Explanation short-term forecasts

The WaterApps website displays the seven day weather forecast provided by meteoblue. These forecasts can help make decisions in farming, for example taking in the harvest before it starts raining. The short term forecasts are quite certain but the farther we look in the future the more uncertain it gets.

The forecast

The weather forecasts consist of temperature, wind and rain. For the temperature there is the actual maximum air temperature and the felt temperature, the temperature on our skin can feel cooler because of wind or warmer because of humidity. For wind there is the wind direction (see figure) and the wind speed. And for rainfall the expected amount of precipitation (rainfall, in millimeters) and the chance this rainfall will come as well as an image of where this rainfall will most likely fall in the 15 kilometer circle from the center of town.

Figure 1: wind directions

Different for every village

For rainfall forecasts it is important to keep in mind that rainfall can be very location specific. If it rains in one village it does not have to rain in the next village. The forecast on this website are specifically for Khulna (Bangladesh) and Ada (Ghana) so if you are not close to these cities the rainfall forecast in your village can be different. It is very important to look at the forecast specific for your village or a place nearby.

How are weather forecasts made?

Short term weather forecasts are made with weather models and what we know of the weather at the moment we make the forecast. Our knowledge of the past and current weather comes from measurements. These have been done for centuries on the ground and now we also have satellites measuring from space. These measurements include rain, temperature, air moisture, wind, air pressure and more. All the historical measurements help us understand how the weather system works and this again helps us to make computer models to predict the weather. This knowledge of weather processes is combined with knowledge of the current weather to make a weather forecast. For example if we know that wind from the west usually brings rain (model) and we know today the wind is coming from the west (knowledge of current weather) we can predict that there will probably be rain tomorrow. A computer with a weather model puts a lot of these pieces of knowledge together and quickly combines it with the measurements of the current weather to make the forecast.

Figure 2: A satellite sees the clouds from space

Models and uncertainty

The forecast that is displayed on the WaterApps website is an average of several weather models. These computer models all calculate the expected weather in a slightly different way. In the figure below, you can see a seven day temperature forecast with temperatures per hour. All the different coloured lines are different models, and you see that they are much closer together for Friday than they are for Wednesday. This means that we are more sure of the expected temperature of Friday than of the temperature of Wednesday, because Wednesday is farther in the future. You can also see that we are much more certain of the day temperature (peaks, maximum temperature, red arrows) than of the night temperature (bottom, minimum temperature, blue arrow).

Figure 3: multi-model temperature forecast for Kuhlna, Bangladesh by meteoblue

Thunderstorms and rainfall in mountainous areas are hard to predict with a weather model because these are caused by very local and quick natural changes.

meteoblue

meteoblue is one of many providers of short term weather forecasts. It provides high quality short term forecasts. The organization is transparent about its forecast development and gives a detailed background to the forecasts.