Weather is a game mechanic in Harvest Moon 64.
There are three states of weather: normal, rain/snow, and typhoon. Each has its own effects on what happens in the game world. The next day's weather can be found by watching Channel 1 on the television. Some characters are also able to tell you the next day's weather.
Normal weather is the most common, and the most desirable. Villagers go on their normal schedule. Animals can be left outside without worry of being fed or getting sick, and cows may enter a happy mood, resulting in an Affection bonus. This weather is also required for some events, such as Karen's event after Saving the Vineyard.
Rain and Snow both have the same effects, they just appear ingame differently. Villagers will go on their "rainy day" schedule, and most animals left outside in this weather will become sick. Using tools will now also increase fatigue by the amount of stamina used by that tool. If you have a girl at a yellow heart, the sick event for that girl may also be triggered.
Typhoon weather occurs only during the Summer. When it hits, you will be unable to leave your house. Any animals left outside run the chance of being killed, crops may be destroyed, and your greenhouse may also be destroyed. If you are good friends with the Harvest Sprites, they will feed your animals for you.