When we're talking about weather in the abstract, we don't use articles. For example, "Snow is common around here," or "I hate rain." But when we're talking about a specific weather system, we sometimes use an article. "A steady rain" refers to the precipitation resulting from that particular storm system in that specific place and time, not rain in general. You can use it with other types of weather patterns, too:
"A fierce wind swept the prairie."
"A dense fog settled over the town."
"A sudden chill came on."