1. Precipitation Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
Hydrology
Precipitation
Atmospheric Moisture
Lecture 3
Week 3
2nd Semester 2008/2009

2. Causes and Mechanisms of Precipitation Formation
Condensation occurs as a result of cooling of the air to a temperature below the saturation point for water vapor.
It can be caused by:
Dynamic or adiabatic cooling.
Mixing of air masses having different temperatures.
Contact cooling
Cooling by radiation

3. Causes and Mechanisms of Precipitation Formation
Source of moisture
Lifting mechanism (orographic or heating)
Phase change from vapor to water - Energy
Small nuclei or dust for droplet formation
Droplets must grow as they fall to earth

4. Causes and Mechanisms of Precipitation Formation

5. Causes and Mechanisms of Precipitation Formation
Condensation nuclei must exist for the formation of cloud droplets
Which can be artificially supplied to clouds to induce precipitation under certain conditions

7. Rainfall Measurement

8. Rainfall Measurement

10. A network of five to ten gages per 100 mi2 is usually required.
Tipping bucket interpreted for total volume and intensity variations
Hyeterograph is a plot of rainfall intensity with time (in/hr or mm/hr)

12. Point Precipitation
In order to predict the rainfall at another point, or to supply missing data for a particular station one may use one of two methods:
–Station Average Method
–Normal Ratio Method
–National Weather Service Method

16. Areal Precipitation
For most hydrologic analyses, it is important to know the aerial distribution of precipitation. (ADP)
Usually average depths for representative portions of the watershed are determined and used for this purpose (ADP).

17. Areal Precipitation
For multiple stations, the average depth of precipitation over the watershed is determined using the information from all stations:
Arithmetic Method
Thiessen Polygon Method
Isohyetal Method
Distance Method

21. Thiessen Polygon

33. Assignment No. 1 Read Section 1.3 from text book
Prob. 1.3, 1.4, 1.5, 1.6, 1.8, 1.15
Submitted on
Prepare a 1-page paper on the use of NEXRAD rainfall for hydrologic prediction
Submitted not later than

34. Check up this paper: