Why is communication possible between two continents at a frequency above the local critical frequency?

The critical frequency is the highest frequency that will be reflected back to Earth when a signal is transmitted straight up (vertically) into the ionosphere. However, most practical long-distance communication involves transmitting at a low, horizontal angle toward the horizon. When a radio wave enters the ionosphere at an oblique or inclined angle, it requires much less bending to return to Earth. This is similar to how a stone skips across the surface of a pond if thrown at a low angle. Because of this geometry, the ionosphere can refract much higher frequencies back to Earth, allowing communication well above the vertical critical frequency.