If it has not started raining, but it is likely to, it’s going to rain(soon) / it’s about to rain. When the first raindrops are falling, or are about to fall, it’s beginning/starting to rain, and you need to grab your brolly. The simplest way to tell others about the falling of rain at the moment is to say that it’s raining. Alternatively, we can use the phrase in the wet, where the wet means the rainy weather. The word rain can be combined with other words, such as it’s raining hard/heavily or it’s raining slightly/a little.
If it’s been raining continuously for a while, you can say that it’s been raining incessantly/constantly. Here are some other situations of this type:
It’s been raining steadily from the moment we arrived.
It’s been raining nonstop most of this week.
It rained solidly for three hours.
It’s been raining steadily and hard much of the afternoon.
We use the above phrases to describe rainfall without interruption. Use them to describe the actual weather conditions and avoid overusing them to exaggerate the situation. If you mean ‘it’s been raining repeatedly’, then a better option is to say that it’s been raining persistently. Intermittent intervals of rain, however, are expressed differently, for example:
The weather’s been awful—it keeps raining every few hours.
Tomorrow will be cloudy with outbreaks (= intermittent intervals) of rain and drizzle (= small, fine, spray-like rain).
If it’s bucketing/throwing it down, then it is raining heavily. We have a similar situation if it’s really pissing (it) down there at the moment, or it’s tipping (it) down, or it’s sheeting down outside, or it’s been teeming down all day (with rain). If the rain is coming down so hard that it splashes back up and hits you from below, it is raining upwards. When the rainstorm is so heavy that the cows shake, it is cow-quaker.
When heavy rain is being blown by the wind, we have driving rain.
The heavens open when it suddenly starts to rain a lot, for example just as we got to the park, the heavens opened.
Mizzle is a fine, light, misty rain. Last night, it was mizzling steadily. The rain grew from a mere sprinkle to a respectable drizzle.
When the rain is slight, it’s only spitting (with rain).
Did you get caught in the shower?