'rain constant' is not a correct and usable part of a sentence in written English.
You could say, "The rain was constant". For example, "We were stuck indoors all day because the rain was constant".
In her "Welle" (1998), museum architecture took on a bruisable quality: a wall rained constant tears.
The rain was constant and the pitch had a damp, mossy-green sheen all too familiar in these parts.
The conditions had deteriorated really throughout the first half, the rain was constant.
The storm's heavy rain and constant thunder may have overpowered the noise of the volcano, and with no systematic warning efforts, the residents of Armero were completely unaware of the continuing activity at Ruiz.
Our simulation experiments show that when fruit removal rate is extremely low, population persistence is enhanced under relatively constant rain conditions, while alternating rain phases of the cyclic scenarios lead to a significant population decrease.
"We had to adjust our algorithm for different cities, like Seattle, which experiences shallow, constant rain, compared to New York, which often sees bursts of rain lasting a few minutes and impacting only a cluster of city blocks.
The constant rain made matters worse.
We were in near-freezing fog and constant rain.
We have had 24 hours of constant rain.
