The arid zone is characterized by an extreme heat and a scarce, variable rainfall. However, differences in weather conditions can be considerable. In most cases, these climatic dissimilarities are a consequence from a gap in temperatures, the season, in which the rain falls, and a degree of aridity.
The term "arid zone" stands for hyper-arid, arid and semi-arid zones, which diverge in a degree of aridity. Dry land areas without vegetation or only with few shrubs belong to the hyper-arid type. The rains here are sporadic and irregular; sometimes, there is no rain during long periods of several years. The local vegetation of arid zones is sparse and encompasses annual and perennial grasses and other herbaceous vegetation, shrubs and small trees. The annual precipitation normally does not exceed more then one hundred up to three hundred millimeters, depending on the type of the arid environment. The semi-arid zone has the native vegetation that is represented by a variety of species, such as grasses and grass-like plants, shrubs and trees. There is much more rainfall, especially during the summer.
Three major types of climate, which greatly affect weather conditions in arid zones, are the Mediterranean climate, the tropical climate and the continental climate. In the Mediterranean climate the rainy season starts in October and lasts up to the end of April or May. Throughout that time the temperatures are mild, but then the hot summer with actually no rainfalls comes. Conversely, in the tropical climate the rainfall can be witnessed only during the summer months, while winters are long and dry. The regularity of the rainy season length is strongly linked to the remoteness of the zone from the Equator. The greater the distance is, the shorter the rainy season is. A vivid example of typically tropical climate is Sennar, Sudan. The wet season here extends from the middle of June until the end of September, followed by a dry season of almost nine months. In the continental climate, the rainfall is distributed regularly throughout the year, though there is a tendency toward a greater summer precipitation.
The arid environment is characterized by three climatic patterns, replacing one another: a relatively "cool" dry season, followed by a relatively "hot" dry period and ultimately by a "moderate" rainy time of the year. It is estimated that temperature fluctuations within these seasons can be very considerable. At some stage in the "cool" dry season, daytime temperatures often record between thirty five and forty five centigrade, and plunge to ten up to fifteen centigrade at night. On the other hand, daytime temperatures can reach forty five centigrade during the "hot" dry season and drop to fifteen centigrade at nighttime. In the period of the wet season, temperatures can range from thirty five centigrade in the daytime to twenty centigrade at night.
In many situations, this diurnal temperature instability limits the potential growth of plant species, giving them a chance for augmentation only during mild temperatures. Extremely high or low peaks can be damaging to plants. Nevertheless, a problem of particularly low temperatures is not very common for the arid environment, which tends to be more familiar to an excessive heat, rather then cold.