Introduction
Water exists in the atmosphere in all three states — gaseous (water vapour), liquid (cloud droplets, rain), and solid (ice crystals, snow, hail). This chapter examines the processes by which water enters the atmosphere, forms clouds, and returns to the surface as precipitation. These processes form the hydrological cycle and are central to weather and climate.
Evaporation and Humidity
- Evaporation is the process by which liquid water converts to water vapour. It requires energy (latent heat) and depends on:
- Temperature (higher temperature = more evaporation)
- Wind speed (wind removes saturated air, allowing more evaporation)
- Humidity of overlying air (drier air = more evaporation)
- Surface area of water
- Humidity is the amount of water vapour in the air:
- Absolute humidity: The actual mass of water vapour in a given volume of air (g/m3). It varies with temperature.
- Relative humidity (RH): The ratio of actual water vapour present to the maximum possible at that temperature, expressed as a percentage. RH = (actual vapour pressure / saturation vapour pressure) x 100.
- Specific humidity: Mass of water vapour per unit mass of moist air (g/kg). Useful because it does not change with pressure or temperature (unlike relative humidity).
Saturation and Dew Point
Air can hold a maximum amount of water vapour at any given temperature — this is called the saturation point. Warm air can hold more water vapour than cold air.
The dew point is the temperature to which air must be cooled (at constant pressure) for it to become saturated (RH reaches 100%). Below the dew point, condensation occurs.
Condensation
- 1.Condensation is the process by which water vapour converts to liquid water. It occurs when:
- 2.Air cools to or below the dew point.
- 3.Condensation nuclei (tiny particles of dust, sea salt, smoke) are present.
- Forms of condensation near the surface:
- Dew: Water droplets that form on cold surfaces (grass, leaves) when air above cools below the dew point at night.
- Frost: If the dew point is below 0 degrees C, water vapour deposits directly as ice crystals on surfaces (freezing dew is not frost).
- Fog: Tiny water droplets suspended in the lower atmosphere, reducing visibility below 1 km. Forms when air near the ground is cooled to the dew point (radiation fog on calm, clear nights; advection fog when warm, moist air moves over cold surfaces).
- Mist: Similar to fog but visibility between 1–2 km; less dense.
- Smog: Mixture of smoke and fog (or photochemical pollutants).
Cloud Formation
Clouds form when air rises and cools to the dew point at altitude. Rising air cools at the dry adiabatic lapse rate (DALR) of ~10 degrees C per 1000 m (unsaturated air). Once saturation is reached, condensation releases latent heat, slowing the cooling to the saturated adiabatic lapse rate (SALR) of ~5–6 degrees C per 1000 m.
Clouds are classified by altitude and appearance:
| Type | Altitude | Description |
|---|---|---|
| Cirrus | High (>6 km) | Thin, wispy ice crystals |
| Cirrocumulus | High | Small, white puffs |
| Altocumulus | Middle (2–6 km) | Layered rolls/patches |
| Altostratus | Middle | Grey sheet, diffuse sun |
| Nimbostratus | Low–middle | Dark rain cloud |
| Stratocumulus | Low (<2 km) | Lumpy grey layer |
| Stratus | Low | Flat grey layer, drizzle |
| Cumulus | Low base, tall | Heaped, fair-weather |
| Cumulonimbus | All levels | Towering storm cloud, thunder |
Precipitation
Precipitation is water falling from the atmosphere in any form. It requires cloud droplets or ice crystals to grow large enough to fall against updrafts.
- Types of precipitation:
- Rain: Liquid water droplets (>0.5 mm diameter). Most common form.
- Drizzle: Very fine droplets (<0.5 mm), falls slowly.
- Snow: Ice crystals aggregate into snowflakes; forms when temperatures are below 0 degrees C throughout.
- Sleet: Rain that freezes before reaching the ground (or a mixture of rain and snow).
- Hail: Balls of ice formed in strong convective storms (cumulonimbus); alternating layers of clear and opaque ice indicate multiple ascents.
- 1.Types of rainfall by mechanism:
- 2.Convectional rainfall: Surface heated strongly; air rises rapidly in convection currents; towering cumulus forms; intense, short-duration thunderstorms. Common near the equator and in tropical interiors in summer.
- 3.Orographic (relief) rainfall: Moist air forced to rise over a mountain range; cools, condenses, rains on windward side; leeward side is a rain shadow. Example: Western Ghats receive heavy monsoon rainfall; Deccan Plateau is drier.
- 4.Frontal (cyclonic) rainfall: Rainfall associated with fronts in mid-latitude cyclones; warm moist air rises over cold air; widespread and steady. Common in Western Europe.
Common mistakes
- Relative humidity can be 100% even when it is not raining — saturation is required for condensation, but precipitation also needs nuclei and droplet growth.
- Frost is NOT frozen dew — it forms by sublimation (vapour directly to ice) when dew point is below freezing.
- The leeward side of a mountain is the rain shadow — it is drier, not wetter.
- Fog is essentially a cloud at ground level — the physics are identical.
Summary
Water enters the atmosphere through evaporation and is measured as humidity. When air cools to the dew point, condensation forms dew, frost, fog, or clouds depending on where cooling occurs. Precipitation forms when cloud droplets grow large enough to fall. Three main mechanisms — convectional, orographic, and frontal — explain the global distribution of rainfall.