Figure 2-15 Altocumulus Clouds

CHAPTER TWO

AVIATION WEATHER

Middle Clouds

In this category, cloud bases form between 6500 and 20,000 feet AGL. The names of the middle

clouds will contain the prefix alto- (Figure 2-15). They are composed of ice crystals, water

droplets, or a mixture of the two.

A special cloud, altocumulus, produces continuous rain, snow, or ice pellets. The cloud base will

extend down to about 1000 feet AGL and fog is often present. Expect poor visibility and low

ceilings with very slow clearing.

Figure 2-15 Altocumulus Clouds

Visibility in middle clouds varies depending on cloud density from 1/2 mile to a few feet.

Turbulence may be encountered in middle clouds. Frequently these clouds are dark and

turbulent enough to make formation flying difficult. Icing is common due to the presence of

super-cooled water droplets. Rain, rain and snow mixed, or snow can be encountered in thick

middle clouds.

Virga is rain or snow that evaporates before reaching the ground and may be encountered below

these clouds.

High Clouds

In this category, cloud bases average 20,000 to 40,000 feet AGL. The names of the high clouds

will contain the prefix cirro- or the word cirrus (Figure 2-16).

High clouds have little effect on flying except for moderate turbulence and limited visibility

associated with dense jet stream cirrus. Since high clouds are composed mostly of ice crystals,

2-18 Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability