Using red or orange color filters can help bring out the dark features. Wratten 25 or 23A filters are often recommended. A blue or violet filter can enhance the cloud features. WHAT TO LOOK FOR The most prominent dark markings to look for are Syrtis Major, Mare Acidalium, Utopia, Mare Erythraem and Sinus Sabeus. These markings vary from year to year and may even show changes during the next two months.
More transient features are the clouds that come and go.
They appear as white spots that often cover the same areas at the same time of the martian day. With larger instruments it is possible to see the clouds that often top the great martian volcanoes in Tharsis. Dust storms can also come and go. They appear as yellowish clouds that start out small, yet may possibly grow to encompass the entire planet. There is no way to predict when or where these dust storms will appear.