Dear Mr. Skilling,
How can a rainstorm or snowstorm travel thousands of miles, constantly depositing
moisture but never running out of it? Why does it not eventually lose steam and dry
up?
John H. Deppen, Portage, Mich.
Dear John,
Your questions rank high among the issues that weather forecasters contend with on
a daily basis. The precipitation-producing capability of a storm system changes
constantly—increasing, decreasing, sometimes ceasing.
The reasons are complex, but one factor stands out: Low-level winds, especially
southerly winds in advance of a low pressure system, constantly deliver "new air" to a
storm system as it moves along. If the moisture content of that new air varies, as it
usually does depending on its source region (like the Gulf of Mexico), the
precipitation capability of the storm system will also vary.
