Precipitation

• Types of precipitation
• Rain, snow, hail, drizzle, glaze, sleet

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• Rain:
• Is precipitation in the form of water drops of size larger than 0.5 mm to 6mm
• The rainfall is classified in to
• Light rain – if intensity is trace to 2.5 mm/h
• Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr
• Heavy rain – above 7.5 mm/hr

• Snow:
• Snow is formed from ice crystal masses, which usually combine to form flakes

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• Hail (violent thunderstorm)
• precipitation in the form of small balls or lumps usually consisting of concentric layers of clear ice and compact snow.
• Hail varies from 0.5 to 5 cm in diameter and can be damaging crops and small buildings.

• Sleet - droplets that freeze once entering the freezing layer of air.

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• Drizzle – fine sprinkle of numerous water droplets of size less than 0.50 mm and intensity less than 1 mm/h.

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• Glaze – when rain or drizzle comes in contact with cold ground at around 0 ˚C, the water drops freeze to form ice coating called glaze or freezing rain.

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Rainfall Measurement

Why do we need to measure rainfall?

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Measurement of precipitation

• Precipitation is expressed in terms of depth to which rainfall water would stand on an area if all the rain were collected on it.

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• Rainfall at a place can be measured by a rain gauge.

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• The rain gauge may be broadly classified into two types.

(1) Non-recording type rain gauge

(2) Recording type rain gauge.

Methods of Measuring Rainfall:�Manual

• Often have a funnel opening into a cylinder gauge.
• Come in a variety of shapes and sizes
• Calculate the rainfall (in mm) by dividing the volume of water collected by the area of the opening of the cup. (The gauge marking often accounts for this).

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Non-recording type rain gauge

• The Symon rain gauge consists of a cylindrical vessel, called metal casing, of 12.7cm diameter, with a base of 21.0cm.

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• The metal casing is fixed vertically to a masonry foundation block of the size 60 cm x 60 cm x 60 cm. A funnel with a circular rim of 12.7 cm and a glass bottle are placed in the metal casing.

• The height of the metal casing is fixed so that the rim is 30.5 cm above the ground surface.

• The glass bottle, called the receiver, is of 7.5 to 10 cm diameter. The rain gauge is kept in open.

• The rain falling into the funnel is collected in the receiver.

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• The receiver, with the rain water in it, is taken out of the metal casing.

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• The rainfall depth is measured with a special measuring glass jar graduated in mm of rainfall.

• It can measure up to 12.5 mm of rainfall.

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• During heavy rains, the rainfall is measured 3 or 4 times in a day.

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• The total rainfall of the day is obtained by adding all the individual measurements of day.

Recording type rain gauge

• The recording type rain gauges have recording arrangement, which gives the rainfall
• intensity and the duration of the rainfall, besides giving the total depth of the rainfall. These
• rain gauges are, therefore, more useful than the non-recording type rain gauges

1. Tipping bucket rain gauge

2. Weighing bucket rain gauge

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3. Float-type rain gauge

Tipping bucket rain gauge

• The tipping bucket rain gauge consists of two small buckets placed below the funnel fitted in a 30 cm diameter receiver .
• The buckets are balanced in an unstable equilibrium about a horizontal axis such that at one time only one bucket remains below the funnel.
• One bucket is always higher than the other. As the rainfall is collected by the receiver, it passes through the funnel to the higher bucket.

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• After a certain small amount of rainfall (usually 0.25 mm) is received by the higher bucket, it becomes unstable and tips to empty itself into a measuring tube placed below it.

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• At the same time, the other bucket comes to the higher position and starts colleting the rainfall.

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• The tipping of the bucket actuates an electric circuit, which causes a pen to make a mark on a chart wrapped around a drum revolved by a clock-driven mechanism.
• Thus each mark on the chart corresponds to a rainfall of 0.25 mm.
• By counting the number of marks and noting the time, the intensity and amount of the rainfall can be determined.

Weighing Bucket Rain Gauge

• The rain is collected by a weighing bucket through a funnel.
• The bucket rests on a weighing platform of a spring or lever balance attached to a weighing mechanism.
• As the bucket is filled with rain water, its weight increases and the weighing platform moves downwards.

• The movement of the weighing platform is transmitted to a pen through a system of links and levers.
• The pen makes a trace of the accumulated amount of rainfall on a chart attached to a drum revolved by a clock driven mechanism.
• The record is continuous and is in the form of the conventional mass curve .

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• The rotation of the drum sets the time scale whereas the vertical motion of the pen records the cumulative rainfall.
• The slope of the mass curve at any point gives the intensity of the rainfall.

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Float type rain gauge

• In the float-type gauge, the precipitation falling on the receiver passes through the funnel and filter and is collected in a float chamber.
• The filter prevents dust, debris, etc, from entering the float chamber.
• The float chamber contains a float with a vertical stem fixed over it.
• A pen is mounted on the top of the vertical stem.

• As the rain is collected in the float chamber, the float rises and the pen moves on a chart fixed on a drum revolved by a clock-driven mechanism.
• The pen thus marks a trace of the cumulative depth of rainfall on the chart.
• The record is in the form of the conventional mass curve, from which the intensity and duration of rainfall can be determined.

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Site for a Rain Gauge Station

1. The site should be in an open space having an area of at least 5.5 m x 5.5 m.

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2. The distance of the instrument from the nearest obstruction should not be less than 30 m or twice the height of the obstruction.

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3. A site that is sheltered from high winds should be chosen.

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4. The rain gauge, as far as possible, should not be installed on the top or the side of the hill. If unavoidable, the site which is best protected from high winds should be chosen.

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5. A fence should be erected around the rain gauge station to protect the gauge from cattle, dogs, etc. However, the distance of the fence should not be less than twice its height.

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6. The gauge must always be mounted firmly so that it cannot be disturbed even by the strongest wind in that region.

7. The gauge should be set as near the ground surface as possible to reduce wind effects. But at the same time, it should be sufficiently high to prevent splashing of surface water into it.

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8. The gauge should have a level horizontal catch surface.