Height of base: 1,000 to 5,000 ft Colour: White upper parts with dark, threatening undersides. Shape: A cumulus-type cloud that goes up very high. When the top of a cumulus reaches great heights, the water droplets are transformed into ice crystals and it loses its clear, sharp outline. At this stage the cloud has become a cumulonimbus. Often, the cloud top spreads out into a distinctive wedge or anvil shape. Other features: Accompanied by heavy showers, perhaps with hail and thunder. By convention Cb is usually reported if hail or thunder occur, even if the observer does not immediately recognise the cloud as Cb; (it may be hidden within layers of other cloud types).