Actually there is not much difference..It has almost the same meaning. You can say "Mart ayı soğuktur" or "Mart ayı soğuk olur" Both indicates a fact. A general fact. But the second one "Mart ayı soguk olur" is more like saying "March happens to be cold" It's like saying that being cold is in March's nature.
If you add "olur", then it means that thing was happened before, and will keep happening. This sentence implies that march was and will be cold. (You can also use the -r suffix on the verb instead of the word "olur" hence they both mean the same thing."