This document discusses precipitation measurement and analysis in hydrology. It defines various forms of precipitation like rain, snow, hail, etc. Factors influencing precipitation formation like cooling of air and water vapor condensation are explained. Methods of precipitation measurement including non-recording and recording rain gauges are described. Techniques for estimating missing precipitation data using arithmetic mean and normal ratio methods are presented. Sources of errors in measurement and how to estimate average precipitation over a basin are also summarized.

1. NUST Institute of Civil Engineering/Engr Sajjad Ahmad1

2. Engineering Hydrology(CE- 235)CHAPTER - 22PRECIPITATIONNUST Institute of Civil Engineering/Engr Sajjad AhmadPRECIPITATION - OUTLINEForms of precipitation

3. Factors influencing precipitation formation

4. Measurement of precipitation

5. Computation of average rainfall over a basin3Engineering Hydrology(CE- 235)NUST Institute of Civil Engineering/Engr Sajjad Ahmad Falling of any form of moisture from atmosphere to ground4Engineering Hydrology(CE- 235)PRECIPITATIONTranspirationEvaporationWater vaporsAtmospheric systemPrecipitationNUST Institute of Civil Engineering/Engr Sajjad AhmadFORMS OF PRECIPITATIONDrizzle

6. Rain

7. Glaze

8. Sleet

9. Snow

10. Snowflake

11. Hail5Engineering Hydrology(CE- 235)NUST Institute of Civil Engineering/Engr Sajjad Ahmad6Engineering Hydrology(CE- 235)DRIZZLEMinute particles of water

12. Drop size < 0.5 mm

13. Intensity < 1.0 mm/hr

14. Do not produce runoff

15. NUST Institute of Civil Engineering/Engr Sajjad Ahmad7Engineering Hydrology(CE- 235)RAINMinimum drop size ≈ 0.5 mm

16. Maximum drop size ≈ 6.25 mm

17. Can produce runoff if

18. Rainfall intensity greater than the rate of infiltration

19. NUST Institute of Civil Engineering/Engr Sajjad Ahmad8Engineering Hydrology(CE- 235)GLAZEIce coating on drizzle or rain drops

20. Ice coating forms when

21. Drops comes in contact with cold surfaces on the ground

22. NUST Institute of Civil Engineering/Engr Sajjad Ahmad9Engineering Hydrology(CE- 235)SLEETDrizzle or rain drops in frozen form

23. Drops freezes while falling through air at subfreezing temperature

24. NUST Institute of Civil Engineering/Engr Sajjad Ahmad10Engineering Hydrology(CE- 235)SNOWCrystals of ice

25. Form due to sublimation

26. NUST Institute of Civil Engineering/Engr Sajjad Ahmad11Engineering Hydrology(CE- 235)SNOWFLAKEIce crystals fused together

27. NUST Institute of Civil Engineering/Engr Sajjad Ahmad12Engineering Hydrology(CE- 235)HAILBalls or lumps of ice

28. Diameter > 5 mm

29. Formed due to alternate freezing and drying

30. NUST Institute of Civil Engineering/Engr Sajjad Ahmad13Engineering Hydrology(CE- 235)FACTORS INFLUENCING PRECIPITATION FORMATIONMechanism of cooling of air

31. Water vapor condensation and formation of droplets

32. Mechanism for growth of cloud droplets

33. Mechanism for accumulation of moisture

34. NUST Institute of Civil Engineering/Engr Sajjad AhmadMECHANISM OF COOLING OF AIRThe warm air near the surface rises carrying moisture/water vapors with it

35. The air cools as it ascends due to lower temperature14Engineering Hydrology(CE- 235)NUST Institute of Civil Engineering/Engr Sajjad AhmadNUST Institute of Civil Engineering/Engr Sajjad Ahmad15

36. Engineering Hydrology(CE- 235)CODENSATION OF WATER VAPORSWater vapors condensed in presence of hygroscopic nuclei

37. These nuclei comes from sea salt or combustion products of sulfurous and nitrous acids

38. NUST Institute of Civil Engineering/Engr Sajjad Ahmad16

39. 17Engineering Hydrology(CE- 235)GROWTH OF CLOUD DROPLETSDrops should be of sufficient size to over come the drag and uplift forces of air

40. Droplet size can grow by two means

41. Coalescence of droplets through collisions

42. Bergeron’s effect

43. NUST Institute of Civil Engineering/Engr Sajjad Ahmad18Engineering Hydrology(CE- 235)BERGERON’S EFFECTIf temp is b/w 10 to 20 F then mixture of water droplets and ice crystals exists in atmosphere

44. Due to higher saturation capacity around the droplets, some water evaporates

45. While due to low saturation capacity around the ice crystals vapors condense and ice crystal size increases

46. NUST Institute of Civil Engineering/Engr Sajjad Ahmad19Engineering Hydrology(CE- 235)ACCUMULATION OF MOISTUREThe quantity of water fall over basin is much more than the amount of moisture over the basin

47. Therefore continuous supply of moisture is required

48. This process is known as convergence

49. NUST Institute of Civil Engineering/Engr Sajjad Ahmad20Engineering Hydrology(CE- 235)CONVERGENCEIt is the process of inflow of moisture from other areas to the basin

50. It can be defined as

51. The net horizontal influx of air per unit area

52. NUST Institute of Civil Engineering/Engr Sajjad Ahmad21Engineering Hydrology(CE- 235)PRECIPITATION CLASSIFICATION BASED ON LIFTING MECHANISMConvectional precipitation

53. Orographic precipitation

54. Cyclonic precipitation

55. Turbulent ascent precipitation

56. NUST Institute of Civil Engineering/Engr Sajjad Ahmad22Engineering Hydrology(CE- 235)CONVECTIONAL PRECIPITATIONNUST Institute of Civil Engineering/Engr Sajjad Ahmad23Engineering Hydrology(CE- 235)CONVECTIONAL PRECIPITATIONNUST Institute of Civil Engineering/Engr Sajjad Ahmad24Engineering Hydrology(CE- 235)CONVECTIONAL PRECIPITATIONNatural rising of warmer, lighter air in colder, denser surroundings

57. In the hot day, the ground surface gets heated, causing the warmer air to lift up

58. the colder air comes to take its place

59. The vertical air currents develop tremendous velocities the vapors condensed and Convective precipitation occurs

60. NUST Institute of Civil Engineering/Engr Sajjad Ahmad25Engineering Hydrology(CE- 235)OROGRAPHIC PRECIPITATIONNUST Institute of Civil Engineering/Engr Sajjad Ahmad26Engineering Hydrology(CE- 235)OROGRAPHIC PRECIPITATIONOrographic precipitation is caused by air masses which strike some natural topographic barriers like mountains, and cannot move forward

61. Hence rise up, causing condensation and precipitation

62. All the precipitation in Himalayan region is because of this nature

63. NUST Institute of Civil Engineering/Engr Sajjad Ahmad27Engineering Hydrology(CE- 235)CYCLONIC PRECIPITATIONCyclonic precipitation is caused by lifting of an air mass due to the pressure difference

64. Cyclonic precipitation may be either frontal or non-frontal cyclonic precipitation

65. NUST Institute of Civil Engineering/Engr Sajjad Ahmad28Engineering Hydrology(CE- 235)FRONTAL PRECIPITATIONIt results from the lifting of warm and moist air on one side of a frontal surface over colder, denser air on the other side