Dust storms on Mars cover large areas and can last more than a month.
The low density of the Martian atmosphere precludes the spectacular effects described in the book and film 'The Martian'. However dust storms on Mars are significant events that must be taken into account for planing a functional settlement.
Local and global dust storms happen fairly frequently on Mars. Planet wide encircling dust storms have a frequency of about once every three years. Practically all large Martian dust storms happen during the latter, warmer part of the Martian year. Some sort of dust storm is expected about the same time each year during the southern hemisphere spring and summer. Most storms initiate from the southern part of the planet, mainly from the Hellas basin and the Acidalia/Chryse regions. Large storms can last about a month, but have also lasted up to six.
The opacity of the Martian atmosphere during a dust storm is an important parameter to evaluate the degradation of performance of solar powered equipment on Mars. The duration of dust storms also affect the required energy storage and the energy management strategies of the settlement. Even during the most severe dust storms, illumination does not drop more than 60% on average. However, there are peaks where illumination drops to 1% that can last at these levels for up to three months (2001 storm, 2018 storm). This means that a settlement operating uniquely on photoelectric Solar energy would be able to maintain itself, but require some form of energy storage. It would need to shut down some of its more energy intensive operations during the storm, and perhaps go into a type of hibernation for the peak periods, as illustrated on Earth by seasonal agriculture in northern countries. As the storms usually last about one month out of 36 month, they should not affect a settlement too seriously most of the time. Planning for severe planet wide storms is required, however, including month long periods every decade or so.
The opacity measure used is Tau, where a tau of 4.7 means an opacity of about 99%. A visualization of the Tau value of the 2018 dust storm is available.
The dust carried by the dust storm is very fine, about 3um per grain. It is essentially composed from the same material as the surface, therefore it is mostly iron oxide and silicates. The dust is very dry and does not agglomerate.