Air Has Weight and Pressure Changes with Height
While climbing a hill the ears feel blocked and breathing feels a little heavier. Air gets thinner as we go up.
Air has weight so it presses on the earth’s surface. This is air pressure. As altitude increases, air pressure decreases.
Key idea 1 : Air has weight — Tie two identical balloons to the two ends of a balance and fill one with air. That balloon tips down, so air has weight. Whatever has weight presses on what lies below. Likewise the atmosphere’s air presses on the earth’s surface. This pressure is not the same everywhere and changes with time.
Key idea 2 : Altitude and air pressure — Air near the ground holds more dust, water vapour and heavy gases. This proportion falls as we go up and the air gets thinner. So pressure falls as altitude rises. It is highest at sea level because gravity pulls air towards the earth.
Key idea 3 : Factors — Three things affect air pressure: the altitude of the place, the air temperature and the amount of water vapour in the air. The average pressure at sea level is 1013.2 millibars. Air presses on our bodies too, but the pressure inside the body balances it so we do not feel it.
- Easy : Pressure is highest at sea level? — Air is denser there.
- Medium : Pressure at a Himalayan peak? — Very low.
- Exam-type : Average sea-level pressure? — 1013.2 mb.
In the figure: Pressure falls with height.
Height up, pressure down.
Air is not weightless; it has weight, which is why it exerts pressure.
Memory clue: Dense below = high pressure; thin above = low pressure.
Spot the difference
| Place | Air | Pressure |
|---|---|---|
| Sea level | Dense | High |
| High mountain | Thin | Low |
Practice questions
- 1How can we show that air has weight?
- 2What is air pressure?
- 3Why does air pressure fall as height rises?
- 4State the factors affecting air pressure.
- 5Where is pressure highest and why?
- 6What is the average sea-level pressure?
- 7Why do we not feel air pressure?
- 8Why is breathing harder while climbing a mountain?
Show answers (try them yourself first)
- 1The air-filled balloon tips down on a balance.
- 2Pressure on the surface due to air’s weight.
- 3The air becomes thinner higher up.
- 4Altitude, temperature and water vapour.
- 5At sea level; the air is dense there.
- 61013.2 millibars.
- 7Pressure inside the body balances it.
- 8The thin air gives less oxygen.
Temperature and Air Pressure
A flying lantern rises once lit because the air inside becomes hot and light.
Where temperature is high, air pressure is low; where temperature is low, air pressure is high.
Key idea 1 : Warm air — Heated air expands, becomes light and rises. When the candle in a flying lantern is lit the air inside heats and the lantern lifts. The same happens in nature. Light warm air near the ground moves skyward and pressure in that area falls.
Key idea 2 : Cold air — When air cools it contracts, becomes heavy and settles. So in cold regions air is pressed towards the ground and pressure is high. If temperature falls, pressure rises. Hence temperature and pressure are inversely related.
Key idea 3 : Temperature zones and pressure belts — As sun’s heat is uneven, temperature differs from the equator to the poles, creating temperature zones. Temperature zones are continuous and spread over wide latitudes; e.g. the temperate zone spans 23°30′ to 66°30′. Pressure belts are narrow and not continuous; high and low pressure regions occur at different places.
- Easy : If temperature rises, pressure? — Falls.
- Medium : Cold air is? — Heavy.
- Exam-type : Why does a lantern rise? — Hot, light air inside.
In the figure: Warm air → low pressure.
Warm = light = low pressure; cold = heavy = high pressure.
Temperature and pressure do not rise together; they change oppositely.
Memory clue: Hot → low; cold → high.
Spot the difference
| Point | Warm air | Cold air |
|---|---|---|
| Weight | Light | Heavy |
| Movement | Rises | Settles |
| Pressure | Low | High |
Practice questions
- 1What happens to air pressure if temperature rises?
- 2What happens to pressure if temperature falls? Why?
- 3Why does a flying lantern rise?
- 4State the difference between temperature zones and pressure belts.
- 5Why does warm air rise?
- 6What is the extent of the temperate zone?
- 7Why does cold air settle down?
- 8Why do you think pressure belts are not continuous?
Show answers (try them yourself first)
- 1It falls.
- 2It rises; air gets heavy and settles.
- 3The air inside becomes hot and light.
- 4Zones continuous and wide; belts narrow and broken.
- 5It expands and becomes lighter.
- 623°30′ to 66°30′.
- 7It contracts and becomes heavy.
- 8Because heat distribution is uneven.
Pressure Belts of the Earth
While flying a kite, air rises at some spots and sinks at others. The earth too has such huge belts.
There are four kinds of pressure belts from the equator to the poles: equatorial low, mid-latitudinal high, sub-polar low and polar high.
Key idea 1 : Equatorial low pressure belt — Between the Tropic of Cancer and the Tropic of Capricorn the sun’s rays can fall vertically, so temperature is high. Air heats, expands, becomes light and rises. As this goes on continuously a low pressure belt forms in the central part between 5° N and 5° S.
Key idea 2 : Mid-latitudinal high pressure belts — Warm air rises and drifts at height towards the poles. At high altitude the cold makes it cool and heavy and it sinks between about 25° and 35° in both hemispheres, forming high pressure belts. This air is dry so no rain falls; hence most hot deserts of the earth lie at these latitudes.
Key idea 3 : Sub-polar low and polar high pressure — Towards the poles the area between two parallels shrinks, so air’s friction with the surface lessens. This and the earth’s rotation throw the air out and a sub-polar low pressure belt forms between 55° and 65°. At both poles temperature stays below 0 °C all year, so the air is cold and polar high pressure belts form between 80° and 90°. With the sun’s position the belts shift 5° to 7° north in Uttarayan and as much south in Dakshinayan; this is the oscillation of pressure belts.
- Easy : 5° N to 5° S is which belt? — Equatorial low.
- Medium : At 25°–35°? — High pressure; deserts.
- Exam-type : At 80°–90°? — Polar high.
In the figure: Belts: low, high, low, high.
0 low, 30 high, 60 low, 90 high.
The 30° high-pressure belt is dry and hence has deserts; not because of cold but because of dry sinking air.
Memory clue: 0–30–60–90: low, high, low, high.
Spot the difference
| Belt | Latitude | Pressure |
|---|---|---|
| Equatorial | 5° N – 5° S | Low |
| Mid-latitudinal | 25° – 35° | High |
| Sub-polar | 55° – 65° | Low |
| Polar | 80° – 90° | High |
Practice questions
- 1How does the equatorial low pressure belt form?
- 2Why does a high pressure belt form at 25°–35°?
- 3Why are hot deserts found at these latitudes?
- 4How does the sub-polar low pressure belt form?
- 5Why is pressure high in the polar regions?
- 6What is the oscillation of pressure belts?
- 7List all four belts with latitudes.
- 8In which direction do belts shift in Uttarayan?
Show answers (try them yourself first)
- 1Vertical sun heats air and it rises.
- 2The cooled air from above sinks.
- 3The sinking air is dry; no rain.
- 4Less friction and rotation throw air out.
- 5Temperature stays below 0 °C.
- 6Belts shifting 5°–7° with the sun.
- 7Equatorial 0°, mid 30°, sub-polar 60°, polar 90°.
- 8Northwards (5° to 7°).
Isobars, Measurement and Effects of Air Pressure
Weather maps show lines like contours. They join places having the same pressure.
A line on a map joining places of equal pressure is an isobar. Air pressure is measured in millibars with an instrument called a barometer.
Key idea 1 : Isobars — Isobars show how pressure is distributed. Close isobars mean pressure changes quickly and winds are strong. Widely spaced isobars mean slow change. Compare their pattern across hemispheres, oceans and continents.
Key idea 2 : Measurement — Air pressure is measured in millibars using a barometer. The average pressure at sea level is 1013.2 millibars. As per the textbook the air column above a person’s head weighs about 1000 kg, yet we do not feel it.
Key idea 3 : Effects — Differences in air pressure give rise to winds. These lead to storms and convectional rain. Pressure also affects breathing; at height low pressure makes breathing harder. Winds blow from high to low pressure, so isobars are valuable in weather forecasts.
- Easy : Pressure is measured in? — Millibars.
- Medium : Instrument to measure pressure? — Barometer.
- Exam-type : Close isobars mean? — Strong winds.
In the figure: Pressure differences make winds.
As per the textbook the weight of the air column over each person’s head is believed to be about 1000 kg, yet it is balanced by pressure inside the body so we do not feel it. (as per the textbook; verify with the book)
Millibar = unit; barometer = instrument.
Isobars are not equal-height lines; they join equal pressure.
Memory clue: Isobar = equal pressure; lines close = strong wind.
Spot the difference
| Idea | Meaning | Use |
|---|---|---|
| Isobar | Line joining equal pressure | Pressure distribution |
| Barometer | Pressure instrument | Measurement |
| Millibar | Unit of pressure | Measure |
Practice questions
- 1What is an isobar?
- 2In which unit is air pressure measured?
- 3What is a barometer?
- 4What happens if isobars are close together?
- 5What are the effects of air pressure?
- 6State the average sea-level pressure.
- 7How does pressure affect breathing?
- 8How are isobars useful in forecasts?
Show answers (try them yourself first)
- 1A line joining places of equal pressure.
- 2Millibar.
- 3An instrument that measures air pressure.
- 4Pressure changes quickly and winds are strong.
- 5Winds, storms, convectional rain, breathing.
- 61013.2 millibars.
- 7Low pressure at height makes breathing harder.
- 8They show where winds will blow.