La Crosse Technology WS-7208TWC Instruction Manual page 16

The weather icons change when the unit
detects a change in air pressure. The icons
change in order, from "sunny" to "partly
sunny" to "cloudy" or the reverse. It will not
change from "sunny" directly to "rainy",
although it is possible for the change to occur
quickly. If the symbols do not change then
the weather has not changed, or the change
has been slow and gradual.
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