Barometers forecast weather based on rate of change of pressure. This does not predict what is happening outside, but rather, if the weather will change in the next 24 to 48 hours due to changes in barometric pressure.

In general, if the rate of change of pressure increases, the weather is improving (sunny). If the rate of change of pressure decreases, the weather is degrading (cloudy or rainy). If the rate of change is relatively steady, it will read partly cloudy or partly sunny (depending on the type of weather station).

The reason the current conditions do not match the forecast icon is because it is a prediction 24-48 hours in advance. In most locations, this prediction is only 70% accurate and it is a good idea to consult the National Weather Service for more accurate weather forecasts. In some locations, this prediction may be less or more accurate.

The National Weather Service (and other weather services such as Accuweather and The Weather Channel) have many tools at their disposal to predict weather conditions, including weather radar, weather models, and detailed mapping of ground conditions.  Thus, these forecasts are significantly more accurate then forecasts based on rate of change of pressure.

You will need to give the weather station enough time to predict changes in weather. This can take up to 7days, depending on where you live. Also, make sure you have calibrated the barometer:

 

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