by Jonathan M. Stern
Although great strides have been made in the development of aircraft structures to withstand encounters with certain hazardous types of weather, and in detection equipment to avoid these hazards, weather still poses a great threat to airplanes of all sizes.
Except to a very limited extent (inducing rain by seeding clouds with silver iodide), we have not figured out how to control weather. Pilots, therefore, are stuck with whatever the weather happens to be at a given point in time and space. Pilots must be able to predict the weather conditions they are likely to meet on a particular flight so that they may avoid placing their aircraft in adverse conditions. Pilots must also be able to recognize when the weather is worse than expected and change route of flight, altitude, or destination to avoid an encounter with hazardous weather conditions.
Thunderstorms can contain the most hazardous aviation weather, including lightning, heavy rain or snow, extreme turbulence, severe icing, strong updrafts and downdrafts, and hail. They may also be associated with wind shear and tornadoes. The Airman's Information Manual, an FAA publication, advises pilots not to fly within 20 miles of a severe thunderstorm or underneath any thunderstorm. Some instrument flying authorities recommend avoiding all thunderstorms by at least twenty miles. Therefore, as a general rule, you should not attempt to pass between two thunderstorms unless they are separated by at least 40 miles.