full text of the classic FAA guide
A well-developed thunderstorm may extend upward through the troposphere and penetrate the lower stratosphere. Sometimes the main updraft in a thunderstorm may toss hail out the top or the upper portions of the storm. An aircraft may encounter hail in clear air at a considerable distance from the thunderstorm, especially under the anvil cloud. Turbulence may be encountered in clear air for a considerable distance both above and around a growing thunderstorm.
Thunderstorm avoidance rules given in chapter 11 apply equally at high altitude. When flying in the clear, visually avoid all thunderstorm tops. In a severe thunderstorm situation, avoid tops by at least 20 miles. When you are on instruments, weather avoidance radar assures you of avoiding thunderstorm hazards. If in an area of severe thunderstorms, avoid the most intense echoes by at least 20 miles. Most air carriers now use this distance as the minimum for thunderstorm avoidance.