When visiting North and West Africa, some weather conditions are not to be taken lightly. In the Sahara, it's of course stiflingly hot, but the main concern is the sandstorms (Haboob) and the dusty winds (like the Harmattan and the Sirocco). Further to Guinea, Ghana,... we are confronted with the rainy season.
Here's some information (we didn't invent it, it's all on the net).

1 : DESERT

Desert winds range from cool evening breezes to major dust and sand storms. Dust devils or whirlwinds often spin over the landscape, but they are harmless and fun compared to occasional storms so thick with sand and dust that you can scarcely breathe or see, and that strip paint off cars and invade the tightest windows and doors to deposit layers of grit everywhere.
Strong dry winds blow at least part of the year, drawing moisture from plants soil and skin. On a hot, gusty day, people perspire up to one quart of water an hour. In severe wind like a Saharan sandstorm, it is literally possible to dry up like a mummy. Imagine yourself in front of a giant hair dryer on its hottest, fastest setting, and you've got the picture.

Haboob

The name Haboob is rooted from the Arabic word "phenomena". This form of monsoon occurs from May until September. It's a dust storm moving at a maximum speed of 80 kph, the average speed being 50 kph. The storm can reach up to heights of 1 km. They last about three hours, in which they change the desert landscape.
The Haboob wind is caused by thunderclouds in a final phase. Downdrafts in a thunderstorm cause gust fronts, descending air hitting the ground and picking up large amounts of dust. The force moves the dust from above.
In the Sahara, there are 2 types of the Haboob. In the south a strong and hot version occurs, creating a common sandstorm. In the north the Haboob is colder and causes less dust storms than in the South.

Sirocco

When a low pressure system above the Mediterranean moves towards the East a Southerly wind comes from Africa. This hot, dry and dusty wind comes from the Sahara desert, where hot air rises and flow North. When the wind blows over the Mediterranean it picks up moisture. When it comes to Southern Europe it causes headaches and insomnia to the people it blows upon because of the high temperature and the humidity of the wind. During a sirocco nights are warmer than 30°C. During the day the temperature rises to 40-45°C.
Sirocco is the Arab word for Easterly, because of the to the East moving low pressure system. The Sirocco is most common during the spring, but it can occur all year. The dust can be spread out to Northern Europe.

Harmattan

An easterly wind, interrupting the (North East) Trade winds off the Atlantic coasts to North Africa, across the Gulf of Guinea and the Cape Verde Islands. It's called Doctor sometimes, because it is supposed to have healthful properties. It occurs in the winter, between November and February. Dust parts cause a haze, which can cover large coastal areas. They can reduce visibility to 45 meters.

2 : THE RAINY SEASON (and other climate properties)

Morocco: November to January
The interior of the country can become stiflingly hot, particularly when the desert winds, known as the sirocco or chergui, are blowing (usually in spring).

Mauritania: almost no rain to be seen, except for the south, with a rainy season between July and October.
The climate is hot and dry. From April to October temperatures can exceed 40°C.

Senegal
North: July to September
South: June to October

The Gambia: July to October
The heat can be relentless in March, with an average of around 36°C most days.

Guinea: June to October, one of the wettest countries in West Africa: Conakry, the capital, receives an amazing 4 mtrs of rain, half of it in July and August.

Mali: June to late September
Rainfall is rare in the north and light in the Sahel.
The coolest part of the dry season is from November to February, although this is when the dusty Harmattan winds blow in from the desert. The other months are less dusty but very hot.

Burkina Faso: March to April. And May to October.
From November to mid-February it is dry and temperatures are moderate, although the dusty Harmattan can be unpleasant.

Benin
South: April to Mid-July
North: June to early October

Togo
South: May to mid-July and mid-September to late October
North: April to October

Ghana
South: April to June and September and October
North: April to October