PRECIPITATION
Week 2 Lecture Notes
Dr.Aysun KOROGLU
ITU CIVIL ENGINEERING DEPARTMENT
 Formation of Precipitation
 Necessary Conditions for Precipitation
 Artificial Rain
 Precipitation Measurement
 Measurement of Rainfall
 Measurement of Snow
 Measurement Errors
 Gage Network
 Analysis of Precipitation Records
 Precipitation Mass Curve and Hyetograph
 Homogeneity of Records
 Filling Missing Observations
 Extrapolating the Records
 Calculation of Areal Average Precipitation
 Areal Distribution of Precipitation
 Precipitation depth-Area-Duration Analysis
 Probable Maximum Precipitation
 Distribution of Precipitation in Time
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 Water falling on earth from atmosphere in liquid or
solid form is called precipitation.
 Rain and snow are the two most important forms of
precipitation hydrologically.
 Main difference between them being that
 falling as rain immediately becomes runoff
 Whereas snow generally melts after a period of time.
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 Necessary Conditions for Precipitation:
Sufficient vapor should exist in that region of the
atmosphere (The main source of vapor in the atmosphere
is oceans, such that 90% of the precipitation on lands
comes from water evaporated from the oceans).
2. Air mass should be cooled (Cooling of the air reduces its
vapor carrying capacity, and water condenses into liquid
form when the saturation point is reached).
1.
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 Necessary Conditions for Precipitation:
3. Condensation must occur (Condensation occurs on
very small pieces of dust-of the order of microns) called
condensation seeds. Such pieces of dust(organic
material, volcanic ash, sulfuric acids and nitric acid, clay
particles, salt and smoke) are always present in the
atmosphere.
 Therefore condensation occurs as soon as the air is
saturated, which forms clouds. The size of water particles
in clouds is 1-100 micron, and therefore they do not fall to
earth’s surface without evaporation but are suspended in
the air.
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 Necessary Conditions for Precipitation:
4.
Drops that are sufficently large(about 1 mm) should be
formed so that they can fall down to earth’s surface
 This occurs when ice crystals on which water vapor condense are
present, or when small drops grows by hitting each other.
 In temperatures below —10°C water vapor condenses on ice
crystals to form large drops when sufficient amount of ice crystals
are present in a cloud, because vapor pressure on ice is lower than
vapor pressure on liquid water.
 This is not possible in warmer clouds, where the only way larger
drops are formed is the collosion of drops condensed on salt nuclei.
 Precipitation may not occur although the first three conditions are
satisfied.
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 Convective precipitation
 Cyclonic(Frontal) precipitation
 Orographic precipitation
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 An air mass near the earth’s surface rises when it is
heated strongly.
 Air is heated
Precipitation
rises
cooles
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 When a warm air mass meets a cold air mass along a
vertical front, warm air will be raised and cold air will
be lowered.
 Cold front precipitation occurs when cold air mass
moves along the front by pushing the warm air mass.
Warm front precipitation occurs when warm mass
moves by pushing the cold air mass
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 A moist air mass will be cooled when it rises to pass
a mountain range and produces orographic
precipitation.
 A positive correlation exists between the elevation
and depth of precipitation in such cases.
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 Precipitation does not occur when the mechanism to form raindrops that
are sufficient size to fall down to earth is not present. In such cases, rain
can be produced artificially.
 Crystals of silver iodine:
 Are scattered over the clouds from airplanes,
 Silver iodine vapor produced by burning it on land surface rises to the
clouds(Water condenses on these crystals at temperatures below
-5°C, and large enough ice crystals are formed.)
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 Crystals of silver iodine:
 1016 number of precipitation seeds are formed from 1 g silver iodine.
 This is the sufficient number of seed that precipitation can occur.
 The concentration of silver iodine is lower than 0.1 mikrogram/liter in rain
drops.
 USA Common Wealth Research Institution has declared that the maximum
concentration of silver iodine in water sources is 50 mikrogram/liter.
 This is first applied in Turkey in 1990 in Istanbul by Istanbul Water and
Sanitation Branch (ISKI) .
 This method was also used in different time intervals in Ankara and Izmir as
well.
.
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 Dry Ice(Solid carbondioxide) :
 Scattering over the clouds to drop the temperature to -40°C so that
water in the clouds form ice crystals.
 Thus ice crystals are not otherwise present in the clouds are
formed artificially(cloud seeding). This is possible only when the
temperature is below -5°C.
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The efficiency of artificial rain depends on such conditions:
Seeding timing
Wind profile,
The velocity of air elevation and other atmospheric conditions,
Super cooled water drops,
Seed concentration,
The collosion of dorps and their direction.
 It is shown that artificial rain can be produced when the conditions are
right, but the results have been the subject of much discussion.
 The rain may start 15 minutes or a couple of hours following the
seeding process.
.
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 Precipitation




is expressed as the depth of
precipitation defined as the height of the water
column that accumulates in a certain time interval on
a horizontal plane.
It is usually given in milimeters. 1 mm of precipitation
is equal to 1 kg/m2.
Recording instruments show the variation of
precipitation in time and supplies information when it
is not possible to approach the instrument.
Non- recording Pluviometers
Recording Pluviographs
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 Pluviometer
:
Non-
recording
instruments
 Any cylinder with vertical sidewall can
be used to measure the rainfall.
 Standart vessels must be used so that
measurements are comparable and
errors are of same magnitude.
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Pluviograph Types
These instruments record the variation of
the precipitation depth with time on paper.
1. Weighing Gage
Rainfall is accumulated in a bucket. As
the bucket gets heavier, it moves a pen
on a rotating paper chart. This gives a
curve
showing
the
variation
of
precipitation depth with downwards
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Pluviograph Types
2. Tipping bucket gage
Rainfall
entering
the
gage
accumulates in a very small bucket,
which is emptied by tippin when it fills,
moves a pen on a chart by certain
amaount, and is replaced by another
bucket.
This gives a step line on the rotating
chart. Each step equaling to 0.3-0.5
mm depth of precipitation.
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Pluviograph Types
3. Float-type gage
As the water level in the vessel rises, a float moves a pen
on a rotating chart. When the vessel is full, it is emptied
rapidly by an automatic siphon.
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Pluviograph Types
4. Pulse counting gage
• When the cups in cylinder vessel is full, the rainfall
fills the other.
• The repositining of the cup generates a pulse.
• Each pulse is equal to 0,2 mm depth of
precipitation .
• These pulses are counted so that the total amount
can be calculated.
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5. Radar
Pluviograph Types
Microwave radar (wavelength 1-20 cm) can be used in precipitation
measurement. Energy of the reflected waves is proportional to the size of
rainfall drops and therefore, to the intensity of precipitation.
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 Measurement of snowfall
 Measurement of snow cover
 Snowfall is measured by gages used for rainfall.
 Non-recording gages and weighing gages can be
used to measure snowfall.
 Antifreeze addititves such as calcium chloride and
ethylene glycol should be used to prevent the
freezing of accumulated snow.
 Gages must be installed at a sufficient distance
above the ground so that they are not buried in the
snow.
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 Water equivalent of snow equals the product of
specific gravity of snow cover and its thickness.
 It varies 100-600 kg/m3
 For new snow- 100, for denser snow 300-600.
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 Most important errors are caused by the wind.
 Wind speed increases with the distance of the
gage above the ground, together with the ration
of precipitation that enters the gage.
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• Another cause of the errors is the reduction of the
precipitation entering the gage by high obstacles such as
buildings and trees. Gages must be placed at a distance
of at least two times the height of such obstacles.
• If the gage is placed too high, the wind effect is stronger.
• Evaporation of the water in the gage must be prevented
by forming a thin layer of oil on the water surface.
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 A network of precipitation measurement gages must
be set up that is dense enough to determine the
areal distribution of precipitation.
 Gages must be placed denser in mountainous
regions affected by air masse comign from the sea,
where the intensity of precipitation varies rapidly with
elevation.
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 World Meteorological Organization reccomends the
optimum density of one gage per 600—900 km2 in
plains, and one gage per 100—250 km2 in
mountainous regions.
 The interval of elevations of gages must be of
recording type to allow the distribution of
precipitation in time to be measured.
 DMI: 545 number of gages are established in Turkey
of the 470 gages are automated.
769.600 km2 of Turkey’s
surface area, of 400 000 km2
are defined as mountainous
area.
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 www.tumas.mgm.gov.tr
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 Precipitation mass curve:
P-t curve
 P= Precipitation depth(mm)
 i= Precipitation depth in unit
time- precipitation density
(mm/hr)
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 Hyetograph:
The curve showing the variation of precipitation intensity with time.
The time interval is chosen respect to the size of the region, usually in the
range 1-6 hours.
Usually the average intesity reduces as the duration increases.
 If the precipitation intensity:
 <2.5mm/hr = Light,
 2.5-7.5mm/hr=Medium
 >7.5mm/hr=Heavy rain
i=Average intensity,
T=Return period,
(The average number of years
between two precipitation
events with intensity of at least i) 30
tp=Duration
 Homogeneity of a precipitation record is analyzed by double
mass curve method to check whether the location, oreintation
and the method of measurement of the precipitation gage has
been changed.
 At least 5 gages must be used to
estimate the average.
 In case the straight line drawn
through the plotted points is
broken, then the year when the
slope of the line changes is the
year when homogeneity is broken.
 To
homogeneize
the
measurements before that date,
they are multiplied by the ratio of
the slopes of the lines before and
after that date.
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 Missing data in a station can be filled in using the records of
neighboring stations.
 NA,NB,NC= Annual average precipitationat stations (mm/year)
 PA,PB,PC= Precipitation observed at stations(mm)
 Nx= Annual average precipitation (mm/year)
 When NA,NB,NC do not differ from Nx by more than 10%, aritmetic
average can be used instead of weighted average of:
 Pi & Di= Readings at 4 stations closest to the
station with missing data and their distances
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to the station with missing data
 In meteorologically homogeneous region, records at
stations that are far enough from each other such
that the precipitation depths are independent, can be
considered together as the record at a single station
along the total years.
 Thus 30-year long records at 10 stations can be
brought together as a 300-year year record at a
single station
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 dA=Element of area
 A= Total area of the region
 The precision increases with the density of the gages
because of the topography, vegetation, lakes and
structures influence the uniformity of precipitation
distribution.
 Arithmetic Average
 Thiessen Method
 Isohyetal Method
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 It can be used in regions smaller than 500 km2 when
the gages are rather uniformly scattered.
 The simplest method is to take the average of the
readings of all instruments
 This method may not give good results in
mountainous regions and heavy rainfall, when
precipitation various rapidly over the area.
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 The area is divided such that each point lies in the subregion of the region of
the gage to which it is closest.
 Drawing their perpendicular bisectors(normals at their midpoints)
 This gives the Thiessen Polygon, where it is assumed that each subregion
formed by the normals is represented by the gage that lies in it.
 Then the average areal precipitation is calculated by a weighted average.
 Ai= Area of the subregion around
the gage i with precipitation Pi
 N=Number of gages
 Can be used for regions of 500-
5000 km2
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Isohyetal Method
 Isohyets(curves of equal precipitation depth) are drawn by joining
the points of equal precipitation.
 Areal average precipitation is then calculated by a weighted
average with weights equal to the average of the isohyets for
areas between the two adjacent isohyets.
 Ai=
The area between two adjacent
isohyets
 Pi = The average of their precipitation
values
 N= Number of gages
 Gives
good results
mountainous regions.
especially
in
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 During a storm average areal precipitation is always smaller
than the point value at the center of the storm.
 Areal average precipitation in an area around the center
decreases as the area increases.
 The ratio (P/Po ) decreases rapidly for a 30-minute storm, but
the decrease is much slower for a 24-hour storm.
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 The recution of the precipitation over an area with size of the
area during a certain storm duration is given by a formula due
to Horton.
 P0= Depth of precipitation at the storm center,
 P=Average precipitation over the area A,
 k,n= Coefficients determined seperately for each storm.
 tp= Storm duration,
 a,m,b= Regional coefficients
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 In some cases it is required to know not only the areal
distribution of precipitation but also its distribution in time:
1.
2.
The area inside each isohyet around each storm center
in the region is measured and average precipitation
depths are calculated.
Mass curves are drawn for each recording gage in the
region.
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6,12 hour precipitation
depth-area curves are
obtained from the regions
inside each isohyet.
4. Areal
distribution
of
precipitation depth for
each duration is plotted
and envelope curves are
drawn.
3.
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 Probable maximum precipitation, defined
as the maximum precipitation depth in the
basin that is physically possible for a
certain duration, is used in the design of
the spillways of large dams that would
cause loss of life when they collapse.
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Precipitation depth-area-duration curves of the basin are
obtained for the heaviest observed storms.
2. The efficiency of the mechanism that converts the water vapor
to rain, the vapor content of the air masses, and the wind
velocities in the observed storms are increased to their
maximum possible values.
3. Also the heavy storms observed in the neighbouring basins
that are meteorologically similar are transported to the basin
taking the meteorological differences into consideration.
1.
This method should not be used for orographic precipitation,
because the precipitation depth varies rapidly in space.
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 Precipitation depth has an annual periodicity because of
the meteorological effects.
 Variation of the precipitation along the year has an effect
on the water budget of the region.
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 www.srh.noaa.gov/
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