Precipitation

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Assignment

Engineering Geology and Hydrology

SUBMITTED TO:

Sir Muhammad Sana_ullah Sani

Institute of Geology

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

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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)

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

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Table of Contents Hydrological Cycle Components of 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


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Arithmetic Mean Method

Thiessen polygon method

References

Hydrological CycleM. Wajid Manzoor (Institute of Geology Punjab University Lahore)

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“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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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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Evaporation:Vaporization is a process by which water is converted into vapors.


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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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Condensation:Condensation is the process by which vapors are changes into liquid

state and is the reverse of evaporation.


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Mechanism of PrecipitationCoalescence Theory 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.Contact cooling:

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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Radiational cooling:

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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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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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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Cyclonic Precipitation: 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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Types of Cyclonic Precipitation1. 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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Orographic Precipitation: 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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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 PrecipitationRainfall measurement:Why do we need to measure rainfall?

• Agriculture

• Horticulture

• Engineers

• Scientists

Precipitation measurement:Rain gages are most commonly used for the measurement of

precipitation.

Types of Rain gaugesThere are two main types of rain gages which are used to measure the

precipitation. These are;


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1. Non recording rain gages

2. Recording rain gages

Non Recording rain 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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Recording rain 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.


https://en.wikipedia.org/wiki/Seesaw

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

https://en.wikipedia.org/wiki/Data_logger

https://en.wikipedia.org/wiki/Vibrating_wire

http://glossary.ametsoc.org/wiki/Recording_rain_gauge

http://glossary.ametsoc.org/wiki/Rain_gauge

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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


https://en.wikipedia.org/wiki/Automated_airport_weather_station

https://en.wikipedia.org/wiki/Acid_rain

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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 theissen polygon 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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P2=20mm, A=15km2

P3=30mm, A=20km2


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Education

Precipitation