The middle dry season begins during the first two weeks of January. It corresponds to the months of January and February. Local people in the region describe the middle dry season as the “harmattan period” or the period during which “the wind blows strongly” (Table 3.3). The monthly characteristics and the daily division are provided below (Appendices G & H).
- Monthly characteristics of the middle dry season
The middle dry season is the driest period of the year. Rainfall is rare and some times non-existent. The middle dry season coincides with the period of harmattan. The harmattan is a continental dry and hot air mass of the northern hemisphere blowing
B
A
southward and covering the whole of West Africa from December to March. It leads to hot and dry weather conditions during the day and cool and humid weather conditions during the night. The ambient air temperature between 6 pm and 9 am is in general low, the relative air humidity is high, and the wind speed is very low, around 0 m/s (Koné et al. forthcoming).
The months of January and February are classified as very dry months. Over the 31-year period (1976-2006), the monthly mean is 4.3 mm for January and 9 mm for February.
During the month of January, the daily air temperature ranges between 81 oF (27
o
C) and 100 oF (38 oC) with a mean of 94 oF (34 oC). During the month of February, the air temperature varies between a low of 93 oF (34 oC) and a high of 102 oF (39 oC) with a daily mean of 98 oF (37 oC).
The lack of rain and the high heat negatively affect air humidity, which decreases over the course of the middle dry season. The daily air humidity ranges between a high of 72% in the beginning of January and a low of 41% in late January. In February humidity levels oscillate between low and high values. They increase from the beginning of February (51%) to mid-February (80%). The general increasing trend of the daily humidity is attributed to a few rains that fall in the region in February and to the beginning of the vegetative cycle and heightened evapotranspiration of plants.
Wind speed during the middle dry season is very high. It tends to increase from early to late January. The wind direction is also constantly changing. The wind speed oscillates during the middle dry season between the low value of 4 m/s and the high value 8.5 m/s. The daily wind speed reaches the value of 10 m/s in the third week of February before decreasing in late February.
The middle dry season burning is mainly linked to the activities of farmers, herders, and hunters. These actors set fire to clear the landscape for visibility purposes and to drive game (Fig 3.8). Areas burned during the early dry season are greater than those burned in the middle dry season (Fig. 3.6 and 3.9). In the middle dry season 2006/07, a total area of 10,487 ha burned (Table 3.6). The majority of fallow field (565 ha) burned during the early dry season while an area of 275 ha burned in the middle dry season. In grass savanna, only 229 ha burned in the middle dry season. The total area for
shrub savanna burned in the 2006/07 middle dry season is estimated at 1,827 ha. Savanna woodland burned is estimated at 1,541 ha. A total area of 4,684 ha for dry forest burned. Gallery forest increasingly burned in the middle dry season (1,931 ha).
Fig. 3.9. Biomass burning in the middle dry season on 1/8/2008
Table 3.6. Areas burned (ha) by vegetation type in the middle dry season
Land cover classes Hectare (ha) Percent (%)
Fallow field 275 3 Grass savanna 229 2 Shrub savanna 1,827 17 Savanna woodland 1,541 15 Dry forest 4,684 45 Gallery forest 1,932 18 Total 10,487 100
Fig. 3.10. Areas burned in the middle dry season between January 11, 2007 and February 27, 2007
- Daily division of the middle dry season
Hourly climate information illustrates the desiccating effects of biophysical factors on the sudanian savanna vegetation. During the period of January 1-14, the air humidity fluctuates between 41% and 71% in the morning (00:00 am-9:00 am), between 15% and 30% in the middle of the day (9:00 am-6:00 pm), and between 30% and 51% after 6:00 pm. The last two weeks of January is a very dry period. The morning humidity level decreases from 60% to 40% between mid-to late-January. It drops below 30% and even reaches 10% some days between 2:00 pm and 4:00 pm, and increases from 20% to 40% from 6:00 pm to 00:00 pm.
Hourly air temperatures during the first half of January are high. They range between 80 oF to 93 oF between 9:00 am and 6: 00pm. The second half of January is slightly cooler with hourly temperatures varying between 80 oF and 90 oF. It gets hotter in February. During the first half of the month, hourly temperatures range from 80 oF to 102
o
F in the middle of the day.
The middle dry season is characterized by high hourly wind speeds. The increase of hourly wind speed reaches its peak in February. The hourly wind speed ranges between 3 m/s and 6.7 m/s, some times reaching a high peak of 7.2 m/s between 2:00 pm and 3:00 pm. The wind that blows in the mornings and in the evenings has a low speed, in general below 3.1 m/s. However, the major characteristic is that the wind blows all the time and constantly changes its direction, favoring the spread of fire when ignited.
In summary, the middle dry season period is the driest and hottest period of the dry season. It is during this period that the biomass reaches its driest state. Fifty-eight household heads deemed that burning occurs in their zone of activity during the middle dry season (Table 3.4). The purpose of the use of fire determines the period of the day at which resources users ignite the bush. The middle dry season ends with the end of the effects of the harmattan. The first rains of the year fall in February, wind speeds decline and the relative air humidity increases.