Before the invention of weather satellites, meteorologists had no more than a few days warning before the arrival of a hurricane. They had to depend on changes in air pressure or ocean waves, or reports from ships to tell if a storm was coming. Today, however, meteorologists watch the breeding grounds of hurricanes closely through satellites. They monitor the growth of storms, and examine their direction of movement. This allows them to warn ships at sea, and give advance warning to coastal areas that the hurricane might hit. When hurricanes get close to North America, they are often visited by the "hurricane hunters." These are Air Force planes whose crews bravely fly through hurricanes to gather detailed information about their characteristics. The planes carry advanced radars, and also drop parachuted weather probes.