Ask what or how to use the weather forecast.

How is used to indicate whether the weather is hot or cold, mainly to explain the degree.

It means that we already know whether the weather is cold or hot, so we only express our feelings about heat or cold.

To use what is to ask what the weather is like, cold or hot, and I have no idea at all.

For example, what's the weather like in Shanghai?

It means you don't know whether the weather is cold or hot. If you want to ask, you will know.

tks！