What is Precipitation?, what are the forms of precipitation?, Measurement of precipitation?

Precipitation:-

• Precipitation includes all forms of water in which they travel from the atmosphere to the earth’s surface.

Forms of Precipitation:-

 1. Rain: -

• When the rain comes in the form of water droplets, the size of the water droplets is 0.6 mm to 6 mm.
• It is the measured in the term of the depth of water.
• Types of Rain:-

• If the depth of rain in a day is more than 2.5 mm, then that day is called a rainy day.

 2. Snow:-

• When rain falls in the form of ice crystals, the density of ice crystals is between 0.06 to 0.15 g/cc. The average density is 0.1 g /cc. 

3. Drizzle:-

• The size of the water droplets is less than 0.5 mm. 

4. Glaze:-

• When water droplets travel from the atmosphere to the surface of the cold earth, the water droplets transform into ice, which is converted into ice, known as glaze. 

5. Sleet:-

• It represents the mixture of a water droplet and ice lumps having a size is less than 6 mm.

6. Hail:-

• When the rain comes in the form of ice lumps, the size of the ice lumps is above 8mm. 

7. Trace:-

• It represents the mixture of a water droplet and ice lumps having a size is less than 0.1 mm.

Measurement of precipitation:-

1) Rainfall Measurement:-

• Measurement of rainfall using rain gauge and (rain gauge also known as pluviometer, udometer, ombrometer, and hyetometer).
• According to IMD (Indian Metrological Department), rainfall should be measure at 8.30 am.
• Site condition for rain gauge:- 

1. The rain gauge must be as near to the ground surface as possible to avoid any obstruction.
2. The rain gauge should be at sufficient height to prevent splashing of the droplet into the device.
3. There should not be any structure at the 30 m radial distance.

• Types of Rain gauge:- 

1. Non-Recording type rain gauge:-

• It is also known as an ordinary rain gauge.
• It is the measured in the term of the depth of water.

     1. Symon’s rain gauge

• In India, Symon’s type non-recording rain gauge use.
• The cylindrical vessel diameter of Symon’s rain gauge is 127 mm or 12.7 cm.
• The glass tube diameter is 7.5 cm to 12 cm.

    2. According to IMD standard rain gauge.

• Measuring jar made by reinforced glass fibre polyester material.

     

     2. Recording type rain gauge:-

• It is measured in terms of the depth of water, rainfall intensity and duration of rainfall.

      1. Tipping bucket type:-

• It is used in a hilly area.

     2. Weighing bucket type:-

• Rainfall and Snowfall are measured in this instrument. 
• It is used in light rainfall area and cold area.

     3. Natural Syphon type:-

• It is also known as float type rain gauge.
• In India, a Natural Syphon type recording rain gauge use.

    

2) Snowfall Measurement:-

• Measurement of snowfall using snow tube.
• It is measured in the depth of water formed after the ice has melted.

Optimum no of rain gauge:-

• It is used to determine the installation of the required rain gauge in a given catchment.
• It is dependent on permissible error.
• Optimum no. of rain gauge:-

   

        Cv = Coefficient of variation

  
        Pm = Mean of rainfall data of an existing set of rain gauge

 
       σ = Standard deviation

 
      %E = Permissible error

Rain gauge Network:-

   A.  As per WMO (World Metrological Organization)

• In flat region:- 1 station = 600 – 900 km2
• In hilly region:- 1 station = 100 – 250 km2
• In Arid zone:- 1 station = 1500 – 10000 km2 

  B.  As per IMD (India Metrological Department)

• In flat area:-1 station = 520 km2
• In areas having average elevation≈ 1000 m:-1 Station = 260-390 km2
• In hilly areas high rainfall :-1 Station = 130 km2

Estimation of Average rainfall over a catchment:-

1. Arithmetic Mean Method:-

• This method is used in small areas.
• Very simple method.
• This method is not used frequently.
• It does not consider rain gauges located outside the catchment. 
• Least accurate method.

 

      2. Thiessen Polygonal Method:-

• In this method, interconnect every rain gauge station and draw a proper perpendicular to each line.
• This method also includes rainfall outside the catchment area.
• It is a more accurate method.

      3. Isohyetal Method:-

• Isohyet:- The line joining points of the equal depth of rainfall is known as isohyet.
• The area between the two isohyets is measured by a Planimeter.
• The different shape is obtained due to different intensity of rainfall.
• It is a more accurate method than Thiessen polygonal method and Arithmetic mean method.
• Time-consuming method.

    

Analysis of rainfall records:-

   Mass curve:-

• It is a graphical representation of the cumulative depth of rainfall and time duration.
• This technique is used in recording type of rain gauge (Float type and Weighing-bucket type rain gauge)

   

  Double mass curve:-

• A curve is a plot of the accumulated rainfall of station ‘X’ against the average accumulated annual rainfall of a group of rain gauge station.
• It is used to find out the consistency of a rain gauge station.

    
  Hyetograph:-

• It is represented as a bar chart.
• A bar chart is a representation between the rainfall intensity and time duration.

   

Also, read about INTRODUCTION OF HYDROLOGY