As you know, the weather is changing moment by moment. 날씨가 좋다 = This weather is good.(at this moment, this is a correct statement.) 날씨는 좋다 = The weather is good.(in the long term, this is a wrong statement.) BTW, 날씨는 좋다 is used with another contrasting sentence. e.g. 날씨는 좋은데 너무 덥다. 날씨는 좋은데 바람이 많이 분다. Read the following. The topic particle, 는/은, is used in cases when we make a general or factual statement whereas 가/이 is not. For example, 치타는 빠르다 = Cheetah is fast 치타는 느리다 = Cheetah is slow (This would be a wrong statement) However, if you visited a zoo and saw a cheetah who seems to move very slowly, you might say, 치타가 느리다 = (That) cheetah is slow So the identifier particle, 가/이, indicates a certain person or thing that the speaker and listener know or are aware of. In this case, it would be that cheetah in the zoo. Here is another example, 바다는 푸르다 = The sea is blue 바다는 까맣다 = The sea is black (In general, this is a wrong statement.) But say, you saw the sea at night and you may exclaim, 바다가 까맣다! = The sea is black! The sea in this sentence is identified as a particular sea at night, and both the speaker and listener know which sea is being talked about. This is not a general statement. Therefore the identifier particle, 가/이, is used in this case. Of course, 바다가 푸르다 is also perfectly acceptable. However, the difference is that the sea in this sentence is also a particular sea that is known by both the speaker and the listener. 바다는 푸르다 = The sea is blue (A general statement) 바다가 푸르다 = The sea is blue (The sea is identified and known by the speaker and listener) It's similar to the way articles are used in English. For example, An apple is red = 사과는 빨갛다 (A general or factual statement about an apple) The apple is red = 사과가 빨갛다 (A particular apple that the speaker identifies and indicates to the listener)

It's because 날씨가 좋다 describes a random event, The phrase is not saying the weather is always or fundamentally good, but that it just happens to be good at the moment. This type of "external" event (i.e. it's not something the speaker was thinking about or prepared for) is described with 이/가. 은/는 on the other hand makes a statement 1) about some entity or concept one has had in one's mind in a more collected and organized way, or 2) contrasting the subject with other things. So 은/는 statement tends to come from the knowledge in your head, whereas 이/가 statements are triggered by an external stimulus. With 은/는, you're sort of taking charge and making a voluntary statement, while with 이/가 you're more like verbally reacting to what's happening around you as a spectator (to exaggerate the difference somewhat). Suppose you saw a three-legged dog. You might react like this (note only the first particle): 1) 개가 다리가 셋이야! => Reflexively says what comes to your mind on seeing it without much thinking. 2) 저 개는 다리가 셋이야. => More "processed" way to say it. You' have now took in the situation more fully, and thus say "저" to describe the situation more clearly. 은/는 is better suited for that. So, 은/는 describes something in a more thoughtful way from one's own perspective. 이/가 is more reactive and transitory, and tends to fill an existing need. For example, if you hear someone asking 누가 할래?, there's an external need (of the asker), and you can satisfy it by answering 내가 (나는 would shift the focus to "me") 날씨는 좋다 is also possible in the #2 sense of 은/는 above (#1 sense is not possible because by definition weather cannot be good or bad in an absolute sense). This phrase implies that the weather is good but that's about the only thing that is good. We might say it with sarcasm when things are going bad, when we're bored, and so on. It's an ironic expression with a nuance of resignation compared with the more straightforward 날씨가 좋다, so you won't hear it as often.