IMAGE - Arcus Cloud, Indiana, USA (17 August 2012)

cwa1970. captured this excellent shot of wall cloud at Crawfordsville, Indiana, USA, today (Friday, 17 August 2012).

A distinct shelf or roll of low cloud that can appear below a powerful cumulonimbus cloud, an arcus ( from the Latin for 'arch' ) is formed by strong downdraughts of cold air, which spread out ahead of the oncoming stormcloud , pushing up layers of warm air nearer the ground. These form dense , horizontal rolls of cloud , some of which ( as the photo shows ) can look quite eerie and menacing .