Tornadoes are violent, rotating columns of air that descend with a familiar funnel (or tube) shape from a cumulonimbus cloud. If the cloud does not reach the surface, it is called a funnel cloud; if it touches water, it is called a water spout. A wide.

Within the tornados funnel-shaped cloud, winds can attain speeds up to 300 miles per hour, while the forward speed of the tornado averages only 40 knots.

Tornadoes occur with isolated thunderstorms at times, but much more frequently they form with thunderstorms associated with fast-moving cold fronts or squall-lines.

Families of tornadoes or tornadic vortices have been observed as appendages of the main cloud, extending 20 miles outward from the area of lightning and precipitation. They may last from a few minutes up to 6 hours. These vortices usually occur on the southern or southwestern flank of the storm. Tornadic vortices can be masked by innocent-looking cumulus clouds trailing the thunderstorm and may or may not be visible to alert the unwary crew member. The invisible vortices may only be evidenced by swirls in the cloud-base or by dust-whirls boiling along the ground, but may be strong enough to cause severe structural damage to the aircraft. Phenomena frequently encountered by naval personnel are waterspouts. Waterspouts fall into two classes: tornadoes over water and fair-weather waterspouts.

The fair-weather waterspout is comparable to a dust devil; it may rotate in either direction, whereas the other type of waterspout rotates cyclonically. In general, waterspouts are not as strong as tornadoes, in spite of the large moisture source and the reduced friction. The water surface under a waterspout is either raised or lowered, depending on whether it is effected more by the atmospheric pressure reduction or by the wind force. There is less inflow and upflow of air in a waterspout than in a tornado. The waterspout does not lift any significant amount of water from the surface. Ships passing through waterspouts have mostly encountered fresh water.

The formation of ice on lift-producing airfoils (wings, propellers, helo rotors, and control surfaces) disrupts the smooth flow of air over these surfaces. The result is decreased lift, increased drag, and increased stall speed of fixed-wing aircraft.

Most aircraft that are normally loaded can fly with icing conditions ongoing and, under normal circumstantes, the danger is not too great. When aircraft are critically loaded, however, icing is extremely important. The formation of ice on some structural parts of an aircraft can cause vibration and place added stress on those parts. For example, vibration caused by a small amount of ice unevenly distributed on a delicately balanced rotor or propeller can create dangerous stress on the system, transmission, and engine mounts.

Learning Objective: Identify the factors necessary for structural ice formation, the various forms of structural ice, the factors that influence the rate of ice accumulation on aircraft, and those regions of the atmosphere where icing is likely to occur.

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