Which of the following is NOT a type of depression in meteorology?

In meteorology, the term "depression" refers to a region of low pressure, which typically influences weather patterns and can lead to various forms of precipitation and storms. Among the types of depressions commonly recognized, "frontal" and "thermal" are well-defined types. Frontal depressions occur at the boundary between two air masses with different temperatures, often leading to severe weather. Thermal depressions are formed due to the heating of air over a surface, such as during the daytime heating of land, which can create localized low-pressure areas.

While "polar" and "tropical" refer to specific regions or climates, "tropical storm" is not classified as a type of depression in meteorological terms. A tropical storm is a more intense weather phenomenon that develops from a tropical depression when wind speeds reach a certain threshold. Therefore, it does not fit the definition of a depression, which is generally a less intense system related to low-pressure areas rather than a specific category of storm. This distinction helps clarify why "polar" is not recognized as a type of depression in meteorology, as it signifies geographic regions rather than a meteorological phenomenon.