13 Cirrus clouds Cirrus clouds usually form above 8,000 m.
They are wispy clouds. Because there is very little water vapour at this height, big thick clouds cannot form. Cirrus clouds are created when water vapour forms ice crystals, which are very thin because of the height at which they form.
: Figure 13.21 Cirrus clouds
(i) Name two types of clouds that form. (ii) Which clouds bring the most precipitation? (iii) Draw a suitable graph to show the difference in the average height of each cloud type.
/ I understand how clouds form and I can identify the various types of clouds.
Precipitation Learning Intentions
In this section, you will learn: zz how rainfall occurs zz about the three basic types of rainfall.
Precipitation Convectional Cyclonic
Frontal Relief
Rain shadow
Precipitation is any form of water – liquid or solid – that falls from the sky. It includes rain, sleet, snow, hail and drizzle.
How rain is formed
Rain is the most common type of precipitation. Rain occurs because air has been forced to rise. As air rises it cools and loses its ability to hold water. Eventually it reaches a point where it is 100 per cent saturated, which means it cannot hold any more water. This is called the dew point and it is above this point that condensation occurs.
Condensation is the process by which the water vapour held in the air is turned back into water droplets that fall as rain.
Types of rain There are three basic types of rain: convectional rain, cyclonic (or frontal) rain and relief rain.
