Hurricanes are powerful weather events that draw energy from warm tropical waters. In the Atlantic ocean these are called hurricanes, but if the same storm were to begin in the Pacific ocean they would be called Cyclones. They often begin as tropical waves, which are low-pressure areas moving through moist tropical regions. To form a hurricane, certain conditions must be met: a pre-existing weather disturbance (usually a tropical wave), warm water (26.5°C or 80°F), thunderstorm activity to provide fuel, and low wind shear (winds that converge in different directions). Even with most of these elements present, a hurricane may not always develop. If the temperatures are too low, or there is not an existing tropical wave, or even if the winds are not converging just so; there won't be a hurricane. As hurricanes move westward, warm ocean air rises, creating low pressure underneath and causing more air to rush in. When wind speeds reach 74 mph, the storm is classified as a hurricane.