UNIT -2
Precipitation
PRECIPITATION:
Precipitation is the fall of water in various forms on the earth from the clouds. The usual forms are rain, snow, sleet, glaze, hail, dew etc.
Before studying the phenomenon of precipitation let us consider water vapors. Air in atmosphere can easily absorb moisture in the form water vapors. The amount of water vapors absorbed by air depends upon the temperature of air, the more is the temperature the more water vapors it can absorb.
The water vapor exerts a partial pressure on the water surface called vapor pressure. The amount do water vapor present in air is indirectly expressed in terms of vapor pressure.
If the evaporation continues, a state of equilibrium is reached when the air is fully saturated with vapor and therefore it cannot absorb more vapors. The vapors then exert a pressure which is known as saturation vapor pressure (es). es increase with increase in temperature.
Let us consider a of parcel of air as temperature T and a vapor pressure (ea) indicated by pt. A. The saturation vapor pressure at that temperature is indicated by pt. B. The intercept BA = (es-ea)is called saturation deficit.
If vapors are added to the parcel of air, the pt. A will move to pt. B when air is fully saturated.
If the parcel of air is cooled at constant pressure but without the addition of more vapors, the pt. Moves horizontally towards pt. D and the air would be saturated when pt. D is reached. At that stage, the air would have a temperature called dew point temperature (T). Cooling of air beyond this pt. Would result in condensation or formation of mint.
If neither the temperature not the pressure remains constant, the water evaporates freely and the pt. Moves to pt. C. In this case, water vapor rises but temperature falls. The temperature at pt C. Is called wet bulb temperature (T). The saturation vapor pressure is indicated by ew.
Air in atmosphere can be cooled by many processes. However, adiabatic soling which occurs by a reduction of pressure through lifting of air masses is the main natural process.
TYPES OF PRECIPITATION
Depending upon the factors responsible for lining and cooling of alit there is following types of precipitation:
(1) Convective precipitation: It occurs due to heating of air. The air close to the earth surface gets heated, and item density decreases. Consequently, the air rises upwards in the atmosphere and it gets cooled adiabatically to form a cloud, precipitation caused by such clouds is called convective precipitation.
(2) Orographic Precipitation: Orographic rainfall occurs due to ascent of air forced by mountain barriers. The mountain barriers lying across the direction of air flow forces the moisture laden air to rise along the mountain slope. It results in cooling, condensation and precipitation.
(3) Cyclonic Precipitation: A cyclone is a large zone of low pressure which is surrounded by circular wind motion. Air tend low pressure zone from and displaces low pressure cyclonic precipitation to move into the surrounding area air upwards. Thus occurs due to displacement of air in upwards direction due to.preasure difference.
(4) Frontal Precipitation: It is a type of cyclonic precipitation. When two contrasting air masses (cold polar air mass and warm westerly air mass) coming from opposite directions converge along a line, a front is formed. The warm wind is lifted upward along this front where as cold air being heavier settles downward.
Because the two types of front (warm and old) have different temperature and density, frontal precipitation occurs when they clash with each other.
Forms of Precipitation
1. Rain: Precipitation in form of water drops of size greater than 0.5 mm and less than 6 mm.
2. Snowfall: The fall of larger snowflakes from the clouds on the ground surface is called snowfall. In fact, snowfall is precipitation of white and opaque grains of ice'. The snowfall occurs when the freezing level is so close to the ground surface (less than 300m from the surface) that aggregation of ice crystals reaches the ground without ground being melted in a solid form of precipitation as snow. Avg density=0.1 gm/cc.
3. Sleet: refers to a mixture of snow and rain but in American terminology sleet means falling of small pellets of transparent or translucent ice having a diameter of 5 mm or less.
4. Hail: consists of large pellets or spheres of ice. In fact, hail is a form of solid precipitation wherein small balls or pieces of ice, known as hailstones, having a diameter of 5 to 50 mm fall downward known as hailstorm. Hails are very destructive and dreaded form of solid precipitation because they destroy agricultural crops and claim human and animal lives.
6. Drizzle: The fall of numerous uniform minute droplets of water having diameter of less than 0.5 mm is called drizzle. Drizzles fall continuously but the total amount of after received on the ground surface is significantly low. Intensity is usually less 0.1 cm/hr.
6 Glaze: It is a form of precipitation which falls as rain and freezes when comes in contact with cold ground at around 0'c. Water drops freeze to form an ice coating also called freezing rain.
CONDITIONS FOR OCCURRENCE OF PRECIPITATION:
Precipitation may occur only when there is some mechanism to cool tastphirc air to bring it to saturation Even saturation of air does not lead to precipitation. Precipitation occur when the following conditions are satisfied
(1) Cooling of air masses.
(2) Formation of clouds due to condensation.
(3) Growth of water droplets.
(4) Accumulation of moisture.
MEASUREMENT OF PRECIPITATION
- The total amount of precipitation on a given area is expressed as the depth of water if accumulated over the horizontal projection of the area. Thus 1 cm of rainfall over a catchment area of 1 km2 represents a volume of water equal to 104 m3.
- Any part of this precipitation, if falling as snow or ice, is to be accounted for in its melted form.
- Since it is not physically possible to catch all the rainfall or snowfall over a drainage basin, it is only sampled by rain gauges whose catch, in a perfect. Exposure, represent the precipitation falling on their respective surrounding areas.
- Terms such as pluviometer. Ombrometer and hyetometer are also s0metimes used to designate a raingauge.
TYPES OF GAUGES:
The various types of precipitation gauges are broadly classified as
(A) Non-recording gauges and (b) recording gauges
Non-Recording Gauges
- The nonrecording gauge extensively used in India is the Symons raingauge. It is installed in an open area on a concrete foundation. The distance of the rain gauge from the nearest object should be at least twice the height of the object. It should never be on a terrace or under a tree. The gauge may be fenced with a gate to prevent animals and unauthorized person’s fromentering the premises.
- Measurements are to be. Made at a fixed time. Every day normally at 08:30 hrs which is considered as the daily rainfall? In case of heavy Fainfall areas, measurements are made as often as possible. However, the last reading must be taken at 8:30 AM. So that last 24 hrs data may be added up to get the rainfall of that day.
Recording Gauges
- The recording gauges produce a continuous plot of rainfall against time and provide valuable short duration data on intensity and duration of rainfall for hydrological analysis of storms. The commonly used recording gauges are:
(a) Tipping bucket type
(b) Weighing type, and
C) Natural siphon type
The weighing type is suitable for measuring all kinds of precipitation (rain, sleet etc.).
Tipping-Bucket Type:
The catch from the funnel falls onto one of a pair of small buckets. These buckets are so balanced that when 0.25 mm of rainfall collects in one bucket, it tips and brings the-other one in position. The tipping actuates an electrically driven pen to trace a record on clockwork-driven chart. The record from tipping bucket gives data on the intensity of rainfall. The main advantage of this type of instrument is that it gives an electronic pulse output that can be recorded at a distance from the raingauge.
Weighing-Bucket Type:
The catch from the funnel empties into a bucket mounted on a weighing cale. The weight of the bucket and its contents are recorded on a clockwork-driven chart. Thai instrument vies a plate of the accumulated rainfall against the elapaed time, i.e. the mass curve of rainfall (accumulated precipitation against time).
Natural Siphon Type:
This type of recording rain-gauge is also known as float type gaugo Hlere the rainfall collected by a funnel shaped collector is led into a float chamber causing a float of 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 dock work mechanism. A siphon arrangement empties the float chamber when the flatheads reached a pre-set maximum level which resets the pen to its zero, level. This type of raingauge is adapted as the standard recording type rain gauge in India
Other Precipitation Measurement Method:
Storage Gauges
In sparsely populated or remote regions such as in a desert or a mountainous terrain, storage gauges are used to measure total seasonal precipitation. These gauges are read monthly, seasonally, or whenever it is possible to inspect the stations.
Telemetering Rain Gauges
These rain gauges are of the recording type and contain electronic units to transmit the data on rainfall to a base station both at regular intervals and on interrogation. The tipping-bucket type rain gauge being ideally suited. Telemetering gauges are of utmost use in gathering rainfall data from mountainous and generally inaccessible places.
Radar Measurement of Rainfall
Radar permits the observation of the areal extent, the location and movement of areas of precipitation in the atmosphere; and certain types of radar equipment can yield estimates of rainfall rates over areas within the range of the radar.
Observations by Satellites
Satellite images can be used for estimating precipitation over areas ranging from the global to the very local scale in real or near-real time. This complements the conventional precipitation measurements in areas of sparse rain gauge networks and can improve the accuracy of estimating precipitation for short time periods (several hours).
SNOWFALL WATER EQUIVALENT
- The water equivalent of a given snow fall is the amount of liquid precipitation contained in that snowfall.
- The water equivalent of the snowfall is determined either by weatherman melting.
- Collected in a non-recording rain-gauge is melted immediately and measured by means of an ordinary measuring cylinder graduated for rainfall measurements.
- Weighing-type recording gauge is also used to determine the water content. Of snowfall
- During periods of snowfall, the funnels of the gauges are removed so that all precipitation can fall directly into the receiver
RAIN GAUGE NETWORK:
- For proper assessment of water resources, a good network of rain gauges is a must. Rain gauge density is expressed as area covered per gauge. For better accuracy, catchment area per gauge should be small
- As per the IS: 4987-1968, the recommended raingauge network density is as follows:
In plains: One Station per 520 km2
Moderately elevated area: One in 260 to 390 km2
(Average elevation up to 1000 m)
Hilly areas: One in 130 km2
According to WMO at least 10 percent of the rain gauge stations should be equipped with automatic (self recording) rain gauges.
ADEQUACY OF RAINGAUGE STATIONS:
Number of rain gauge stations in a given catchment must be sufficient so that the error in precipitation measurement is not more than acceptable value.
Number of raingauge stations for area to give necessary average rainfall with certain percentage of error can be obtained as follows
Step 1: Calculate mean rainfall. Pm
Pm=(p1+p2+........+pn)/n
Where, p1,p2,....,pn are rainfall recorded by each stations and n is the total number of raingauge stations in the catchment.
Step 2: Calculate the standard deviation, 6n-1
Step 3: Calculate coefficient of variation, Cv,
Step 4: Optimal number of stations, N,
N=(Cv/ )2
Where, is the allowable degree of error in the estimation of mean rainfall (in%)
Step 5: Additional number of raingauge station required = N- n.
In routine hydrological investigation, error of estimate should not exceed 10%
NORMAL PRECIPITATION:
The normal rainfall is the average value of rainfall of a particular date, month or year over a specified 30-year period (like normal rainfall of both March or normal rainfall of January or yearly rainfall). The 30-year normal are recomputed every decade to account for change in environment and land use, because these factor may affect the amount of rainfall on that area.
Normal rainfall is used to find out the missing data of certain raingauges.
AVERAGE ANNUAL RAINEALL:
The amount of rain collected by a rain gauge in the last 24 hrs is called daily rainfall and the total amount collected inl1year is called annual rainfall. Average annual rainfall is the average value of annual rainfall values for the last 35 years.
PREPARATION OF DATA:
Before using the rainfall records of a station it is necessary to 1st check the data for continuity and consistency. Continuity means availability of continuous record of previous rainfall and consistency means that rainfall data of previous years should be consistent with the present environmental and land use conditions (like if there is a jungle in a particular area which did not exist 15 years ago then previous records will not be consistant with current record).
ESTIMATION OF MISSING DATA:
Sometimes a station has a break in record due to absence of observer or failure of the instrument. It is then necessary to estimate that missing data. To estimate the data, three or more stations close to this station are selected. Following are the different ways of calculating the missing data.
Arithmetic Mean Method
If the normal precipitation at each of these selected stations is within 100fthat for the station with missing data, then simple arithmetical mean of the precipitation of those stations will give the value of the missing station.
Px = p1 + p2 +p3 +.......pm/m
Normal Ratio Method
If normal precipitation at any of these selected stations is above,10% of that for station with missing data then,
Where, P1 =Precipitation of 1st station
N1 = Normal precipitation of the 1st station
m = No. Of additional station chosen
Px = precipitation (missing data)
Nx = Normal precipitation of the station at which data is missing.
INCONSISTENCY OF RECORDS:
Some of the common causes for inconsistency of the records are
- Shifting of raingauge station to a new location
- Neighborhood of the station undergoing a marked change
- Replacement of are old instrument by new one
- Change of observer or method of observation.
- Change in the ecosystem due to calamities such as forest fire landslides etc.
- Occurrence of observational error from a certain date
Inconsistency of record is corrected by using double mass curve technique. Thus on correction, the previous record becomes consistent with the present day environmental and land use condition.
DOUBLE MASS CURVE TBCHNIQUE:
- To draw this curve, a group of station (say 10) is taken as base station in the neighborhood of the problematic station X.
- The accumulated rainfall of station X (PX) and accumulated values of average of group of base stations (PAV) are calculated starting from the latest record.
- The values of Px as ordinate and PAV as abscissa are plotted for available data of rainfall.
- In the plot, if a break in the slope is observed. It indicates à change in precipitation of station X.
- The values of precipitation at X beyond the break point are corrected based on the slope of both the lines.
- A change in slope is normally taken as significant only where it persiste for more than 5-yrs.
Pcorr X = Px × (Mc/Ma)
Where,
Me = corrected slope of double mass curve and Ma = original slope of double mass curve and Correction factor =Mc /Ma =c/s .
PRESENTATION OF RAINFALL DATA:
Rainfall data is presented in the form of
(a) Mass curve (b) Hyetograph (c) Moving average
Mass Curve of Rainfall
The mass curve of rainfall is a plot of the accumulated precipitation against time, plotted in chronological order. Records of float type and weighing bucket type gauge are of this form. Mass curves of rainfall are very useful in extracting the information on the duration and magnitude of a storm. Intensities at various time intervals in a storm can be obtained from the slope of the curve.
Average intensity in lst storm (A-B) =20/20 = 1 mm/hr
No rainfall during BC
Average intensity in 2nd storm (C-D) =30/20= 1.5 mm/hr
No rainfall during DE
Hyetograph
A hyetograph is a plot of the average intensity of rainfall against the time interval. The hyetograph is derived from the mass curve and is usually representeda8 a bar chart. The area under a hyetograph represents the total precipitation received in the period. Hyetograph is more preferred than mass curve since it is convenient to determine the area of hyetograph.
Moving Average
If we plot point rainfall (rainfall collected at raingauge station) with time in chronological order the fluctuations will be large in the time series of rainfall. From this it will be difficult to determine the trend of the rainfall. Thus a moving average plot is made which smoothens out the fluctuations in time series and help in determining the trend of rainfall.
To find out moving average, for say 3 years, average of rainfall of lst, 2nd and 3rd yrs is plotted against 2nd yrs. Average of 2nd, 3rd and 4th yr is plotted against 3rd yr. And so on.
Similarly for 5 yr moving average, av. Of rainfall of lst, 2nd, 3rd, 4th and 5th yr is plotted against 3rd yr, avg. Of 2nd ,3rd ,4th,5th,and 6th ye is plotted against 4th ye and so on.
CALCULATION OF AVERAGE DEPTH OF PRECIPITATION OVER A CATCEHMENT:
The precipitation over a catchment is actually measured as point values at a finite number of precipitation stations (Raingauge station).
However, hydrological analysis requires knowledge not of point rainfall but of the rainfall over an area, such as over a catchment.
To convert the point rainfall values at various stations into an average rälueoyer catchment, several methods are available.
(1)Arithmetical-mean method, (2) Thiessen-polygon method (3) Isohyetal method
Arithmetie Mean Method:
- The arithmetic mean method gives equal weights to all the rain gaugs. Apart from being quick and easy, it yields fairly accurate results if the rain gauges are uniformly distributed and are under homogeneous climate. Under normal situation this method is least accurate method
- This method doesn't take into account the raingauge located outside the catchment. As per this method:
Pm = (P1 +P2 +......+Pn)/n
Pm is the average rainfall in the catchment
Pi is the rainfall magnitude at the ith station inside catchment
n is the number of raingauges in the catchment
Thiessen Polygon Method
This method considers the representative. Area for each rain gauge also considering the rain gauge present outside the catchment. Representative area can also be thought of as the areas of influence of each raingauge.
These areas are found out using the following steps:
1. Joining the rain gauge station locations by straight lines to form triangles.
2. Bisecting the edges of the triangle to form the so-called "Thiessen polygons".
3. Calculate the area encloset around each rain gauge stations bounded by the polygon edges (and the catchment boundary, wherever appropriate) to find out the area of influence corresponding to the rain gauge.
Pm is the average rainfall in the catchment.
Pi is the rainfall magnitude at the it station
Ai is the area of influence for the it station
A is the total area of the catchment.
Wi is weight of station i
- Weighted average rainfall is calculated by this method.
- Polygon needs to be calculated only once for a given distribution of rain gauge network. Thus once the weight of a station is fixed there after calculation is easy
- The new polygon is required to be redrawn when, due to addition or deletion of rain gauges to the network, weight of each station changes.
- This method takes care of non uniform distribution of raingauge.
- However the variability in rain fall due to elevation differences is not taken care of (i.e. topographical influence are not taken care of).
- Hence polygon method is reliable only for plin grepe and inhyetal method is used for both plain and hilly areas
- This method is more accurate than arithmetic method and takes care of raingauges located outside the catchment also.
Isohyetal Method:
- Isohyetal is a line joining points of equal rainfall magnitude. The area between two adjacent isohyets is determined by a planimeter
- This is the most accurate method.
- Topographic influence is taken into account.
- New isohyets have to be made for each rainfall event. Average rainfall is
- Average rainfall is calculated as
Pm =( PijAij)/A
Pm is the average rainfall in the catchment
Pij =(Pi +Pj )/2
Aij is the area between two succesaive isohyets Pi and Pj.
A is the total area of the catchment.
Area A1 and A2 falls between two isohyets each. Hence, these areas may be thought of as corresponding to rainfall depth of (P1+P2)/2 and (P2+ P3)/2 respectively. For Area A1 we would expect rainfall to be different from P1 but if no record of rainfall is available beyond the catchment, A1 may be assume to correspond to P1 only. If however, rainfall record of area beyond catchment is available we can find the rainfall depth for area,A1 by interpolation. Similar logic applies for area A4.
DEPTH AREA DURATION RELATIONSHIP:
- Depth of rainfall at a raiogauge station is called point rainfall. To convert the point rainfall datas to areal rainfall data, [i.e. to find out how much of rainfall will occur over various areas) Depth area duration curve is used.
- Find out the depth'arga duration curve for a rainfall of particular duration in a catchment, rainfall of that duration is selected and followings steps are taken:
Step 1: From the data of rainfall at different place of a catchment for a particular storm, isohyetal map is prepared.
Step 2: Area between the isohyets are determined using planimeter.
Step 3: Area between two isohyets is multiplied by the average of the corresponding isohyets and hence volume of rainfall in that area is found.
Step 4: Cumulative volume and cumulative area are calculated.
Step 5: Cumulative volume is divided by cumulative area to know the average depth of rainfall over the cumulative area for the chosen duration of rainfall. The same procedure is repeated for other durations also.
Step 6: Average depth for a particular duration is plotted against the cumulative area to obtain the DAD curve for that duration of rainfall
Thus average depth of rainfall for a particular area can be calculated from DAD curve.
For a given duration of rainfalls (say 6-hr rainfall) column (1), (2), (3) Will be known and from this (4). (5). (6), (7) are calculated. Thus curve is plotted with (7) as ordinate and (5) as abscissa.
DAD curve can also be obtained from empirical equation like
P=Poe-KAn
P= Average depth in catchment over an area A (km2)
P0= Highest amount of rainfall in catchment at the storm centre
K, n = constant for a given region. They can be obtained by regression analysis.
But it is unlikely that the storm centre coincides with the raingauge station. Hence exact determination of P0 is not possible.
However, P0 can be calculated from the assumption. That highest rainfall over a raingauge in the catchment is the average depth over an area of 25 km2. Hence
Pmax =P0 e-(k×25n)
From this P0 can be calculated.
In a typically depth area curve depth decrease with increase in area. Similarly analysis of rainfall of larger duration for given area indicates that depths rainfall increases as the duration increases.
Now average depth is plotted against cumulative area to get depth area curve for a particular duration.
MAXIMUM DEPTH AREA DURATION CURVE:
In the design d hydraulic structures, we need to determine the design food. The design flood will be determined from the design storm. To find out the design storm we need to know the maximum rainfall of a particular duration over a particular area. This is found from Max DAD Curve.
Due to various storms of a particular duration (.e. Say various storms of 24 hr duration) depth area curve is found. Then area is plotted war. To Max depth of rainfall corresponding for the various storms of 24 hr duration. This is max DAD curve.
When the above maximum depth of rainfall is plotted w.r.to. area we get maximum DAD curve. Max. DAD curve can also be plotted in terms of point rainfall percentage with area
Point rainfall percentage means rerate depth over area is how much percentage of point rainfall recorded by rain gauge.
Frequency of Point Rainfall:
In much hydraulic design, we need to know what the chance of recurrence of a particular rainfall is or what is the probability that a particular rainfall will not be exceeded during the design life of the structure.
- Such information is obtained from the frequency analysis of the point rainfall data.
- Rainfall data when arrangedin.chronological order constitutes a time-series. One may prepare time series like annual series or monthly series etc. of extreme value of an event (like extreme value of 24-hr rainfall). For example if data for maximum magnitude of 24 hr rainfall in a year is collected year over year, we get annual series of extreme value of 24 hr rainfall.
- The purpose of the freqtueney analysis of an annual series is to obtain a relation between the magnitude of the event and it probability of exceedence.
- If the annual extreme series is arranged in descending order of method magnitude and each position given a number 1 to N, 1 being given to the lst i.e. largest value and N-given to least or last value. Then probability P of a rainfall at position 'm' being equaled or exceeded is given by weibull's formula as
P= m/ (N+1). (Plotting position approach)
Where, N=no allotted to the last position i.e. no. Of yr. Of record.
- Recurrence interval or return period is the average time period after which the particular rainfall value is likely to be equaled or exceeded.
Recurrence interval or return period is given by
T= 1/P = (N+1)/m
Once the probability of exceedence has been determined one can algd find out certain other probabilities like:
(a) Probability of non occurrence= (1-P) =q
(b) Probability of occurrence of event, r, times in n successive yes=nCrprqn-r
(c)Probability of the event not occurring at all in all n-successive yrs=nC0p0qn =qn
(d) Probability of the event occurring at least once in n successive yrs =1-qn =1-(1-p)n
% dependable flow = (100 x p) %
INTENSITY DURATION FREQUENCY CURVES:
Intensity duration frequency curve estimates the rainfall intensities of different durations and recurrence interval. These curves are used by engineers for risk assessment of dams, bridges, water drainage system, storm sewers, runoff canals etc.
- They can also be used as a prediction tool to identify when a certaigrainfall rate or a specific volume of flow will recur in the future that will create flooding have in an are
- IDF curve i.e. most often used to express the verity of a single rainfall event
- Curves are used in design with the assumption that past rainfall statistics continue to represent rainfall statistics into the future.
- A simple use of IDF curve is illustrated n follows. Suppo8e one has to determine the design discharge of a storm sewer with a consideration that its return period is not less than 5 yrs. i.e. the risk of storm sewer getting overloaded is acceptable once inner
- As per rational formula, runoff is maximum when the rainfall has duration, equal to or more than the time of concentration of the catchment i.e. the time when entire catchment starts contributing to the discharge at outlet.
Hence critical rainfall intensity is 'determined from IDF curve corresponding to duration equal to time of concentration and frequency or return period equal to 5 yes.
From this intensity (ic), using rational formula Qdesign=CiA
Runoff
(C= runoff coefficient, A= Area of catchment)
Design discharge is calculated.
IDF curve is commonly expressed as: i=KTx/ (D+a) n
Where I = intensity; D = duration; T= return period
K, a, x, n are constants for a given catchment.
PROBABLE MAXIMUM PRECIPITATION:
The probable maximum precipitation (PMP) is defined as the greatest or extreme rainfall of a given duration that is physically possible over a station or basin.
DAD of PMP is usually derived by taking the results of maximum depth area duration analysis and adjusting them for most favorable hydrometer logical conditions that is possible to maximize the rainfall. From this DAD of PMP is obtained.
PMP be can be statistically estimated as:
PMP =P +K
P =Mean annual rainfall series; K= Frequency factor; 6= Standard deviation of series
PMP is used for design of large hydraulic structures like spillway of large dams such that there is virtually no probability of failure.
Standard Project Storm (SPS):
For design of major and intermediate structure SPS iguanid SPS is the greatest storm that may reasonably be expected without modifying the rainfall data for favorable by drometerological conditions as was done in PMP.
Max Rainfall Observed (in any part of the world)
Pm=42.16 D0.475
Pm =extreme rainfall depth (cm); D =Duration in (hr)
- Average annual rainfall is 300–650 millimetres (11.8–25.6 in), but is very unreliable; as in much of the rest of India, the southwest monsoon accounts for most precipitation.
- Rainfall is measured by means of rain gauges. (ii) Recording type gauges. In the non-recording type rain gauge, the total rainfall in a particular period can be obtained. Observations are taken at the end of 24 hours period or at lesser intervals during heavy rains and the rainfall in the previous period are recorded.
- The highest recorded one-day point rainfall of 1036 mm at Cherrapunji was for long considered as the highest for an Indian site.
- The rains are more or less intense and long-lasting depending on area, but generally the wettest period is from July to September, except in the southeast, where the retreating monsoon continues until the end of the year.
- The longest stretch any location has seen measurable precipitation (rain/snow) is 79 days near Otis, Oregon, in the winter of 1997-98. Alaska's record of 88 consecutive days with measurable precipitation was set in Ketchikan in 1920.
