As sunshine gives way to clouds, a little-known process is occurring thousands of feet up in the sky. Rain is forming. All of the necessary ingredients have come together perfectly at this moment: moisture, unstable air, even dust particles. Rain is the lifeblood of Earth. We rely on it for drinking water, agriculture, and healthy ecosystems that sustain the diversity of life on Earth.

The first step in rain formation is nucleation. In biology, the nucleus is the ____1____ center of the cell, usually near the center. In rainfall formation, a nucleus or ‘nucleation point’ is any tiny object in the air that helps water in the air condense into a droplet. This nucleation point could be dust particles, smoke, ____2____, or any other tiny object floating around in the air. Basically, moisture needs something to cling onto if it will have any chance of becoming rain. 

As the formation of rain continues, rising motion and mixing in the gathering clouds sends _____3_____ droplets moving in all directions. They frequently bounce into each other. Each time that two droplets collide, they combine to form one larger droplet. This process of _____4_____ and growth continues until suddenly, the droplet has grown too heavy to continue floating in the clouds. Once that happens, the droplet falls to the ground as rain!

How Rain Differs From Hail, Snow, and Sleet

Sometimes this process includes a few more steps that lead to other kinds of precipitation. In larger, more energetic _____5_____, the rising motion in the clouds is so great that the rain droplets continue rising due to upward motion within the storm, growing larger and larger. If these raindrops are sent high enough in the sky, they may reach altitudes that are below 32 degrees Fahrenheit (0 degrees Celsius). We all know what happens at that temperature; water freezes. This is how _____6_____ forms. As the rain freezes over _____7_____ feet in the air, it will eventually collect too much ice mass to remain suspended in the air. Then, it must fall to the ground as hail. 

If the entire cloud is below freezing from top to bottom, the process of nucleation and condensation occurs in frozen format, creating ____8____. If the clouds are above freezing but air closer to the surface is just below freezing, water droplets created in the clouds will freeze during their fall down to Earth. This is how ____9____ is formed. As you can see, there are several ways that precipitation can form. Moisture is precious for life on Earth. Subtle differences in _____10_____ and energy can lead to very different outcomes in weather.

Measuring Rain

The most important thing to know when measuring rainfall is the amount of time during the measurement period. Rain is often measured _____11_____ or once per day. When studying climate, or long-term weather patterns, annual and seasonal rainfall is measured by adding up all of the daily totals for the desired period. Measurement is done with a gauge that meets the standards used by weather scientists, known as _____12_____. The gauge should have an opening that is at least four inches wide. In official automatic weather stations, a Tipping ____13_____ Rain Gauge is used, which also has an opening that is 8 inches wide. This type of gauge never needs to be emptied, and the amount of rainfall (and even the rate at which the rain is falling) can be read automatically. An electronic pulse is generated each time the volume equal to 0.01 inch of water collected in one of the small buckets causes it to tip. These measurement devices can be expensive, but luckily there are free alternatives that you can make with just a few short steps.

You can measure rainfall by making your own reliable gauge using recycled materials. Take a bottle with smooth sides, such as a two-liter soda bottle, and cut off the upper third. Turn it upside down on the top of the bottle to create a ____14____. Stick a ruler on the side of the bottle and fill the bottle of water up to the zero mark on the ruler, which should be above the bumps at the bottom of the bottle. The bumps at the bottom of a soda bottle would otherwise affect the measurement. Place the rain gauge outside away from buildings and trees. Follow a schedule to check and record the water level each day. You can even share your observations with the local weather station!