Rain Gauge

Here's how to make this type of rain gauge:

Materials:

Procedure:

Take a plastic bottle, cut off the top at the widest point.
The top is inverted and placed inside the bottom of the bottle, with the two cut edges flush.
Find your baseline, where the bottom end of the bottle comes to a consistent circumference, and mark your baseline. You will fill the bottom with water to the baseline, because most bottles have irregular corrugation towards the bottom of the bottle.
Measure and mark centimeter increments up the side of the bottle.
Fill the bottom with water to the baseline.
Place somewhere with full exposure to precipitation, with support to prevent it getting jostled or blown over.

Barometer