Cumulonimbus clouds – thunder clouds that have built up from cumulus clouds. Their bases are often quite dark. These clouds can forecast some of the most extreme weather, including heavy rain, hail, snow, thunderstorms, tornadoes and hurricanes.
Stratus – dull greyish clouds that stretch across and block the sky. They look like fog in the sky. Stratus cover is also called overcast. If their bases reach the ground, they become fog. They can produce drizzle or fine snow.
Stratocumulus – low, puffy and grey, forming rows in the sky. They indicate dry weather if the temperature differences between night and day are slight. Precipitation is rare, but they can turn into nimbostratus clouds.
Nimbostratus – dark grey, wet-looking cloudy layer so thick that it completely blocks out the Sun. They often produce precipitation in the form of rain and/or snow. Precipitation can be long lasting.
Cloud roads (or streets) are clouds that follow the wind. Grandmaster navigator Mau Piailug called them “the road to the wind”.
Wayfinder navigators use clouds to work out where the wind is coming from or if it changes direction (so they can trim their sails accordingly). For example, they might look for cloud roads – puffs of cloud that come up from the far end of the horizon to form a ‘road’ in the sky. Like smoke from a haystack, cloud roads follow the wind. A cloud road indicates the wind is coming from the horizon. If the road is straight, the wind is steady – but if you see the road curve, it means that the wind direction will change. The way the road curves will tell you the new direction. Meteorologists call this kind of phenomenon ‘cloud streets’.