La Crosse Technology DCF-77 Instruction Manual page 11

For every sudden or significant change in the air pressure, the weather
icons will update accordingly to represent the change in weather. If the
icons do not change, then it means either the air pressure has not
changed or the change has been too slow for the Weather station to
register. However, if the icon displayed is a sun or raining cloud, there
will be no change of icon if the weather gets any better (with sunny icon)
or worse (with rainy icon) since the icons are already at their extremes.
The icons display weather forecasts in terms of getting better or worse,
and not necessarily sunny or rainy as each icon indicates. For example,
if the current weather is cloudy and the rainy icon is displayed, it does
not mean that the product is faulty because it is not raining. It simply
means that the air pressure has dropped and the weather is expected to
get worse but not necessarily rainy.
Note :
After setting up, readings for weather forecasts should be disregarded for
the next 12-24 hours. This will allow sufficient time for the Weather
Station to collect air pressure data at a constant altitude and therefore
result in a more accurate forecast.
Common to weather forecasting, absolute accuracy cannot be
guaranteed. The weather forecasting feature is estimated to have an
accuracy level of about 75% due to the varying areas the Weather
Station has been designed for use in. In areas that experience sudden
changes in weather (for example from sunny to rain), the Weather
Station will be more accurate compared to use in areas where the
weather is stagnant most of the time (for example mostly sunny).
If the Weather Station is moved to another location significantly higher or
lower than its initial standing point (for example from the ground floor to
the upper floors of a house), remove the batteries and re-insert them
after about 30 seconds. By doing this, the Weather Station will not
mistake the new location as being a possible change in air-pressure
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