depression or cyclone is a region of the atmosphere in which the pressures are lower than those of the surrounding region at the same level

Equinoxes occur twice a year, when day and night are of equal length, and

solstices occur twice a year, marking the longest and shortest days of the year.

Equinoxes occur around March 20 (vernal equinox) and September 23 (autumnal equinox).

Solstices occur around June 21 (summer solstice) and December 22 (winter solstice).