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What Makes it Rain?

Recommended for ages  8+

Rain is an important part of the Earth’s endless precipitation cycle.  ; however, a few ingredients are needed for precipitation to form – moisture, instability (see our “Hot Air Rises, Cold Air Sinks” activity to learn more about instability), and a lifting mechanism. In the United States and most of North America, the most common sources of moisture come from the Pacific and Atlantic (Gulf of Mexico) oceans. Air is considered unstable if it continually rises and sinks when warm air mixes with cold. Unstable air masses occur when warm moist air is near the surface, with cold, dry air above it. 

The most common lifting mechanism for thunderstorms is a cold front, which initiates upward air motion from the surface. When warm, moist air is lifted from the surface into a colder, more dry layer of air, the warmer air will cool as it rises higher into the atmosphere. After some time, the water vapor will condense into water droplets and form what’s called a cumulonimbus cloud (or a thunderhead). These droplets will continue to rise until the point where the weight of the water is greater than what can be supported by the atmosphere below, and then they begin to fall toward the ground as precipitation.

The Water Cycle

The Water Cycle (also called the Hydrologic Cycle) is the continuous circulation of water throughout the Earth-atmosphere system, or the motion of the water from the ground to the atmosphere and back again. Two of the essential processes are evaporation and precipitation. Evaporation is the process of changing liquid water into water vapor from plants, animals, and bodies of water, transferring it into the atmosphere via energy from the sun or Earth. Another type of evaporation is called evapotranspiration, which occurs when liquid water evaporates from the surface of plants into the atmosphere. Once water vapor enters the atmosphere, it can return to Earth via the many forms of precipitation like rain, hail, sleet, and snow.

Clouds & Cloud Formation

Clouds are visible aggregates of tiny particles of water vapor, ice, or sometimes both that form when water vapor condenses in the atmosphere. The two ingredients needed for cloud formation are water and nuclei. Water is always present in the atmosphere; however, the molecules are usually too small to bond together to form cloud droplets, so in come the nuclei. Nuclei are objects or surfaces that help bond the molecules together. Hygroscopic nuclei (meaning they attract water molecules) are smoke particles, ocean spray, or dust. This means every cloud droplet has a speck of dirt, dust, or salt combined with a water molecule!

When the droplets get too heavy to stay suspended in the cloud, they fall to the Earth as rain. If the air in the cloud is below freezing (32 °F or 0 °C), ice crystals form. If the air from the cloud to the ground is also freezing or below, the droplets become snow. However, if the layers of atmosphere within and below the cloud alternate between above and below-freezing temperatures,  other kinds of precipitation may form, like hail or sleet.

Now that you know how it forms, let’s catch it when it falls! Here’s a simple activity to make your own rain gauge which you can use to compare rainfall with your Tempest Weather System data.

Measuring Rainfall


  • Tempest Weather System
  • Empty plastic soda bottle
  • Permanent marker
  • Ruler
  • Scissors


  1. Carefully cut around the body of the plastic bottle, about 5cm down from the top. 
  2. Turn the top upside-down and place it inside the bottle, making sure it was pushed down and flush with the edges.
  3. Using your ruler, draw lines around the bottle at regular intervals. We suggest using centimeters to better measure light rain.
  4. Place the bottle outside, in a location where it can collect rain (not under a tree or awning) and where it will not blow away.
  5. After a steady rain, check the water level in your homemade rain gauge. Compare it to the rain data from your Tempest. How do they compare?
  6. Make a line graph to chart the rain measurements from your Tempest and your homemade rain gauge
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