Map from Google - Eritrea

The climate of Eritrea is tropical desert on the coast and the eastern plain (zone 1 on the map), mild and semiarid in the mountainous belt (zone 2 on the map), and tropical and semi-arid in the south-west (zone 3). From June to September, the country is influenced by the southwest monsoon coming from Ethiopia, which brings a bit of rain, especially in the central and southern inland part.
Climates in Eritrea
Along the coast of the Red Sea, (zone 1 on the map), the climate is desert and hot all year round.
In Massawa, winter, from December to March, is hot and sunny: daytime temperatures are around 30 °C (86 °F). Summer, from May to September is oppressive, with daytime temperatures about 40 °C (104 °F), but with peaks of 45 °C (113 °F), and the humidity of the sea that aggravates the feeling of heat. Here are the average temperatures.
Average temperatures - Massawa
MassawaJanFebMarAprMayJunJulAugSepOctNovDec
MassawaJFMAMJJASOND
Min (°C)202122242527292827252221
Max (°C)292932343740414039363331
Min (°F)687072757781848281777270
Max (°F)8484909399104106104102979188

From October to April, there can be some rain, usually not abundant, but the amount of sunshine is good. In summer, some cloud banks can pass, and rare rains occur as part of the southwest monsoon. On average, only 180 millimetres (7 inches) of rain per year fall. Here is the average precipitation.
Average precipitation - Massawa
MassawaJanFebMarAprMayJunJulAugSepOctNovDecYear
MassawaJFMAMJJASONDY
Prec. (mm)332588409114163333184
Prec.(in)1.310.30.30.200.40.40.20.61.31.37.2
Days54311011023425

In the southern part of coast, in Assab (or Aseb) the climate is similar, but it's even more arid: in an average year, just 60 mm (2.4 in) of rain fall.
In the Dahlak Archipelago, the summer temperatures are a bit lower than on the coast, but the higher humidity makes the heat unbearable.
In the central inland area, we find the Danakil Depression, where there are salt lakes and salt mines; here the weather is scorchingly hot all year round, with temperatures around 40 °C (104 °F) even in winter.

In the mountainous belt that runs from north to south (zone 2 on the map), the climate is mild due to the altitude, and is also moderately rainy, at least in the centre-south, while the northern part is desert. The highest peak, Emba Soira, reaches 3,000 metres (9,800 ft) of altitude and is located in the south.
In the capital Asmara (or Asmera), the climate is pleasant, being located at 2,300 metres (7,500 ft) above sea level. Daytime temperatures are around 22/23 °C (72/73 °F) in winter, a little higher from March to June, and then they drop a bit in July and August, which are the only really rainy months. Night temperatures are cool in summer, and very cool or even cold in winter. Here are the average temperatures.
Average temperatures - Asmara
AsmaraJanFebMarAprMayJunJulAugSepOctNovDec
AsmaraJFMAMJJASOND
Min (°C)67910111111119986
Max (°C)232425252525222223222222
Min (°F)434548505252525248484643
Max (°F)737577777777727273727272

Throughout the year, about 500 mm (20 in) of rain fall; the rains are almost entirely absent from October to March, a few short showers occur from April to June and again in September, July and August, which as mentioned are the two very rainy months. The sunshine amount is great from October to February, good in spring, and low in July and August, when the sky is often cloudy. All in all, in Asmara, the climate is good from October to May, and probably the best months are October and November.
Here is the average precipitation.
Average precipitation - Asmara
AsmaraJanFebMarAprMayJunJulAugSepOctNovDecYear
AsmaraJFMAMJJASONDY
Prec. (mm)33153040401751502515206522
Prec.(in)0.10.10.61.21.61.66.95.910.60.80.220.6
Days22357678533354

Some mountainous areas located north-east of Asmara receive more rain, therefore are greener: here even forests are found.

In the western part of Eritrea (zone 3 on the map), the climate is desert in the north, and semi-desert or semi-arid in the south, where a bit of rain from June to September occur, related to the summer monsoon.
In Agordat (or Ak'ordat), located at 600 metres (2,000 feet) above sea level, 260 mm (10.2 in) of rain per year fall, concentrated from June to September. Here is the average precipitation.
Average precipitation - Agordat
AgordatJanFebMarAprMayJunJulAugSepOctNovDecYear
AgordatJFMAMJJASONDY
Prec. (mm)1104925809535631260
Prec.(in)0000.20.413.13.71.40.20.1010.2
Days11113467421132

The temperature is high in winter, with more than 30 °C (86 °F) even in January, but nights are quite cool; the temperature becomes torrid in spring, touching 40 °C (104 °F) in May, while it decreases with the arrival of the monsoon, but at the cost of an increase in humidity. Between mid-September and November, the hot weather returns, with highs about 35/37 °C (95/99 °F), although a little less intense than in spring. Here are the average temperatures.
Average temperatures - Agordat
AgordatJanFebMarAprMayJunJulAugSepOctNovDec
AgordatJFMAMJJASOND
Min (°C)141314182222202120211916
Max (°C)313335394037323135373533
Min (°F)575557647272687068706661
Max (°F)88919510210499908895999591

In the south, the landscape is still arid, but the summer rains are more abundant, so much so that precipitation exceeds 700 mm (27.5 in) per year, allowing the survival of a greater amount of shrubs and trees.

Asmara

The Red Sea is warm all year round, and in summer is one of the warmest seas in the world, as you can see in the following table.
Sea temperature - Assab
AssabJanFebMarAprMayJunJulAugSepOctNovDec
AssabJFMAMJJASOND
Sea (°C)252627283031313132312826
Sea (°F)777981828688888890888279

When to go

The best time to visit Eritrea runs from December to February, because it is the least hot and the driest. It must be said that in some regions it is still a torrid period, particularly in the Danakil, but also in the entire western part at low altitudes, where, however, the heat is more bearable and nights are cool, especially in December and January (in February the temperature already starts to increase a little).

What to pack

In winter: in Massawa, the Dahlak Islands and the northern coast: light clothing, a light sweatshirt for the evening, possibly a light raincoat or umbrella; for the reef, equipment for snorkeling, water shoes or rubber soled shoes. In Danakil, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), desert turban, hiking shoes. In Asmara and the highlands, spring/autumn clothes, jacket and sweater for the evening, sun hat, sunscreen. In the western inland areas, light clothes, desert turban, sweatshirt and light jacket for the evening.

In summer: on the coast, tropics-friendly, lightweight clothes; for the reef, equipment for snorkeling, water shoes or rubber soled shoes. In Danakil, loose fitting, light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), desert turban, hiking shoes. In Asmara and the highlands, spring/autumn clothes, light jacket and sweater for the evening, raincoat or umbrella. For the western inland areas, light clothes, light raincoat.