1. What's the weather like today?

2. How is the weather today?
3. Whhat's today's forecast?
4. How is it outside?
(Are they all interchangeable? are they all the same and grammatical?)

#3 is different, because it involves making a prediction. The others just require an observation.

1,2,4 are all the same, and colloquially acceptable, though maybe not strictly grammatical.

#1 follows the rules of syntax.

I guess, #1,2 could also be interpreted as observing the current conditions and extrapolating to the rest of the day.

3. Correct the spelling of "what's."

For most part, these mean the same thing, although 3 has a different meaning. In the other sentences, you ask what the weather looks like on that day (perhaps to another person). In number 3, you ask about the prediction of the weather for that day.

Hmmm. "what's" is a valid contraction for "what is"

what's your problem?

:-)

There are two hs in 3.

oops - my bad. Just read the first line.

1. "What's the weather like today?" - This question is asking for a description or report of the current weather conditions.

2. "How is the weather today?" - This question is asking for an assessment or opinion on the current weather conditions.

3. "What's today's forecast?" - This question is specifically asking for a prediction or outlook of the weather for the entire day.

4. "How is it outside?" - This question is asking for an assessment of the weather conditions outside at the present moment.

While the four questions have similar intentions, there are slight differences in their meaning and usage.

1. "What's the weather like today?" and "What's today's forecast?" are more commonly used to inquire about the overall weather conditions for the day, while taking into consideration any changes or possible variations throughout the day.

2. "How is the weather today?" and "How is it outside?" express a more immediate interest in the current weather conditions at the specific time the question is being asked.

All four sentences are grammatically correct and, to some extent, interchangeable depending on the context and preference of the speaker. The choice of question might depend on the specific information the person is looking for or the manner in which they prefer to ask.