The Rift Valley
Apart from its northern and southern extremes (see below), the Rift Valley is subject to the same weather patterns as the north, the only differences being that the long rains begin earlier (late April), last longer (October), and that it is slightly warmer.
The Northern Historical Circuit
The northern historical circuit constitutes a 3000 km round trip, beginning in Addis Ababa and moving clockwise through Debre Markos, Bahir Dar, Gondar and Axum – its western flank – and peaking in Adigrat, before moving south again, through Mekele, Lalibela, Woldia and Kombolcha, and finishing back in Addis. There are plenty of places in between and off track, including – for example -the Simien Mountains, the Blue Nile Falls, Lake Tana and the Mesket area. While most of these are served by either tarmac or good dirt roads, a few areas of interest are accessed only by dirt tracks and paths, requiring that travellers use 4 x 4 vehicles, mules or - in some cases - foot.
The northern historical circuit as a whole is best visited between October and March, the long rains beginning in June, becoming increasingly heavy through July and August, before thinning out towards the September. Temperatures average around 16 °C. However, when looked at in more detail, the climatic picture for the northern circuit makes for more complicated viewing: the area is vast, the west wetter than the east, the north-east much drier, local temperatures and levels of precipitation dictated by altitude. Experienced travellers can pick and choose places of interest throughout the year – even in July and August.
At 2,400m, Addis Ababa is the world’s highest capital. Temperatures are extremely constant - 16 to 18°C - and precipitation levels range between a paltry 10mm in November to a reasonable but by no means impossible 280mm in August. Clearly, it is possible and - given its status and the fact that it is here that journeys to destinations throughout Ethiopia begin and end - necessary to visit Addis throughout the year, though July, August and some of September would be the months in which to avoid it rain-wise.
After Addis Ababa, and moving clockwise, the next place of real significance is Bahir Dar, from which it is possible to visit Lake Tana’s island monasteries and the Blue Nile Falls. At an altitude of 1840m, Bahir Dar is lower than Addis, and therefore warmer, with temperatures that range between 18 and 22°C. Precipitation levels rise significantly in June (185mm), peak at 430mm in July, are in the top 300s in August, fall to 300mm in September, and continue to hover at just below 10mm in October, all of which makes November through to April the best months to visit.
North - but roughly on the same line of longitude - is Gondar. At 2,120m, temperatures here are the same as those found in Bahir Dar, but the levels of precipitation peak in July and August at just over 300mm, making it slightly more manageable, but only just. Both middle to late June and early September are still very wet, May and October the buffer months, making the dry season reasonably short.
East and further north still, Axum is 2,100m high and - with an annual temperature range of 12°C to 18°C - a tad cooler than Gondar. However, the monthly pattern with regards to levels of precipitation changes, with the 200mm mark breached only in July. Either side, levels drop rapidly to just over 50mm, making Axum very easy to visit between late September and mid June - a significantly longer on-season, therefore, than the likes of Bahir Dar and Gondar.
At Adigrat (2,400m), the northern most point of the circuit, the mid-year rainfall spike is even less pronounced than it is in Axum, with just July and August managing to break into the hundreds (approximately 140mm and 155mm respectively), while the rest of the year sees levels range between next to nothing - 5mm - in February and 55mm in April. Again, temperatures are fairly constant, ranging from an average of just under 13°C in December to 19°C in April.
At Mekele, 2,500m high and a quarter of the way down the eastern flank, things are wetter, but only in July (200mm) and August (210 mm). They are also warmer (16-20°C), making October through to early June a perfectly reasonable time to visit.
Finally, there is Lalibela, half way between Adigrat and Addis Ababa. At 2630m, the average temperature range is approximately 14-18°C, and rainfall levels rise fairly gently between March and June (from 40mm to 70mm), before shooting up to the 260s in July and August. September is still reasonably wet (110mm), making October the beginning of a dry (ish) season that runs right through to mid June.