Weather forecasts for 3 and 10 days are the most common. You can also see forecasts for 5, 7 and 14 days. But what does these numbers actually mean? Could we trust a forecast for more than two weeks or even a month? Let's deal will all of these common durations.
A weather forecast is the prediction of weather conditions that will occur in the future. There are several methods to do this.
The most modern and accurate method to predict the weather is numeric modeling. There are well-known physical laws that together can describe how the air moves and how conditions in every spot change.
Meteorologists use special observation stations to collect weather data all over the world. Then they use special computers that use those physical laws to turn past and current weather conditions into the weather forecast.
The forecast is always about the time in the future. There is no weather forecast in the past. (But there is, for example, a weather history, which can also be used to predict the state of the atmosphere in the future.)
So let's learn what does different weather forecast durations actually mean:
3 days is the usual time period of a middle-range weather forecast. Speaking about outdoors, this time period is long enough to plan some activities, but at the same time, it’s short enough to have high forecast accuracy.
That comes from synoptic meteorology: most cyclones and anticyclones live for about a week. And approximately 3 days is the period one particular cyclone or anticyclone influences one particular spot. The weather conditions can be predicted with the highest accuracy in this case. Of course, these are just mean values, and variability all over the world is very high.
How exactly accurate is the forecast for 3 days? The mean value is about 90%. But in different cases, the accuracy can vary. For example, in mountain areas, it’s much lower than on planes. You should also remember that severe weather events are predicted much worse than calm weather.
Photo: courtney-kenady / unsplash
10 days is the usual time period of a long-range weather forecast. This time period is long enough to plan outdoor activities even for the next weekend. And it’s the longest forecast to have at least minor accuracy. What’s the matter?
From the physical laws that describe the atmosphere dynamics, meteorologists know that the atmosphere “forgets” its current state in 14 days. That means that no matter what’s happening with the weather now, in 14 days the current situation can change in any direction.
14 days are called the “theoretical limit” of weather prediction. We can’t predict the weather for more than 14 days. But our methods of weather prediction are still imperfect, so the real limit is a bit lower and equals approximately 10 days. This is where this number of the days comes from.
So, 10-days forecast is one of the two main periods in weather prediction, together with a 3 days weather forecast. It gives the longest possible forecast but unlike the 3 days forecast it has a lower success rate and should be used cautiously.
How accurate is the forecast for 10 days? It is logical to assume that the longer the forecast period, the less accurate it is. It is very important to know, understand and remember this speaking about weather forecasting. It can be lower for mountain areas, as well. Forecast for 10 days can be used only for the primal understanding of the conditions. So use also weather history.
Photo: kay-liedl / unsplash
The forecast for 1 day is the most accurate with a success rate of about 96—98%. You can trust it even in difficult conditions because most future changes in weather can be noticed at least 1 day before they happen. Remember that heavy rain is the hardest event to predict.
Weather forecast for 5 days has approximately 80% accuracy. It is similar with 3 days forecast because it has a length less than the mean one of cyclones and anticyclones.
The accuracy of 7 days forecast is 70%. It is similar with 3 days and 5 days forecast because it also has a length less than the mean one of cyclones and anticyclones.
Due to physical reasons, the forecast for 14 days is the theoretical limit to predict the weather. Starting with this time any prediction has equal chances to happen or not to happen. The forecast is random.
The period of such a weather forecast is longer than the theoretical limit of the prediction. So the forecast for such a long time shows the way everything could be but can’t show how everything is going to be. It could only be used to check weather conditions that are possible to happen in the area.
The same goes for the weather forecast for a month.
Photo: federico-persiani / unsplash
The 3-day forecast suits very well for any sport you can do on flat terrain like city biking, but also in the lake, sea or ocean: kite- and windsurfing, surfing, SUP...
In the mountains, it’s useful for understanding of weather conditions. But once again: don’t rely on it entirely. The weather in mountains can change frequently, significantly, and unpredictably. Extreme weather events (like thunderstorms and so on) can often be predicted only hours before they happen on any territory.
If you do sports in the mountains — hiking, cycling, paragliding, skiing and snowboarding, and others, check the latest forecasts if you do your sport in a place where such events are possible — at least for a day.
The 10 day-forecast is equally suitable for any sport or outdoor activity.
But you shouldn’t rely on it in particular values of temperature, wind speed, etc. at 100%. It can be used for a general understanding of the situation: the temperature is rising, the wind is strengthening, and so on.
Check also the latest forecasts for extreme weather, if such a weather is possible in your area.
In Windy.app you could find both of the most common forecasts durations — for 3 days and for 10 days. And, of course, for today and tomorrow.
In the app you can do it in two places: on the spot and on the Wind and Precipitation Map. Let's take 10 days and iOS as an example:
1. Open any spot in the app and find the weather forecast bar.
The colors mean wind speed:
2. Choose the day you want to have the forecast for by moving the bar to the right.
3. At the same time choose a weather profile for your favorite sport or outdoor activity to have the preset of additional weather parameters.
At the end of the list you will also find wind speed / gust colors chart.
4. You can also change the forecast step: 1 hour or 3 hours.
5. Remember the Weather Archive? Move the bar to the left to get weather data for the past 10 days, so you now have the weather picture for almost a month.
1. Go to Wind and Precipitation Map.
2. Here you have the wind and precipitation forecast for the world for the same 10 days with 3 hours step (it could be for 1 hour on the spot). Move the days and choose the hours at the bottom of the screen to see the forecast for the next few days. The choose between wind and precipitation forecast click on the wind gust icon.
There is also a very useful Weather Archive feature, which shows you the historical weather data for each day and month for the past 8 years (2019-2012). Use it for weather forecasting for longer periods, then 10 days. There is also a Wind Archive feature. Learn more about it.
Text: Evgeniy Petrov. Cover photo: holly-mandarich / Unsplash
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