CHAPTER 11: PRECIPITATION

1. DEFINITION OF PRECIPITATION

Precipitation is any form of liquid or solid water that falls from a cloud and reaches the Earth’s surface.

Main forms of precipitation include:

• Drizzle
• Rain
• Snow
• Sleet
• Snow pellets or graupel
• Hail

Formation of Precipitation

Clouds consist of extremely small water droplets. An average cloud droplet has a diameter of approximately 0.02 mm and an almost negligible rate of fall.

Precipitation develops through the following process:

1. Small cloud droplets collide with other droplets.
2. The droplets combine and grow in size.
3. They eventually become too heavy to be supported by the cloud’s updraughts.
4. They fall towards the ground as precipitation.

Important: Cloudy weather does not always produce precipitation. Clouds may remain in the atmosphere for several days without producing rain or snow.

PART A: TYPES OF PRECIPITATION

2. LIQUID PRECIPITATION

3. SOLID AND MIXED PRECIPITATION

PART B: CLOUD CLASSIFICATION

5. WARM CLOUDS AND COLD CLOUDS

Warm Clouds

A warm cloud remains entirely above 0°C.

Characteristics:

• Contains liquid-water droplets
• Does not contain ice crystals or supercooled water
• Produces precipitation mainly through collision-coalescence
• Common in tropical and maritime areas
• Produces drizzle or rain
• Does not create a structural icing hazard while its temperature remains above 0°C

Exam Rule: If the cloud top does not reach the freezing level, it is a warm cloud.

Cold Clouds

A cold cloud extends partly or completely into air below 0°C.

It may contain:

• Ice crystals
• Supercooled water droplets
• Water vapour

Examples include:

• Cumulonimbus
• Towering Cumulus
• Nimbostratus
• Altostratus
• Supercooled Stratocumulus

Cold-cloud precipitation mainly develops through the ice-crystal or Bergeron process.

6. SUPERCOOLED WATER DROPLETS

A supercooled water droplet is a liquid-water droplet that remains unfrozen at a temperature below 0°C.

Important points:

• Supercooled droplets are abundant between approximately 0°C and -15°C.
• Small liquid droplets may exist at temperatures approaching -40°C.
• They freeze rapidly after striking an aircraft.
• They are the principal cause of structural airframe icing.
• They can freeze onto ice crystals to form snow pellets or graupel.
• They contribute to hail formation inside CB clouds.

Icing in Different Clouds

CU, TCU and CB

• Strong updraughts carry liquid water above the freezing level.
• Moderate to severe icing may occur.
• CB contains the greatest combination of icing, turbulence and precipitation hazards.

AS and NS

• May contain supercooled droplets and ice crystals.
• Prolonged flight through these clouds can produce significant icing.
• Serious icing may occur approximately 2,000-5,000 ft above the freezing level.

SC

• Mainly contains water droplets.
• Droplets may become supercooled below 0°C.
• Usually produces light to moderate rime-ice conditions.
• Precipitation is normally drizzle, freezing drizzle or snow grains.

7. CLOUDS AND THEIR PRECIPITATION

Essential Cloud Rules

• ST produces drizzle.
• NS produces widespread continuous or intermittent precipitation.
• CU and CB produce showers.
• CB may produce hail.
• CC and CS produce no precipitation reaching the ground.

PART C: PRECIPITATION-FORMATION THEORIES

8. COLLISION-COALESCENCE PROCESS

The collision-coalescence process explains precipitation formation in warm clouds.

It is also called:

• Coalescence theory
• Capture effect
• Giant-drop process

Process

1. A warm cloud contains droplets of different sizes.
2. Larger droplets fall faster than smaller droplets.
3. The larger droplets collide with smaller droplets.
4. The droplets combine or coalesce.
5. Repeated collisions increase their size.
6. The drops eventually become too heavy to remain suspended.
7. They fall as drizzle or rain.

Essential Conditions

• Cloud temperature above 0°C
• Liquid-water droplets of different sizes
• Sufficient cloud depth and water content
• Weak or moderate vertical currents

Key Exam Points

• Cloud type: Warm cloud
• Main location: Tropical and lower latitudes
• Process: Larger drops collect smaller drops
• Result: Drizzle or rain

9. GIANT-NUCLEUS THEORY

The giant-nucleus theory is a maritime form of the collision-coalescence process.

Process

• Sea spray supplies large salt particles.
• Salt particles act as hygroscopic condensation nuclei.
• Hygroscopic nuclei attract moisture.
• Water vapour condenses easily around them.
• Relatively large cloud droplets form rapidly.
• These droplets initiate collision-coalescence.
• Rain can develop even from clouds with limited vertical growth.

Key Exam Points

• Main location: Maritime areas and coastlines
• Main nuclei: Salt particles
• Property of salt: Hygroscopic
• Precipitation: Drizzle or rain
• Related theory: Collision-coalescence

Memory Rule: Maritime cloud + salt nuclei = Giant-nucleus theory

10. ICE-CRYSTAL OR BERGERON PROCESS

The Bergeron process explains precipitation formation in cold clouds.

Essential Conditions

The cloud must contain:

• Ice crystals
• Supercooled water droplets
• Subfreezing temperatures

Fundamental Principle

At the same subfreezing temperature:

Saturation vapour pressure over water is greater than saturation vapour pressure over ice.

Process

1. Supercooled water droplets exist beside ice crystals.
2. The higher vapour pressure around liquid droplets causes them to evaporate.
3. Water vapour moves towards the ice crystals.
4. Vapour deposits directly onto the ice crystals.
5. Ice crystals grow at the expense of the supercooled droplets.
6. They may also grow by collecting supercooled droplets.
7. When heavy enough, the particles fall.
8. They reach the surface as snow or melt into rain.

Key Exam Points

• Cloud type: Cold cloud
• Temperature: Below 0°C
• Required particles: Ice crystals and supercooled droplets
• Main principle: SVP over water > SVP over ice
• Result: Snow or rain

11. COMPARISON OF PRECIPITATION THEORIES

One-Line Revision

• Warm cloud = Collision-coalescence
• Cold cloud = Bergeron process
• Maritime cloud = Giant salt nuclei

PART D: CHARACTER AND INTENSITY

12. CONTINUITY OF PRECIPITATION

Showers

Showers are associated exclusively with convective clouds.

Characteristics:

• Clouds: CU and CB
• Short duration
• Sudden beginning and ending
• Rapid changes in intensity
• Localized occurrence

Intermittent Precipitation

Intermittent precipitation:

• Falls from layer clouds
• Contains short breaks
• Repeatedly starts and stops
• May continue over a long period

Continuous Precipitation

Continuous precipitation:

• Falls from layer clouds
• Has no breaks
• Continues for at least one hour
• Is widespread and comparatively steady
• Commonly falls from Nimbostratus

Comparison

13. INTENSITY OF PRECIPITATION

Precipitation intensity is the rate at which precipitation falls.

Rate-Based Classification

Important: Rain is measured in millimetres per hour, while snow intensity is based on accumulation in centimetres per hour.

METAR and TAF Intensity Symbols

Do Not Confuse

• Type: Drizzle, rain, snow or hail
• Intensity: Light, moderate, heavy or violent
• Character: Showery, intermittent or continuous

PART E: IMPORTANT WEATHER PHENOMENA

14. CLOUDBURST

A cloudburst is a sudden and exceptionally heavy fall of rain, usually of the shower type.

Characteristics:

• Associated with deep convective clouds, particularly CB
• Sudden beginning
• Very high rainfall intensity
• Short duration
• Localized occurrence
• May trigger flash flooding

15. FLASH FLOOD

A flash flood is a flood that rises rapidly with little or no advance warning.

It commonly occurs when:

• An intense thunderstorm becomes stationary or moves slowly
• Excessive rain falls over a small area
• The ground cannot absorb the water
• Drainage channels become overwhelmed
• A cloudburst occurs over steep or confined terrain

Relationship

Cloudburst → Excessive localized rainfall → Flash flood

16. VISIBILITY IN PRECIPITATION

Heavy or blowing snow generally causes the greatest reduction in horizontal visibility.

Approximate visibility values:

• Heavy snow: 50-200 m
• Heavy drizzle: May reduce visibility to approximately 500 m
• Drizzle generally produces visibility between approximately 500-3,000 m

Key Exam Point

The form of precipitation most likely to cause the greatest reduction in visibility is snow, particularly heavy or blowing snow.

PART F: CLOUD DEVELOPMENT AND ATMOSPHERIC STABILITY

17. DIURNAL DEVELOPMENT OF CONVECTIVE CLOUDS

Convective clouds form when the surface is heated and rising thermals develop.

Daily Development

1. Morning solar heating warms the surface.
2. The surface heats the lower atmosphere.
3. Thermals begin to rise.
4. Rising air cools adiabatically.
5. Condensation begins at the lifting condensation level.
6. Small fair-weather CU clouds form.
7. Continued heating increases instability.
8. CU may grow into TCU or CB during the afternoon.

Cloud-Base Variation

As the surface temperature rises:

• The temperature-dew-point spread generally increases.
• Air must rise farther before becoming saturated.
• The lifting condensation level becomes higher.
• The convective cloud base rises.

Forecasting Significance

Fair-weather CU observed during the morning may indicate:

• Increasing convection
• Low-level thermal turbulence
• TCU or CB development later
• Possible afternoon thunderstorms

Maximum convective cloud development normally occurs near the period of maximum surface temperature, generally during the middle or late afternoon.

18. COLD AIR MOVING OVER A WARM SURFACE

When cold or polar air moves over a warmer surface:

1. The air is heated from below.
2. The environmental lapse rate steepens.
3. Atmospheric instability increases.
4. Vertical currents develop.
5. CU and CB clouds may form.
6. Showery precipitation may occur.

A common example is polar maritime air moving over a warmer sea.

Important Contrast

• Cold air over warm surface = Instability, CU/CB and showers
• Warm moist air over cold surface = Cooling from below and advection fog

19. TURBULENCE CLOUDS

Turbulence clouds form through mechanical mixing, normally within the friction layer.

Common turbulence clouds include:

• Stratus
• Stratocumulus
• Altocumulus at higher levels

Formation

1. Sufficient wind produces mechanical turbulence.
2. Turbulence mixes the air vertically.
3. The environmental lapse rate approaches the dry adiabatic lapse rate.
4. Condensation occurs if the mixed layer becomes saturated.
5. A stable layer or inversion prevents further vertical development.

Characteristics

• Large horizontal extent
• Limited vertical development
• Relatively flat cloud top
• Frequently capped by a temperature inversion
• Typical thickness of approximately 1,000-2,000 ft

A wind speed greater than approximately 10 kt can support the mechanical mixing required for low turbulence-cloud formation.

20. ANTICYCLONIC WEATHER

An anticyclone is characterized by:

• Surface divergence
• General subsidence
• Stable atmospheric conditions

Effect of Subsidence

• Descending air is compressed.
• Compression produces adiabatic warming.
• Relative humidity decreases.
• Vertical cloud development is suppressed.
• CU and CB formation becomes unlikely.
• Skies are usually clear above the subsidence inversion.

Weather Below the Inversion

Moisture, dust, smoke and pollutants may become trapped below the inversion.

Possible conditions include:

• Stratus
• Stratocumulus
• Haze
• Mist
• Fog
• Poor visibility

Exam Rule: Anticyclones suppress convective cloud. If cloud is present, it is usually low ST or SC trapped below an inversion.

PART G: RAINFALL DISTRIBUTION

21. RAIN-SHADOW EFFECT

A rain-shadow region lies on the leeward or downwind side of a mountain range.

Formation

1. Moist air approaches a mountain barrier.
2. It is forced to rise along the windward slope.
3. Rising air expands and cools.
4. Condensation, cloud and precipitation develop.
5. The air loses much of its moisture.
6. Drier air descends along the leeward slope.
7. Descending air is compressed and warms adiabatically.
8. Relative humidity falls and cloud formation is suppressed.

Foehn Effect

The warming and drying of descending air on the leeward side is known as the Foehn effect.

22. RAIN SHADOW OF THE WESTERN GHATS

During the Southwest Monsoon from June to September:

• Moisture-bearing winds approach from the west or southwest.
• They rise along the western slopes of the Western Ghats.
• The west coast and windward slopes receive heavy rainfall.
• Air descends and dries on the eastern side.

Exam Answer

The rain-shadow region of the Western Ghats lies to the east of the mountain range.

23. RAIN SHADOW OF THE EASTERN GHATS

During the Northeast Monsoon from October to December:

• Winds collect moisture while crossing the Bay of Bengal.
• Moist air reaches the eastern coast.
• The eastern or seaward slopes receive rainfall.
• Air descends towards the western interior.

Exam Answer

The rain-shadow region associated with the Eastern Ghats lies towards the west or interior side.

24. DIURNAL AND SEASONAL TROPICAL RAINFALL

Tropical Land Areas

• Strong surface heating produces convection.
• CU, TCU and CB develop.
• Rainfall and thunderstorm activity usually reach a maximum during the afternoon.
• Tropical summer rainfall generally exceeds winter rainfall.

Tropical Coastal Areas

Many tropical coastal regions experience maximum rainfall at night or during the early morning.

Mechanism

1. Land cools faster than the sea at night.
2. Higher pressure develops over land.
3. A land breeze blows offshore.
4. Cool air moves over relatively warm water.
5. The air becomes unstable and gains moisture.
6. Offshore convergence produces rising motion.
7. CU and CB clouds develop.
8. Showers may drift back towards the coast.

Katabatic winds from nearby mountains can strengthen the offshore flow.

Example: Sumatras

The Sumatras of the Malacca Straits are violent nocturnal thundery squalls.

• They develop at night.
• Katabatic flow moves over warm water.
• Convergence and instability produce thunderstorms.
• Maximum development often occurs near dawn.

Northeast India

Some parts of Northeast India may also experience maximum rainfall at night or in the early morning because of local topography, mountain winds and convergence.

25. TROPICAL SEASONAL RAINFALL AND ITCZ

In tropical regions, particularly between approximately 10° and 20° latitude, rainfall is generally maximum during the summer or high-sun period.

Cause

• The ITCZ follows the seasonal movement of the Sun.
• It moves into the summer hemisphere.
• Surface convergence increases.
• Warm, moist air rises.
• CU and CB clouds develop.
• Heavy showers and thunderstorms occur.

During winter:

• The ITCZ moves away.
• Dry trade winds may dominate.
• Subtropical high-pressure systems produce subsidence.
• Rainfall decreases.

Key Exam Point

Tropical seasonal rainfall maximum = Summer or high-sun period

26. PRECIPITATION IN TEMPERATE LATITUDES

Temperate precipitation is mainly associated with:

• Travelling depressions
• Polar-front systems
• Frontal convergence and uplift

These systems are generally more frequent and vigorous in winter because the temperature contrast between polar and tropical air masses is stronger.

Therefore:

• Rain or snow is often maximum during winter.
• Temperate maritime western regions commonly have wet winters.
• Mediterranean regions characteristically have wet winters and dry summers.
• Some continental interiors may have a summer convective maximum.

27. WINTER PRECIPITATION IN JAMMU AND KASHMIR

Jammu and Kashmir and the Western Himalayas receive their maximum precipitation during winter, particularly in January and February.

Cause

• Western Disturbances embedded in the mid-latitude westerlies
• Frequent frontal or occluded systems
• Uplift over the Himalayan region

Results

• Rain at lower elevations
• Snowfall at higher elevations
• Significant winter precipitation over the Western Himalayas

This is an important exception to the general summer-monsoon rainfall maximum experienced over much of India.

PART H: MEASUREMENT AND WEATHER MODIFICATION

28. RAIN GAUGE

A rain gauge is a meteorological instrument used to measure the amount of liquid precipitation received over a specified period.

Types of Rain Gauges

1. Standard Rain Gauge

• Collects rainwater in a container.
• Rainfall is measured using a graduated cylinder.

2. Tipping-Bucket Rain Gauge

• Contains a small bucket mechanism.
• The bucket tips after collecting a fixed quantity of rain.
• Each tip is recorded electronically.
• Measures rainfall amount and intensity.

3. Weighing Rain Gauge

• Measures the weight of accumulated precipitation.
• Converts the increase in weight into rainfall amount.
• Provides a continuous record.

4. Optical Rain Gauge

• Uses laser or infrared beams.
• Detects precipitation particles interrupting the light beam.
• Can estimate the size and number of drops.

Uses

• Weather forecasting
• Climate studies
• Agricultural planning
• Hydrological monitoring
• Flood forecasting
• Rainfall-intensity measurement

29. RAINY DAY

A day is classified as a rainy day when the total rainfall recorded during 24 hours is 2.5 mm or more.

Formula

Rainy day = Rainfall ≥ 2.5 mm in 24 hours

30. ARTIFICIAL RAIN OR CLOUD SEEDING

Artificial rainmaking is formally known as cloud seeding.

Common Seeding Agents

• Silver iodide
• Dry ice or solid carbon dioxide
• Hygroscopic particles in suitable warm clouds

Process

1. Seeding material is introduced into a suitable cloud.
2. The particles act as condensation or ice-forming nuclei.
3. Water droplets or ice crystals develop around them.
4. Cloud particles grow in size.
5. When sufficiently heavy, they fall as precipitation.

Applications

• Increasing precipitation in drought-affected areas
• Supporting water-resource management
• Modifying suitable cloud systems
• Helping clear atmospheric pollution under suitable conditions

Important: Cloud seeding cannot produce rain from a clear sky. A suitable cloud with sufficient moisture must already be present.

31. ARTIFICIAL FOG DISPERSAL

Artificial fog dispersal is most successful in cold or supercooled fog.

Cold-Fog Seeding

• Dry ice is introduced into the fog.
• Supercooled droplets freeze into ice crystals.
• Ice crystals grow and fall out.
• A temporarily clear area develops.

Warm-Fog Dispersal

Methods include:

• Heating
• Mechanical mixing
• Hygroscopic-particle injection

These methods are generally:

• Expensive
• Short-lived
• Less effective than cold-fog seeding

Key Exam Point

The most successful artificial fog-dispersal method is seeding cold or supercooled fog.

PART I: FINAL EXAM REVISION

32. HIGH-YIELD ONE-LINERS

• Precipitation is liquid or solid water falling from a cloud and reaching the ground.
• An average cloud droplet has a diameter of approximately 0.02 mm.
• Drizzle droplets are approximately 0.2-0.5 mm in diameter.
• Drizzle mainly falls from Stratus.
• NS produces continuous or intermittent rain or snow.
• CU and CB produce showers.
• CB produces moderate to heavy rain, snow or hail showers.
• CC and CS produce no precipitation reaching the ground.
• Snow may reach the surface when the temperature is below approximately +4°C.
• The mean temperature between cloud base and surface should be below 0°C for favourable snowfall.
• Sleet commonly means a mixture of rain and snow.
• In aviation terminology, sleet refers to transparent ice pellets.
• Warm clouds produce precipitation through collision-coalescence.
• Cold clouds produce precipitation through the Bergeron process.
• Giant-nucleus theory applies mainly to maritime areas.
• Salt particles are hygroscopic nuclei.
• SVP over water is greater than SVP over ice below freezing.
• Supercooled droplets are the main cause of structural aircraft icing.
• Showers are short and associated with convective clouds.
• Continuous precipitation lasts for at least one hour without a break.
• Heavy or blowing snow produces the greatest reduction in visibility.
• A cloudburst is an exceptionally heavy convective rain shower.
• A cloudburst may produce a flash flood.
• Cold air moving over a warm surface becomes unstable.
• Warm moist air moving over a cold surface may produce advection fog.
• Anticyclonic subsidence suppresses convective cloud.
• Tropical land rainfall is generally maximum in the afternoon.
• Many tropical coastal areas have a night or early-morning rainfall maximum.
• Tropical seasonal rainfall is generally greatest in summer.
• Temperate frontal precipitation is often greatest in winter.
• Western Disturbances produce winter precipitation in Jammu and Kashmir.
• The windward side of a mountain is wet.
• The leeward side is the rain-shadow region.
• The rain shadow of the Western Ghats lies to the east.
• The rain shadow of the Eastern Ghats lies towards the western interior.
• A rainy day records at least 2.5 mm of rainfall in 24 hours.
• Violent rain or hail showers exceed 50 mm per hour.
• Artificial rainmaking is called cloud seeding.
• Silver iodide and dry ice are common seeding agents.

33. MASTER MEMORY TABLE

34. IMPORTANT ABBREVIATIONS

FINAL FIVE-LINE REVISION

Warm cloud = Collision-coalescence
Cold cloud = Bergeron process
Heap cloud = Showers
Layer cloud = Continuous or intermittent precipitation
Windward = Wet; Leeward = Rain shadow