**2. Study area**

The study area is made up of the Sudan and Sahel Savanna zones in Nigeria which covers about one-third of its entire land area. It occupies from latitude 10°N to latitude 14°N of the Equator and from longitude 4°E to longitude 14°E of the Prime Meridian (**Figure 1**). The region lies south of the Sahara Desert, extending from the Sokoto plains on the west through the northern parts of the plains of Hausaland to the Chad Basin on the east [32].

The climate of the study area is the tropical wet/dry type, also referred to as Aw. The annual mean rainfall for the zone varies from less than 500 mm in the extreme northeastern part to approximately 1323 mm in the southern part [33, 34]. The wet

**Figure 1.** *The Sudano-Sahelian Zone of Nigeria (Adapted from [31]).*

*Meteorological Drought and Temperature in Sudano-Sahelian Region of Nigeria… DOI: http://dx.doi.org/10.5772/intechopen.100108*

season occurs from April to October with the highest rainfall amount in August, while the dry season is from November to April [35, 36]. The intensity of rainfall is high in July and August, from about 60 mm hour<sup>1</sup> to 99 mm hour<sup>1</sup> [37]. The rainfall pattern is variable over space and time with inter-annual variability of between 15% and 20% [37, 38].

The climate of the study area is influenced by three air-masses, namely: the tropical maritime (mT); the tropical continental (cT), and the equatorial easterly. The mT and cT meet along a sloping surface known as the Intertropical Discontinuity (ITD). The equatorial easterly is an unstable cool air mass from the east in the upper troposphere along the ITD [32, 34]. The equatorial easterly sometimes undercut the mT or cT air masses to result in line squalls or dust devils respectively [39, 40]. The location of the ITD depends on the season in association with shortperiod fluctuations. Overall, it is located to the north of the Sudan-Sahel region in July and August, thereby making the area to be completely under the effects of the mT air mass. The reverse is the case from October to May when the ITD is located south of the region causing the cT air mass to prevail over the region [32].

The movement of the ITD is not regular, it varies according to the season. Its retreat towards the south is rapid than its northward progress. Its northward movement is at about 160 km per month while that of the southward retreat is at about 320 km per month [41]. This explains the gentle onset of the rainy season in the study area and northern Nigeria as a whole and its sudden end [42]. The mean highest air temperature occurs during the hot season from March to May while the mean lowest air temperature occurs during the cold season from December to February.

Temporal occurrence of temperature is steadier than that of rainfall, however, the highest diurnal range of temperature is in the dry season. The highest air temperature occurs either in April or May and the lowest between December and February [43]. The mean atmospheric relative humidity is between 70% and 90% during the rainy season and between 25–30% during the dry season [44]. Evapotranspiration is high year-round, the highest amount occurring during the dry season [45]. Sunshine hours depend on latitude and season. The mean annual number of sunshine hours in the study area is high in contrast to the southern part of the country, with very high values in the extreme northeast part. For example, Nguru has a mean annual total of 3177 hours [46]. Overall, values are high during the dry season in the area as a result of low cloudiness compared to the rainy season [47].

The entire study area is covered by Savanna vegetation and classified into Sudan and Sahel with the density of trees and other plants decreasing towards the north. The Sudan and Sahel zones are occasionally together referred to as the Sudano-Sahelian Ecologically Zone (SSEZ).
