Precipitation , Its types , Forms And Measurement methods.

1. 1
M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Assignment
Engineering Geology and Hydrology
SUBMITTED TO:
Sir Muhammad Sana_ullah Sani
Institute of Geology
University of the Punjab Lahore

2. 2
M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
GROUP MEMBERS
Farhad Jaffer Chishti (02)
Naseer Ahmed (08)
Muhammad Wajid Manzoor (09)
Tariq Mehmod (21)
Muhammad Azhar Mushtaq (29)
Shujha Nasir (36)
Usman Ali (37)

3. 3
M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Table of Contents
 Hydrological Cycle
 Componentsof Hydrological Cycle
 Precipitation
 Evaporation
 Evapotranspiration
 Condensation
Mechanism of Precipitation
 Coalescence Theory of Precipitation
 Mechanism of Cooling
Precipitation Forms
 Rain
 Snow
 Hail
 Drizzle
 Freezing fog
Types of Precipitation
 Convective precipitation
 Cyclonic precipitation
 Orographic precipitation
 Measurement of Precipitation
Rainfall Measurements

4. 4
M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Arithmetic Mean Method
Thiessen polygon method
References

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Hydrological Cycle
“The phenomenon of cyclic movement of water from the
atmosphere to the land and then back to the sea and then again to
the atmosphere”

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Components of Hydrologic Cycle
 Precipitation
 Evaporation
 Evapotranspiration
 Condensation
Precipitation:
Any form of water, such as rain, snow, sleet, or hail that falls to the
earth's surface is precipitation.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Evaporation:
Vaporization is a process by which water is converted into vapors.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Evapotranspiration:
Evapotranspiration is the sum of evaporation and plant transpiration
from the Earth's land and ocean surface to the atmosphere.
Evaporation accounts for the movement of water to the air.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Condensation:
Condensation is the process by which vapors are changes into liquid
state and is the reverse of evaporation.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Mechanism of Precipitation
CoalescenceTheory of Precipitation:
This theory assumes that droplets form when the dew point is
reached.
Mechanism of Cooling:
Precipitation occurs as a result of cooling of water-vapor-laden air to
its dew point or below.
 Condensation of vapor can occur from:
 Dynamic or Adiabatic, cooling
 Frontal conditions
 Contact cooling
 Radiation cooling
Dynamic or Adiabatic cooling:
This occurs when air circulation carries air to higher elevations. The
air might be elevated by thermal updrafts or by circulating into low-
pressure system.
Frontal conditions:
This provide an atmospheric environment favorable for mixing of two
air masses of different temperatures. An advancing cold front results
in mixing conditions along contact of cold & warm air.
Contactcooling:
It occurs when warm moist air is blown across a cold surface.
Those areas are noted for high amounts of precipitation from this
condition.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Radiationalcooling:
It occurs when clear nights provide a favorable environment for the
radiation of heat from the earth.
During day, isolation warms the atmosphere & the earth.
Precipitation Forms:
Rain:
Liquid precipitation, in the form of drops greater than 0.5mm in
diameter
Snow:
Aggregate of ice crystals that form into flakes below freezing
temperature.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Hail:
It is dense precipitation ice that is at least 5mm in diameter. Soft hail
is more white & less dense having air bubble.
Drizzle:
Liquid precipitation that reaches the surface in the form of drops less
than 0.5mm in diameter.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Freezing fog:
Fog composed of super cooled water drops.
Precipitation Types
 Convective precipitation
 Cyclonic precipitation
 Orographic precipitation
Convection:
 It is caused by thermal uplifting of moisture.
 Precipitation occurs in the form of intense showers of short
duration.
 The areal extent of such rains is small (generally less than 10km in
diameter).
 This generally occurs in tropics & rarely in arctic regions.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
CyclonicPrecipitation:
 It occurs as sequence of warm air in low pressure system, where it
is elevated & circulated over cold air.
 A cyclone is a low pressure region with circular wind motion.
 A cyclone storm is usually smaller.
 Intensity is light to medium.
 Causes most of the precipitation.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Types of Cyclonic Precipitation
1.Tropical cyclones:
Is a wind system associated with an intense depression?
2.Anticyclones:
High pressure regions of large areal extent
Winds are of moderate speed.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
OrographicPrecipitation:
 Occurs as a result of mechanical lifting of warm moist air.
 It is caused by moist air masses that strike natural topographic
barriers like mountains, causing them rise upwards, causing
cooling, condensation and precipitation.
 The precipitation must be stationary as it occurs at an elevation
along topography where dew drops occurs.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
 The leeward side receives very little precipitation whereas the
windward side receives good rain.
 Its duration is long
 Its intensity is medium to high
 The size of orographic precipitation might not cover a particularly
large area.
Measurement of Precipitation
Rainfall measurement:
Why do we need to measure rainfall?
• Agriculture
• Horticulture
• Engineers
• Scientists
Precipitationmeasurement:
Rain gages are most commonly used for the measurement of
precipitation.
Types of Rain gauges
There are two main types of rain gages which are used to measure the
precipitation. These are;

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
1. Non recording rain gages
2. Recording rain gages
Non Recordingrain gauges:
It is a rain gage which does not provide the distribution of amount of
precipitation in a day. It simply gives the amount of precipitation after
24 hours (daily precipitation).

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
Recordingrain gauges:
There are three main types of recording rain gauges
1. Tipping bucket rain gauge
2. Float type rain gauge
3. Weighing type rain gauge
1. Tipping bucket rain gauge:
The tipping bucket rain gauge consists of a funnel that collects and
channels the precipitation into a small seesaw-like container.
After a pre-set amount of precipitation falls, the lever tips, dumping
the collected water and sending an electrical signal.
An old-style recording device may consist of a pen mounted on
an arm attached to a geared wheel that moves once with each signal
sent from the collector.
In this design, the wheel turns the pen arm moves either up or down
leaving a trace on the graph and at the same time making a loud click.
Each jump of the arm is sometimes referred to as a 'click' in reference
to the noise.
The chart is measured in 10-minute periods (vertical lines) and
0.4 mm (0.015 in) (horizontal lines) and rotates once every 24 hours
and is powered by a clockwork motor that must be manually wound.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
2. Floattype rain gauge:
A class of rain gauge in which the level of the collected rainwater is
measured by the position of a float resting on the surface of the water.
This instrument is frequently used as a recording rain gauge by
connecting the float through a linkage to a pen that records on a clock-
driven chart.
3. Weighing type rain gauge:
A weighing-type precipitation gauge consists of a storage bin, which
is weighed to record the mass.
Certain models measure the mass using a pen on a rotating drum, or
by using a vibrating wire attached to a data logger.
The advantages of this type of gauge over tipping buckets are that it
does not underestimate intense rain, and it can measure other forms of
precipitation, including rain, hail and snow.
These gauges are, however, more expensive and require more
maintenance than tipping bucket gauges.
The weighing-type recording gauge may also contain a device to
measure the quantity of chemicals contained in the location's
atmosphere.

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
This is extremely helpful for scientists studying the effects of
greenhouse gases released into the atmosphere and their effects on the
levels of the acid rain.
Some Automated Surface Observing System (ASOS) units use an
automated weighing gauge called the AWPAG (All Weather
Precipitation Accumulation Gauge).
Arithmetic Mean Method:
 Simplest method for determining areal average.
P1=10mm
P2=20mm
P3=30mm

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
 Gauges must be uniformly distributed.
 Gauges measurement should not vary
about the mean.
Theissen polygon method:
 Any point in the water shade receives the same amount of rainfall
as that the nearest gauge.
 Rainfall recorded at a gauge can be applied to any point at a
distance halfway to the next station in any direction.
 Steps in theissenpolygon method:
1. Draw lines joining the adjacent gauges.
2. Draw perpendicular bisector to the lines created in step 1.
3. Extend the lines created in step 2 in both directions from the
representative areas for gauges.
4. Compute representative area for each gauge.
5. Compute the aerial average by using the following formula:
Let
P1=10mm, A=12km2

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)
P2=20mm, A=15km2
P3=30mm, A=20km2

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M. Wajid Manzoor
(Institute of Geology Punjab University Lahore)