Sorry I missed this - I've been away for a few days and am still catching up.
Starting with a. 'Drizzle' is indeed uncountable, and does not use an indefinite article, but if you look at what you have written you will see that we have specified what kind of drizzle - 'light drizzle' and therefore we use the indefinite article. When we specify a type of uncountable noun we can usually use both plurals and 'a'.
For examples 'a hot coffee would be really good right now'; 'the waters of the Red Sea'.
With the next question, one of the best ways of deciding if something is abstract or concrete is that if you can drop it on your foot, it is concrete.
This generally defines a first order nominal. Since third order nominals are abstract, the problem comes with things like railway timetables in the sentence 'railway timetables are subject to change'. Here the timetables are both the abstract idea of timetables and the concrete ones. However you can only drop the concrete ones on your foot, so applying this usage again helps you to see whether you are using an abstract or material object in your sentence.