A weather app compresses a huge amount of information into one icon and one number. That is convenient for deciding whether to take an umbrella to work, but it is a poor basis for deciding whether to drive three hours for a sunny day. This guide explains what sits behind the icon and how to read a forecast well enough to make travel decisions.
Where forecasts come from
Every forecast starts with a numerical weather model: a computer simulation of the atmosphere, started from millions of observations from weather stations, balloons, aircraft, ships and satellites. Large centres such as the European Centre for Medium-Range Weather Forecasts (ECMWF) and national weather services run these models several times a day. Apps and websites then take the model output, sometimes combine several models, and adjust it for local conditions.
Two consequences matter for you:
- Different apps can disagree because they use different models or blend them differently. Disagreement is itself useful information: it means the atmosphere is hard to predict that day.
- Forecasts change every time a new model run arrives, typically every 6 to 12 hours. A forecast checked on Monday for Saturday is an early draft, not a final answer.
For the next few hours, forecasters also use nowcasting: extrapolating what radar and satellite show right now. For "will this shower reach me in the next hour", a radar map beats any model.
Rain probability: the most misunderstood number
A "40% chance of rain" does not mean it will rain for 40% of the day, nor that 40% of the area will be wet. It means that, on days with conditions like these, measurable rain (usually at least 0.1–0.2 mm) falls at that location during the period in roughly four cases out of ten.
That makes rain probability a statement about whether it rains at all, not how much or for how long. To judge impact, pair it with two other numbers:
- Rain amount (mm). Under 1 mm over a day is a passing sprinkle. 1–5 mm is noticeable showers. Above 10 mm is a properly wet day.
- Timing. An hourly view often shows that a 60% daily chance is concentrated in a two-hour afternoon window, leaving a dry, bright morning.
A day with 70% probability but 0.5 mm expected can be perfectly good for a city walk. A day with 30% probability of a thunderstorm dropping 15 mm is a much bigger risk for a mountain hike.
Why the cloud icon is misleading
Daily icons are usually chosen from the "most significant" weather of the day. One shower at 16:00 can turn a mostly sunny day into a rain-cloud icon. When sun matters to you, look instead at:
- Sunshine hours, the forecast number of hours of direct sun. This is the single best measure of a bright day, and it is what SunFinder compares between places.
- Cloud cover (%) by hour. Below about 30% feels sunny; 30–70% is a mix of sun and cloud; above 80% is grey unless the cloud is thin and high.
Also be aware that low cloud and fog are hard for models to predict. Coasts in spring and early summer, and valleys and lakes in autumn and winter, often stay grey longer than forecast.
Temperature: highs, lows and what you will feel
The daily high is typically reached in mid-afternoon, and the low just before sunrise. For deciding how a day will feel, the high matters most; for camping or evening plans, look at the low as well.
The forecast refers to air temperature in the shade, about 2 m above ground. In direct sun and out of the wind, you will feel several degrees warmer. In wind, or near cold water, you will feel colder. Many apps show a "feels like" value that combines temperature with wind and humidity.
Near the sea, remember that coastal temperatures are moderated by the water. A seaside town can be 5–8 °C cooler than a city 30 km inland on a spring afternoon.
Wind: look at gusts, not just averages
Average wind speed tells you how breezy it will feel; gusts tell you what might go wrong. Gusts are short bursts that are often 30–50% stronger than the average, and more in hilly terrain or between buildings.
Rough guide for outdoor plans:
- Under 20 km/h: comfortable almost anywhere.
- 20–40 km/h: noticeable; beaches become sandy and cool, cycling into the wind is hard work.
- Above 50 km/h gusts: uncomfortable on exposed coasts and ridges; ferries and small boats may be affected.
Direction matters too. An onshore breeze cools a beach, while the sheltered side of a headland or mountain can be much warmer and sunnier.
UV index
The UV index measures the strength of sunburn-causing radiation. A value of 3 or more means protection is sensible, 6–7 is high and 8+ is very high. UV depends on sun angle rather than temperature, so a cool, clear day in May can have higher UV than a hazy day in August. Thin cloud blocks less UV than people expect. Our guide to the UV index goes into detail.
How far ahead can you trust a forecast?
Forecast skill decreases steadily with time. As a rule of thumb for Europe:
- 1–2 days: reliable for temperature and generally for whether a day is dry or wet; exact shower timing can still shift.
- 3–5 days: good for the overall pattern (high pressure and sun versus a frontal system and rain), less precise locally.
- 6–10 days: useful for trends only. Treat a sunny icon on day 8 as "possibly good", not a promise.
- Beyond 10 days: close to climate averages. Use typical monthly weather instead.
Confidence also depends on the weather pattern. A large, stable high-pressure system can be forecast well a week ahead. A slow-moving low with thunderstorms can be uncertain even for tomorrow. This is why SunFinder labels results with a confidence level and is more cautious about days further away. Our guide to long-range forecast reliability explains the details.
A five-point check before a trip
- Sunshine hours at the destination versus at home. A difference of 3 or more hours a day is noticeable.
- Rain amount and timing, not just probability.
- Daytime high and whether it is on the coast or inland.
- Wind gusts, especially for beaches, ridges and ferries.
- How far ahead the day is, and whether different sources agree.
Then re-check the evening before you leave. If the picture is still good, go and enjoy it.