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Chapter 4 (cont.). Precipitation. How does precipitation form? Why do some clouds generate precipitation and others do not? What factors determine the various types of precipitation?. To answer these questions, let’s start by examining the sizes of typical cloud and raindrops.

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chapter 4 cont

Chapter 4 (cont.)



How does precipitation form?

Why do some clouds generate precipitation and others do not?

What factors determine the various types of precipitation?

To answer these questions, let’s start by examining the sizes of typical cloud and raindrops.


1. Growth by condensation

Fog and cloud drops form by the aggregation of water vapor molecules by condensation on hygroscopic cloud condensation nuclei (CCN), a sub-set of the aerosol.

Initially, small droplets grow rapidly in an air parcel that is cooling, but as they become larger, their rate of growth decreases rapidly, so that this process (condensational growth) becomes too slow to produce raindrop size water drops. (It would take days!)

The excess water vapor grows mainly on other activated CCN


2.Growth by collision and coalescence.

In this process, large drops fall through smaller drops, and collect many of those in their path, thereby, growing even larger.

This mechanism works because large drops fall more rapidly than small drops.


What determines the fall speed of drops?

There are two forces that act on a falling object, namely

(1) the force of gravity which tends to accelerate the object towards the Earth’s surface, and

(2) the frictional drag caused by the air resistance.

When these two forces are exactly balanced, the object falls at a steady, constant speed, the terminal velocity.

Terminal velocities of larger drops are greater than those of small drops.


Collision and Coalescence Mechanism of Raindrop Growth

Updrafts in clouds tend to hold falling drops aloft. Drops with terminal velocities less than the updraft velocity are swept up higher into the cloud.

Drops only fall from the cloud when their terminal velocity exceeds the updraft velocity.


Collision and Coalescence Mechanism of Raindrop Growth

Growth by collision and coalescence is enhanced by:

A wide spectrum of drop sizes, which therefore implies a range of drop terminal velocities.


Collision and Coalescence Mechanism of Raindrop Growth

Growth by collision and coalescence is also enhanced by:

A high concentration of drops.

Strong cloud updrafts which hold the drops aloft in the cloud and give them more time to grow.

The collision and coalescence process explains the production of precipitation in “warm” clouds.


Only relatively shallow clouds that do not extend high into the troposphere contain nothing but water drops.

e.g. stratus cloud


Deep clouds extend up into regions of the troposphere where temperatures are well below freezing.

These ‘cold’ glaciated clouds consequently contain ice, as well as supercooled water drops.


Ice Formation in Cold Clouds

Small, pure water drops do NOT freeze at 0°C.

As the temperature cools below 0°C, larger water drops tend to freeze first, before the smaller drops.

As smaller water drops require colder temperatures to freeze, the drop size of supercooled water drops in a cold cloud tends to decrease with height.

Spontaneous nucleation of ice (homogeneous freezing) only occurs at very low temperatures. Most ice formation in clouds results from the action of Ice Nuclei.


The saturation vapor pressure increases rapidly with increasing temperature

How does the saturation vapor pressure vary with temperature below 0C?



3. The Bergeron Process

90% RH

When supercooled liquid water drops and ice particles co-exist in the same air parcel, the liquid drops near ice “feel” the drier shell around ice, and they start to evaporate.

This water vapor deposits on the ice particles as fast as it evaporates from the water drops. The net result is that the ice crystals grow at the expense of the water drops.

This ice crystal growth process, which promotes the rapid growth of ice crystals, is known as the Bergeron process. This process is the dominant precipitation forming mechanism in most places.

100% RH

90% RH

100% RH

90% RH

100% RH


Ice Particles and the Formation of Precipitation

Ice particles sometimes collide and stick together, forming a larger particle. This process is called aggregation and leads to the formation of ice crystal aggregates, commonly known as snowflakes.

Some collisions result in splintering of the ice crystals, thereby forming ice crystal fragments, which in turn act as embryonic ice crystals.

Supercooled cloud droplets sometimes collide with ice crystals and snowflakes. They freeze, and stick to their surface. The resulting ice crystal is said to be rimed.


Rime Ice

Rime forms when supercooled water fog and cloud drops impact a cold object. It not only leads to the growth of graupel and hail but can form on objects at the surface.


Cloud and small drizzle drops are roughly spherical. As falling drops become larger, they flatten on their underside and look

more like a ham-burger bun. This results from the higher air pressure under the falling drop.

Types of Precipitation: Rain

Falling raindrops are usually wrongly depicted by artists as being shaped like teardrops.


Types of Precipitation: Rain

Virga are falling streaks of drizzle and rain that evaporate before reaching the ground.

Donna Charlevoix U. of Illinois


Types of Precipitation: Rain

Drizzle is composed of drops of diameter less than 0.5 mm.

Raindrops have diameters between 0.5 mm and about 5 mm.

Showers fall from convective (cumiliform) clouds.

Continuous rain invariably falls from stratiform clouds (e.g., nimbostratus).


Single ice crystals form in an infinite number of shapes. All however exhibit hexagonal symmetry.

Most of them fall into three main categories, namely

columns (hollow and solid)

plates (thick and thin)


Types of Precipitation: Snow and Ice

The type of crystal that forms reflects its growth environment (primarily temperature).

Plates grow at 0 to -4C, -10 to -12C, and -16 to -22C Dendrites grow at -12 to -16C Columns at -4 to -10C and -22 to -50C


Sector Plates

Rasmussen & Libbrecht, 2003



Rasmussen & Libbrecht, 2003


More dendrites

Rasmussen & Libbrecht, 2003



Rasmussen & Libbrecht, 2003


Types of Precipitation: Snow and Ice

Snow Grains are the ice equivalent of drizzle. They are opaque, white ice particles of diameter <1 mm. They are usually associated with stratus clouds.

5 km

Snow pellets (graupel) are opaque, white ice particles formed by riming. Their diameter is 2 – 5mm. They fall from cumiliform clouds.

3 km

1 km

Hail and hailstones are transparent or partially opaque ice particles of diameter 5 – 140 mm. They form by riming of both graupel and frozen drops. To grow larger than 5 mm, a hailstone must make several up-and-down cycles in the cloud.


The Giant Coffeyville Hailstone!

Diameter: 14 cm (5.5”); Weight: 0.7 kg (24 oz)

Cumulonimbus are the only clouds with sufficiently strong updrafts to produce hail.


Types of Precipitation: Snow and Ice

When below-cloud temperatures are below freezing, falling snow reaches the ground.


Types of Precipitation: Snow and Ice

When surface temperatures are significantly above freezing, falling snow melts about 1000 ft (300 m) below the freezing level, reaching the surface as rain. This reflects the time required to acquire sufficient latent heat to effect melting.


Types of Precipitation: Freezing Rain

Freezing rainoccurs when rain or drizzle freezes on contact with a frozen surface.


Types of Precipitation: Freezing Rain

Freezing rain is hazardous because it coats surfaces with a very slippery layer of clear ice. The ice can bring down power lines and cause both personal and other accidents.


Types of Precipitation: Freezing Rain

Ice storms can break down trees, cause livestock to slip and fall, and can even cause suffocation due to ice clogging their nostrils. Birds have been found frozen to trees, with their beaks frozen shut, and unable to fly due to the ice build-up on their wings.


Types of Precipitation: Freezing Rain

Freezing rain occurs in the Front Range area when cold-air is dammed up against the mountains


Types of Precipitation: Sleet

Sleet consists of small ice pellets formed by the re-freezing of small drops and/or partially melted snowflakes. Re-freezing occurs when the drops fall into a deep cold layer at the surface.