All of us in Tamil Nadu know that the north east monsoon(NEM) is a major contributor to the rainfall of the state. I quote the Indian Meteorological Department here-- " For Tamilnadu this is the main rainy season accounting for about 48% of the annual rainfall. Coastal districts of the State get nearly 60% of the annual rainfall and the interior districts get about 40-50% of the annual rainfall."
Also we know that during weak monsoon condition(September-Early October) when there is ample sunshine, the lower levels of the atmosphere are warm which gives rise to convection current. The air goes up leading to the formation of clouds, resulting in rainfall in some interior regions of the state. Again I quote IMD here- "The rainfall over south peninsula towards the end of southwest monsoon season is mainly in the interior districts and it generally occurs in the afternoon , evening or early part of the night. As the season advances the rainfall is mainly in the coastal districts with the interior districts getting less rain."
Let us walk through distinct regions of Tamil Nadu one by one and find out more about the wettest months. The rainfall figures are in millimeters(mm). The top 2 wettest months are indicated in brackets for every region.
1. North Coastal Tamil Nadu (November--October)
This is one of the most rainiest regions of the state or even in the entire country outside West Coast and East India. It is home to the dry evergreen forests and hosts the Chennai(Madras) metropolis. It is also known for its hot weather during summer and a wet flooding weather during the winter. The following table gives the rainfall(in mm) for three months viz September,October and November.
This is the region where NEM effect is more pronounced than any other region of the state and hence clearly November tops the charts followed by October throwing no surprises. September too is pretty decent but no where close to the other two months.
2. North Interior Tamil Nadu (October--November)
By north interior TN, we mean the interior places covering west of Chennai and east of Salem. This region is one of the hottest regions in Tamil Nadu. Its quite intolerable in summer, enjoyable after August, quite cold in winter. It covers districts like Tiruvallur,Kanchipuram,Vellore, Tiruvannamalai and Villupuram.
As we can see these places are dominated by the ending South West Monsoon and North East Monsoon with October and November being the wettest months (in that order). It is also notable that Vellore has almost equal rainfall in all the 3 months.
3. North West Tamil Nadu (October--September)
This region is a relatively pleasant region. Besides being primarily agrarian , it is also decently forested. It is located in a relatively higher altitude giving it a reasonably salubrious climate compared to most regions of TN. However, it is not a high rainfall zone. This region is also known for a fairly wet May making it a summer destination. It hosts the non-western ghat hill stations of TN like Yercaud and Yelagiri.
This region along with neighboring Bangalore experiences a very good September-early October due to active convective currents due to warming up of lower levels of atmosphere. However the effect of NEM is limited in this region. So, the wettest months are October and September(in that order).
4. West Tamil Nadu (October--November)
One of the most prosperous regions of Tamil Nadu outside Chennai, West TN hosts municipal corporations like Coimbatore,Erode and Tirupur. It also hosts famous hill stations like Ooty,Coonoor,Kotagiri and Valparai. Although it is one of the driest regions of the state(or even the country), people get eluded by the pleasant overcast and chill weather. However the hilly regions receive good rainfall due to orographic effects and local T-storms.
Despite its proximity to the western ghats, the rain shadow effects are more pronounced in this area virtually ruling out any heavy rain in the South West Monsoon season(including September). It has to rely highly on the NEM. Thus as with other interiors October is the wettest month followed by November.
5. Central TN (October--Indeterminate)
This region forms the central part of TN. It is primarily drained by Kaveri and its distributeries. This region is known for its hot climate. Trichy is its principal city.
Although October is the rainiest month in this region(as always), it displays considerable variance for the second wettest month. September is the second wettest month of Trichy. October for Thanjavur and November for Ariyalur.
6. South Tamil Nadu (October--November)
The region south of Trichy till Tirunelveli comprises of South TN. This area is primarily drained by the Vaigai.
Madurai is its principal city. It also hosts the famous hill station of Kodaikanal.
October again tops for this region followed by November. Due to its proximity with Trichy, Karaikudi experiences Trichy like Climate with second wettest month being September. Kodaikanal has a great North East Monsoon.
7. Deep Southern Tamil Nadu (November--October)
This region comprises of Tirunelveli, Tuticorin and Kanyakumari districts. The Tirunelveli and Tuticorin districts have one of the worst South West Monsoon in the Indian Republic(worse than West Rajasthan).
If not for the North East Monsoon(NEM), these districts would be fully arid deserts. Thanks to NEM, this region is saved. This region throws an interesting information. Apart from North Coastal TN, this is the only region where November is the wettest month followed by October.
Conclusion
We see that October and November months that coincide with the North East Monsoon season are the wettest months of Tamil Nadu state as a whole. However, it is interesting to note that the north west part of the state has September as the second wettest month.
Also we know that during weak monsoon condition(September-Early October) when there is ample sunshine, the lower levels of the atmosphere are warm which gives rise to convection current. The air goes up leading to the formation of clouds, resulting in rainfall in some interior regions of the state. Again I quote IMD here- "The rainfall over south peninsula towards the end of southwest monsoon season is mainly in the interior districts and it generally occurs in the afternoon , evening or early part of the night. As the season advances the rainfall is mainly in the coastal districts with the interior districts getting less rain."
Let us walk through distinct regions of Tamil Nadu one by one and find out more about the wettest months. The rainfall figures are in millimeters(mm). The top 2 wettest months are indicated in brackets for every region.
1. North Coastal Tamil Nadu (November--October)
This is one of the most rainiest regions of the state or even in the entire country outside West Coast and East India. It is home to the dry evergreen forests and hosts the Chennai(Madras) metropolis. It is also known for its hot weather during summer and a wet flooding weather during the winter. The following table gives the rainfall(in mm) for three months viz September,October and November.
This is the region where NEM effect is more pronounced than any other region of the state and hence clearly November tops the charts followed by October throwing no surprises. September too is pretty decent but no where close to the other two months.
2. North Interior Tamil Nadu (October--November)
By north interior TN, we mean the interior places covering west of Chennai and east of Salem. This region is one of the hottest regions in Tamil Nadu. Its quite intolerable in summer, enjoyable after August, quite cold in winter. It covers districts like Tiruvallur,Kanchipuram,Vellore, Tiruvannamalai and Villupuram.
As we can see these places are dominated by the ending South West Monsoon and North East Monsoon with October and November being the wettest months (in that order). It is also notable that Vellore has almost equal rainfall in all the 3 months.
3. North West Tamil Nadu (October--September)
This region is a relatively pleasant region. Besides being primarily agrarian , it is also decently forested. It is located in a relatively higher altitude giving it a reasonably salubrious climate compared to most regions of TN. However, it is not a high rainfall zone. This region is also known for a fairly wet May making it a summer destination. It hosts the non-western ghat hill stations of TN like Yercaud and Yelagiri.
This region along with neighboring Bangalore experiences a very good September-early October due to active convective currents due to warming up of lower levels of atmosphere. However the effect of NEM is limited in this region. So, the wettest months are October and September(in that order).
4. West Tamil Nadu (October--November)
One of the most prosperous regions of Tamil Nadu outside Chennai, West TN hosts municipal corporations like Coimbatore,Erode and Tirupur. It also hosts famous hill stations like Ooty,Coonoor,Kotagiri and Valparai. Although it is one of the driest regions of the state(or even the country), people get eluded by the pleasant overcast and chill weather. However the hilly regions receive good rainfall due to orographic effects and local T-storms.
Despite its proximity to the western ghats, the rain shadow effects are more pronounced in this area virtually ruling out any heavy rain in the South West Monsoon season(including September). It has to rely highly on the NEM. Thus as with other interiors October is the wettest month followed by November.
5. Central TN (October--Indeterminate)
This region forms the central part of TN. It is primarily drained by Kaveri and its distributeries. This region is known for its hot climate. Trichy is its principal city.
Although October is the rainiest month in this region(as always), it displays considerable variance for the second wettest month. September is the second wettest month of Trichy. October for Thanjavur and November for Ariyalur.
6. South Tamil Nadu (October--November)
The region south of Trichy till Tirunelveli comprises of South TN. This area is primarily drained by the Vaigai.
Madurai is its principal city. It also hosts the famous hill station of Kodaikanal.
October again tops for this region followed by November. Due to its proximity with Trichy, Karaikudi experiences Trichy like Climate with second wettest month being September. Kodaikanal has a great North East Monsoon.
7. Deep Southern Tamil Nadu (November--October)
This region comprises of Tirunelveli, Tuticorin and Kanyakumari districts. The Tirunelveli and Tuticorin districts have one of the worst South West Monsoon in the Indian Republic(worse than West Rajasthan).
If not for the North East Monsoon(NEM), these districts would be fully arid deserts. Thanks to NEM, this region is saved. This region throws an interesting information. Apart from North Coastal TN, this is the only region where November is the wettest month followed by October.
Conclusion
We see that October and November months that coincide with the North East Monsoon season are the wettest months of Tamil Nadu state as a whole. However, it is interesting to note that the north west part of the state has September as the second wettest month.
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