Sunday, June 20, 2010

Rainfall characteristics of Kerala

Kerala can be divided into three unique rainfall regions, each region having a similar covariance structure of annual rainfall. Stations north of 10.N (north Kerala) fall into one group and they receive more rainfall than stations south of 10.N (south Kerala). Group I stations (table 2) receive more than 65% of the annual rainfall during the south-west monsoon period, whereas stations falling in Group II (table 3) receive 25-30% of annual rainfall during the pre-monsoon and the north-east monsoon periods. Group III stations (table 4) with IIIrd factor as the primary loading, i.e., IIIrd factor greater than Ist and IInd factor.
The meteorology of Kerala is profoundly influenced by its orographical features, however it is difficult to make out a direct relationship between elevation and rainfall. Local features of the state as reflected in the rainfall distribution are also clearly brought out by the study

Abstract of Heaviest Rainfall places in Kerala
Neriamangalam 451 cm
Kutiyadi 416
Vythiri 396
Peermade 392
Ponmudi 391
Quilandi 379
Kasargod 355
Hosdurg 351
Karikode 349
Munnar 346
Vellor 345
Irikkur 345

There is a lack of direct relationship between the height of a station and its rainfall from this study. The spatial variability of mean annual precipitation depends upon the topographic factors like exposure of the station to the prevailing wind, elevation, orientation, and slope of the mountain.The study is, however, limited to the number of rainfall stations available and a better network of stations in the region will reflect more local-scale phenomena. The results are useful for mesoscale and synoptic scale atmospheric modelling.

More Rainfall data & the detail study can be found in


  1. Dear Pradeep,
    Among the other major criteria for heavy rainfall regions, orography plays a important role. There has been studies relating to orographic effect and station elecation.,of course with prevailing wind direction and low level jet [LLJ]. It is noteworthy to mention here that around 850 hPa level [Height:1.5km]and preferably around 1.0 to 1.3 km a.m.s.l is said to be idle place for orographic ascent copious rainfall. [In addition to that if there is a system in the near by ocean (Bay or Arabian Sea) again it also influences the rainfall] Low Level Jet [LLJ] is another important factor for the heavy rainfall which is absent now in this JUNE 2010 SWM period.
    As you pointed out micro level study of rainfall in carefully identified raingauge network which represents all meteorologically vantage location will help to evolve a better understanding about the variation of rainfall in space and time.

  2. Dear Kaneyan,

    The Orographic criteria is also taken under this study as the third factor in the factor analysis. There are 8 stations that receive very heavy annual rainfall (> than 336 cm) and are located on the windward slope of the Ghats. On the windward side of mountains, moist air is forced up the slope, where it cools and condenses, leading to precipitation. These peaks also provide convection points of instability. This instability will be triggered when the orographic lifting is strong enough to force air parcels to ascend to their level of free convection. Thus, the heavy rainfall over the windward slopes depends not only on the elevation of the station but also on the wind velocity perpendicular to the mountain range and on the moist static stability

    Vythiri(945 m) and Kuttiyadi(20 m) lie on the windward slope of the Western Ghats (figure 4) and receive very heavy rainfall during the southwest monsoon even though both the stations are situated at markedly different elevations.

    There are high altitude stations that receive below normal rainfall. Manathavady, a high altitude station (900 m), also situated on the windward slope of the mountains in north Kerala, receives comparatively less rainfall than its neighbouring stations during the monsoon season.

    Similarly, while Nerimangalam (200 m) receives very heavy rainfall during south-west monsoon, Kumily(1140 m),situated on the windward valley of the Anamalai, receives below normal rainfall during the season ( figure 5). Insolation heats the peaks much faster than the valleys below which are shaded by the mountains.

    A similar example is of Vandanmettu in Anamalai, a high altitude station (900 m) lying in a valley which receives relatively less of rainfall

    I am not sure if Orographic Uplift is the only reason for heavy rainfall in Kerala as the study clearly explains....

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  3. Dear Pradeep,
    While giving weight age for orographic rise there is yet another point to ponder with. That is the heavy rainfall region should be a valley like place and should have covered in almost all the direction with mountain slopes of course,except from the wind blowing direction. Devala,Chinna Kallar, Munnar,upper Kothiyar etc are such places where mountain coverage is in almost all direction leaving the place look like as a valley. Further Low Level Jet [LLJ] must be there to enhance the rise. When ever the striking wind is strong obviously these places experience heavy to very heavy rainfall [>25cm]
    Last year Ketty recorded a whooping 82 cm one day rainfall during NEM period. However there is dispute over the measurement of rain by revenue authority. The raingauge is situated in Ketty Railway station and it was flooded during the night. Ketty is a valley and orographic rise and forced ascent is reasoned for the heavy rainfall.

  4. Pradeep7:44 PM

    Thanks for your views. You have any data of orographic rainfall in india.
    Like which towns get influenced by it. Eg.cherrapunji