The onset of a drought can be gradual and subtle. It can take weeks, months, or even years for the full effects of long-term inadequate rainfall to become apparent. The end of a drought can also be difficult to determine. While a single rainstorm will provide short-term relief from a drought, it might take weeks or months before levels of precipitation return to normal. The start and end of a drought are often only clear in hindsight. Earthquakes start when tectonic plates shift and stop when they finally settle down. Foreshocks and aftershocks may sometimes be experienced, though. A landslide, which is the movement of a mass of rock, debris, or earth down a slope due to gravity, can be triggered by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors. It ends when the unstable mass of earth comes to settle down at the end of the slope. Hurricanes are large, swirling storms with winds of 119 kilometres per hour (74 mph) or higher. They start out as a tropical disturbance where rain clouds build over warm ocean waters; then, grow into a tropical depression and gather speed. They die out gradually after they drift from the warm water surface towards land.