Weather is a factor that determines what items may be used or where your Tamagotchi may go at any given time. Weather is based on the current given month, as well as the time of day. Weather ranges from sunny, to storming, to snowing. In strong rainstorms, the Tamagotchi will refuse to use the Door function. On partially cloudy days, several Happy Items can be found, including the Rainbow, Four-Leaf Clover, and Heart-Shaped Cloud.
More about the Happy Items and how to obtain them can be found HERE.
There are many different weather animations to be found on this version that were not included in the latter ones.
When there is snow, you can use the shovel and build a snowman.
When there is rain, you can use the umbrella.
* Source: Tamagotchi Wikia