The photo above features an amazing roll cloud over Calgary, Alberta as seen on the morning of June 18, 2013. Roll clouds are a type of arcus cloud often associated with turbulent weather. As is the case here, they sometimes look like a horizontal tornado. Although these cylindrically shaped clouds look quite fierce and may be observed to roll about their horizontal axis, they don’t usually generate dangerous winds. Roll clouds are typically found behind outflow boundaries but unlike shelf clouds are detached from any close-by cumulonimbus cloud.