Water in the Atmosphere

(d) Oxygen
Reason: While water vapour is crucial for weather, oxygen is biologically essential for the survival of human beings (respiration).

(c) Evaporation
Reason: Evaporation is the process by which water transforms from liquid to gaseous state (vapour). Condensation is the reverse (vapour to liquid).

(d) Saturated air
Reason: When air cannot hold any more water vapour at a given temperature, it is called saturated.

(a) Cirrus
Reason: Cirrus clouds form at high altitudes (8,000 – 12,000m) and are composed of ice crystals, appearing feathery or fibrous.

The three common types of precipitation are:

• Rainfall: Water droplets (liquid form).
• Snowfall: Ice crystals (solid form) when temperature is below $0^\circ C$.
• Hail: Hard ice pellets formed in cumulonimbus clouds.

Relative Humidity (RH) is the ratio of the actual amount of water vapour present in the air to the maximum amount of water vapour the air can hold at that temperature. It is expressed as a percentage.
$$RH = \frac{\text{Actual Moisture}}{\text{Moisture Capacity}} \times 100$$

Water vapour is primarily derived from the evaporation of oceans and water bodies on the surface of the earth. Since the source is at the bottom, the concentration is highest near the surface and diminishes rapidly as we go higher due to gravity and lower temperatures condensing the vapour.

Formation: Clouds are formed by the condensation of water vapour around hygroscopic nuclei (dust particles) at considerable heights. This happens when rising air cools adiabatically.
Classification (by height):

• High Clouds: Cirrus, Cirrostratus, Cirrocumulus.
• Middle Clouds: Altostratus, Altocumulus.
• Low Clouds: Stratocumulus, Nimbostratus.
• Vertical Clouds: Cumulus, Cumulonimbus.

[Image of cloud classification chart]

The global rainfall pattern is influenced by latitude, pressure belts, and mountains:

• Equatorial Zone: Receives the highest rainfall (heavy convectional rain) due to the ITCZ and intense heating.
• Subtropical Highs ($20^\circ-30^\circ$): Rainfall decreases. The western margins of continents are dry (Deserts) due to offshore Trade Winds and cold currents, while eastern margins receive rain from Trade Winds.
• Middle Latitudes ($35^\circ-40^\circ$): Western margins receive rain from Westerlies (Mediterranean/Marine West Coast), decreasing towards the interior.
• Polar Regions: Very low precipitation (mostly snow) due to extremely cold air holding little moisture.
• General Trend: Rainfall is generally higher over oceans than land and higher in coastal areas than continental interiors.

[Image of world annual precipitation map]

Forms of Condensation: Condensation occurs when the temperature of air falls below the dew point. Forms include Dew, Frost, Fog, Mist, and Clouds.

1. Dew:

• Process: When moisture forms directly on cooler surfaces (leaves, stones) as water droplets.
• Conditions: Requires clear skies, calm air, high relative humidity, and cold nights.
• Key Factor: The Dew Point must be above freezing point ($0^\circ C$).

2. Frost:

• Process: When moisture forms on cold surfaces in the form of minute ice crystals.
• Key Factor: The Dew Point must be at or below freezing point ($0^\circ C$).
• Effect: It is harmful to crops.

(Guidance for Student Activity)
To track extreme rainfall (1st June – 31st Dec):

• Season: This period covers the Southwest Monsoon (June-Sept) and Retreating Monsoon/NE Monsoon (Oct-Dec).
• What to look for: News headlines about “Cloudbursts” (e.g., in Himachal/Uttarakhand), “Floods” (e.g., Assam/Bihar), or “Cyclones” (hitting Odisha/Andhra/Tamil Nadu coasts in Oct-Nov).
• Compilation: Create a scrapbook pasting snippets and marking the locations on a physical map of India to see the spatial pattern of monsoon intensity.