Today I learn that A tropical depression is designated when the first appearance of a lowered pressure and organized circulation in the center of the thunderstorm complex occurs. A surface pressure chart will reveal at least one closed isobar to reflect this lowering.When viewed from a satellite, tropical depressions appear to have little organization. However, the slightest amount of rotation can usually be perceived when looking at a series of satellite images. Instead of a round appearance similar to hurricanes, tropical depressions look like individual thunderstorms that are grouped together. One such tropical depression is shown here.The rotation of a tropical storm is more recognizable than for a tropical depression. Tropical storms can cause a lot of problems even without becoming a hurricane. However, most of the problems a tropical storm cause stem from heavy rainfall.Hurricanes are Earth's strongest tropical cyclones. A distinctive feature seen on many hurricanes and are unique to them is the dark spot found in the middle of the hurricane. This is called the eye. Surrounding the eye is the region of most intense winds and rainfall called the eye wall. Large bands of clouds and precipitation spiral from the eye wall and are thusly called spiral rain bands.