A practical guide to reading Sri Lanka's weather on Windy Reading Windy

Before you read anything

Three things to check first

Every colour on the map is the answer to a question you may not have asked. Check these three things before you read anything into it. Two of them live in corners of the screen most people never touch.

This is why a screenshot of Windy on its own tells you very little. If a colleague sends you one, ask which model and which hour.

The layer menu

Fifty layers. You need about eight.

Open More layers… and Windy throws nearly fifty options at you, most of them built for pilots, sailors and energy traders. Here they all are, sorted by whether they are actually any use to you in Sri Lanka. Tap any one to see what it does.

Sorted for a Sri Lankan tourism business. A pilot or a yacht crew would sort them completely differently, and a few of the "ignore" ones exist only because Windy is used worldwide; there is no snow to forecast here.

Rain

A millimetre of rain is a litre of water on every square metre

That is all the number means: how deep the water would sit if none of it drained away. What matters for your day is how fast it arrives. Drag the slider and see the difference.

There is a separate layer called Rain accumulation that adds the rain up over days. Useful for "has the road washed out" questions. Useless for "will the 4pm walking tour get wet".

Where the rain falls

The wet side faces the wind

This is the part no weather app will tell you, and it explains more about Sri Lankan rainfall than anything else. The central highlands rise to over 2,500 m. When wet monsoon air hits them it has to climb, and climbing air cools and dumps its water on the slope facing the wind. By the time it comes down the other side it is dry.

Drag the wind direction and watch the wet side swap.

That is also why "it is raining in Sri Lanka" is close to a meaningless statement. Across June to August, Ratnapura has rain on about two days in three, while Jaffna has rain on fewer than one day in ten. Annual totals run from about 900 mm in the driest lowlands to over 5,000 mm on the western slopes, more than five times the rain, on an island 430 km long.

Wind

Knots mean nothing until you have seen what they do

Wind is measured in knots on Windy by default. The number is meaningless on its own, so here is what each one actually does to a palm tree, a flag and the sea. Drag it.

Sri Lanka's Department of Meteorology issues bulletins in kilometres per hour, not knots. A red marine advisory is issued at roughly 60 to 70 km/h, which is 32 to 38 knots, and fishing gets suspended in the named sea areas when it happens. If Windy is showing you that on your stretch of coast, the official answer is already no.

Gusts are what break things

The wind number is an average, usually over ten minutes. A gust is a short burst well above it. Marquees collapse, umbrellas invert and boats get into trouble on the gust, never on the average. Windy gives gusts as a separate layer, and the gap between the two is the thing to look at.

Out at sea in steady conditions, gusts run about 1.2 to 1.3× the average. Near the coast and over land, more like 1.4 to 1.7×. When a thunderstorm is about, the gust can be several times the average, because it is cold air crashing down out of the cloud and spreading sideways. That is the one that catches people out.

The sea

Two numbers decide whether the boat goes out

If your business is inland (hill country, the cultural triangle, safari, city events), the rest of this section is about boats. Pick up again at Direction, which matters wherever you are: get the wind the wrong way round and you send guests to the wrong half of the island.

One: the wave you get is bigger than the height shown

Wave layers show significant wave height. That is the average of the biggest third of the waves, so it already sits above the typical wave you will see.

Over a few hours the largest single wave is about twice that number. So when Windy paints 2 m, expect an occasional 3.5 to 4 m wave. That occasional one is what swamps a boat or knocks a guest off a rock, and doubling the printed number is the whole trick.

Two: height is how it looks, period is how it feels

Wave period is the seconds between one wave and the next. It lives in the detail panel, so you have to click the sea to see it, and it matters more than height. Two seas can both say 1.5 m and be completely different days.

Watch the pink bar. That is a 10 m boat, drawn at the same scale as the water. Keep the height where it is and drag the period. The waves barely change height, but the boat goes from climbing over every one to being lifted gently by something far bigger than itself. That is why 1.5 m at 14 seconds is a pleasant trip and 1.5 m at 5 seconds is a bucket day.

Direction

Wind is named for where it comes from, not where it goes

A "southwest wind" blows out of the southwest and travels towards the northeast. Get this backwards and you send guests to exactly the wrong coast. On the map the little moving dots travel the way the air is going, so they appear to head the opposite way from the name. Both are correct.

The detail view

Click the map. This is where the real forecast is.

The coloured map is for finding where the weather is. Once you have your spot, click it. The panel that slides up, which Windy calls the meteogram, holds far more than the map ever shows, including the two numbers that decide most of your calls: wave period, and the gap between wind and gusts.

Below is a real April day on the south coast, hour by hour, drawn the way Windy draws it. Drag across it, or use the slider under it. It starts glassy, the sea breeze builds through the middle of the day, and at 5pm a thunderstorm arrives. Everything this guide has taught you is visible in it.

The main question

Working out if it will rain at your place, and how much

Start with the uncomfortable bit. Rain is the worst thing a weather model does. Since 1985 forecasts of pressure and upper-air temperature have roughly halved their errors and wind has improved by about a third, but rainfall has barely moved. In the American forecast centre's best year on record, a one-day forecast of where at least 25 mm of rain would fall scored 0.38, on a scale where 1.0 is perfect.

That is a reason to read it the way a forecaster does: trust the pattern, distrust the pixel.

The method, in order

1. Which season, and which side of the hills are you on? Before you open anything, you already know a lot. In the southwest monsoon the western slopes and southwest coast get the rain and the east is dry. In December to February it is the other way round. Get this right and you have done half the work.
2. Open Rain accumulation for the week's shape. Pick a window (12 hours, 24 hours, 2, 4 or 8 days) and slide it along. You are looking for whether this is a wet spell or a dry one, not for detail.
3. Switch to Rain, thunder and scrub the timeline through one day. Each frame is millimetres over a 3-hour block. Watch when the colour arrives and when it leaves.
4. Click your exact spot and read the meteogram. This is where the numbers are. The rain bars give you millimetres for your point instead of a colour you have to guess at. It shows 3-hour blocks by default; switching to hourly is a paid feature, so a single bar can still hide a short violent burst.
5. Switch models. Rain is where models disagree most, so this matters more here than anywhere else. If ECMWF says 4 mm and GFS says 40 mm, what you have is a coin toss.
6. Inside two hours, stop forecasting and start looking. Open Weather radar. It shows rain that is actually falling right now, measured by radar rather than predicted. Sri Lanka has no working weather radar of its own yet, so most of the island sits outside any real coverage. Only the far north, around Jaffna and Mannar, is close enough to the Indian radar at Karaikal to show anything you can trust. Everywhere else, open Satellite and use your own eyes.

How much is that, really?

Windy gives you millimetres. Sri Lanka's Department of Meteorology issues its warnings in millimetres too, so you can translate a forecast straight into the language of an official warning. Drag this to see where a number lands.

The landslide numbers come from the National Building Research Organisation (renamed the National Building Research Institute in 2025, though almost every warning you will read still says NBRO), which sets its thresholds over two periods. Across a full day, 75, 100 and 150 mm mark watch, alert and evacuation. Over a single hour, 75 mm reaches evacuation on its own, without the day's total ever going near 150. That hourly one you will not spot on Windy: its rain bars are often three-hour blocks, so an hour that violent gets averaged away before it reaches you. They apply to the ten landslide-prone districts: Kalutara, Galle, Hambantota, Nuwara Eliya, Matale, Kandy, Kegalle, Ratnapura, Matara and Badulla. If your property or your guests' route is in one of those, this is the number that matters more than any other on this page.

When it will not stop

It has been raining for days. Is this easing or getting worse?

This is a different question from "will it rain", and it has a much better answer. Flooding is rain that fell somewhere else, hours or days ago, now arriving down a river or collecting in a low spot.

A property in Gampola goes under because of rain that fell in the hills above it, not because of rain over Gampola. The Welipenna interchange on the Southern Expressway floods from water draining off inland, and when that happens the entry and exit ramps close while the carriageway itself keeps running. Traffic heading south towards Galle and Matara carries on; anyone joining or leaving at Welipenna uses Dodangoda or Kurundugahahetekma instead, so the transfer runs late rather than failing. The sky above you is often the least useful thing to look at.

The lag between the rain falling and the river rising is the useful part. Once the water is on the ground it moves downstream at a speed nobody can change, so the river is telling you about water that already exists rather than water a model expects. That is a different kind of warning from a forecast, and how many hours it buys you depends entirely on which river you are on. Your job is to work out whether the catchment upstream is still being fed.

A teaching model: it shows how rainfall becomes river level, so you can see the shape of the problem. No real river behaves exactly like it; for yours, use the Irrigation Department's gauge readings.

Why the third day of rain is far worse than the first

Drag "rain already fallen" up and down and watch the runoff rate change. On dry ground most rain soaks in and only a small share reaches the river. On ground already soaked, there is nowhere left for it to go and most of it runs straight off.

The same 100 mm can be an ordinary wet day or a flood, depending entirely on what happened the week before. This is why Rain accumulation is the layer that matters here, not the hour-by-hour rain layer. Set it to 4 or 8 days and look at what the hills upstream have already taken.

The number that connects Windy to an official warning

Here is the useful link most people never make. A 2009 study of Sri Lanka's early warning system, run by the Irrigation Department with JICA, worked out what a rainfall trigger for Ratnapura would look like: 100 mm of rain in 24 hours at any station in the catchment for an advisory, and 150 mm in 24 hours, or the Ratnapura gauge reaching its trigger level, for a warning. Treat that as a sense of scale rather than a switch. It was a proposal rather than a rule the Department operates, and it was keyed to a minor flood level at Ratnapura that the bulletins have since moved. What it does give you is a rainfall number, in the same units Windy gives you.

The number differs by river. On the Kelani, the Department's own standing order puts 125 mm in 24 hours as the point at which it starts checking levels for a minor flood. Find the one for your river. Either way the rain figure is the early hint and the gauge is the decision: the Department's bulletins are graded on river level, not rainfall.

So when you see a forecast putting that much rain over the hills above your town, you have something solid to go on. You are looking at roughly the amount that puts the official system on alert, a day or two before anything is issued. That is the whole point of learning to read this.

What to check, and when

• Windy, 1 to 3 days out. Is heavy rain forecast over the hills that feed your river? Use Rain accumulation over 24 hours or more, and look upstream, not overhead.
• The Department of Meteorology rain advisory. Amber means be prepared, red means take action. This usually comes first.
• The Irrigation Department river bulletin. Every gauge is graded Alert, then Minor Flood, then Major Flood, with a column showing whether the level is rising or falling. The trend column matters as much as the level.
• NBRO for landslides if you or your guests' route is in the hill districts. Its levels run on 75, 100 and 150 mm of rain in a day, or 75 mm in a single hour.
• Reservoir gate announcements if you are on a controlled river. Nothing in a weather forecast will tell you this.

How much warning you actually get

This differs enormously by river, and it is the most practical thing to know about your own location. From one real set of warnings issued on the same day for the same rainfall:

If you are on a fast river, the rain forecast is your warning system; by the time the river is rising it is already too late to move much. If you are on a slow one, you have a day or two, and the official bulletin is the better guide. Kelani floods usually drain within two to three days.

Is this getting worse every year?

Carefully: there is no single national trend in Sri Lanka's annual rainfall. The direction depends on where you are and which decades you count. What has been changing more consistently is the shape of it, with heavy rain arriving in shorter, harder bursts.

The shape of the rain is also only part of why a given storm floods more than it used to. Building on floodplains and filling in flood storage have cut how much water the ground and the drains can take, and in Sri Lanka that change is better documented than the rainfall one.

The practical consequence for you is simple and does not require settling any scientific argument. "We have never flooded here" is weaker evidence than it used to be. If your planning is based on what the last twenty years did, treat that as a floor rather than a ceiling, and check the warnings even when your own experience says you are fine.

How far ahead

When the models disagree, that is your answer

Windy draws one clean number for every hour of the next two weeks. Day twelve looks exactly as confident as tonight, which is misleading. The fix is free: switch models and see how far apart they are. Close together means the weather is settled. Spread out means nobody knows yet, and staring at one model harder will not help.

The limits

To the American model, all of Sri Lanka is 164 squares

A model sees the world as a grid of boxes, and it knows only one wind, one temperature and one rainfall for each whole box. Anything smaller than a box is invisible to it. Drag this and watch the island turn into steps.

Weligama Bay is about 2 km wide. Even the finest model available here has boxes more than four times wider than the entire bay, and Sri Lanka has no local high-resolution model on Windy, no 1 to 2 km option like Europe gets. Nine kilometres is the finest resolution available here today. The map still draws a smooth, confident colour over your beach, because it fills in the gaps between boxes to look nice. That smoothness carries no information.

Sri Lanka by month

Somewhere in Sri Lanka is having good weather right now

That is the useful way to think about the monsoon. The wind reverses by about 180 degrees twice a year, so the two halves of the island take turns. When the southwest is wet, the east is in its dry season. When the east is wet, the southwest is dry.

For ten months of the year there is a dry side to send people to. A forecast that looks terrible where you are standing can be a perfect day 100 km away, and knowing which way to point guests is the most useful thing you can do with a bad forecast. October and November are the exception: in those two months rain can fall on both coasts at once, so switching sides stops working.

Pink shading marks the exposed side. These are typical patterns from Sri Lanka's Department of Meteorology and from operators; individual years vary a lot. Use them to plan, then check the live forecast for the actual date.

Making the call

Turning a forecast into a decision

Sri Lanka publishes no official wind or wave limit for tourist boats. The tourism authority requires you to assess the weather in your own procedures but sets no number. So the only official trigger is the Met Department's own advisory, when it tells the fishing fleet not to go out in your sea area, that is your answer too.

• A red advisory means stop. It tells naval and fishing communities not to venture out until further notice, and fishing gets suspended. Recent ones have been issued for gusts around 60 to 70 km/h, which is 32 to 38 knots. Your boat is smaller than theirs.
• Watch the gap between gust and average. Gusts of 1.4 to 1.7 times the average are ordinary; that is just wind tumbling over ground. It is when the gust reaches about twice the average that a storm is in the mix, and the average is the number that will not show it.
• Read the period alongside the height. 1.5 m at 14 seconds is a big, slow, comfortable swell. 1.5 m at 5 seconds is the trip where everyone is sick.
• Decide at 48 hours. Take the booking with a weather clause. Make the real call two days out, when the models have stopped arguing.

Test yourself

Make the call

Four real situations. Each one shows you what Windy would show you, and nothing else. Decide, then see whether you would have got it right.

Practice

A one-minute routine

1. Check which model and which hour you are looking at.
2. Drag the timeline through the whole day. You are looking for when things change, not for one moment.
3. Switch models. If they agree, trust it. If they spread apart, that is your real answer: the weather is still undecided.
4. Click your exact spot and read the detail panel. Wave period and the gust gap are the two numbers the map did not show you.
5. Do the two conversions: double the wave height for the biggest wave, and divide gust by average for the squall signal.
6. Ask what the model cannot see here (the afternoon sea breeze, one storm cell, the shelter of your own bay) and fill those gaps with the radar layer and your own eyes.

Every word explained

Glossary

Everything the guide used, in one place.