Before the advent of Mars orbiting spacecraft, astronomers had a sketchy view of the Red Planet's ever-changing weather. Though Mars is one of the nearest planets to Earth, the distance between it and Earth varies greatly as the two planets circle the Sun like a fast and slow racetrack cars where Earth "laps" Mars approximately every two years. Dust storms are most likely to erupt when it is spring and early summer in the Martian southern hemisphere, a time when Mars is closest in its orbit to the Sun. (Unlike Earth, Mars has a very elliptical orbit so its distance form the Sun varies more widely and this is a bigger factor in climate variability.) Local dust storms, regional obscurations and discrete "yellow" clouds on Mars have been reported throughout the last century; however, it is the planet-encircling and global dust storms that have captured our attention in the age of spacecraft exploration of Mars.
The emerging discovery of Mars great dust storms:
1796 - "yellow clouds" on Mars - as opposed to fleecy whitish water-ice clouds —first noted by astronomer H. Flaugergues.
1920 - Patchy yellow clouds observed and photographed at Lowell Observatory, AZ.
1956 - Observations of the first suspected globe-encircling dust storm on Mars
1965 - The first flyby of Mars (Mariner 4) sends back dust-free photographs of a narrow swath of the planet.
1969 - Mariners 6 & 7 fly by Mars to photograph clear and cool atmospheric conditions
1971 - The first Mars orbiting spacecraft—Mariner 9—arrives to find the planet already shrouded in dust - the first definitive planet-wide storm ever seen. Tall volcanic peaks poke above the dust, revealing themselves as high mountains rather than circular basins.
1973 - Ground-based telescopes detect another planet-encircling storm just one Mars year after the global storm viewed by Mariner 9.
1977 - Viking spacecraft watch two major Martian dust storms in succession from Mars orbit and, for the first time, from the surface of Mars.
1982 - Viking Lander 1 detects what appears to be another major dust storm, just weeks before the loss of communications ends its nearly 7 year observational record on the surface of another planet.
1994 - Ground-based microwave monitoring of Mars atmospheric temperatures indicate a planet-encircling storm in progress, the first such planet-wide storm detected since Viking.
1997 - Mars Global Surveyor begins aerobraking at Mars, and finds planet-wide warming events can be produced by even a regional dust storm.
2001 - A near-global, unusually early dust storm erupts in a few days, after Hubble Space Telescope makes a Mars opposition photograph showing seasonal dust activity in Hellas and at the North Pole. Unprecedented details of the blossoming storm are followed by the Mars Global Surveyor, which takes daily all-planet images and measures atmospheric temperatures.