A visible collection of particles of ice and water held in suspension above the ground. Clouds form when air becomes saturated and water vapour condenses around nuclei of dust, smoke particles, and salt. Four main categories of clouds are recognized: nimbus clouds, which produce rain; stratus clouds, which resemble layers; cumulus clouds, which resemble heaps; and cirrus clouds, which resemble strands or filaments of hair. These names are further modified by an indication of cloud height: strato - low level clouds; alto - middle level clouds; cirro - high level clouds. Fog can be considered as cloud close to ground level.