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more information: weather & whether
Cool means cold, but not too cold. However here, April doesn't mean that the weather is cool, she means that the weather is good, and she likes it.
hot but not too hot
Heaps is a slang word. It means a lot or very.
Example: He's heaps good-looking.
Raining is the -ing form, the continuous tense, of the verb 'to rain'.
'It's raining' means that right now, there is rain. 'It's not raining' means there is no rain right at that moment.
more information: present continuous tense
The adjective from the verb 'to rain' is rainy. It describes the type of weather.
Example: I don't like rainy days.
Adding a 'y' to weather nouns is a common way of turning them into adjectives to describe the dayís weather.
Example: Itís stormy. It's sunny. It's windy. Itís cloudy.
not too bad
average or satisfactory