Quick answer
The first inter-monsoon is the period between the end of the northeast monsoon and the start of the southwest monsoon, roughly March to mid-May. With light, variable winds, the land heats up each day and the moisture rises into afternoon and evening thunderstorms, mostly over the southwest and the hills. Mornings are usually clear, it is the hottest time of year in the lowlands, and the sea is often calm on both coasts.
Key takeaways
- The first inter-monsoon runs roughly from March to the onset of the southwest monsoon in late May.
- Rain comes as afternoon and evening thunderstorms, not as days of steady monsoon rain.
- The southwest and the hills get the most storms; the east coast and the north the fewest.
- It is the hottest and most humid period of the year in the lowlands, peaking in April.
- Seas on both the southwest and east coasts are often calm at the same time.
What the first inter-monsoon is
The inter-monsoon periods are the gaps between Sri Lanka's two monsoons. In March, the northeast wind dies away. The southwest wind will not arrive until late May. In between, the winds are light and come from different directions on different days, and the weather is driven by the sun rather than by a prevailing wind.
The sun is nearly overhead in April. It heats the land quickly every morning, warm, moist air rises, and by mid-afternoon it has built into towering cumulonimbus cloud. The cloud releases its water as a thunderstorm, often with lightning and heavy rain for an hour or two, and the sky clears again by night or by the next morning.
Meteorologists call this convective rain. For visitors, it means a daily rhythm that can be planned around.
- →Usual period: March to mid or late May.
- →Wind: light and variable.
- →Rain type: afternoon and evening thunderstorms.
Where the storms fall
The storms are not spread evenly. They form most readily over the hills, where the land is high and the air is forced upwards, and over the wet southwest, where there is most moisture. Colombo, Galle, Kandy, Ratnapura and the western hill country get the most rain, and Colombo averages roughly 200 to 250 mm in April.
The east coast and the north get far fewer storms. Trincomalee averages roughly 50 to 70 mm in April, and Jaffna is drier still. The Cultural Triangle sits in between, with occasional storms in the late afternoon.
- →Most storms: Colombo, Galle, Kandy, Ratnapura, Nuwara Eliya.
- →Some storms: Sigiriya, Dambulla, Polonnaruwa.
- →Fewest storms: Trincomalee, Pasikudah, Jaffna.
The heat
The first inter-monsoon is the hottest time of year in the lowlands. Without the cooling wind of either monsoon, and with the sun overhead, daytime highs reach about 32 to 34 °C on the coasts and 34 to 35 °C in the dry zone, and humidity often stays above 80 per cent. April is usually the peak.
Even the hills feel warmer. Kandy reaches about 31 °C, and Nuwara Eliya about 21 °C by day, which is why its April season draws visitors from Colombo. Nights in the lowlands stay warm, around 25 to 26 °C, so air conditioning matters more than in January.
Heat exhaustion is a real risk for active visitors. The usual advice applies: start early, drink often, rest in the middle of the day and wear a hat.
Calm seas on both coasts
The most useful feature of the first inter-monsoon for beach lovers is the sea. With no monsoon wind pushing swell onto either coast, the water is often calm on the south coast and the east coast at the same time. In March and April, Mirissa and Trincomalee can both be swimmable in the same week.
The sea is also at its warmest, roughly 29 °C. Storms can stir it up for a few hours, and in late April the first southwest swell can reach the south coast, but in general this is the best window of the year for a two-coast beach trip.
- →Both coasts often calm in March and April.
- →Sea temperature roughly 29 °C.
- →First southwest swell possible in late April.
Planning a trip in the first inter-monsoon
The daily rhythm is the key. Plan climbs, walks and safaris for the morning, travel or rest in the early afternoon, and keep evenings flexible for storms. On the beach, leave the water and open sand when thunder is heard.
The New Year holidays in mid-April are busy in the hills, so book early. Lankan Stays & Trails plans first inter-monsoon trips with early starts and often adds the east coast as a second beach.
Frequently asked questions
What is the first inter-monsoon in Sri Lanka?
It is the period between the northeast and southwest monsoons, roughly March to mid or late May. Winds are light and variable, and the weather is driven by daytime heat, which builds into afternoon and evening thunderstorms. It is the hottest period of the year in the lowlands.
When is the first inter-monsoon season in Sri Lanka?
Roughly from March, when the northeast monsoon fades, until the southwest monsoon arrives, usually in the second half of May. April is the peak of the season, with the most heat and the most frequent storms in the southwest and the hills.
Does it rain every day in Sri Lanka in April?
Not everywhere. In the southwest and the hills, afternoon or evening storms are common on many days in April, but mornings are usually dry. On the east coast and in the north, storms are much less frequent and many days are completely dry.
Why is April so hot in Sri Lanka?
In April the sun is almost directly overhead, and there is no monsoon wind to bring cooler air or cloud cover in the mornings. Highs reach about 32 to 35 °C in the lowlands with humidity above 80 per cent, which makes it the hottest-feeling month of the year.
Is the sea calm during the first inter-monsoon?
Usually, yes. With no monsoon wind pushing swell onto either coast, the south and east coasts are often calm at the same time in March and April. Storms can stir the sea briefly, and swell can start to build on the south coast in late April.
Is the first inter-monsoon a good time to visit Sri Lanka?
It can be, for travellers who handle heat well. The sea is calm on both coasts, the hills are lively in April, and mornings are clear for sightseeing. The trade-offs are high humidity and frequent afternoon storms in the southwest and hills.
How does Lankan Stays & Trails plan trips in March and April?
Trips in March and April are planned with sightseeing early in the day, rest or travel in the early afternoon and flexible evenings. Hotels with air conditioning and pools are chosen in the plains, and the east coast is often added as a second beach.