As you analyze the mountain weather before your flight, pay spe› cial attention to the position of the highs and lows to give clues to  the wind speed potential. For example, in the winter, a high often  sets up over the Four Corners area in southwest Colorado. This is  often coupled with a low pressure centered in eastern Colorado or  western Kansas. The result can be very strong westerly winds and•  dangerous turbulence in the high terrain for days at a time.  When the wind speed is above  about 25 knots and flowing perpen› dicular to the ridge lines, the air  flow can form waves, much like  water flowing over rocks in a  stream bed. The waves form down  wind from the ridge line and will  be composed of very strong up and down drafts, plus dangerous  rotor action under the crests of the waves. If enough moisture is  present, lenticular clouds can form to give a visual indication of the  wave action. These clouds are reported in the remarks section of  hourly sequence reports as ACSL (altocumulus standing lenticular)  or CCSL (cirrocumulus standing lenticular).  Just as the flow through a carburetor speeds up in the restriction of  the throat, wind flowing through the narrow restriction of a moun› tain pass will also speed up. When the winds are forecasted above  about 20 knots, be aware that the speed in passes may cause turbu› lence and drafts that should be avoided.  As the wind blows moist air upslope, it will cool and may form  clouds.  If,  as is often the case winter, the air is stable, the clouds  will stay close to the mountain, forming a "cap" cloud. However, if  the air is unstable, as is usually the case in the summer, this initial  lifting will be enough to start convection and result in thunder› storms forming.  Microbursts have received much space in the aviation and popular  press in the pasl several years because of their implication in many  serious airline accidents. Many light aircraft accidents have also  6  Microbursts  Orographic Lifting  Winds Through  Passes  Mountain Wave  High and Low Pressure Patterns 