure changes much faster upwards (about 1 mb in 10 m) than sideways. On sea-level maps, isobars are often hundreds of kilometres apart.
• So the upward (vertical) pressure gradient force is much larger than the horizontal one.
• But gravity pulling the air down almost exactly cancels this upward push.

• So there are no strong upward winds. Winds blow mostly sideways, from high to low pressure.

• Vertical air movement is weak. It happens mainly through heating and convection (warm air rising), as in thunderstorms and the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of both hemispheres meet).

Where it occurs

• Khardung La, Ladakh (India):
• This high mountain pass has a pressure of about 650 mb, roughly two-thirds of sea-level pressure.
• The thin air holds less oxygen, so visitors may get breathless or suffer altitude sickness.

• They must acclimatise, which means staying a day or two at a lower height so the body can adjust (Class 7).

• India Meteorological Department (IMD):

• It draws its mean sea-level pressure charts in hPa.

• It marks a low-pressure area when there are two or three closed isobars at 2 hPa intervals [4].

• Equator vs poles: the daily rise and fall of pressure is largest near the equator and smallest towards the poles [2].

• Equatorial belt: the ITCZ is a low-pressure zone where rising air causes convection.
• Pacific Ocean (Tahiti–Darwin): pressure differences measured at sea level between these two places define the Southern Oscillation, which is linked to El Nino and La Nina.
• Aircraft everywhere: altimeters use aneroid barometers, because pressure falls with height in a known way [3].

Don't confuse with

• Station pressure vs sea-level pressure: station pressure is the raw reading at the station's own height. Weather maps use sea-level-reduced pressure, so the effect of height is removed.
• Vertical vs horizontal pressure gradient force: the vertical force is the larger one. Yet winds are mostly horizontal, because gravity balances the vertical force (hydrostatic balance).
• Mercury barometer vs aneroid barometer: the mercury type uses a liquid column and is used to check other barometers. The aneroid type has no liquid, uses a metal box with the air removed, and is used in aircraft altimeters.
• Millibar vs pascal: 1 mb = 1 hPa = 100 Pa. A millibar equals a hectopascal, not a pascal.

Prelims Hooks

• Standard sea-level pressure = 1,013.25 mb = 1,013.25 hPa = 760 mm of mercury = 29.92 inches of mercury [2].
• Near the surface, pressure falls about 1 mb per 10 m. This rate gets smaller with height. It is not constant.
• Order in the standard atmosphere: sea level 1,013.25 > 1 km 898.76 > Khardung La ~650 > 5 km 540.48 > 10 km 265 (mb).
• About half the mass of the atmosphere lies below 5.5 km (NCERT). NOAA gives 5.6 km [2].
• Pressure is highest around 10 a.m./10 p.m. and lowest around 4 a.m./4 p.m.. The daily range is largest near the equator [2].
• Trap: IMD marks a low when there are 2–3 closed isobars at 2 hPa intervals on sea-level-reduced charts, not on raw station pressure [4].

Mains Points

• Pressure falls with height, and this affects life in high regions:
• Gravity packs air into the lowest layers, so half the air lies below about 5.5 km.
• At places like Khardung La in Ladakh (~650 mb), there is less oxygen. Tourists and troops must acclimatise, and people at high altitude face health risks.

• Engines and aircraft perform less well in thin air. Aircraft altimeters work only because pressure falls with height in a known way.

• Sea-level reduction is the base of weather forecasting:

• Only sea-level-reduced isobars show the true horizontal differences in pressure that drive winds.
• IMD uses these charts to spot lows, depressions and cyclones, and to track the monsoon trough.

• The Tahiti–Darwin pressure seesaw (the Southern Oscillation) is measured at sea level and helps track El Nino and La Nina.

• Hydrostatic balance explains why winds are mostly horizontal:

• The pressure belts, the trade winds and the westerlies are all driven by small horizontal differences in pressure.
• Air moves up or down only through convection, fronts and lifting over mountains. This matters for explaining thunderstorms and rainfall in the ITCZ.

Related concepts

• Millibar
• Vertical variation of pressure

Read more

• Atmospheric pressure and its vertical variation