On November 19, 1996, at 1750 hours Pacific standard time, a flight attendant sustained a serious foot injury after encountering some clear air turbulence (CAT) near Bishop, California, during cruise at flight level (FL) 290 on United Flight 2188 from San Francisco, California, to Las Vegas, Nevada. The aircraft, a Boeing 737-500, N916UA, was not damaged, nor were there any injuries to the other 112 persons onboard. According to the operator, the captain illuminated the fasten seat belt sign and instructed the flight attendants to take their seats. The injured aft flight attendant was in the process of securing a carrier door in the aft galley when the turbulence was encountered. After arriving in Las Vegas, she was taken to a local hospital where it was determined that her right foot was broken. Another flight attendant stated that shortly after the captain made the announcement, the aircraft dropped sharply. He was holding on to something and didn't get hurt. According to the operator, there were several reports of moderate or greater turbulence throughout central and southern California at flight levels under FL300. A "CAT ALERT" was in effect for the area at the time of the accident with some mention of mountain wave activity east of the Sierra Mountains. This alert was on the Weather Briefing Message (WBM) for Flight 2188. The captain was monitoring the flight conditions of other aircraft in the area and chose the smoother ride at FL290 due to turbulent rides at the higher altitudes. When the captain was advised of turbulent conditions ahead, he made the announcement to the cabin crew to take their seats. United Airlines procedures for crew action to take when turbulence weather conditions are anticipated are to turn on the Fasten Seat Belt sign and make a PA announcement. This permits the flight attendants to tie down serving carts and stow other loose items before taking their seats. If turbulence is encountered unexpectedly, the fasten seat belt sign will be cycled on and off three or four times and a PA announcement advising the passengers and flight attendants to be seated immediately.