WHAT IS PRECIPITATION?
Precipitation is a general term for the fall or downpour of water from the atmosphere to the
Earth’s Surface. Precipitation still has multiple types and variations as to how it affects the Earth’s
surface.
Infiltration is precipitation that soaks into the soil.
Runoff is precipitation that does not soak into the soil but instead moves on the Earth’s surface
toward streams.
Why is it important?
Precipitation’s importance stems from the needs of different organisms which makes water one
of the vital components in sustaining life. It provides the Earth’s surface a source of water for
lakes, rivers, and other bodies of water to refill, and recharges underground aquifers.
How does precipitation form?
This part of the hydrologic cycle occurs when a body of moisture is sufficiently cooled to the point
it becomes saturated in water droplets and ice crystals while in the process of condensation.
SIMPLE EXPLANATION:
The cooling of air may occur in different ways such as meeting of air masses that are completely
different temperatures, or by coming into contact with cold objects such as the ground. The most
important component of precipitation is the uplift of air, or otherwise known as condensation. As
air increases in temperature, it rises as it is lighter than the cooler air surrounding it. As it rises,
its pressure decreases which in turn allows it to expand and cool down. During a certain point in
this process, its ability to hold water is reduced resulting in the air to become saturated
(condensation). Formation of clouds does not necessarily mean that precipitation will occur. Some
clouds are associated with dry weather or light rain, while others indicate heavy rainfall.
There are four conditions that must be present for the production of precipitation
1.
2.
3.
4.
Atmosphere must have moisture
Must be sufficient nuclei (tiny particles) present to aid condensation
Weather conditions must be good for condensation of water vapour to take place
The product of condensation must reach the earth
EXPANDED EXPLANATION:
All forms and types of precipitation originate from clouds. These clouds contain water vapour and
cloud droplets which can simply be described as droplets of condensed water that are too small
to be precipitated. However, these same droplets are what forms visible clouds since clouds
themselves are continually evaporating and condensing. Condensed water in clouds doesn’t
actually fall as precipitation as the droplets’ speed aren’t enough to overcome the updraft speeds
that keep the clouds afloat.
For precipitation to occur, the tiny water droplets’ inside clouds must condenses on small particles
such as dust, salt, or smoke. A raindrop is formed as these provide a core for precipitation to form
over as water will continue to condense over it. This process happens for a long time since millions
of droplets are required just to form a single raindrop.
The Bergeron-Findeisen Process is another mechanism that pertains to the formation of
precipitation. This process explains how a precipitation sized raindrop is created through the rapid
growth of ice crystals by taking water vapour from a cloud. Once these crystals have formed, they
can either melt and fall as rain, or remain as ice crystal and fall as snow.
What are the different forms of precipitation?
1. Drizzle – are tiny liquid water drops which ranges between 0.1 to 0.5 mm in diameter. It
is also known as mist. This form of precipitation usually fall from the low strata’s and rarely
exceeds speeds of 1mm/hr. This is normally produced by low strati-form clouds and
stratocumulus clouds. Drizzle evaporates quickly before reaching earth because of its
small size
2. Rain - Consists of liquid drops mostly > 0.5 in diameter. Rain is reported in three
intensities.
a. Light: for rates of fall up to 2.5mm/hr
b. Moderate: From 2.5 to 7.5 mm/hr
c. Heavy: Over 7.5 mm/hr
3. Glaze - It is ice coating formed on exposed surface by freezing of super cooled water
deposits by rain or drizzle. Its specific gravity is as high as 0.8 to 0.9. When rain or drizzle
comes in contact with cold ground at around 0° C, the water drops freeze to form an ice
coasting called glaze.
4. Rime - It is white opaque deposit of ice granular mass or less separated by trapped air
and formed by rapid freezing of super cooled water drops. Its specific gravity may be as
low as 0.2 to 0.3.
5. Snow - It is composed of ice crystals formed by the process of sublimation. The average
specific gravity of snow is 0.1. The density of freshly fallen snow varies greatly - ranges
from 0.06 to 0.15 gm/ cm3 (average density 0.1 gm/cm3). An amount of 185 to 500mm of
snow is generally required to equal 25mm of liquid water.
6. Snowflake - Bigger particles of snow formed by combination of snow crystals in
atmosphere and fall as heavy masses. They may reach several centimetres in dia.
7. Hail - Precipitation in the form of ball of ice produced in convective clouds. Hailstones may
be spherical, conical or irregular in shape and from 5 to 125mm in dia. They are composed
of alternative layers of glaze and Rime. Their specific gravity may reach 0.8. The largest
hailstone in US fell of size 44cm circumference and 776g weight.
8. Sleet - Consists of transparent globular solid grain of ice formed by freezing of raindrops
or freezing of largely melted ice crystals fully through a layer of sub-freezed air, near the
earth surface. These are simply frozen raindrops. As it falls toward the ground, it goes
through a layer of warmer air somewhere between the cloud and the ground. If that layer
is thick & warm enough, the snowflake melts and becomes a rain drop. That liquid rain
drop falls back into colder air (temperature below 0° C or 32° F), and refreezes into a
small, hard ice pellet.
Sources
 https://www.usgs.gov/special-topics/water-science-school/science/precipitation-andwatercycle#:~:text=Rain%20and%20snow%20are%20key,drinks%20to%20plants%20and%2
0animals.
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https://www.lordgrey.org.uk/~f014/usefulresources/aric/Resources/Teaching_Packs/Key
_Stage_4/Weather_Climate/06.html#:~:text=There%20are%20three%20main%20ways,
as%20rain%2C%20hail%20or%20snow.
https://www.weather.gov/lot/hydrology_education_watercycle#:~:text=Precipitation%20is
%20water%20that%20falls,the%20Earth%27s%20surface%20in%20streams.
https://www.aboutcivil.org/mechanism-of-precipitation.html
https://energyeducation.ca/encyclopedia/Precipitation
https://education.nationalgeographic.org/resource/precipitation