just common sense and applies to tethering as  well. For crossing powerlines downwind of the  launch site, a good rule of thumb is to allow at  least 100 feet of horizontal separation between  the lines and the balloon for each knot of wind  speed. For example, in a 1 0-knot surface wind,  the launch site should be at least 1,000 feet  upwind.  But  be  conservative  in  your  estimates-1500 feet would be even better. The  actual crossing should be made with several  hundred feet of vertical clearance. It is even  advisable to cross the line in an ascent. Pay  attention to how heavily loaded the balloon is  and quickly assess your balloon’s ability to  climb under the currect conditions. Overloaded  balloons tend to be sluggish and very slow  reacting. Don’t forget-an average balloon has  over two tons of inertia!  Do  not attempt landings upwind of  powerlines without employing the  above  distance minimums. Better yet, it just makes  good sense not to make landings upwind of  powerlines at all; instead, cross them at a safe  altitude and select a more appropriate landing  site. If winds are gusty, thermic or otherwise  unstable, be especially cautious. Under such  conditions, the direction and altitude of the  balloon can change abruptly and  safety  margins can dwindle in seconds.  In level flight, keep a constant vigil for  powerlines. They are not easy to see from the  air, especially when viewed against a  mixed(~"  background of earth and vegetation. If the wind\  dies and the balloon is suddenly becalmed over •--› powerlines, climb to a higher altitude to find a  wind that will move the balloon aside. If no  such lateral movement is discovered, carefully  use a drop line and an experienced ground crew  tci pull the balloon out of danger. Keep -in mind  that hemp and nylon, under certain conditions,  have been known to conduct over 500 volts.  Whenever ballooning  in  the vicinity of  powerlines, use every precaution possible.  Don’t take chances. Don’t tempt the destruc› tive potential (no  pun  intended) of such  innocent-looking wires. Things can happen very  quickly and a not-so-bad situation can rapidly  deteriorate into a gruesome ordeal. Balloonists  right now are pushing their luck. Flights are  getting sloppy and accidents are happening.  For your own safety, the safety of your trusting  passengers, and the reputation of the great  sport of ballooning, be more careful.  Raven Owners Newsletter  To avoid tragic weather-related experiences  in ballooning, it helps to have some knowledge  of summer thunderstorms and thermals. Never  attempt the destructive potential of either  phenomenon.  At 12,000 to 16,000 feet, small cumulus  (building) clouds of summer flourish and grow  rapidly to large towering (20,000 to 60,000 feet)  cumulonimbus thunderstorm clouds. Surpris› ingly, an-afternoon thermal or small orographic  disturbance can trigger the development of a  thunderstorm by initiating the :rise of warm  moist air. Most severe thunderstorm develop› nient occurs along or just ahead of summer  cold fronts. They materialize in a squall line  several hundred miles long at times. Invariably,  there is a continuous line of strong gusty  winds,  violent  updrafts and  downdrafts  sometimes in excess of several thousand feet  per minute. wind shear turbulence, thunder roll,  ’lightning strokes, hail, heavy rainfall, and even  flash floods. Some of the more ferocious  storms also spawn tornadoes. Obviously, these  are not conditions suitable for balloon flying.  It should be noted that there is still much  unknown about just how thunderstorms  operate. For instance, scientists still do not  fully understand what causes lightning and its  close association with rain. Some think light› ning •serves as a catalyst in the formation of  rain while others believe that the rapid descent  of charged rain or hail generates lightning.  Here in the U.S., winds gusting to 60 and 70  mph are oiien observed, usually moving from  west to east or from northwest to southeast.  Radar studies show the thunderstorm life  cycle, from start of growth through maturity to  dissipation, to be very short and somewhat  complex. It is difficult to forecast and com- 6  Summer Safety in the Storm and Thermal Season 