1. Cloud Types and Precipitation
Chapter 18.3
Cloud Types and Precipitation

2. Types of Clouds
Clouds are classified based on their form and height. There are 3 basic forms – cirrus, cumulus, and stratus.
Cirrus – high, white, and thin. They have a feathery appearance.
Cumulus – rounded individual cloud masses. They have a flat base and rising domes.
Stratus – sheets or layers that cover much or all of the sky. There are no individual clouds.

3. There are also 3 heights: high, middle, and low.
High clouds are above 6000 meters.
Middle clouds will be between and 6000 meters.
Low clouds are below 2000 meters.

4. High Clouds
There are 3 types of high clouds: cirrus, cirrostratus, and cirrocumulus.
Cirrocumulus are fluffy masses.
Cirrostratus clouds are flat layers.
All high clouds are made of ice crystal and will be thin and white. There is a low amount of water vapor in these clouds. They will usually not produce precipitation. They can warn of stormy weather.

5. Middle Clouds These clouds have the prefix alto as part of the name.
Altocumulus clouds – made of rounded masses that are large and dense.
Altostratus clouds make a white to grayish sheet covering the sky, the sun or moon will appear as a bright spot.
There may be a little snow or light drizzle with these clouds.

6. Low Clouds
Stratus clouds are uniform, fog-like clouds that cover much of the sky. They may produce light precipitation.
Stratocumulus clouds will be stratus clouds with a scalloped bottom.
Nimbostratus clouds are the main precipitation makers.

7. Clouds of Vertical Development
There may be clouds that don’t fit in the other categories. They can cover all of the height levels.

8. Fog
There is no physical difference between a cloud and fog. The difference is the method and place of formation. Fog is the result of radiation cooling of the movement of air over a cold surface.
Fog can also form when enough water vapor is added to the air to bring saturation.
Fog is defined as a cloud with its base at or very near the ground.

9. How Precipitation Forms
For precipitation to form, cloud droplets must grown in volume about 1 million times.
Bergeron process – needs supercooled water – this is water that is still liquid below 0˚C. Supercooled water easily freezes if it lands on a solid object. Freezing nuclei are materials that have a crystal form that matches ice. It can cause supercooled water to freeze.
Supersatured air is air that is saturated with respect to water.
These 2 processes can cause snowflakes to form. These snowflakes can melt before they reach the ground.

10. Forms of Precipitation
The type of precipitation that reaches the Earth’s surface depends on temperature in the lowest few kilometers of the atmosphere.
Rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm.
At very low temperatures light, fluffy snow crystals form.
Sleet is the fall of small particles of clear- to-translucent ice.
Glaze – freezing rain, happens when raindrops are supercooled.
Hail is made by cumulonimbus clouds.