TEMPERATURE INVERSION

1. NORMAL LAPSE RATE

• Under normal atmospheric conditions, temperature decreases with increase in height in the troposphere.
• The average rate of decrease is:
• 6.5°C per 1000 metres or about 1°C per 165 metres.
• This is called the normal lapse rate or average vertical temperature gradient.
• Example: If temperature at sea level is 30°C, then at 1000 m it will be around 23.5°C.

2. WHAT IS TEMPERATURE INVERSION?

• Sometimes, instead of decreasing, temperature increases with height.
• This reversal of the normal lapse rate is called Temperature Inversion or Thermal Inversion.
• In this condition:
  • Cold air remains below and warm air lies above.
  • Thus, lower layers are colder and upper layers are warmer.
• Hence, temperature inversion is also called negative lapse rate.

3. WHY TEMPERATURE INVERSION OCCURS

• It occurs mainly due to:
  • Radiational cooling of the ground,
  • Calm and stable air,
  • Subsiding air in high pressure areas,
  • Movement of air masses,
  • Valley topography.
• It is generally short-lived but very common, especially in winter.

4. IDEAL CONDITIONS FOR TEMPERATURE INVERSION

• Long nights so that heat loss is more,
• Clear skies for free radiation,
• Calm and stable air so no vertical mixing takes place,
• Dry air,
• Snow covered surface in cold regions.

5. EFFECTS OF TEMPERATURE INVERSION

A. Fog and Smog Formation

• Fog is a cloud formed at ground level.
• In cities, fog mixes with smoke and becomes smog.
• Example: London smog of 1952 killed about 4000 people. Delhi NCR faces severe smog in winters.

B. Health Problems

• Breathing difficulty, asthma, bronchitis, eye irritation and other respiratory diseases increase.

C. Transport Hazards

• Low visibility leads to road accidents, train delays and flight diversions.

D. Agricultural Damage

• Frost damages wheat, barley, mustard, potato and vegetables.
• Sugarcane in North India develops Red Rot disease.
• Crops in valley bottoms suffer more.

E. Effect on Vegetation and Settlement

• Valley bottoms are colder and foggy, upper slopes are warmer.
• Hence orchards, settlements and hotels are built on upper slopes.
• Examples: Alps, Himalayas, Brazil coffee plantations.

F. Effect on Cloud Formation

• Strong inversion prevents vertical growth of clouds, so convection and rainfall are suppressed.

G. Effect on Diurnal Temperature Range

• Day and night temperature difference becomes very small.

6. TYPES OF TEMPERATURE INVERSION

1. Frontal Inversion

• Occurs due to meeting of warm and cold air masses in temperate cyclones.
• Warm air overrides cold air, producing inversion.
• It has a slope, cloudy and humid conditions and is unstable.

2. Valley Inversion or Air Drainage Inversion

• Occurs in mountain valleys.
• Cold dense air flows down the slopes and collects in the valley bottom.
• Upper layers remain warmer.
• Very strong in middle and high latitudes and in Himalayan and Alpine regions.

3. Ground Inversion or Surface Inversion

• Most common type.
• Occurs on clear, calm and cold nights.
• Ground cools rapidly, air near ground becomes colder than air above.
• If air cools below dew point, fog forms.
• Disappears after sunrise.

4. Subsidence Inversion or Upper Surface Inversion

• Occurs in high pressure areas.
• Air slowly sinks, gets compressed and heated.
• Upper air becomes warmer than lower air.
• Common over subtropical oceans and over continents in winter.

5. Marine Inversion

• Occurs near coastal areas.
• Cold air from ocean moves inland and flows below warmer land air.
• Common in spring season and western coastal regions.

7. ECONOMIC AND HUMAN SIGNIFICANCE

• Valley bottoms are colder, foggier and more polluted.
• Hence agriculture, orchards, settlements and hotels are built on upper slopes.
• Examples: Himalayas, Alps, Brazil coffee belts.

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Subject: Geography

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