Weather disturbances in the trade winds, known as easterly waves, are another source of cloud development and precipitation in the tropics. Easterly waves develop first as a weak disturbance in the atmosphere usually because of the presence of localized warmer sea temperatures. This is seen on a weather map as a wave in the isobars. On the eastern side of the wave, convergence occurs forming thunderstorms (divergence on the western side). If the convergence is strong enough the storm system may evolve into a hurricane.