Geography: Class 112 — Chapter 10: Water in the Atmosphere

• Humidity: Water vapour present in the air, expressed quantitatively in various ways.
• Absolute Humidity: The actual weight of water vapour per unit volume of air, expressed in grams per cubic metre ($g/m^3$). It varies from place to place and depends on temperature.
• Relative Humidity: The percentage of moisture present in the atmosphere compared to its full capacity at a given temperature. It is greater over oceans and least over continents.
• Saturated Air: Air containing moisture to its full capacity at a given temperature, making it incapable of holding additional moisture.
• Dew Point: The temperature at which saturation occurs for a given sample of air.

Evaporation is the process by which water is transformed from liquid to gaseous state. Heat is the primary cause of evaporation.

• Latent Heat of Vaporisation: The amount of heat energy required to convert a unit mass of liquid into vapour without a change in temperature.
• Factors increasing evaporation: Increase in temperature, low moisture content (dry air), and increased movement of air (wind).

Condensation is the transformation of water vapour into water, caused by the loss of heat. When moist air cools, its capacity to hold water vapour decreases until excess vapour condenses.

• Hygroscopic Condensation Nuclei: Microscopic particles like dust, smoke, and ocean salt that absorb water and facilitate condensation in free air.
• Condensation occurs when: (i) air temperature is reduced to its dew point with constant volume, (ii) both volume and temperature are reduced, or (iii) moisture is added through evaporation.

• Dew: Moisture deposited as water droplets on cooler surfaces of solid objects (stones, grass, leaves). Ideal conditions: clear sky, calm air, high relative humidity, long cold nights. Dew point must be above freezing point.
• Frost: Forms on cold surfaces when condensation occurs below freezing point ($0^\circ\text{C}$). Moisture is deposited as minute ice crystals. Dew point must be at or below freezing point.

• Fog: A cloud with its base at or near the ground, reducing horizontal visibility to less than 1 km. Forms when a large quantity of water vapour condenses suddenly on fine dust particles.
• Mist: Similar to fog but contains more moisture, with each nucleus having a thicker layer of moisture. Limits visibility to between 1 km and 2 km. Frequent over mountains.
• Smog: A condition where fog is mixed with smoke, common in urban and industrial centres.

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

• Rainfall: Precipitation in liquid form.
• Snowfall: Precipitation in the form of fine flakes of snow (hexagonal crystals) when temperature is below $0^\circ\text{C}$.
• Sleet: Frozen raindrops or refrozen melted snow-water, forming small pellets of ice.
• Hail: Solid rounded pieces of ice (hailstones) with concentric layers, formed by rainwater passing through colder layers.

Air heated by the surface becomes light and rises in convection currents. As it rises, it expands, cools, and condenses to form cumulus clouds. Results in heavy, short-duration rainfall with thunder and lightning, common in equatorial regions and summer afternoons.

Occurs when a saturated air mass is forced to ascend over a mountain barrier. As it rises, it expands, cools, and condenses, causing heavy rain on the windward slope. On crossing the peak, the air descends, warms up, and its moisture capacity increases, leaving the leeward slope dry (Rain-shadow area).

Rainfall associated with cyclonic activity and the convergence of different air masses (frontal zones).

• Rainfall decreases steadily from the equator towards the poles.
• Coastal areas receive more rainfall than continental interiors.
• Oceans receive more precipitation than landmasses.
• Between $35^\circ$ and $40^\circ$ N & S, rainfall is heavier on eastern coasts; between $45^\circ$ and $65^\circ$ N & S, westerlies bring heavier rain to western margins.