Envoy Air flight 3788 encountered turbulence during initial descent into Chicago O’Hare International airport (ORD), Chicago, Illinois. The flight was a regularly scheduled passenger flight from Des Moines International airport (DSM), Des Moines, Iowa, to ORD. As a result of the turbulence, one flight attendant (FA) sustained serious injuries. The airplane was not damaged, and the flight continued to ORD without further incident. The cabin crew reported that before the flight, the captain briefed that there would be “nothing serious”, but they could expect some “bumps along the way”. The captain reiterated the same message twice during the flight. The first officer recalled that the cabin crew had been advised to use caution and left the decision to provide service during the flight at their discretion. FA2, (who was injured) had reviewed the weather conditions before the flight, and after the flight had experienced “some bumps”, ultimately decided not to perform a service and remained seated until the initial descent. The flight crew reported an uneventful take off and climb but decided to keep the fasten seatbelt sign on for the duration of the flight. Upon observing a “buildup of weather” on their route, the flight crew decided to climb to flight level (FL) 310 (31,000 ft above mean sea level) to stay on top of the developing weather. Upon reaching FL310, air traffic control (ATC) advised them to descend to 11,000 ft at pilot’s discretion. ATC further advised that other aircraft were making their way through the weather with “no complaints”. After initiating the descent, the captain advised the cabin crew to prepare for arrival and FA2 got up to recheck that the galley was secure. During the descent, the flight crew observed a “small cumulous cloud” (which was not towering) ahead. As the airplane descended into the cloud it encountered moderate turbulence at an altitude of 26,650 ft. The turbulence caused the airspeed to increase to an “overspeed” before the cabin crew used the touch control steering (TCS) to manually slow the aircraft down to the normal operating speed. The flight crew reported that the overspeed lasted no more than 2 seconds, at an airspeed of 321 knots. FA2 reported that the turbulence was “severe”, and it occurred before she could return to her seat. While attempting to sit, her left foot went out from under her and she “slammed down” to the floor, where she remained for the rest of the flight. Upon being notified of the injury, the flight crew advised company operations and requested paramedics at the gate upon arrival. FA2 was treated at the gate and then taken to the hospital via ambulance and was diagnosed with a fractured left ankle. A review of weather records revealed light to extreme precipitation in the accident area below about 25,000 ft. A vertical profile of the radar reflectivity indicated that the precipitation intensity decreased with altitude and was generally stratified and not characteristic of deep updrafts. Infrared satellite imagery indicated the coldest cloud tops near the accident location were about -25°C, which along with an atmospheric model created from weather balloon sounding data, indicates cloud tops at a height of about 26,000 ft msl. This data suggests a relatively weak convective environment up to about 25,000 ft msl. There were no active significant meteorological information (SIGMETS) for the accident area. The atmospheric model identified some vertical shearing of the horizontal wind (speed sheer) between 25,000 and 30,000 ft msl. Wind speed calculations varied from 41 knots at 25,000 ft, and increased with altitude to 90 knots at 30,000 ft. Given the weak convective environment and the likelihood of wind speed shear, the airplane likely encountered clear air turbulence. The Federal Aviation Administration’s advisory circular AC 00-30C defines clear air turbulence as “sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.” This term is commonly applied to higher altitude turbulence associated with wind shear. The most comprehensive definition is high-altitude turbulence encountered outside of convective clouds. This includes turbulence in cirrus clouds, within and in the vicinity of standing lenticular clouds and, in some cases, in clear air in the vicinity of thunderstorms. There were two pilot reports within a 40 nautical mile radius of the accident area. The first was about ½ hour after the accident and reported moderate chop at FL310, the second was 15 minutes later and reported consistent light chop and occasional light turbulence.