The Thunder Cloud
Cumulonimbus or CB
The CB Cloud is normally associated with the Cold Front However on hot days an unstable atmosphere can trigger one of these clouds off on its own. The highest risk time is normally between midday and 3 PM.. These clouds represent every bad weather thing you can think of. The clouds can rise to well above the cruise heights of most aircraft depending how close they are to the Equator. Modern airliners can pick these clouds up on weather radar to a range of 300 miles. Pilots will try and get Air Traffic Control Clearance to pass one of these clouds on the upwind side by at least twenty miles. If this cannot be arranged the pilot will put the seat belt sign on and slow the aircraft and on some aircraft can adjust the autopilot to relax the the autopilot stiff resistance to movement. They will then have to just ride the bumps in the cloud.
Hot air from the engines is pushed down the leading edge of the wings to stop ice from forming. There have been many accidents as a result of these clouds around airports at the time of arrival. Often heavy showers of rain sleet or snow will fall. Hailstones also. Instances where aircraft have landed and then aquaplaned off the runway with water on the runway not giving wheels grip to slow down or steer following torrential downpour have occured.. On final approach the microburst has caused crashes by the sudden downdraught forcing an aircraft into the ground short of the runway. Or a tailwind making the landing distance longer following windshear and the aircraft has over ran the runway.
Below shows the wind flow from one of these clouds. The red lines show warm air being sucked into the cloud and rising. As a result of that cold air rushes down and out to replace it Imagine now an aircraft flying from left to right. The pilot will have a tailwind then a headwind then air which could force straight down at a terrific speed. (Know as a microburst) then a tailwind then a headwind again. All this is in a short time over a short distance with hail rain sleet slow and poor visibility.Sometimes this cloud can hide itself in with other clouds this is known as an embedded thunderstorm. It can be very difficult to see in this case. Sometimes a whole group of these clouds can form in an area.
The life of a CB cloud is about 40 minutes normally. Pilots should consider holding off at an airport if one of these clouds are in the vicinity of the airport. Lightning and hailstones can come from these clouds. The cloud starts its life as a towering cumulus convection cloud but may continue to rise into a cumulonimbus cloud there is also ice in the clouds.
Eventually the cold air forced out cools the surface air and the cloud collapses into Stratus clouds. The CB cloud may have some rolling cumulus clouds near it as well. Rain is normally the heaviest in the first 5 minutes. Structural failure to aircraft has also been know to happen when flying in these clouds.