In continuation of our tropical classification series, we are going to explore tropical depressions. In case you missed this first part of our series on tropical cyclones, you can learn more about tropical invests by following the link below.
If an invest continues to develop and strengthen into a closed low-level circulation with sustained winds of 22 to 34 knots (25 to 38 mph), it becomes a tropical depression.
The name depression comes from the fact that the center of circulation contains low pressure or “depressed” air.
On satellite, the developing storm still looks disorganized, but examining a satellite loop will reveal a developing counter-clockwise rotation. Tropical depressions on satellites look like a cluster of individual thunderstorms.
What is in a Number?
Tropical depressions are numbered according to the total number of systems that have reached tropical depression status in that tropical cyclone season. For example, let’s say there have been 5 storms in a season that have strengthened to tropical depression status or greater. If a new tropical depression formed, it would become Tropical Depression 6. If the depression continued to strengthen to tropical storm status, it would then receive the 6th name on the World Meteorological Organization’s seasonal list.
A storm that fails to reach tropical depression status, does not count toward the total number of storms for that hurricane season.
For a real-world example let’s take an example from the 2021 Atlantic Hurricane Season. We had 15 storms so far that had reached tropical depression status or greater in the 2021 season. When the new depression formed, it became the 16th storm of the 2021 Atlantic Hurricane Season.
Storms that reach tropical depression status can either weaken to a remnant low or continue to strengthen to tropical storm status or greater.