Nimbus means rain-bearing, and alto means high. The following are some of the more common clouds used to predict weather in three categories – high-level, mid-level and low-level clouds.
The bases of these clouds form at about 6200 metres above sea level. They are usually composed of ice crystals.
Cirrus clouds – thin, wispy clouds strewn across the sky in high winds. A few cirrus clouds may indicate fair weather, but increasing cover indicates a change of weather (an approaching warm front) will occur within 24 hours. These are the most abundant of all high-level clouds.