Atmosphere and Weather

What to remember

• The atmosphere has five layers. Troposphere (weather), stratosphere (ozone), mesosphere, thermosphere (ionosphere) and exosphere.
• Pressure differences drive winds. Air moves from high to low pressure, and Earth's rotation bends it (Coriolis effect). The result is planetary winds, local winds and monsoons.
• Weather systems come from air masses, fronts and storms. Cyclones and anticyclones, tropical and temperate, bring rain and extreme weather; Andhra's coast is exposed to tropical cyclones.

Composition and structure

The atmosphere is a layer of gases held by gravity. By volume dry air is about 78% nitrogen, 21% oxygen, about 1% argon, and small amounts of carbon dioxide, neon, helium, ozone and others. Water vapour varies from near 0 to about 4%. Dust and salt particles act as nuclei for cloud formation.

The boundary between layers is named with "pause": tropopause, stratopause, mesopause. The tropopause is higher at the Equator than at the poles.

Heating of the atmosphere

Solar energy arrives as short-wave radiation (insolation). Earth gives out long-wave terrestrial radiation, which heats the air from below. The air is warmed mainly by Earth's surface, not directly by the Sun.

• Heat budget: the energy received equals the energy lost, so Earth does not keep heating.
• Albedo: the share of sunlight reflected; high for snow and clouds, low for forests.
• Greenhouse effect: gases such as carbon dioxide, methane and water vapour trap long-wave heat. It keeps Earth warm enough for life; extra greenhouse gases cause global warming.
• Inversion of temperature: temperature rises with height for a layer, usually on clear, calm winter nights in valleys; it causes fog and traps pollution.

Atmospheric pressure

Atmospheric pressure is the weight of air above a point. It is measured by a barometer in millibars (mb) or hectopascals; standard sea-level pressure is about 1013 mb. Pressure falls with height and is shown on maps by isobars.

Pressure belts:

• Equatorial low (doldrums) near 0°–5°: thermal, due to heating.
• Sub-tropical highs (horse latitudes) near 30°N and 30°S: dynamic, due to sinking air.
• Sub-polar lows near 60°N and 60°S.
• Polar highs near the poles: thermal, due to cold.

Winds

Wind is horizontal movement of air from high to low pressure. Speed is measured by an anemometer, direction by a wind vane. Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (Ferrel's law).

Planetary (permanent) winds:

In the southern oceans the westerlies are strong and called the "Roaring Forties", "Furious Fifties" and "Screaming Sixties".

Seasonal winds: The monsoon winds of South Asia reverse their direction with season. In summer the south-west monsoon blows from sea to land; in winter the north-east monsoon blows from land to sea. Andhra Pradesh and Tamil Nadu receive much rain from the retreating (north-east) monsoon in October to December.

Local winds:

• Land and sea breezes: sea breeze by day, land breeze by night.
• Mountain and valley breezes.
• Hot local winds: Loo (north India), Foehn (Alps), Chinook (Rockies, "snow eater").
• Cold local winds: Mistral (France).
• In India, the pre-monsoon "mango showers" and the Nor'westers (Kalbaisakhi) of Bengal.

Humidity, clouds and precipitation

Humidity is the water vapour in the air. Relative humidity is the actual vapour as a percent of what the air can hold at that temperature. Saturated air is at its dew point.

Clouds: cirrus (high, wispy), cumulus (puffy, fair weather), stratus (layered), nimbus (rain-bearing).

Types of rainfall:

• 1. Convectional: heated moist air rises; common in the tropics.
• 2. Orographic (relief): air forced up mountains; windward side wet, leeward side dry (rain shadow).
• 3. Cyclonic (frontal): warm and cold air meet.

Air masses and fronts

An air mass is a large body of air with similar temperature and humidity. Names: continental (dry), maritime (moist), polar (cold), tropical (warm).

A front is the boundary where two different air masses meet.

• Cold front: cold air pushes under warm air; short, heavy rain.
• Warm front: warm air rises over cold air; long, light rain.
• Stationary and occluded fronts also occur.

Cyclones and anticyclones

Temperate (extratropical) cyclones form along fronts between polar and westerly air in mid-latitudes. They move west to east.

Tropical cyclones form over warm oceans (above about 26–27°C), at least 5° from the Equator, and need Coriolis force. They have a calm centre called the eye. Names by region: cyclone (Indian Ocean), hurricane (Atlantic), typhoon (China Sea), willy-willy (Australia). Winds blow anticlockwise in the northern hemisphere and clockwise in the southern. The Bay of Bengal produces strong cyclones, especially in October-November, and the Andhra coast is vulnerable. Storm surge and heavy rain cause most damage. Warnings are issued by the India Meteorological Department.

Anticyclones: high pressure at the centre; winds move out clockwise in the north; clear, dry weather.

Thunderstorms and tornadoes: Thunderstorms form from strong convection with lightning; tornadoes are narrow, violent whirls. Western disturbances are winter storms from the Mediterranean that bring rain to north-west India.

Monsoon, weather forecasting and the Indian setting

The Indian monsoon is explained by the differential heating of land and sea and by the shift of the Inter-Tropical Convergence Zone (ITCZ) northward in summer. The jet stream (a narrow band of very fast upper-air winds) and the temperature of the Indian Ocean also affect its onset. The south-west monsoon normally bursts first on the Kerala coast around the beginning of June. A weak or break in the monsoon causes dry spells. The Western Ghats receive heavy rain on their windward side, leaving much of Rayalaseema in a rain shadow.

Seasons in India: winter (December to February), summer or hot season (March to May), the south-west monsoon (June to September), and the retreating monsoon (October to November). Andhra's coastal districts feel humid heat in summer, and heavy cyclonic rain in the post-monsoon season.

Weather and climate: Weather is the state of the atmosphere at a place over a short time; climate is the average of many years. The elements are temperature, pressure, wind, humidity and precipitation.

Forecasting tools: weather balloons, radar, satellites (the INSAT series) and automatic weather stations. The India Meteorological Department issues forecasts and cyclone warnings, using colour-coded alerts.

Climate change link: Rising greenhouse gases, loss of forests and burning of fuels increase warming and may affect rainfall patterns and cyclone strength. Mitigation includes cleaner energy and afforestation.

Ozone depletion: Chlorofluorocarbons (CFCs) destroy ozone in the stratosphere and let more ultraviolet reach Earth. The Montreal Protocol (1987) controls substances that deplete ozone.

Exam traps

• 1. Weather happens in the troposphere; the ozone layer is in the stratosphere.
• 2. Mesosphere is the coldest layer; the thermosphere has the highest temperatures.
• 3. Trade winds blow from the sub-tropical high to the equatorial low.
• 4. Coriolis: right in the north, left in the south.
• 5. Cold front brings short heavy rain; warm front brings long gentle rain.
• 6. Tropical cyclone has an eye; temperate cyclone does not.
• 7. Loo is hot (India); Mistral is cold (France).
• 8. Barometer measures pressure; anemometer measures speed.

One-liners

• 1. Nitrogen forms about 78% of dry air.
• 2. The ozone layer is in the stratosphere.
• 3. The ionosphere reflects radio waves.
• 4. Normal lapse rate is about 6.5°C per km.
• 5. Sea-level pressure is about 1013 mb.
• 6. Isobars join places of equal pressure.
• 7. Trade winds blow from the north-east in the Northern Hemisphere.
• 8. The eye of a cyclone is calm.
• 9. The Chinook is a warm wind of the Rockies.
• 10. Orographic rain falls on the windward side.
• 11. Western disturbances come from the Mediterranean.
• 12. Cumulonimbus clouds bring thunderstorms.

Practice questions

1. Which gas forms the largest share of dry air by volume?

   1. Argon
   2. Oxygen
   3. Nitrogen
   4. Carbon dioxide
   Answer

   C. Nitrogen

   Nitrogen is about 78 percent of dry air.

2. Weather phenomena occur mainly in the

   1. Thermosphere
   2. Mesosphere
   3. Stratosphere
   4. Troposphere
   Answer

   D. Troposphere

   The troposphere holds most air and water vapour.

3. The ozone layer is found in the

   1. Mesosphere
   2. Exosphere
   3. Stratosphere
   4. Troposphere
   Answer

   C. Stratosphere

   It absorbs ultraviolet radiation.

4. The coldest layer of the atmosphere is the

   1. Stratosphere
   2. Thermosphere
   3. Mesosphere
   4. Troposphere
   Answer

   C. Mesosphere

   Temperatures fall to the lowest values here.

5. Radio waves are reflected back to Earth by the

   1. Ozone layer
   2. Troposphere
   3. Tropopause
   4. Ionosphere
   Answer

   D. Ionosphere

   The ionosphere lies in the thermosphere.

6. The instrument used to measure atmospheric pressure is the

   1. Rain gauge
   2. Barometer
   3. Hygrometer
   4. Anemometer
   Answer

   B. Barometer

   Anemometer measures wind speed.

7. Lines joining places with equal atmospheric pressure are called

   1. Isotherms
   2. Isohyets
   3. Contours
   4. Isobars
   Answer

   D. Isobars

   Isotherms join equal temperature; isohyets equal rainfall.

8. The doldrums are associated with

   1. Sub-polar high pressure
   2. Sub-tropical high pressure
   3. Equatorial low pressure
   4. Polar high pressure
   Answer

   C. Equatorial low pressure

   Heated air rises and winds are calm.

9. Trade winds blow from the sub-tropical high to the

   1. Equatorial low
   2. Sub-polar low
   3. Sub-tropical high
   4. Polar high
   Answer

   A. Equatorial low

   They are steady easterlies.

10. In the Northern Hemisphere trade winds blow from the

    1. North-west
    2. North-east
    3. South-west
    4. South-east
    Answer

    B. North-east

    Coriolis deflects them to the right.

11. The Roaring Forties are strong

    1. Monsoons of India
    2. Easterlies of the Arctic
    3. Westerlies of the Southern Hemisphere
    4. Trade winds of the Equator
    Answer

    C. Westerlies of the Southern Hemisphere

    There is little land in the south to slow them.

12. The Loo is a

    1. Cold wind of France
    2. Warm wind of the Rockies
    3. Cold wind of the Alps
    4. Hot, dry wind of north India
    Answer

    D. Hot, dry wind of north India

    It blows in summer over the plains.

13. The Chinook is a warm wind blowing over the

    1. Rocky Mountains
    2. Andes
    3. Himalayas
    4. Alps
    Answer

    A. Rocky Mountains

    It melts snow and is called the snow eater.

14. The Mistral is a

    1. Monsoon wind in India
    2. Cold wind in France
    3. Hot wind in Australia
    4. Warm wind in Canada
    Answer

    B. Cold wind in France

    It blows down the Rhone valley.

15. Rain falling on the windward side of mountains is called

    1. Cyclonic rainfall
    2. Convectional rainfall
    3. Orographic rainfall
    4. Frontal fog
    Answer

    C. Orographic rainfall

    Air is forced up the slope.

16. Convectional rainfall is most common in

    1. Equatorial regions
    2. Desert coasts
    3. Polar regions
    4. High plateaus
    Answer

    A. Equatorial regions

    Strong heating makes moist air rise.

17. The calm centre of a tropical cyclone is called the

    1. Trough
    2. Eye
    3. Front
    4. Ridge
    Answer

    B. Eye

    It has low pressure and light winds.

18. Tropical cyclones in the Northern Hemisphere blow

    1. Clockwise
    2. In straight lines
    3. Upward only
    4. Anticlockwise
    Answer

    D. Anticlockwise

    The Coriolis force deflects the winds to the right.

19. A tropical cyclone usually needs sea surface temperature of at least about

    1. 10°C
    2. 15°C
    3. 26-27°C
    4. 5°C
    Answer

    C. 26-27°C

    Warm water supplies the heat and moisture.

20. Tropical cyclones of the Atlantic Ocean are called

    1. Hurricanes
    2. Cyclones
    3. Typhoons
    4. Willy-willies
    Answer

    A. Hurricanes

    Typhoon is used in the China Sea.

21. A cold front usually brings

    1. No rain at all
    2. Short, heavy rain
    3. Permanent snow
    4. Long, light rain
    Answer

    B. Short, heavy rain

    Cold air pushes under warm air.

22. Western disturbances bring winter rain to north-west India and originate from the

    1. Arabian Sea
    2. Pacific Ocean
    3. Bay of Bengal
    4. Mediterranean region
    Answer

    D. Mediterranean region

    They are extratropical systems.

23. Temperature inversion most often occurs on

    1. Cloudy monsoon days
    2. Clear, calm winter nights
    3. Hot desert noons
    4. Windy summer afternoons
    Answer

    B. Clear, calm winter nights

    The ground cools and chills the air above it.

24. The normal lapse rate in the troposphere is about

    1. 6.5°C per km
    2. 20°C per km
    3. 0.5°C per km
    4. 1°C per km
    Answer

    A. 6.5°C per km

    Temperature falls with height.

25. Why do cyclones weaken after reaching land?

    1. The Coriolis force becomes zero
    2. They gain heat from the soil
    3. They meet more oxygen
    4. They lose the moisture supply from warm sea water
    Answer

    D. They lose the moisture supply from warm sea water

    Without warm ocean water the energy source is cut off.

26. Why is the troposphere deeper at the Equator than at the poles?

    1. The Equator has more ozone
    2. Winds are weaker at the poles
    3. Strong heating causes air to rise to greater heights
    4. The Moon pulls the air upwards
    Answer

    C. Strong heating causes air to rise to greater heights

    Convection is intense in the tropics.

27. The earth's surface heats the air mainly through

    1. Direct short-wave insolation
    2. Long-wave terrestrial radiation
    3. Ozone absorption
    4. Moonlight
    Answer

    B. Long-wave terrestrial radiation

    The atmosphere is largely transparent to incoming short-wave radiation.

28. Albedo refers to

    1. The weight of air above a place
    2. The speed of wind
    3. The humidity of a cloud
    4. The share of sunlight reflected by a surface
    Answer

    D. The share of sunlight reflected by a surface

    Snow has a high albedo.

29. The Andhra coast receives much of its rain from the

    1. North-east (retreating) monsoon
    2. South-west monsoon only
    3. Western disturbances
    4. Mistral
    Answer

    A. North-east (retreating) monsoon

    October to December rainfall is important in coastal Andhra.

30. Consider the statements. 1. The ozone layer lies in the troposphere. 2. The ozone layer absorbs ultraviolet radiation.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Ozone lies in the stratosphere; only 2 is correct.

31. Consider the statements on winds. 1. It deflects winds to the right in the Southern Hemisphere. 2. Coriolis force deflects winds to the right in the Northern Hemisphere.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    In the south it deflects to the left.

32. Consider the statements on cyclones. 1. Tropical cyclones form over warm oceans. 2. They have no eye.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    They have a calm eye.

33. Consider the statements on fronts. 1. A warm front brings sudden heavy rain. 2. A cold front brings short and heavy rain.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    A warm front brings long, gentle rain.

34. Consider the statements on atmospheric pressure. 1. Pressure decreases with height. 2. Isobars join places of equal temperature.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Isobars join places of equal pressure.

35. Consider the statements on monsoon winds. 1. They blow from the sea to the land in winter. 2. They reverse direction with the season.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    In winter they blow from land to sea.

36. Consider the statements on layers. 1. The mesosphere is colder than the thermosphere. 2. The ionosphere is part of the thermosphere.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

37. Consider the statements on rainfall. 1. Orographic rain is heavy on the leeward side. 2. Convectional rain is typical in the tropics.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Windward side is wet; leeward is the rain shadow.

38. Consider the statements on anticyclones. 1. They bring clear weather. 2. They have low pressure at the centre.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    They have high pressure at the centre.

39. Match wind with type: (1) Loo (2) Chinook (3) Mistral (4) Sea breeze

    1. Warm, cold, hot, daytime offshore
    2. Cold, hot, warm, night offshore
    3. Hot, cold, warm, night onshore
    4. Hot, warm snow-eater, cold, daytime onshore
    Answer

    D. Hot, warm snow-eater, cold, daytime onshore

    Standard local winds.

40. Match layer with feature: (1) Troposphere (2) Stratosphere (3) Mesosphere (4) Thermosphere

    1. Coldest, ionosphere, weather, ozone
    2. Ionosphere, coldest, ozone, weather
    3. Weather, ozone, coldest, ionosphere
    4. Ozone, weather, ionosphere, coldest
    Answer

    C. Weather, ozone, coldest, ionosphere

    Standard layer features.

41. Match instrument with use: (1) Barometer (2) Anemometer (3) Hygrometer (4) Rain gauge

    1. Rainfall, humidity, wind speed, pressure
    2. Wind speed, pressure, rainfall, humidity
    3. Humidity, rainfall, pressure, wind speed
    4. Pressure, wind speed, humidity, rainfall
    Answer

    D. Pressure, wind speed, humidity, rainfall

    Standard meteorological instruments.

42. Match: (1) Trade winds (2) Westerlies (3) Polar easterlies (4) Doldrums

    1. 30° to 60°, calm Equator, poles to 60°, 30° to Equator
    2. Poles to 60°, 30° to Equator, calm Equator, 30° to 60°
    3. Sub-tropical high to Equator, 30° to 60°, poles to 60°, calm Equator
    4. Calm Equator, poles to 60°, 30° to 60°, 30° to Equator
    Answer

    C. Sub-tropical high to Equator, 30° to 60°, poles to 60°, calm Equator

    The belts of planetary winds.

43. Water vapour in the air as a percentage of the amount the air can hold is

    1. Dew point
    2. Absolute humidity
    3. Albedo
    4. Relative humidity
    Answer

    D. Relative humidity

    It depends on the temperature.

44. The cloud type that is high, thin and wispy is

    1. Stratus
    2. Nimbus
    3. Cirrus
    4. Cumulus
    Answer

    C. Cirrus

    Cirrus clouds are made of ice crystals.

45. The greenhouse gas that traps long-wave heat includes

    1. Carbon dioxide
    2. Argon
    3. Nitrogen
    4. Neon
    Answer

    A. Carbon dioxide

    Carbon dioxide, methane and water vapour trap heat.