Talking About The Future: Will vs. Be Going to

• Express future actions decided at the moment of speaking (Immediate Decisions)

I'll have salad now.

• Express a prediction based on personal opinions or experiences (Predictions without Evidence)

I think United will win the game.

• A future fact

The sun will rise tomorrow.

• To make a promise, an offer, a threat or refusal

A promise

I promise I won't tell anyone you broke the window.

An offer

I'll take you to the airport tomorrow.

A threat

I'll tell your parents what you did.

A refusal

No, I won't cook your dinner, you can cook it yourself.

• Express future plans decided before the moment of speaking (Prior Plans)

I'm going to visit my aunt next Friday.

• Express a prediction based on present evidence (Predictions with Evidence)

Look at those black clouds. It is going to rain.

• Something is about to happen

Get back! The bomb is going to explode.

Both Will and Be Going to can be used for making future predictions without having a real difference in meaning
Examples:
I think it will be foggy tomorrow. = I think it is going to be foggy tomorrow.