Precipitation presentation

2. Definition
All types of moisture reaching the surface of earth from
atmosphere.
Precipitation is the basic input to the hydrology.
Factors determining
precipitation or the
amount of atmospheric
moisture over a region
Climate
Geography
Ocean surfaces is the
chief source of moisture
for precipitation

3. Forms of precipitation

4. Rain
Rain is the most common type of
precipitation in our atmosphere. Rain is
when liquid droplets fall to the surface of
the Earth.
There are two different forms of rain,
either in the form of
showers
drizzles
Showers are heavy, large drops of rain
and usually only last a period of time.
Drizzles however usually last longer
and are made up of smaller droplets
of water.
Rain can either be formed as ice crystals
melt or it can be smaller water droplets.
Light
I = 2.5mm/hr
Moderate
I = 2.8-7.6mm/hr
Heavy
I > 7.6 mm/hr

5. Snow
Snow is the second most common precipitation in
the North East.
Snow forms when water vapor turns directly into ice
without ever passing through a liquid state. This
happens as water condenses around an ice crystal.
Density of freshly fallen
snow varies between
125-500mm of snow
required to equal 25mm of
liquid water
Average density (specific
gravity) = 0.1

6. Hail
Hail is created when moisture and wind are together. Inside
the cumulonimbus clouds ice crystals form, and begin to fall
towards the surface of Earth. When this starts to happen
wind gusts start to pick up the ice crystals pushing them up
high into the clouds. As they start to fall down again they
continue to grow in size. A wind gust might catch the hail
stone again which will push it back up into the cloud. This
whole process gets repeated several times before the hail
stone becomes so big that it is too heavy for the wind to
carry so it must fall towards Earth.
Shapes of hail particles
2.Spherical
3.Conical
4.Irregular
Diameter range 5 to 125 mm
Specific gravity = 0.8
Average density (specific gravity) =
0.1

7. Fog
There are four main types of fog,
radiation fog
advection fog
upslope fog
evaporation fog
There is really no different between fog and the
clouds that are high in the sky. In simple terms
fog is; a cloud that has formed near the surface of
the Earth.

8. Dew
The small drops of water which can be found on cool
surfaces like grass in the morning.
This is the result of atmospheric vapor condensing on
the surface in the colder night air.
Dew Point is the temperature in which condensation
starts to take place or when dew is created.

9. Mist / Drizzle
Mist is a bunch of small droplets of water which are in the
air. This occurs with cold air when it is above a warm
surface, for example water.
Fog and mist are very similar, the only difference is their
visibility.
If you cannot see 1 kilometer or less you know you're dealing
with fog.
You can see visuals through mist and it is more haze looking
than a thicker substance.
Diameter range between 0.1
and 0.5 mm/hr

10. Glaze
Glaze is the ice coating, generally clear and
smooth, formed on exposed surfaces by the
freezing of super cooled water deposited by rain or
drizzle.
Specific gravity may be as high as 0.8-0.9

11. Rime
Rime is the white opaque deposit of ice granules
more or less separated by trapped air and formed
by rapid freezing of super cooled water drops
impinging on exposed objects.
Specific gravity may be as low as 0.2-0.3

12. Sleet
Sleet consists of transparent, globular, solid grains
of ice formed by the freezing of raindrops or
freezing of largely melted ice crystals falling
through a layer of sub freezing air near the earth’s
surface.

14. Lapse rate
In general, a lapse rate is the
negative of the rate of
temperature change with altitude
change, thus:
where γ is the lapse rate given in
units of temperature divided by
units of altitude, T = temperature,
and z = altitude. Average lapse
rate is about 2°C/1000ft
The lapse rate is defined as the rate of decrease with height
for an atmospheric variable. The variable involved
is temperature unless specified otherwise.

The terminology arises from the word lapse in the sense of a
decrease or decline; thus, the lapse rate is the rate of decrease
with height and not simply the rate of change. While most
often applied to Earth's atmosphere.
Altitude (m)
Temperature (C)

15. Formation of precipitation
Moisture is always present
in the atmosphere, even on
the cloudless day.
Saturation however does
not necessarily lead to
precipitation.

16. Necessary mechanism to form Precipitation
1. Lifting mechanism to cool the air
2. Formation of cloud elements
(Droplets/Ice crystals)
3. Growth of cloud elements
4. Sufficient accumulation of cloud elements

17. 1. Lifting mechanism to cool the air
Lifting mechanism gives the three main types of
Precipitation.
Cyclonic Precipitation (Frontal /non Frontal)
Convective Precipitation
Orographic Precipitation

18. Cyclonic Precipitation
(Frontal/Non frontal)
Frontal precipitation results when the leading edge( front) of
a warm air mass meets a cool air mass. The warmer air mass is
forced up over the cool air. As it rises the warm air cools,
moisture in the air condenses, clouds and precipitation result.

19. Convective Precipitation
Convectional precipitation results from the heating of the earth's
surface that causes air to rise rapidly. As the air rises, it cools and
moisture condenses into clouds and precipitation

20. Orographic Precipitation
It results when warm moist air of the ocean is forced to rise by
large mountains. As the air rises it cools, moisture in the air
condenses and clouds and precipitation result on the windward
side of the mountain while the leeward side receives very little.
This is common in British Columbia.

21. Formation of cloud elements
(Droplets/Ice crystals)
For droplets, hygroscopic nuclei ,small particles
(0.1-10µm) having affinity for water must be
available in upper troposphere.
For ice crystals, Freezing Nuclei are required
Source of condensation nuclei are particles of sea
salts, products of sulphurous and nitric acid
Source of freezing nuclei are clay minerals, usually
kaolin, silver iodide etc

22. Growth of cloud elements
For occurrence of precipitation over an area it is necessary that cloud
elements must be grown in size to over come
Coalescence of cloud droplets
Cloud droplets are usually smaller than 50µm in diameter, due to different
diameters of droplets they fall with varying fall velocities. As the bigger cloud
elements are heavier , having more fall velocity, hence they collide with
smaller droplets. Smaller droplets join the bigger droplets and in this way the
size of cloud droplets increases.
Co-existence of cloud droplets & ice crystals
If in a layer of clouds there is mixture of water droplets and ice crystals. As the
saturation vapour pressure over ice is lesser than over water. As a result of this
difference , there results evaporation of water drops and condensation of
much of this water on ice crystals. Causing their growth and ultimate fall
through clouds. The ice crystals will further grow as they fall and collide with
water drops.

23. Growth of droplets and ice crystals
For the occurrence of precipitation over an area
necessary conditions are :
Cloud elements must increase in size until their
falling speeds exceed the ascending rate of air
Cloud elements should be large enough in size not to
get evaporated completely before reaching the ground

24. Measurement of Precipitation
1. Amount of precipitation
2. Intensity of precipitation
3. Duration of precipitation
4. Arial extent of precipitation

25. Measurement Methods
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

26. RAIN GAGES
Rain gages are most commonly used for the
measurement of precipitation, both in terms of rain
fall and snow.

27. Types of rain gages
There are two main types of rain gages which are used
to measure the precipitation. These are;
1. Non recording rain gages
2. Recording rain gages

28. 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).

29. 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)

30. Types of recording Rain gauges
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

31. 1. Tipping bucket gauges
A tipping bucket rain gauge is used for
measurement of rainfall. It measures the rainfall
with a least count of 1 mm and gives out one
electrical pulse for every millimeter of rainfall

32. 2. Weighing type gauges
It consists of a storage bin, which is weighed to record the
mass. It weighs rain or snow which falls into a bucket, set
on a platform with a spring or lever balance. The
increasing weight of the bucket and its contents are
recorded on a chart. The record shows accumulation of
precipitation.

33. 3. Float recording gauges
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

34. Errors in precipitation measurement
by Rain 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.

35. Measurement of snow
In case of snow fall following two properties of more
interest are measured.
1. Depth of snow at a particular place in mm/inches
2. Equivalent amount of water in mm

36. 1. Depth of snow
Depth of snow fall at a particular place can be
measured by the following methods.
a. Standard rain gauges without collectors
b. Snow gauges
c. By scratching snow packs

37. Depth of snow methods
Standard rain gauges can also be used for measuring
the snow depth, with some alterations in the
arrangement of rain gauges, particularly, the
collectors are not used
On a paved surface with snow over it, scratching that
snow layer with some scrapper helps to measure the
depth of snow fall with a tape. Visual observation and
with help of measuring tape helps to measure the
depth of snow

38. Snow gauges
A snow gauge is a
type of instrument
used to measure the
solid form of
precipitation.

39. Equivalent water in snow
Snow Water Equivalent (SWE) is a common snow
pack measurement. It is the amount of water
contained within the snow pack. It can be thought of
as the depth of water that would theoretically result if
you melted the entire snow pack instantaneously.
Equipment used is;

Standard rain gages without receivers
Weighing type rain gages
Snow gages

40. Measurement of equivalent
amount of water in a snow pack
The equivalent amount of water in a snow pack can
be measured by
1. Heating
2. Weighing
3. Adding measured amount of hot water

41. 1. By Heating
The equivalent amount in mm of water can be
obtained by heating the cylinder. it will melt the snow
and the depth of the liquid water can be measured
with a measuring stick but this approach is adjustable
because some water may get evaporated during the
heating.

42. 2. By Weighing
Weight is measured either by weighing type rain gauges or by using a snow
gauge.
W=W
1
-W
2

W
1
= weight of snow + empty cylinder
W
2= Weight of empty cylinder
W= Weight of snow

By using weight volume relationship

Γ = Weight/ Volume

Γ = W/ A.h

h = W/A Γ
Where,

h = Equivalent amount of water in snow.

43. c. By scratching snow packs
A measured amount of hot water is added into the
cylinder which will melt the snow. Now measure the
total depth of water in the cylinder “h
1
”

h = h
1
-h
2

Where,

h
2 = measured amount of hot water
h = equivalent amount of water

44. 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.

45. 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.

46. Interpretation Of Precipitation Data
Interpretation of missing precipitation data includes;

1. Estimating missing precipitation data at a station
2. Checking inconsistency in particular data at a
station
3. Averaging precipitation over an area

47. 1. Estimating missing precipitation data
at a station

Missing precipitation data is estimated by two
commonly used methods.
Arithmetic Mean Method
Normal Ratio Method (NRM)

48. Arithmetic Mean Method
Arithmetic mean method is used when normal annual
precipitation is within 10% of the gauge for which
data are being reconstructed. This method is least
accurate however.

Where:
Pm = precipitation at the missing location
Pi = precipitation at index station I
N = number of rain gauges

49. Normal ratio method (NRM)
Normal ratio method (NRM) is used when the normal annual
precipitation at any of the index station differs from that of the
interpolation station by more than 10%. In this method, the
precipitation amounts at the index stations are weighted by the ratios
of their normal annual precipitation data in a relationship of the form:
Where:
Pm = precipitation at the missing location
Pi = precipitation at index station
Nm = average annual rain at ‘missing data’ gauge
Ni = average annual rain at gauge
N = number of rain gauges

50. Checking inconsistency in a
particular data record at a station
By a technique called Double Mass Curve Analysis.
It is used to check the consistency of many kinds of
hydrologic data by comparing date for a single station
with that of a pattern composed of the data from
several other stations in the area
The double-mass curve can be used to adjust
inconsistent precipitation data

51. Double Mass Curve Analysis
The theory of the double-mass
curve is based on the fact
that a plot of the two
cumulative quantities
during the same period
exhibits a straight line so
long as the proportionality
between the two remains
unchanged, and the slope of
the line represents the
proportionality. This
method can smooth a time
series and suppress random
elements in the series, and
thus show the main trends
of the time series.

52. Averaging precipitation over area
It is the amount of precipitation which can be
assumed uniform over an area. If the average
precipitation over an area is known than total rain
volume of water can be computed for that area.
Rain volume = P
avg
× A

53. Methods for computing Average
precipitation over an area
There are some widely used methods to compute
average precipitation over an area, but the most
common of these used are:
Arithmetic mean method
Theissen polygon method
Isohytal method

54. Arithmetic Mean Method
Arithmetic mean method is used when normal annual
precipitation is within 10% of the gauge for which
data are being reconstructed. This method is least
accurate however.

Where:
Pm = precipitation at the missing location
Pi = precipitation at index station I
N = number of rain gauges

55. Theissen Polygon Method
Divide the region (area A)
into sub-regions centred
about each rain gauge;
Determine the area of each
sub-region (Ai) and
compute sub-region
weightings (Wi) using: Wi
= Ai/A
Compute total aerial
rainfall using Rainfall
recorded at each station is
given a weight age based
on the area closest to the
station.

56. Theissen Polygon Method
Consider a catchment area with say, 3
rain gauge stations. Let there be 3
stations outside the catchment,
but in its neighborhood.
Catchment area is drawn to scale
and position of these 6 stations is
plotted on it. Stations are joined
so as to get a network of triangles.
Perpendicular bisectors are drawn
to each of the sides of these
triangles. These bisectors form a
polygon around each station. If
the boundary of catchment cuts
the bisectors, then boundary is
taken as outer limit of polygon.
These bounding polygons are
called Thiessen Polygons. The area
of these polygons is measured
with a planimeter or by grid
overlay

57. 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);

58. Isohytal Method
Compute aerial rain using Isohyets –
It is a line joining points of equal rainfall
magnitude.
The catchment area is drawn to scale and
the rain gauge stations are marked on
it. The recorded rainfall values for
which aerial average is to determined
are marked at the respective stations.
Neighboring stations outside the
catchment are also considered. Taking
point rainfall values as the guide,
isohyets of different rainfall values are
drawn (similar to drawing contours
based on spot levels.
The area between adjacent isohyets is
measured using a planimeter. If
isohyets go out of the catchment, the
catchment boundary is used as the
bounding line.
It is assumed that the average value of
rainfall indicated by two isohyets acts
over the inter isohytal area