Precipitation – Types & Forms

Introduction

Precipitation is a vital component of the hydrological cycle, wherein water droplets or ice crystals fall from the atmosphere to the Earth’s surface. Here, we explore the various forms and types of precipitation in detail.

Precipitation Formation

• Water Droplets: Formed through condensation in the atmosphere.
• Ice and Snow Formation: Result from water vapor freezing in the atmosphere and forming ice crystals or snowflakes.

Forms of Precipitation

Liquid Forms

• Drizzle:
  • Drops of water with a diameter of less than 0.5mm.
  • Typically falls from low stratiform clouds, occasionally from cumulus clouds.
• Rain:
  • Drops of water with a diameter greater than 0.5mm.
  • Larger raindrops often originate as snow at higher altitudes.
  • Biggest raindrops usually from cumulonimbus clouds.
• Freezing Rain/Drizzle:
  • Liquid rain/drizzle that freezes on contact with the ground and vegetation.
  • Occurs during warm fronts ending cold spells, posing dangerous road conditions.

Solid Form

• Diamond Dust:
  • Tiny ice crystals falling from a clear sky, requiring very cold weather.
  • Rare in Britain but common in polar regions.
• Snow Grains:
  • White opaque particles less than 1mm in size, falling from low stratus or stratocumulus clouds.
• Snow:
  • Solid precipitation in the form of ice crystals or pellets.
  • Often formed from gas to solid directly.
  • Branched into flakes in warmer conditions.
• Glaze:
  • Rain that instantly freezes upon collision with solid objects.

• Sleet:
  • Small raindrops that freeze during descent, reaching the ground as small pellets of ice.

• Hail:
  • Transparent or opaque particles, rounded or irregular pellets.
  • Produced in cumulonimbus clouds due to active turbulence and vertical air currents.
  • Varies in size, with larger hailstones forming in heavy thunderstorms.
  • Growth occurs by collecting moisture from supercooled cloud droplets.
  • Small hail consists of snow pellets encased in a thin layer of clear ice, common in showery weather in Britain.
• Others:
  • Solid precipitation from cumulonimbus clouds, producing striking cloud features such as fallstreaks or virga when they don’t reach the surface.

Exploring Types of Rainfall

Precipitation is the result of moisture in the atmosphere being lifted to cooler heights, where it condenses into water droplets or ice crystals, eventually falling to the Earth’s surface. This process is often preceded by cloud formation and is a consequence of air being uplifted.

Different Mechanisms of Upliftment

• Self Upliftment: Due to temperature factors.
• Forced Upliftment: Such as upliftment along mountain slopes, especially on the windward side.
• Cyclonic Upliftment: Occurs due to surface air convergence, particularly in low-pressure systems.
• Upliftment at the Fronts: Associated with the meeting of air masses at weather fronts.

Types of Rainfall

Convectional Rainfall

• Occurrence: Most common in regions intensely heated, either during the day in tropics or in summer in temperate interiors.
• Mechanism:
  • Heated earth’s surface leads to rising moist air, forming cumulonimbus clouds.
  • Afternoon heating leads to well-developed convectional systems.
• Characteristics:
  • Intense downpours often accompanied by thunder and lightning.
  • May not penetrate soil due to intensity, draining off quickly.
  • Equally heavy in temperate regions with occasional thunderstorms.

Orographic or Relief Rain

• Cause: Moist air forced to ascend mountain barriers.
• Occurrence: Best developed on windward slopes of mountains.
• Process:
  • Air cools and condenses as it rises due to expansion at higher altitudes.
  • Condensation forms clouds and eventually rain.
• Effect: Rain shadow area on leeward slopes experiences little or no precipitation.

Cyclonic Rainfall

• Independence: Not associated with relief or convection, purely with cyclonic activity.
• Cause: Convergence of different air masses with different temperatures.
• Mechanism:
  • Cold, dense air tends to remain close to the ground, while warm, lighter air rises over it.
  • Convergence leads to ascent, cooling, and condensation, resulting in cyclonic or frontal rain.
• Occurrence: Can happen anywhere, associated with cyclones in tropical regions and depressions in temperate regions.

Frontal Rainfall

• Similarity: Shares similarities with cyclonic rainfall.
• Occurrence: Typically observed in mid and high latitudes.
• Phenomenon: Associated with the meeting of contrasting air masses at weather fronts.