Windy shows weather forecasts as coloured maps. This guide explains what the colours mean, how much to believe them, and when to move the boat trip. You need no weather background at all. Every technical word is explained the first time it appears — tap any to see what it means.
This is Windy's colour scale for wave height. Look at the numbers: 0.5, 1, 1.5, 2 — then it jumps to 6, then 9. The scale is not evenly spaced. Half the colours cover the first two metres, because that is where nearly all real sea conditions sit. So a small colour change at the calm end is a bigger real change than a big colour change at the red end.
Start here
Behind the map sit a few big computer programs called . They are run by weather agencies in Europe, America and elsewhere. Windy takes their numbers and paints them onto a map you can scroll through.
Two things follow from that, and both are useful.
It chops the air and sea into a grid of boxes, fills each box with today's real measurements from satellites, weather balloons, ships and buoys, then uses the laws of physics to work out what each box does a few seconds later. Then it repeats that, millions of times, until it reaches next week.
Nobody is looking up what happened on similar days in the past. It is a physics calculation, run fresh, usually two to four times a day. When you hear ECMWF or GFS, those are the European and American versions.
Before you read anything
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.
RIGHT EDGE
Wind, rain, waves, temperature, cloud and about forty more. They are completely different things. The one that opens by default is not always the one you want.
BOTTOM RIGHT
A small row of names: ECMWF, GFS, ICON, ACCESS‑G3. Tap it and the map redraws. If the picture changes a lot, that is worth knowing.
BOTTOM STRIP
The timeline. Windy opens on the nearest time step, which may not be now. Drag it. A morning that looks fine at 9am can be a thunderstorm at 5pm, and a still picture hides that completely.
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.
Rain
That is all the number means: how deep the water would sit if none of it drained away. What matters for your day is not the total but how fast it arrives. Drag the slider and see the difference.
moderate
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".
The sun heats the land all morning. Warm wet air rises, cools, and its water turns back into droplets, which releases heat, which pushes it up faster still. The result is a tall cloud that dumps its water in a short burst.
Over Sri Lanka this runs on a clock. Lightning peaks around 4:30pm and the heaviest rain around 5:30pm, with most strikes between 4pm and 6pm. That is why an outdoor event at 5pm in April is a different proposition from the same event at 11am.
Timing from published Sri Lankan lightning and rainfall studies. It is the typical pattern, not a promise.
Have a guess first
Windy shows 40% for rain tomorrow. What does that mean?
B. A percentage is the chance of rain happening. The millimetres are how much. They are two separate numbers and people mix them up constantly. A 40% chance of 50 mm and a 40% chance of 1 mm look identical in the percentage.
Wind
Wind is measured in 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.
Force 4 — Moderate breeze
Sri Lanka's Department of Meteorology issues bulletins in kilometres per hour, not knots. A red marine advisory is issued at roughly 60–70 km/h, which is 32–38 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.
The wind number is an average, usually over ten minutes. A 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.
Same afternoon, two numbers
Out at sea in steady conditions, gusts run about 1.2–1.3× the average. Near the coast and over land, more like 1.4–1.7×. 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.
Have a guess first
Windy shows 10 knots average and 27 knots gusting, on a morning that otherwise looks calm. What is it telling you?
C. The average is smoothed over a wide area and a long period, so a squall crossing it barely moves that number while the gust figure jumps. A light-wind morning with a huge gust number is the classic signature of an afternoon storm building. Check the Thunderstorms layer next.
The sea
If your business is inland — hill country, the cultural triangle, safari, city events — nothing in this section applies to you. Skip to The detail view below.
Wave layers show . It is not the average wave and it is not the biggest one. It is the average of the biggest third of the waves.
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.
Have a guess first
Windy shows 1.5 m for tomorrow's whale-watching trip. What is the biggest wave you should plan for?
B. The number already leans towards the big end, so the biggest wave in a few hours runs to roughly double it. Half your waves will be smaller than 1.5 m and something near 3 m will turn up while you are out there. That is usually the difference between a boat full of happy guests and a boat full of seasick ones.
is the seconds between one wave and the next. It is not on the map colour — 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.
about three waves in view
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 4 seconds is a bucket day.
Direction
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
The coloured map is for finding where the weather is. Once you have your spot, click it. The panel that slides up 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.
tap a number
How far ahead
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.
drag across the chart
| Ahead | What it is good for |
|---|---|
| Today | Reliable. Though a storm that has not formed yet is still invisible — for the next few hours use the radar and satellite layers, not the forecast. |
| 1–3 days | Solid enough to commit money to. Confirm bookings, order the extra staff, decide the boat. |
| 4–7 days | Trend only. "Windy midweek" is fair. "18 knots at 3pm Thursday" is not. |
| 8–14 days | Barely better than knowing the season. Do not let a guest cancel on it. |
Have a guess first
A guest asks on Sunday about a boat trip the following Saturday. ECMWF says 12 kt, GFS says 26 kt, ICON says 17 kt. What do you tell them?
B. ECMWF does score best on average, but that is not the same as being right this Saturday. A 14‑knot spread is the models telling you the weather has not decided. C is the worse mistake: averaging turns pure disagreement into a confident-looking number, and 18 kt might be the one answer no model actually supports.
The limits
A model does not see the world as a photograph. It sees it as a , 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.
ECMWF — 9 km boxes
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 1–2 km option like Europe gets. Nine kilometres is the best you will ever have. The map still draws a smooth, confident colour over your beach, because it fills in the gaps between boxes to look nice. That smoothness is decoration, not knowledge.
The afternoon sea breeze. Land heats faster than sea, so air flows in off the water from late morning, building to maybe 15–25 knots by mid-afternoon and dying at sunset. At night it reverses and blows gently offshore. This sits on top of whatever the monsoon is doing and the coarse models handle it poorly.
Individual thunderstorms. A storm cell is 2–5 km across. The model can tell you the day is stormy; it cannot tell you the storm hits your beach at 4:40pm.
Shelter. Headlands, reefs and bays change everything at scales the grid cannot represent. Unawatuna is calm in conditions that close Hikkaduwa, because of a reef the model does not know exists.
Local measurements. Sri Lanka's Met Department runs 23 stations reporting every three hours and just one weather balloon site, at Colombo. It says itself that this is not enough to pick up smaller weather systems. The models feeding Windy have thin local data to work from here.
Have a guess first
You zoom right in on your own bay and the colours get smoother and more detailed. What has actually happened?
B. Zoom is a drawing setting. The numbers underneath are still one value per box, blended smoothly so the map looks good. This is the single most misleading thing about every weather map, not just Windy.
Sri Lanka by month
This is the single most useful thing to know here, and it is why a forecast that looks terrible can still mean a perfect day — just on the other side of the island. The wind reverses by about 180 degrees between the monsoons, so when one coast is blown out, the other is glassy.
Pink shading marks the exposed side. These are typical patterns from Sri Lanka's Department of Meteorology and from operators, not guarantees — individual years vary a lot. Use them to plan, then check the live forecast for the actual date.
MAY–SEPTEMBER
Wind from the southwest. West and south coasts take it; the east and northeast are calm and open for business. Galle, Mirissa and Hikkaduwa go quiet on the water. Arugam Bay, Trincomalee and Kalpitiya are at their best.
OCTOBER–NOVEMBER
The awkward one. Rain across the whole island, the year's most frequent squalls, and the main cyclone window for the north and east. No coast is reliably sheltered. Build slack into schedules and keep refund terms generous.
DECEMBER–FEBRUARY
Wind from the northeast. Now the east coast takes it and the southwest is sheltered. Mirissa whales, south-coast surf and diving are all on. Trincomalee dive schools mostly close.
MARCH–APRIL
Light, shifting winds, and the hottest time of year across the west, south, hills and cultural triangle. Also the peak for lightning — nearly a third of the island's annual strikes fall in these two months, mostly between 4pm and 6pm. Move late-afternoon outdoor events under cover.
Making the call
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.
Have a guess first
A guest points at a calm-looking gap in the breaking waves and says it looks like the safest place to swim. What is it?
C. Waves push water onto the beach and it has to escape somewhere. It escapes through a narrow channel, 10 to 30 m wide, and that channel has fewer breaking waves, so it looks calm. It can run seaward faster than an Olympic swimmer. A rip does not pull you under — it carries you out while you stay on the surface. The way out is to float and swim along the beach, not back towards it.
Practice
| You see | You should think |
|---|---|
| Waves 2 m | Biggest wave near 3.7 m |
| Period 5 s | Short choppy sea. Uncomfortable. |
| Period 14 s | Long swell from far away. Big but smooth. |
| Gust ÷ wind above 1.5 | A storm is in the area |
| 60–70 km/h | 32–38 knots. Met Department red advisory territory. |
| Models 30% apart | Not a forecast yet. Decide later. |
Every word explained
Everything the guide used, in one place.