Precipitation and its forms Center For Integrated Mountain Research , Punjab University Pakistan

2. PRECIPITATION
PRESENTED BY:
Maha Sabri
(M.SC-Part1)
Mountain Conservation And Watershed
Management.

3. PRECIPITATION
• Introduction
• Forms of precipitation
• Mechanism of precipitation
• Classification
• Measurement of precipitation
• Types of rain gauges

4. Precipitation
• Definition:
• Fall of moisture from atmosphere in any form.
• Precipitation is any product of the
condensation of water vapours that fall under
gravity.
• It is an important input to hydrology.
• Floods and droughts are directly related to
precipitation.

5. Precipitation
• Factors determining precipitation are
• Climate
• Geography
• Ocean surfaces is the chief
• source of moisture for
• precipitation.

6. FORMS OF PRECIPITATION
• LIQUID PRECIPITATION
• Drizzle
• Rainfall
• FROZEN PRECIPITATION
• Snow
• Hail
• Glaze
• Sleet
• Snowflakes

7. Forms of precipitation

8. Forms of precipitation

9. Forms of precipitation
• hail

10. Mechanism of precipitation

11. Mechanism of precipitation
• All precipitation originates from parcels of
moist air that have been cooled below dew-
point temperature.
• Lifting mechanism to cool the air.
• Formation of cloud elements(droplets/ice
crystals)
• Growth of cloud elements.
• Sufficient accumulation of cloud elements.

12. Convective precipitation
It results from heating of earth’s surface that
cause air to rise rapidly. As the air rises it cools
and moisture condenses into clouds and
PRECIPITATION.

13. Cyclonic precipitation
(frontal/non frontal)
• when two air masses due to contrasting
temperatures and densities clash with each
other ,condensation and precipitation occur at
the surface of contact.

14. Orographic precipitation
• When heavily moisture laden air stops due to
topographic conditions (mountains)and
precipitation occurs on the windward side of
the mountain while leeward side receives very
little .just like in Himalayan region.

15. Measurement of precipitation
• 1. Amount of precipitation
• 2. Intensity of precipitation
• 3. Duration of precipitation
• 4. Arial extent of precipitation

16.  Measurement of precipitation (Rain and Snow)
can be done by various devices. These measuring
devices and techniques are;
 Rain Gauges
 Snow Gauges
 Radars
 Satellites
 Scratching of snow packs
 Water equivalent in snow packs
Measurement Methods

17. RAIN GAUGES
Rain gauges are most commonly used for the
measurement of precipitation, both in terms
of rain fall and snow.

18. Types of rain gauges
1. Non recording rain gauges
2. Recording rain gauges

19. 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

20. Recording rain gauges
• These rain gauges are also called integrating
rain gauges since they record cumulative
rainfall. In addition to the total amount of
rainfall at a station, it gives the times of onset
and cessation of rains (thereby gives the
duration of rainfall events)

21. • There are three main types of recording rain
gauges
• 1. Float type rain gages
• 2. Tipping bucket type rain gages
• 3. Weighing type rain gages
Types of recording Rain gauges

22. 2.Tipping Bucket Rain-gauge
• Consists of 30 cm dia sharp edge
receiver.
• At the end of receiver funnel is
provided.
• Under the funnel a pair of buckets are
pivoted (the central point which
balances) in such away that when one
bucket receives 0.25 mm (0.01”) of
rainfall it tips (to fall or turn over),
discharging its contents into reservoir
bringing other bucket under funnel.
• Tipping of bucket completes an
electric circuit causing the movement
of pen to mark on clock driven
revolving drum which carries a
record sheet

23. Weighing Bucket Rain-gauge
• • Consists of a receiver
• bucket supported by a
• spring or lever balance or
• any other weighing
• mechanism.
• • The movement of bucket
• due to its increasing
• weight is transmitted to a
• pen which traces the
• record on a clock driven
• chart.

24. Float Type Rain-gauge
• The rise of float with increasing catch of rainfall is
recorded. Some gauges must be emptied manually
while others are emptied automatically using self
starting siphons. In most gauges oil or mercury is the
float and is placed in the receiver, but in some cases
the receiver rests on a bath of oil or mercury and the
float measures the rise of oil or mercury displaced by
the increasing weight of the receiver as the rainfall
catch freezes. Float may get damaged by rainfall catch
freezer

25. Errors in precipitation measurement
by gauges
 Instrumental errors
 Errors in scale reading
 Dent in receivers
 Dent in measuring cylinders
 About 0.25mm of water is initially required to wet the surface of
gauge
 Rain gauges splash from collector
 Frictional effects
 Non verticality of measuring cylinders (10° inclination gives 1.5%
less precipitation)
 Loss of water by evaporation
 Leakage in measuring cylinder
 Wind speed reduces measured amount of rain in the rain gauges.

26. Average precipitation over an area
• • In order to compute the average rainfall over a basin or
• catchment area, the rainfall is measured at a number of raingauge
stations suitably located in the area.
• • The no. of rain-gauge stations depends upon the area and
• distribution of rainfall.
• • If a basin or catchment area contains more than one raingauge
station, the computation of average rainfall may be
• done by the following methods:
• 1. Arithmetic average method.
• 2. Thiessen polygon method.
• 3. Isohytel method.

27. 1.Arithmetic average method.
• Precipitation over area area is arithmetic average of gauge
precipitation values.
• Simplest method but only applicable when basin area is flat.
• All stations with in practical limit uniformly distributed over
area.
• Rainfall is also nearly uniformly distributed over area.
• P1,P2…….Pn=no of precipitation
• N=no of gauges
• I=stations

28. 2.Thiessen polygon method.
• This method is a more common method of
weighing the rain gauge observation according
to the area.
• Also called Weighted Mean Method.
• Accurate than arithmetic average method.
• Average rainfall can be computed by the
following
expression.

29. 2.Thiessen polygon method.
• 1. Join the adjacent rain-gauge stations.
• 2. Construct the perpendicular bisectors of each of these lines.
• 3. The polygon formed by the perpendicular bisectors around
• a station encloses an area encloses an area which is every
• where closer to that station than to any other station.
• 4. Find the area of each of these polygons, shown hatched in
• the figure.
• 5. Compute the average precipitation using the given formula.

30. 2.Thiessen polygon method.

31. 2.Thiessen polygon method.

32. 3.Isohytel method
An isohyet is a line, on a rainfall map of the basin, joining
places of equal rainfall readings.
• An isohyetal map showing contours of equal rainfall presents
a more accurate picture of the rainfall distribution over the
basin.
• Average rainfall can be computed by the following

33. Isohytal method
 Plot gauge locations on a map;
 Subjectively interpolate between rain amounts
between gauges at a selected interval;
 Connect points of equal rain depth to produce lines
of equal rainfall amounts (isohyets);

34. Isphytel method

35. Radar Measurements
A weather radar is a type of radar used to locate precipitation,
calculate its motion, estimate its type (rain, snow, hail, etc.),
and forecast its future position and intensity. Weather radars
are mostly Doppler radars, capable of detecting the motion of
rain droplets in addition to intensity of the precipitation. Both
types of data can be analyzed to determine the structure of
storms and their potential to cause severe weather.

36. Satellite Measurements
A weather satellite is a type of
satellite that is primarily used to
monitor the weather and
climate of the Earth.These
meteorological satellites,
however, see more than clouds
and cloud systems, like other
types of environmental
information collected using
weather satellites.

37. Weekly precipitation outlook over
Pakistan by PMD