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A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than 400 millimetres (16 in). A common definition distinguishes between true deserts, which receive less than 250 millimetres (10 in) of average annual precipitation, and semideserts or steppes, which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in).

Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.

A cactus is a member of the plant family Cactaceae, within the order Caryophyllales. The plural of cactus varies; the Latin cacti, the English cactuses and the uninflected plural cactus are all used. Cacti have a variety of uses: some species are used as ornamental plants, others are grown for fodder or forage, others for food (particularly their fruit). Cochineal is the product of an insect that lives on some cacti.

Most cacti live in habitats which are subject to at least some degree of drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines reduce air flow close to the cactus and provide some shade, both of which help to prevent water loss. Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch; areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multi-petaled.

In the absence of leaves, enlarged stems carry out photosynthesis. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Cactus stems also store water, often being ribbed or fluted which allows them to expand and contract easily. Cacti come in a wide range of shapes and sizes. The tallest free-standing cactus is Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity. The smaller cacti usually have globe-shaped stems, combining the highest possible volume with the lowest possible surface area. Many cacti have a short growing season and a long dormancy and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb up to 3,000 liters (790 US gal; 660 imp gal) of water in 10 days.^{[citation needed]}

Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.

The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon in the west. The ray path of light from the setting Sun is highly distorted near the horizon because of atmospheric refraction, making the sunset appear to occur when the Sun’s disk is already about one diameter below the horizon. Sunset is distinct from dusk, which is the moment at which darkness falls, which occurs when the Sun is approximately eighteen degrees below the horizon. The period between sunset and dusk is called twilight.

Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise at least one day of the year, when the polar day or the polar night persist continuously for 24 hours.

Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the Sun and the surrounding sky.