- The amount of moisture in the air
- Water vapour present in the atmosphere → 0–4%
- Instrument to measure humidity: Hygrometer
Absolute Humidity
- Amount of water present per unit volume of air
- Measured in grams per cubic metre
Relative Humidity
- Actual vapour pressure ÷ saturation vapour pressure at the same temperature ×100
- Relative humidity of saturated air = 100%
Specific Humidity
- Mass of water vapour per unit mass of moist air
- Measured in grams per kilogram
⚡ Quick Recall
Absolute Humidity = per unit
volume · Specific Humidity = per unit
weight — the volume vs. weight distinction is a common exam trap.
- Condensation: vapour → liquid water; direct vapour → ice is deposition.
- Dew point: temperature reached by cooling air to saturation at constant pressure/moisture content; RH =100%.
| Form | Key Details |
|---|
| Dew | Water droplets on surfaces – forms when temperature drops below dew point at night |
| Frost / Hoar Frost | Ice crystals on surfaces – forms when temperature is below 0°C |
| Fog | Tiny water droplets suspended near ground – reduces visibility below 1 km |
| Mist | Less dense than fog; visibility 1 km or more, though below clear-air visibility |
| Smog (Smoke + Fog) | Visibility-reducing air pollution; name originally smoke+fog, but photochemical smog need not contain fog. |
| Clouds | Condensed water vapour at higher altitudes |
⚠ Exam Trap
Operational distinction:
fog <1 km visibility;
mist ≥1 km with reduced visibility compared with clear air.
- Form due to the cooling of air through an adiabatic process and falling below dew point
- Instrument to measure cloud direction and speed: Nephoscope
Low-Level (Below 2,000 m)
- Nimbostratus: usually classified at middle level but extends into low/high levels; persistent rain/snow and obscured Sun.
- Stratus: Continuous rain/drizzle, layered, blue/grey colour
Middle (2,000–6,000 m)
- Altostratus: Thin, sheet-like • appears before rain/snow
- Altocumulus: Small, round, wavy • indicates instability, potential thunderstorms
High-Level (Above 6,000 m)
- Cirrus: high, fibrous ice-crystal clouds; precipitation usually does not reach ground. Other high genera: Cirrostratus, Cirrocumulus.
Vertical Development
- Cumulus: Dome-shaped, cauliflower-like • fair-weather cloud
- Cumulonimbus: Massive, towering • thunderstorms, hail, heavy rain
⚠ Exam Trap
Cumulus/Cumulonimbus usually have low bases and can span several levels. Principal high-cloud genera are
Cirrus, Cirrostratus and Cirrocumulus.
- Rainfall: Falling of water droplets on Earth's surface
- Precipitation: Falling of moisture in liquid or solid form
| Form | Details |
|---|
| Drizzle | Very small, fine water droplets |
| Snowfall | Precipitation in the form of ice crystals / snowflakes |
| Hail | Large chunks of ice that fall from cumulonimbus clouds |
| Sleet | Regional usage: ice pellets in U.S. meteorology; rain–snow mix in British/Indian usage |
⚡ Quick Recall
Hail always forms in
Cumulonimbus clouds — the only cloud type strong enough to keep ice pellets suspended long enough to grow.
1. Convectional Rainfall
- Sun heats Earth's surface → air near the ground becomes warm and rises
- Rising air cools, condenses into clouds, and rain falls
- Occurs in equatorial and temperate regions
- Thunderstorm with heavy rainfall, mostly after noon
2. Orographic Rainfall — Distinct from the Monsoon System
- Occurs when moist air is forced to rise over a mountain or elevated landform
- Windward slope (facing the wind) – Maximum Rainfall
- Leeward slope (opposite side) – Less rainfall – Rain Shadow region
3. Cyclonic / Frontal Rainfall
- Associated with weather systems (cyclones or fronts)
- Widespread and steady rainfall
- Turbulence occurs when warm and cold air masses meet
- At the front, rising air causes clouds and rainfall
⚠ Exam Trap
Convectional Rainfall mostly occurs
after noon (due to peak surface heating); Orographic Rainfall's timing depends on when moist winds hit the mountain — don't assume all rainfall types follow the same daily pattern.
| Fact | Details |
|---|
| Instrument for humidity | Hygrometer |
| Instrument for cloud direction/speed | Nephoscope |
| Relative humidity of saturated air | 100% |
| Dew Point | Saturation temperature reached by cooling at constant pressure/moisture content (RH 100%) |
| Smog | Air pollution; historic smoke+fog origin, while photochemical smog need not contain fog |
| Rain-bearing cloud | Nimbostratus / Cumulonimbus clouds |
| Clear weather indicator cloud | Cumulus clouds |
| High Ice-Crystal Cloud; Precipitation Usually Not Reaching Ground | Cirrus clouds |
| Rainfall after noon – which type? | Convectional Rainfall |
| Rain shadow region – which rainfall? | Orographic / Mountain Rainfall – Leeward side gets less rain |
⚡ Quick Recall
Hygrometer measures humidity · Nephoscope measures cloud direction/speed — two commonly confused meteorological instruments.