Precipitation
Introduction
Precipitation is the water that falls from the atmosphere in either liquid or solid
form. It results from the condensation of moisture in the atmosphere due to the cooling
of a parcel of air. The most common cause of cooling is dynamic or adiabatic lifting of
the air. Adiabatic lifting means that a given parcel of air is caused to rise with resultant
cooling and possible condensation into very small cloud droplets. If these droplets
coalesce and become of sufficient size to overcome the air resistance, precipitation in
some forms results.
Forms of Precipitation
Factors Affecting the Type, Properties, and Behavior of Precipitation
• topography/region/space
• temporal/time
Meteorological Factors: weather elements that affect precipitation
• wind, temperature, humidity, pressure in the volume region enclosing
the clouds and ground surface at the given place
The term precipitation denotes all forms of water that reach the earth from the
atmosphere. Precipitation is the basic input in hydrology.
For precipitation to form:
• The atmosphere must have moisture
• There must be sufficient nuclei present to aid condensation
• Water conditions must be good for condensation of water vapor to take place
• The products of condensation must reach the earth
Forms of Precipitation
a. Rain
• Rain is the most common type of
precipitation in our atmosphere.
Rain is when liquid droplets fall to
the surface of the earth. The term
rainfall is used to describe
precipitations in the form of water
drops.
• There are two different forms of rain either showers or 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.
b. Snow
• Snow is the second most common precipitation.
• Snow consists of ice crystal which usually
combine to form flakes.
• Snow forms when the water vapor turns
directly into ice without ever passing
through a liquid state. This happens as
water condenses around ice crystal.
c. Hail
• It is a showery precipitation in the form of irregular pellets or lumps of
ice of size ranging from 5 to 125 mm. Hails occur in violent thunderstorms
in which vertical currents are very strong
• Hail is created when moisture and wind are together. Inside the
cumulonimbus clouds ice crystals forms, and begin to fall towards the
surface of earth.
Shapes of hail particles
o Spherical
o Conical
o Irregular
d. 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.
e. 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
f. Mist
• 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.
g. Glaze
• Glaze is the ice coating, generally clear and smooth, formed on exposed
surfaces by freezing of super cooled water deposited by rain or drizzle.
5. Weather Systems for Precipitation
Front - A front is the interface
between two distinct air masses.
Under certain favorable conditions
when warm air mass and cold air mass
meet, the warmer air mass is lifted
over the colder one with the
formation of a front.
Cyclone - A cyclone is a large low pressure region with circular wind motion. Two types
of cyclones are recognized: tropical cyclones and extratropical cyclones
Types of Cyclone
a. Tropical
• Typhoon: Southeast Asia;
cyclone: India; hurricane:
USA
• A tropical cyclone also called
cyclone , hurricane in USA
and typhoon in South-East
Asia, is a wind system with an
intensely strong depression.
• The normal areal extent of a
cyclone is about 100 – 200 km
in diameter.
• The isobars are closely
spaced and the winds are
anticlockwise in the northern
hemisphere.
• The center of the storm,
called the eye, which may extend to about 10 – 50 km in diameter.
b. Extratropical cyclones
• These are cyclones formed in locations outside the tropical zone.
Associated with frontal system, they possess a strong counter-clockwise
wind circulation in the northern hemisphere.
• The magnitude of precipitation and wind velocities are relatively lower
than those of a tropical cyclone.
c. Anticyclones
• These are regions of
high pressure,
usually of large
extent.
Anticyclones cause
clockwise wind
circulation in
northern
hemisphere.
• Winds are moderate
speed, cloudy and
precipitation conditions exist
Types of Cyclone
a. cyclonic precipitation – lifting of air converging into a low pressure area (also
known as cyclone)
b. warm front precipitation – warm air advancing upward over a colder air mass;
has a slow rate of ascent
c. cold front precipitation – warm air forced upward by an advancing cold air mass;
the leading edge of the cold air mass is a cold front; faster rate of ascent; rainfall is
showery in nature or high precipitation rate
d. convective precipitation – rising of warmer, lighter air in colder, denser
surroundings; there is a change in temperature such as unequal heating at the surface
or unequal cooling at the top of the air layer; may experience a scattered rain showers
and cloud bursts
e. orographic precipitation –mechanical lifting of air mass over mountain barriers
Artificially Induced Precipitation = conducted to modify and control weather condition
Cloud Seeding/Modification = a type of artificially induced precipitation to dissipate
cloud or stimulate precipitation
seeding agents: dry ice and silver iodide
2 general approaches in cloud seeding
static: 1 artificial nucleus per liter of cloud air
dynamic: massive seeding
Classification of Measuring Precipitation
a. yield point data – commonly uses rain gauge since the area of considered
in measuring the precipitation is about 20cm only; done in small regions
*point precipitation analysis = analysis of data from a single gauge
b. areal data – uses radar where the area covered for measurement is at
around 2.5 km2
Measurement of Precipitation
1. Rainfall - Precipitation is expressed in terms of the depth to which rainfall water
stand in an area if all the rain were collected on it. Thus 1 cm of rainfall over a
catchment area of 1sq.km represents a volume of water equal to 10x104 cu.m.
The precipitation is collected and measured in raingauge.
Terms such as pluviometer, ombrometer and hyetometer are also
sometimes used to designate a raingauge.
Recording Gauges produce a continuous plot of rainfall against time to
provide valuable data of intensity and duration of rainfall for hydrological
analysis of storms.
2. Consideration for setting up a raingauges
To enable the catch of raingauge to accurately represent the rainfall in an
area surrounding the raingauge, standard settings are adopted:
a. The ground must be level and in the open and the instrument must present a
horizontal catch surface.
b. The gauge must be set as near the ground as possible to reduce wind effects but
it must be sufficiently high to prevent splashing, flooding, etc.
c. The instrument must be surrounded by an open fenced area of at least 5.5m by
5.5m. No object should be nearer to the instrument than 30m or twice the height of
the obstruction.
3. Categories of raingauges
• Non Recording Raingauges
o Simons’ Gauge - It essentially consists of a circular collecting
area of 12.7cm(5inches) diameter connected to a funnel
• Recording Raingauges - Produce a continuous plot of rainfall against
time and provide valuable data of intensity and duration of rainfall
for hydrological analysis of storms
o Tipping-Bucket Type
o Weighing-Bucket Type
o Natural-Syphon Type (Float-Type Gauge)
Types of Recording Gauges
1. Tipping bucket – not applicable for snow
• 30.5 cm size raingauge
• the catch form the funnel falls onto
one of pair of small buckets
• it measures the rainfall with at
least count of 1 mm and gives out
one electrical pulse for every mm of
rainfall
b. Weighing type gauge – records
the weight of the snow or rain
• In this raingauge the catch
from the funnel empties
into a bucket mounted on a
weighing scale.
• The weight of the bucket
and its contents are
recorded on a clock – work
– driven chart.
c. Natural – Siphon type
• Also known as float-type
gauge.
• The rainfall collected by a
funnel – shaped collector is
led into a float chamber
causing a float to rise. As
the float rises, a pen
attached to the float
through a lever system
records the elevation of
the float on a rotating
drum driven by a clock –
work mechanism.
d. DOST PAG-ASA Automated Rain Gauge (ARG)
developed to gather and record the amount of
rainfall over a set of period of time and
automatically sends the data to a central based
station on a predetermined interval basin
• rainfall data are sent wirelessly through the cellular network as a
text message or Short Messaging System (SMS)
• It is designed to be rugged and standalone, the station can be
deployed even in the harshest remote areas and can operate
continuously, as it gets power from the sun, backed up by the
internal rechargeable battery
Preparation of Data
Estimation of Missing Data
Given the annual precipitation values, P1, P2, P3, … Pm at a neighboring M stations 1,2,
3,…, M respectively, it is required to find the missing annual precipitation P x at a station
X not included in the above M stations. Further, the normal annual precipitations N 1,
N2, … Ni … at each of the above (M + 1) stations including station X are known.
If the normal annual precipitations at various stations are within about 10% of
the annual precipitation at station X, then a simple arithmetic average procedure is
followed to estimate Px, thus
If the normal precipitations vary considerably, then Px is estimated by weighing
the precipitation at the various stations by the ratios of normal annual precipitations.
Normal Ratio Method
Example: The normal annual rainfall at stations A, B, C, and D in a basin are 80.97,
67.59, and 76.28 and 92.01 cm respectively. In the year 1975, the station D was
inoperative and the stations A, B and C recorded annual precipitations of 91.11, 72.23
and 79.89 cm respectively. Estimate the rainfall at station D in that year.
Solution:
Stations A B C D
Normal Annual Rainfall
80.97 67.59 76.28 92.01
(cm)
Recorded Annual
91.11 72.23 78.89 -
Precipitations (cm)
92.01 91.11 72.23 79.89
PD = 𝑥( + + )
3 80.79 67.59 76.28
= 99.48 cm
Presentation of Rainfall Data
Mass Curve of Rainfall
The mass curve of
rainfall is a plot of the
accumulated precipitation
against time, plotted in
chronological order. A
typical mass curve rainfall
at station during a storm is
shown in the figure.
Hyetograph
A hyetograph is a plot of the
intensity of rainfall against the time
interval. The hyetograph is derived
from the mass curve and is usually
represented as a bar chart. It is a very
convenient way of representing the
characteristics of a storm and is
particularly important in the
development of design storms to predict extreme flood. The area under a hyetograph
represents the total precipitation received in the period.
Point Rainfall
Point rainfall, also known as station rainfall refers to the rainfall data of a station.
Depending upon the need, data can be listed as daily, weekly, monthly, seasonal or
annual values for various periods. Graphically these data are represented as plots of
magnitude vs. chronological time in the form of a bar diagram.
Mean Precipitation over an Area
Arithmetical-Mean Method
If P1 + P2, …, Pi, …Pn are the rainfall values in a given period in N stations within a
catchment, then the value of the mean precipitation P bar over the catchment by the
arithmetic – mean method is
Thiessen-Mean Method
In this method rainfall recorded at
each stations are given a weightage
on the basis of an area closest to the
stations. The procedure of
determining the weighting area is as
follows: the catchment area is
drawn to a scale and the positions of
the six stations marked on it.
Stations 1 – 6 are joined to form a network of triangles.
Perpendicular bisectors for each of the sides of the triangle are drawn. These bisectors
form a polygon around each stations. The boundary of the catchment, if it cuts the
bisector is taken as the outer limit of the polygon. Thus for station 1, the bounding
polygon is abcd. For station 2, kade is taken as the bounding polygons. These bounding
polygons are called Thiessen polygons.
If P1 + P2, …, P6 are the rainfall magnitudes recorded by the stations 1,2, …, 6
respectively, and A1, A2, … A6 are the respective areas of the Thiessen polygons, then
the average rainfall over the catchment P bar is given by
Thus in general for M stations,
𝐴1
The ratio is called the weightage factor for each station.
𝐴
Example: Using the data given below, estimate the average precipitation using Thiessen
Polygon method.
Stations Area(km2) Precipitation (mm) Area x Precipitaion (km2xmm)
A 72 90 6480
B 34 110 3740
C 76 105 7980
D 40 150 6000
E 76 160 12160
F 92 140 12880
G 46 130 5980
H 40 135 5400
I 86 95 8170
J 6 70 420
TOTAL 568 1185 69210
Σ 𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 𝑥 𝐴𝑟𝑒𝑎
Average Precipitation = ΣArea
69210
= 568
= 121.8 mm
Isohyetal Mehod
An isohyet is a line joining points of equal rainfall magnitude. In the isohyetal method,
the catchment area is drawn to scale and the raingauge stations are marked.
The area between two adjacent isohyets are then determined with a planimeter. If the
ishyets go out of catchment, the catchment boundary is used as the bounding line. The
average value of the rainfall indicated by two isohyets is assumed to be acting over the
inter-isohyet area. Thus P1, P2, … Pn are the values of isohyets and if a 1, a2, … an-1 are
the inter-isohyet areas respectively, then the mean precipitation over the catchment
of area A is given by
Example: the isohyets due to a storm in a catchment were drawn and the area of the
catchment bounded by isohyets were tabulated below.
Isohyets (cm) Area ( km2)
Station – 12.0 30
12.0 – 10.0 140
10.0 – 8.0 80
8.0 – 6.0 180
6.0 – 4.0 20
Average value Fraction of Weighted P
Isohyets Area ( km2)
of P (cm) total area (cm)
12.0 12.0 30 0.0667 0.8004
12.0 – 10.0 11.0 140 0.3111 3.4221
10.0 – 8.0 9.0 80 0.1778 1.6002
8.0 – 6.0 7.0 180 0.4 2.7
6.0 – 4.0 5.0 20 0.0444 0.222
Total 450 1.0 8.7447
Mean
8.74 cm
Precipitation
