Normal lapse rate

Geography glossary

Also called: Lapse rate, Environmental lapse rate, Effect of altitude on temperature · Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 7, Ch 3 "Climates of India"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

Normal lapse rate (also called the environmental lapse rate) is the rate at which air temperature falls as you go up in still air in the troposphere: about 6.5°C per 1,000 m, or about 1°C for every 165 m.

It explains why high places are colder than low places at the same latitude, such as Darjiling and Agra. It is also the standard used to judge whether air is stable or unstable, and that decides whether the sky stays clear or storms form.

Explanation

Why temperature falls with height

  • The air is heated from below, not from the top.
  • Sunlight (insolation) passes through the air and heats the ground first.

  • The ground then warms the air above it by conduction, convection and long-wave (terrestrial) radiation.

  • Higher up, you are farther from this heat source.

  • So each higher layer of air gets less heat from the ground.

  • Higher air is thinner.

  • It holds less water vapour and dust.

  • So it traps less heat and stays colder.

  • Result: in normal conditions it is warmest at the surface, and temperature drops steadily as you climb, by about 6.5°C for every kilometre.

Normal lapse rate vs adiabatic lapse rates

  • The normal lapse rate describes the still air around us. It is a measure of how temperature changes from one height to the next.
  • An adiabatic lapse rate is different. It is the rate at which a rising parcel (bubble) of air cools only because it expands. "Adiabatic" means no heat comes in from the air around it and none goes out.
  • How a rising parcel cools:
  • Air rises → air pressure falls with height → the parcel expands.
  • Expansion uses up the parcel's own energy → the parcel cools.
  • When air sinks, the opposite happens: it is squeezed and warms up.
Rate Value Why
Normal (environmental) lapse rate 6.5°C per km Real temperature of the still air around the parcel
Dry adiabatic lapse rate ~9.8°C per km Unsaturated air cools only by expanding
Saturated (wet) adiabatic lapse rate ~4–7°C per km Condensation releases latent heat (hidden heat stored in water vapour), which slows the cooling

Stability: comparing the air with the rising parcel

  • Stable air:
  • The surrounding air cools more slowly than the rising parcel.
  • The parcel soon becomes colder and heavier than the air around it, so it sinks back.

  • The sky stays clear, or there are only flat layer clouds.

  • Unstable air:

  • The surrounding air cools faster than the parcel, so the environmental lapse rate is steep.
  • The parcel stays warmer and lighter than the air around it, so it keeps rising.
  • This builds tall cumulonimbus clouds, thunderstorms and pre-monsoon storms.

When the rule breaks: temperature inversion

  • In a temperature inversion, temperature rises with height for a while instead of falling.
  • Conditions it needs:
  • Long, clear winter nights, when the ground loses heat quickly and cools the air touching it.

  • Valleys, where cold, heavy air drains down the slopes on calm nights.

  • Result: valley floors become colder than the slopes and get frost and fog. The air becomes very stable.

Where it occurs

  • Everywhere in the troposphere (the lowest layer of the atmosphere). The 6.5°C per km figure is an average.
  • Darjiling vs Agra (India, both ~27°N):
  • Darjiling (West Bengal, about 2,000 m) averages about 4°C in January.
  • Agra (Uttar Pradesh, on the plains) averages about 16°C in January.

  • The gap of about 12°C comes mostly from height, not latitude.

  • Hill stations have cool summers because they are high up:

  • Shimla (Himachal Pradesh)
  • Ooty (Nilgiris, Tamil Nadu)

  • Shillong (Meghalaya)

  • Leh (Ladakh, about 3,500 m) has very cold winters, with monthly means well below 0°C. Its height adds to its dryness and its distance from the sea (Class 9, Table 3.3).

  • Himalaya and Western Ghats: altitude makes these ranges cooler than the plains around them.
  • Unstable air (steep lapse rate): the Kal Baisakhi (Nor'westers) of West Bengal are pre-monsoon thunderstorms.
  • Inversions (rule reversed): Himalayan valley floors on calm winter nights get frost and fog. This is why orchards are planted on the slopes instead.

Don't confuse with

  • Dry adiabatic lapse rate: about 9.8°C per km. It is the cooling of a rising parcel of unsaturated air. The normal lapse rate (6.5°C per km) is the temperature of the still air around it. Exams often offer 9.8°C as a trap answer for the "normal lapse rate".
  • Saturated (wet) adiabatic lapse rate: about 4–7°C per km. It is lower than the dry rate because condensation releases latent heat.
  • Temperature inversion: the opposite of the normal lapse rate. Temperature rises with height. It happens on long, clear winter nights and in valleys.
  • Latitude effect: temperature also changes from south to north (Thiruvananthapuram is warmer than Amritsar in winter). Darjiling and Agra lie at the same latitude, so their difference is due to altitude, not latitude.

Prelims Hooks

  • Normal (environmental) lapse rate = 6.5°C per 1,000 m, or about 1°C per 165 m. It is not 9.8°C, which is the dry adiabatic rate.
  • Order by rate of cooling, highest to lowest: dry adiabatic (~9.8°C/km) > normal lapse rate (6.5°C/km) > saturated adiabatic (~4–7°C/km).
  • The saturated rate is lower than the dry rate because condensation releases latent heat.
  • Darjiling (~4°C) and Agra (~16°C) in January lie at the same latitude (~27°N). The difference is due to altitude.
  • Temperature falls with height because the air is heated from below by the earth's surface, not directly by the Sun.
  • Hill station–state pairs: Shimla–Himachal Pradesh, Ooty–Tamil Nadu (Nilgiris), Shillong–Meghalaya.

Mains Points

  • Altitude as a control of temperature in India:
  • The lapse rate makes the Himalaya and the Western Ghats cooler than the plains at the same latitude.
  • This creates hill stations with cool summers (Shimla, Ooty, Shillong) and very cold high deserts such as Leh.

  • Together with latitude and continentality, it explains why heat waves mainly hit the north-west and central plains, not the hills (GS-I).

  • Lapse rate and atmospheric stability as a weather control:

  • If the environmental lapse rate is steeper than the rising parcel's rate, the air is unstable. This brings cumulonimbus clouds, thunderstorms and Nor'westers (Kal Baisakhi) in West Bengal.

  • When the rate is reversed (temperature inversion on winter nights and in valleys), the air is stable. This traps fog, frost and smog near the ground, which links temperature with pollution and crop damage (GS-I, GS-III).

  • Uses in agriculture and settlement:

  • Inversions turn valley floors into frost pockets that damage tea, fruit and vegetables.
  • So in the Himalaya, orchards and villages are placed on the slopes instead (GS-I).

Related concepts

Read more

Sources

  1. 1Class 7, Ch 3 "Climates of India"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 7 "Composition and Structure of Atmosphere"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature" (primary)