Map from Google - Ethiopia

In Ethiopia, the climate varies with altitude, from the hot and arid climate of the lowlands, to the cool climate of the plateau. Lying just north of the Equator, it experiences little variation in temperature throughout the year.
The rains are determined by the southwest monsoon, which affects the country from June to September, but it only affects some areas, namely the plateau and the mountain slopes exposed to the south-west, while in the south-east of the country there are two rainy periods, though less intense, usually from March to May and from October to November; in some years the rains don't occur, causing droughts in this area.

In the Ethiopian Plateau (Zone 1 on the map), the weather is cool, mild or pleasantly warm, depending on the altitude, with a season characterized by scarce rains from November to February, and a rainy season from June to September, preceded by a period, from March to May, when some showers and thunderstorms may occur in the afternoon. The annual rainfall ranges from 1,000 to 2,200 millimetres. The rains occur as downpour or thunderstorm, usually in the afternoon or in the evening, even during the monsoon season, when, however, the cloudiness is more widespread.
The capital Addis Ababa is located at 2,300 metres above sea level, though the different districts of the city range from 2,100 to 2,700 metres, and has a mild climate, with cool nights, even cold from November to February, when the minima drop below 10 °C, and pleasantly warm days, around 23/25 °C, except in July and August, at the height of the rainy season, when the maxima drop to about 20 °C. The period from March to May, as often happens in Ethiopia, is the warmest of the year, albeit of a few degrees.
Here are the average temperatures.
Average temperatures - Addis Ababa
Addis AbabaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)8910111110101010977
Max (°C)242425242523212122232323

Throughout the year, 1,200 mm of rain fall, with a maximum from June to mid-September, which is the only remarkably rainy period. From November to February, there is little rain, and rare showers occur, while from March to May, the afternoon showers become a bit more frequent, occurring for 6/7 days per month. The sunshine amount in Addis Ababa, from November to March is good, and just acceptable in July and August, when the rainfall occur one day out of two.
Here is the average precipitation.
Average precipitation - Addis Ababa
Addis AbabaJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)1645708595135280295190251561257

The other cities on the plateau have a climate similar to that of Addis Ababa, with variations related to the altitude and position. The rainiest part of the plateau is the western one: in Gore, 2,100 mm of rain per year fall, and it rains a lot from April to October included. In Gondar, located in the north-west at an altitude of 2,000 metres, the temperature is slightly higher than in Addis Ababa, and rainfall amounts to 1,100 mm per year, distributed the same way as in the capital. In Bahir Dar, at 1,800 metres above sea level, 1,400 mm of rain fall, of which even 430 mm in July; here the showers are enhanced by the waters of Lake Tana, from which the Blue Nile originates; the river gives rise to spectacular waterfalls near Tissisat.
Jimma (or Jima) is located at 1,700 metres above sea level in the south-west, where the rainfall amounts to 1,500 mm per year, of which more than 100 mm per month from April to September. Here is the average precipitation.
Average precipitation - Jimma
Prec. (mm)4050801801502202302101908540351510

Due to the lower altitude, the temperatures in Jimma are higher than in the capital, but here also are pleasant. Here are the average temperatures.
Average temperatures - Jimma
Min (°C)121313131313131313121212
Max (°C)272828272725242425262727

Harar (Harer) is located in the east at the edge of the plateau, at no great distance from Dire Dawa (see below), but it is cooler because it is located at 1,850 metres above sea level, besides it's located on a different slope. Here, 860 mm of rain per year fall, with a rainy period in which precipitation goes from 100 to 140 mm per month, which runs from April to September.

Blue Nile Falls

In the arid or semiarid areas surrounding the Ethiopian plateau (zone 2 on the map), the rainfall amount is lower than 800 mm per year, and is generally located at a lower altitudes, with a few exceptions such as Mek'ele (Makale), located at 2,000 metres of altitude in the north (Tigray region): here 700 mm of rain per year fall, with only two very rainy months, July and August. Despite the altitude, here from April to June it can be a little hot, with highs about 27/28 °C.
But the heat is intense especially below 1,000 metres, and in particular in the dry period.
The narrow strip west of the plateau, receives the south-west monsoon, but here its effects are less pronounced than on the plateau; here the heat is intense in the dry season, from October to mid-May, with a peak between April and the beginning of May, when the temperatures reach normally 40 °C, then the rains come and the temperature drops a bit: therefore it's difficult to find a good period to visit this area. In addition, the basins of the rivers Baro and Sobat, on the border with South Sudan, are marshy and unhealthy.
In the far south, in the area of Lake Turkana, and in the south-east (see Moyale), a scorching heat dominates throughout the year, with irregular rains, concentrated in two periods: from March to May and from October to November. Here, unlike in the rest of the country, the best time (or the least bad...) is from June to August, because it is, albeit slightly, the least hot. Even the Somali Region (Ogaden), in the south-east, is hot and semi-desert. Here are the average temperatures of Gode, in the south-east.
Average temperatures - Gode
Min (°C)212123242323232323232120
Max (°C)353737363534333436353535

Here the rains do not reach 250 millimetres per year, and are distributed in two rainy seasons, April-May and October-November. Here is the average rainfall.
Average precipitation - Gode
Prec. (mm)021075352151570252245

In the east, we find Dire Dawa, located at 1,200 metres, on the north slope, overlooking the torrid zone: here it's hot for most of the year, with a maximum in June, when highs of 35 °C are normal, while in winter it's warm during the day, but the nights are quite cool. Here are the average temperatures.
Average temperatures - Dire Dawa
Dire DawaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)151518192021181718171516
Max (°C)282931323335323132333128

Here, 670 mm of rain per year fall, with two relatively rainy periods, from March to May and July to September, and two dry periods, from October to February and in June. Here is the average precipitation.
Average precipitation - Dire Dawa
Dire DawaJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)20357010560309513075301510670

In northern Ethiopia, in the Afar Region (zone 3 on the map), the climate is desert and hot throughout the year. In particular, in the Danakil Depression, where some salt lakes such as Lake Afrera are found, the climate is particularly hot, also because of the low altitude, which comes down to 125 metres below sea level; here a stable and overwhelming heat reigns throughout the year. The abandoned settlement of Dallol, in the far north, is considered the hottest place in the world on average, with a daily temperature of 35 °C all year round: lows are around 30 °C and highs around 40 °C. On the slopes of the volcano Erta Ale, the dark basalt rock heats up particularly, absorbing sunlight and creating a hellish environment.

Dallol, Ethiopia

From the Ethiopian Plateau, several mountain peaks rise, including Ras Dashen, the highest of the country with its 4,550 metres, Abuna Yosef, 4,260 m, Tullu Deemtu, 4,389 metres, and Batu, 4,307 m, the last two in the Bale Mountains. In the Bale Mountains, several interesting ecosystems are found: up to 2,800 metres the forest, replaced at higher altitude by a barren mountain vegetation, able to withstand the cold of the night, while over 4,000 m, the constant cold prevents the growth of almost all kind of vegetation, so that there's a cold desert. On the contrary, Ras Dashen in the north and Abuna Yosef in the east, are located in the semi-arid area.

When to go

The best time to visit Ethiopia goes from November to February, being the driest and coolest almost everywhere. In the arid areas, at low altitude this is still a hot period, with torrid days, but with generally pretty cool nights. The worst months are July and August, due to rainfall on the plateau, as well as April and May because of the heat in the semi-arid areas, while the Danakil Depression is steadily hot and inhospitable throughout the year.

What to pack

In winter: in Danakil, the Afar region and the south-east, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), desert turban, hiking shoes, sweatshirt for the evening at hill altitude. In Addis Ababa and the plateau, spring/autumn clothes, light for the day, sun hat, jacket, sweater and hat for the evening; for the high mountains, above 3,000 metres, warm clothes, warm jacket, gloves, scarf.

In summer: in Danakil, the Afar region and the south-east, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), desert turban, hiking shoes. In Addis Ababa and the plateau, spring/autumn clothes, jacket, raincoat and umbrella; for the high mountains, above 3,000 metres, warm clothes, warm jacket, gloves, scarf.