WEATHER 177 



steadily forward with the area of low pressure. On 

 account of their great height they consist of minute ice 

 crystals. If the cloud spreads out in a thin sheet, it 

 is called a cirrostratus. When we look at the sun or 

 moon through such a cloud, we have the familiar " ring 

 around the moon " or " ring around the sun " phenom- 

 enon. Such a ring is called a halo and is formed by the 

 refraction of the light in passing through the ice crystals. 



FIG. 148. Cirrus Clouds. 



Stratus clouds are frequently seen near the horizon in 

 the late afternoon or early morning. They occur in long 

 narrow bands (Figure 149) or strata. Nimbus is the name 

 applied to the dark gray rain cloud. These clouds usually 

 occupy the whole central portion of a cyclonic storm and 

 often have an area of many thousand square miles. Their 

 height above the surface of the earth varies with condi- 

 tions, but they are often but a few hundred feet above 

 our heads. In winter they are especially large and dense 



