Desert terrain exists in warm, temperate, and cold climates, but all deserts share one common trait: little rain. The three categories of desert terrain are tundra (cold desert), rocky deserts (often temperate), and sandy deserts (often warm).
Tundra differs from the other desert categories in two important ways. Because snow and ice cover much of the landscape, it's easy to find water. During the height of summer, the permafrost thaws to a depth of a foot or so, turning the landscape into a vast field of mud. The muddy tundra affects movement and skill use as the shallow bogs described in Marsh Terrain, although there's little standing water.
The table below describes terrain elements found in each of the three desert categories. The terrain elements on this table are mutually exclusive; for instance, a square of tundra might contain either light undergrowth or an ice sheet, but not both.
Light Undergrowth: Consisting of scrubby, hardy bushes and cacti, light undergrowth functions as described for other terrain types.
Ice Sheet: The ground is covered with slippery ice. It costs 2 squares of movement to enter a square covered by an ice sheet, and the DC of Acrobatics checks there increases by 5. A DC 10 Acrobatics check is required to run or charge across an ice sheet.
Light Rubble: Small rocks are strewn across the ground, making nimble movement more difficult. The DC of Acrobatics checks increases by 2.
Dense Rubble: This terrain feature consists of more and larger stones. It costs 2 squares of movement to enter a square with dense rubble. The DC of Acrobatics checks increases by 5, and the DC of Stealth checks increases by 2.
Sand Dunes: Created by the action of wind on sand, dunes function as hills that move. If the wind is strong and consistent, a sand dune can move several hundred feet in a week's time. Sand dunes can cover hundreds of squares. They always have a gentle slope pointing in the direction of the prevailing wind and a steep slope on the leeward side.
Other Desert Terrain Features: Tundra is sometimes bordered by forests, and the occasional tree isn't out of place in the cold wastes. Rocky deserts have towers and mesas consisting of flat ground surrounded on all sides by cliffs and steep slopes (as described in Mountain Terrain). Sandy deserts sometimes have quicksand; this functions as described in Marsh Terrain, although desert quicksand is a waterless mixture of fine sand and dust. All desert terrain is crisscrossed with dry streambeds (treat as trenches 5 to 15 feet wide) that fill with water on the rare occasions when rain falls.
Stealth and Detection in the Desert: In general, the maximum distance in desert terrain at which a Perception check for detecting the nearby presence of others can succeed is 6d6 × 20 feet; beyond this distance, elevation changes and heat distortion in warm deserts makes sight-based Perception impossible. The presence of dunes in sandy deserts limits spotting distance to 6d6 × 10 feet. The scarcity of undergrowth or other elements that offer concealment or cover makes using Stealth more difficult.
Bad weather can range from minor precipitation to a serious storm. The weather can include lightning strikes and cause floods, landslides, and other natural hazards.
Whether from a lack of water, a plague, or hostile magic, the plants and wildlife in the area are suffering. A blight lasts for 1d4+2 weeks. During a blight, Survival DCs to get along in the wild increase by +5.
In a desert, PCs can attempt a DC 15 Perception check to notice potentially deadly or dangerous fumes before coming to harm. Otherwise, they must succeed at a DC 15 Fortitude save or take 1d4 points of Constitution damage and be nauseated for 10 minutes.
A dust devil is a whirlwind not associated with a storm, particularly in a region with little or no topsoil. Treat a dust devil as a duststorm, sandstorm, or tornado.
Powerful desert storms are known as khamaseen. These blasts of hot wind can last days, sweeping across the desert and carrying the dunes before them until the land is changed beyond recognition. Khamaseen are capable of swallowing whole towns, uncovering ancient ruins, and scouring skin from the bones of anyone foolish enough to be trapped in one. Though they resemble the sandstorms encountered in other deserts the violent khamaseen aren't simply mere meteorological phenomena—they're seasonal sandstorms made truly dangerous after being affected by spirited conflict between various elemental chieftains of wind and fire that dwell in deserts. Once set in motion, a khamaseen can be as unpredictable and as tempestuous as the warring elementals themselves.
A khamaseen storm reduces visibility to 1d6 × 5 feet and imposes a –8 penalty on Perception checks. The storm's blasting sands deal 1d6 points of nonlethal damage and 1d3 points of fire damage per hour of exposure. A khamaseen leaves behind 1d6 inches of dust and sand per hour it rages over a specific location. A single khamaseen can last anywhere from 1d4 hours to 1d3 days, and historians report a rare few lasting a week or even longer, generally resulting in a completely different landscape left in the wake of its shifting sands.
Mirages are naturally occurring optical illusions that result from light refraction and produce the appearance of false images on the horizon. Though they are not magical, mirages function as illusion (glamer) spells, generally blur and hallucinatory terrain. Unlike magical illusions, mirages cannot be dispelled, though some of them can be disbelieved. Mirages take two primary forms: the traditional mirage and the phenomenon known as heat haze.
Traditional mirages are long-distance phenomena in which hazy images appear on the distant horizon, often in the shapes of rock formations, flat pools of reflective water, or oases and city walls. These mirages pose the greatest threat to travelers suffering from dehydration (whose desperation may persuade their minds to believe in an otherwise obvious mirage), or to those using the desert's few landmarks as navigation aids. When navigating in the desert, the existence of a mirage imposes a –2 penalty on Survival checks to keep from getting lost.
Because mirages are created by light refraction and not magical manipulation of the environment, they're very difficult to distinguish from true structures or terrain features without the aid of magical divination or trial and error. Spells such as true seeing, which reveal things as they truly are, indicate a mirage's true nature but can't reveal what the mirage is obscuring because of the distance (generally several miles) at which the phenomena are observed.
Outside of magical means of detecting a mirage, a character must interact with the illusion to disbelieve it. A character who makes a successful DC 15 Survival check while observing a mirage can estimate the perceived distance to the image. When the character has traveled the estimated distance toward the mirage, he may attempt a DC 10 Will save to disbelieve the illusion.
Alternatively, a character casting reveal mirage may attempt to disbelieve a mirage or similar effect as though he had interacted with the illusion regardless of the distance.
Heat haze appears in areas where the air close to the surface of the desert is heated to a significantly higher temperature than the air above it. During the day's hottest points, when hot air rises and cooler air above it sinks, this causes the air to appear to shimmer. This type of mirage can be seen from as little as 30 feet away. In conditions of heat haze, creatures beyond 30 feet appear indistinct, as though affected by the blur spell, and gain concealment against ranged attacks. Effects and abilities that would allow a character to ignore this concealment, such as blindsight and true seeing, negate this effect. Heat haze has no effect on navigation and cannot be disbelieved.
A sandstorm reduces visibility to 1d10 × 5 feet and provides a –4 penalty on Perception checks. A sandstorm deals 1d3 points of nonlethal damage per hour to any creatures caught in the open, and leaves a thin coating of sand in its wake. Driving sand creeps in through all but the most secure seals and seams, chafing skin and contaminating carried gear.