Raindrops are formed when the cloud droplets grow big enough to fall out of the clouds. Most of the rain that falls in the winter, and even a lot of it that falls in the summer, is from melting of snowflakes as they fall through warmer air.
Rainfall is measured by the depth of water that falls on a level surface without soaking in. Rainfall is measured with a rain gauge. A basic rain gauge is nothing more than a cylindrical container, like a metal can, with a flat bottom. The only problem is to get an accurate measurement of the depth of water that has fallen. Accurate rain gauges are arranged so that the water that falls into the container is funneled into a much narrow container inside. That way, the height of the water is magnified, and is easier to read.
If you live in a part of the United States where it snows in winter, you can easily measure the snow depth with a ruler. The best time to make the measurement is right after the snow stops falling. The measurement can be tricky, because wind can blow snow from one place to another. The best place to measure snow depth is on level ground far away from buildings and trees.