Rain

Water condensed from atmospheric vapor and falling in drops.

Explanation

Rain begins with condensation, which forms clouds. Water vapor rises with the hot air and forms clouds. If the air temperature in a cloud is well below freezing, the water vapor may immediately change to solid. Most clouds that form precipitation have very small ice crystals in them, while others have water droplets or a mixture of the two. Within the cloud, the droplets grow in size by attracting surround droplets. (This is deposition) Then, because of their growing weight, some particles fall or rise and collide with one another. (This is coalescence) Through these processes, the water vapor grows large enough to fall to the ground. If there are rising air currents in the cloud, the droplets may be swept upward, bumping into ice crystals to create even bigger and heavier particles.

What You'll Need:

• jar with lid (1 liter)
• ice cubes

Directions:

1. Pour enough water into the jar to cover the bottom.
2. Turn the jar lid upside down on top of the jar.
3. Put 3-4 ice cubes inside the lid.
4. Observe the underside of the lid for 10 minutes as water drops form.

What's Happening?

Some of the liquid water in the bottom of the jar evaporates (changes to gas) and condenses, then changes back to liquid when it hits the cool lid. The same thing happens when the vapor rises from streams, lakes, and oceans, and hits the cooler upper air to make clouds.

Explanation

Inside the cloud, the droplets collide and merge together. Eventually they become too big and heavy to float in the air, so they must fall down to the earth. Larger rain drops form when water condenses onto ice crystals and evaporates from droplets at the same time, causing it to grow quickly and fall. On the way down, it collects any other droplets and smaller crystals it finds. You can see how rain collects by looking at the experiment below.

What You'll Need:

• coffee can lid
• eye dropper
• pencil

Directions:

1. Fill the eye dropper with water.
2. Hold the plastic lid in your hand, bottom side up.
3. Squeeze as many drops of water as will fit on the lid.
4. Quickly turn the lid over.
5. Move the drops of water together by using the point of the pencil.

What's Happening?

The drops are coming together to form larger drops, and then falling, because the water molecules attract each other. Each molecule has a positive and a negative side, so the positive side of the molecule attracts the negative side of another. Just like in clouds, the tiny water droplets on the lid join to form heavier drops, which then fall.

Liquid precipitation can be divided into three basic types based on the size of the droplet. Below is a description of these types.

• Mist: This falls through fog and stratus clouds. The droplets are just large enough to be felt on the skin, and generally range in size from .002 to .02 inch in diameter. The water content of a fog or cloud that produces mist is quite low.
• Drizzle: Small drops fall from thicker patches of fog and stratus clouds. These drops, ranging in size from .008 to .02 inch in diameter, are small and numerous. It can fall steadily for several days, gently saturates the ground with water.
• Rain: These drops have a minimum diameter of .02 inch, and some are even greater than .3 inch. Rainfall is heavier and more concentrated than drizzle, usually amounting to .2 inch per hour. A heavy rain may produce more water per hour. Rain usually comes from nimbus or cumulus clouds.

Besides just normal rain, there are also other forms.

• Freezing Rain, or glaze: Rain clouds are warmer than the ground below them, so the falling precipitation freezes when it strikes solid objects to create a clear coating of ice everywhere the rain falls. Usually a rush of warm air melts the ice before it damages trees and plants, but it can make road surfaces dangerous. When a lot of freezing rain falls, it is called an ice storm. This year, Canada had an ice storm because of El Niņo.
• Sleet: This begins as liquid rain, and freezes as it falls. Often, it mixes with snowflakes that have melted and refrozen. Sleet hits hard, pelting windowpanes and sometimes damaging plants, but it is fortunately only the size of a drizzle.

Water is always circulating between the sea and the sky in a cycle called the water cycle. Water we use usually ends up in rivers, lakes, or the sea, where it evaporates in the heat of the sun to fill the lower layers of the atmosphere with water vapor. Some of this vapor is carried by the rising air currents until it cools and condenses into clouds of water droplets and ice. Then, once the drops grow big enough and the clouds grow cold enough, the water falls to the Earth's surface as rain and snow. Some of this runs back to the sea, and some is trapped in reservoirs and supply taps to let the cycle continue.