Atmospheric pressure and its vertical variation

Detail

What atmospheric pressure is

• Atmospheric pressure is the weight of a column of air over a unit area. The column runs from mean sea level to the top of the atmosphere.
• Why it is highest at the surface:
• Gravity pulls air towards the Earth.
• So the lowest layers are squeezed by all the air above them.

• Air is therefore 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.

• In other units, sea-level pressure is about 760 mm (30 inches) of mercury, 1,013.2 mb or 101,320 pascals [3]. On US charts the standard value is written as 29.92 inches of mercury [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.
• The two balance, so we feel nothing.

How pressure is measured

• Barometer: the instrument that measures atmospheric pressure. There are two main types: mercury and aneroid [3].

Barometer	How it works	Where it is used
Mercury barometer	The air pressure holds up a column of mercury in a glass tube. The height of the column is measured. Readings are corrected for local temperature and gravity [3].	Observatories. It is also the standard used to check and set 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 air pressure changes [3].	Portable instruments and aircraft altimeters (instruments that show height), because it is small and easy to carry [3].

• Pressure also changes during the day:
• It is lowest around 4 a.m. and 4 p.m. and highest around 10 a.m. and 10 p.m. [2].
• This daily rise and fall is largest near the equator and gets smaller towards the poles [2].

Units: millibar and hectopascal

• Millibar (mb): the unit long used for air pressure on weather maps.
• 1 mb = 1 hectopascal (hPa) = 100 pascals (Pa). The pascal is the SI unit of pressure. It is named after Blaise Pascal (1623–1662) [2].
• 1,013.25 mb = 760 mm of mercury.
• 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].
• The India Meteorological Department (IMD) 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]. An isobar is a line on a map joining places with equal pressure.

Reducing pressure to sea level

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

• This height effect would hide the real horizontal pattern of highs and lows.

• The fix: each station's pressure is reduced to sea level. This means it is corrected to the value it would have at mean sea level, before the map is drawn.

• Sea-level pressure is defined as a pressure value found by the theoretical reduction of barometric pressure to sea level [3].
• Result: isobars on a sea-level map show only the horizontal differences in pressure. These differences are what drive winds.

Vertical variation of pressure

• Vertical variation of pressure: the change in air pressure as you go up through the atmosphere.
• Near the ground, pressure falls fast, by about 1 mb for every 10 m of height.
• The rate is not constant:
• Higher up, the air is thinner (less dense).
• Each 10 m of height then holds less air.

• So the fall in pressure for every 10 m becomes smaller as you rise.

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

Standard atmosphere (NCERT Table 9.1)

Level	Pressure (mb)	Share of sea-level pressure	Temperature (°C)	Average pressure fall in the layer below
Sea level	1,013.25	100%	15.2	—
1 km	898.76	~89%	8.7	~1.1 mb per 10 m (0–1 km)
5 km	540.48	~53%	–17.3	~0.9 mb per 10 m (1–5 km)
10 km	265.00	~26%	–49.7	~0.55 mb per 10 m (5–10 km)

• Reading the table:
• Pressure falls more slowly as height increases.

• Temperature falls about 6.5 °C per km in the lower layers (15.2 °C at sea level to 8.7 °C at 1 km). This is the normal lapse rate, meaning the rate at which temperature drops with height.

• About half of all the air lies below 5.5 km (NCERT). NOAA puts this 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 is higher in warm air and lower in cold air, because warm air is less dense [2].
• Pattern of the fall: pressure and density drop exponentially with height. This means they halve roughly every 5.6 km. They fall to about 37% (1/e) every 7.64 km, a distance called the scale height [5].
• So pressure is about half at ~5.5 km, about a quarter at ~11 km, and so on.

High altitude in India: Khardung La (Class 7)

• Khardung La is a high mountain pass in Ladakh. Pressure there is about 650 mb, roughly two-thirds of sea-level pressure.
• Effect on people:
• Thinner air means less oxygen in each breath.
• Visitors can get breathless or develop altitude sickness.
• They must acclimatise, which means staying a day or two at a lower height so the body can adjust.

Hydrostatic balance: why there are no strong upward winds

• Pressure gradient force: the push on air from a place of higher pressure towards a place of lower pressure.
• Vertical vs horizontal:
• Pressure changes far faster upwards (about 1 mb in 10 m) than sideways. On sea-level maps, isobars are often hundreds of kilometres apart.

• So the vertical pressure gradient force (pushing air upwards) is much larger than the horizontal one.

• Hydrostatic balance: the upward pressure gradient force is almost exactly cancelled by gravity pulling the air down.

• Result:
• There are no strong upward winds blowing into space.
• Winds are mostly horizontal, from high to low pressure.
• Vertical motion is weak. It happens mainly through heating and convection, 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).

Prelims Hooks

• Standard sea-level pressure = 1,013.25 mb = 1,013.25 hPa = 760 mm of mercury = 29.92 inches of mercury.
• 1 mb = 1 hPa = 100 Pa. The pascal is the SI unit, named after Blaise Pascal.
• Near the surface, pressure falls about 1 mb per 10 m of height. The rate is not constant and becomes smaller higher up.
• Pressure ordering 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 (NOAA: 5.6 km).
• Aneroid barometer = no liquid, uses a capsule with the air removed. It is used in aircraft altimeters. The mercury barometer is used to check aneroid barometers.
• Trap: the vertical pressure gradient force is larger than the horizontal one. There are still no strong vertical winds, because gravity balances it (hydrostatic balance).
• Weather maps use sea-level-reduced pressure, not raw station pressure. IMD marks a low with 2–3 closed isobars at 2 hPa intervals.
• Daily 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.

Mains Points

• Why pressure falls with height, and what it does to life:
• Gravity squeezes air into the lowest layers, so half the air lies below about 5.5 km.
• At high places in Ladakh, such as Khardung La (~650 mb), there is less oxygen.

• This means acclimatisation for tourists and troops, health risks for people at high altitude, and lower engine and aircraft performance. Aircraft altimeters work because pressure falls with height in a known way.

• Reducing pressure to sea level is the base of weather analysis:

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

• Pressure differences measured at sea level also define the Southern Oscillation (the Tahiti–Darwin pressure seesaw linked to El Nino and La Nina).

• Hydrostatic balance explains why winds are mainly horizontal:

• Large-scale circulation, including the pressure belts, the trade winds and the westerlies, is driven by small horizontal pressure differences.
• Vertical motion is limited to convection, fronts and lifting by mountains.