Ethiopia is a landlocked country (no access to the see) since Eritrea proclaimed its independency back in 1991. Unlike what most of us thought of Ethiopia, it is a territory rich on water, with 14 major rivers (including the Nile), numerous lakes and springs waters (indeed, I recently read that Ethiopia has the largest water reserve in Africa!). Sad to know that drought is still a problem in a country that sits in such a rich soil.
Regarding its climate, you will generally find yourself in a pretty warm country situated near the Ecuador, but cooler than others that are at similar latitude, mainly due to the highlands where the country rests on. The main cities are located at elevations between 2000-2500 meters (Addis, the capital, is at 2400 meters high, 7900 ft), which means a big weather relief most of the time.
The moment you go to lower altitudes you will obviously get warmer. Ethiopia is a very diverse country that has different types of climates, from oceanic to tropical savanna, semi-arid, desert, to tropical, so it is not a bad idea to plan your travelling clothes depending on what places are you going to visit (see the map aside).
Regarding rainfalls, we identify 2 main seasons: the rainy season during the summer (mid June-mid September) with heavy rains for many hours of the day (but not all day long though), and dry season the rest of the year with sporadic rains at times. Temperature is more or less constant. There are a couple of small rainy seasons that come at random weeks somewhere between Feb-April (even though climate change is modifying this trend every year and it is hard to say if the rain will catch you on those dates). This is important to know in case you travel by car, as some rural roads may be inaccessible (not the main ones) and also for your footwear (lots of mud on un-asphalted roads).
Talking about temperature, expect a warm sunny climate during the day and a bit chilly during the night. The coolest months are December – January. If you are planning to walk around during the day, sunscreen is is advisable. A single shirt and shorts would be enough for the day and a jersey and long jeans for the night (nothing thicker than that). On rainy season, nice footwear and waterproof clothes are good friends too. You can check rainfall and temperature charts by city through this link: https://en.climate-data.org/country/249/#example0. I’m attaching Addis graph as an example:
Serious warning regarding swimming!: In Ethiopia, like in many other African, Asian and South American countries, there is a disease known as bilharzia, which can be spread by contact with fresh water contaminated with the parasites released from infected snails; this can damage the urinary track and intestines and has some nasty symptoms. When I first got here I was told not to swim anywhere except lake Langano which I was told was bilharzia-free. However, lately I have been told of other places that seem to be free from the disease (Babogaya lake in Debre Zeyt or Wonchi, for example). I would suggest that no matter how tempting a lake might seem, you do some research before you throw yourself into the water! And don’t just ask around as locals will dump themselves there without hesitation!