Map from Google - Eritrea

The climate of Eritrea is tropical desert on the coast and in the north-west (zone 1 on the map), mild and semiarid in the mountainous belt (zone 2 on the map), tropical and semi-arid in the south-west (zone 3 on the map). From June to September, a bit of rain occurs, especially in the southern and central inland part, due to the southwest monsoon.
Climates in Eritrea
Along the the coast of the Red Sea, (zone 1 on the map), the climate is desert and hot all year round.
In Massawa the winter from December to March is hot and sunny: daytime temperatures are around 85 °F. Summer, from May to September is oppressive, with daytime temperatures about 105 °F, but with peaks of 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 (°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 bodies can pass, and rare rains occur as part of the southwest monsoon. On average, only 7 inches of rain per year fall. Here is the average precipitation.
Average precipitation - Massawa
MassawaJanFebMarAprMayJunJulAugSepOctNovDecYear
MassawaJFMAMJJASONDY
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 a year just 2.4 in of rain fall. Here is the average precipitation.
In the Dahlak Archipelago, summer temperatures are a bit lower than on the coast, but the high humidity makes the heat unbearable.
In the central inland areas 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 105 °F even in winter.

In the mountainous belt that runs from north to south (zone 2 on the map), the weather is mild due to the altitude, and is also moderately rainy, at least in the center-south, while the northern part is desert. The highest peak, Emba Soira, reaches 9,900 feet of altitude and is located in the south.
In the capital Asmara (or Asmera), the climate is pleasant, being located at 7,500 feet above sea level. Daytime temperatures are around 72/73 °F in winter, a little higher from March to June, and then down a bit in July and August, which are the only really rainy months. The night temperatures are cool in summer, and very cool or even cold in winter. Here are the average temperatures of Asmara.
Average temperatures - Asmara
AsmaraJanFebMarAprMayJunJulAugSepOctNovDec
AsmaraJFMAMJJASOND
Min (°F)434548505252525248484643
Max (°F)737577777777727273727272

Throughout the year, about 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, after the already mentioned two very rainy months, July and August. 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. (in)0.10.10.61.21.61.66.95.910.60.80.220.6
Days22357678533354

Some mountainous areas located north-east of Asmara are rainier, and therefore greener: here there are even woods.

In the western part of Eritrea (zone 3 on the map) the climate is desert in the north, semi-desert or semi-arid in the south, where a bit of rain from June to September occur, related to the summer monsoon of neighboring Ethiopia.
In Agordat (or Ak'ordat), at 2,000 feet above sea level, 10 in of rain per year fall, concentrated from June to September. Here is the average precipitation.
Average precipitation - Agordat
AgordatJanFebMarAprMayJunJulAugSepOctNovDecYear
AgordatJFMAMJJASONDY
Prec. (in)0000.20.413.13.71.40.20.1010.2
Days11113467421132

The temperature is high in winter, with more than 85 °F even in January, but nights are quite cool; the temperature becomes torrid in spring, touching 104 °F in May, while it decreases a little with the arrival of the monsoon, but at the cost of increased humidity. Between mid-September and November, the hot weather returns, with highs about 95/100 °F.
Here are the average temperatures of Agordat.
Average temperatures - Agordat
AgordatJanFebMarAprMayJunJulAugSepOctNovDec
AgordatJFMAMJJASOND
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 28 inches 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 (°F)777981828688888890888279

When to go

The best time to visit Eritrea runs from December to February, because it is less hot and more dry. 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 the nights are cool, especially in December and January (in February the temperatures already start to increase a little).

What to pack

In winter: in Massawa, Dahlak Island 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-colored clothing, light and long shirt and pants of natural fibers (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-colored clothing, light and long shirt and pants of natural fibers (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.