Map from Google - Yemen

In Yemen, the climate is tropical desert on the coast and in the lowlands, while it becomes mild and moderately rainy on the western plateau. During summer, from June to September, the southwest monsoon brings some rainfall on the western highlands, while in the rest of the country it brings widespread cloudiness but almost no rains.

The coast of the Red Sea (called Tihamah) has a desert climate, hot all year round, with highs around 28/30 °C in winter and 38/40 °C in summer. In summer, the humidity from the sea makes the heat sweltering. In Al Hudaydah only 80 millimetres of rain per year fall. The sunshine amount, good all year round, decreases a bit in summer due to the cloudiness brought by the southwest monsoon. Despite the few rains, in this region unhealthy and malarial areas are found, due to the rivers that descend from the mountains and drain into wetlands.
Here are the average temperatures of Al Hudaydah.
Average temperatures - Al Hudaydah
Al HudaydahJanFebMarAprMayJunJulAugSepOctNovDec
Al HudaydahJFMAMJJASOND
Min (°C)181821242628292726232019
Max (°C)293031343637383737353230

The Red Sea is warm enough for swimming all year round, and in summer it gets very warm, as you can see in the following table.
Sea temperature - Al Hudaydah
Al HudaydahJanFebMarAprMayJunJulAugSepOctNovDec
Al HudaydahJFMAMJJASOND
Sea (°C)262627283031323232312927

The islands of the Red Sea (Hanish, Kamaran, Perim) are also desert, hot in winter and very hot in summer.

The coast of the Gulf of Aden has a similar climate, but during summer it's a bit less hot, especially in the eastern part, due to the southwestern monsoon, which is felt more, bringing more often cloudy skies, even though the rains are still very scarce, as you can see from the following table, regarding the rainfall in Al Mukalla.
Average precipitation - Al Mukalla
Al MukallaJanFebMarAprMayJunJulAugSepOctNovDecYear
Al MukallaJFMAMJJASONDY
Prec. (mm)7414135044113359
Days1122101100009

In Aden, the daytime temperature in June and July is about 37 °C, and in August and September is about 36 °C, while in Al Mukalla it's "only" 34 °C in June and July, with a steady breeze blowing and the clouds often covering the sun, even though the rains are sporadic.
Average temperatures - Al Mukalla
Al MukallaJanFebMarAprMayJunJulAugSepOctNovDec
Al MukallaJFMAMJJASOND
Min (°C)191921232526252526222120
Max (°C)272729313234333232313028

The temperature of the Gulf of Aden is similar to that of the Red Sea, although in summer it heats up less, due to the wind and the cloudiness related to the monsoon.
Sea temperature - Al Mukalla
Al MukallaJanFebMarAprMayJunJulAugSepOctNovDec
Al MukallaJFMAMJJASOND
Sea (°C)262627283031302930292726

The western plateau has a mild climate because of the high altitude, but also because it receives moderate rains from the summer monsoon. In Taiz, 760 mm of rain per year fall, and in Ibb even 1,000 mm. The landscape is green in the south-west facing slopes, and with the terrace system the cultivation is possible even on declines. On the highest peaks, above 3,000 metres, during winter sometimes it can snow.
The capital Sana'a is located at 2,200 metres above sea level, on the eastern side of the plateau, the one that receives less rainfall, and in particular it's protected by Jabal an Nabi Shu'ayb, the highest mountain in Yemen with its 3,666 metres, however here 240 mm of rain per year fall, concentrated from July to September, with a maximum in August of 95 mm: precipitation is therefore typical of a semi-desert climate. Some rains may actually occur also in spring, from March to May, here as in the rest of the plateau. Here is the average precipitation.
Average precipitation - Sanaa
SanaaJanFebMarAprMayJunJulAugSepOctNovDecYear
SanaaJFMAMJJASONDY
Prec. (mm)19735133459530111241
Days02132136200020

In Sana'a, winter is pleasantly warm and sunny during the day, with highs around 21/23 °C from November to February, but at night the temperature drops quite a bit, around 10 °C and sometimes below. During summer the temperature is warm: the average is about 24 °C. As we said, in summer the sky is often cloudy, and sometimes a few downpours or thunderstorms can occur.
Average temperatures - Sanaa
SanaaJanFebMarAprMayJunJulAugSepOctNovDec
SanaaJFMAMJJASOND
Min (°C)101215161819201918131010
Max (°C)222324242628282726232221

Sana

The central-eastern part of Yemen (formerly South Yemen), consists of a coastal range and a plateau which declines from 1,000 to 400 metres from west to east, on the border with Oman; this is a desert area, although some summer rains are possible along the coastal range. In the far north, there is also a low altitude area occupied by the southern tip of the Rub 'al Khali, the sandy desert of southern Arabia. Here summer is very hot, with maxima steadily above 40 °C, but a bit of cloudiness linked to the summer monsoon can come to these parts too.
The island of Socotra, which is located between the Arabian Sea and the Indian Ocean, has a tropical desert climate. Socotra is hot all year round, with daytime temperatures around 28/30 °C in winter, rising to 32/33 °C between April and June, and dropping again to 30 °C in summer, due to the monsoon.
Average temperatures - Socotra
SocotraJanFebMarAprMayJunJulAugSepOctNovDec
SocotraJFMAMJJASOND
Min (°C)182021232525232323212019
Max (°C)282930323332302930313028

In Socotra the rains are very scarce, at least in the lowlands, while the slopes of Mount Hajhir are greener, and host a xerophile vegetation which is able to withstand the drought. However, from June to September, the island is affected by the cloudiness associated with the monsoon, and can receive a few showers and is often windy; in addition, it sometimes can be affected by tropical cyclones, coming from the east. Cyclones can also touch the easternmost portion of mainland Yemen, reaching more easily the neighbouring Oman. During winter, Socotra may experience some short showers, associated with the northeast Asian monsoon, which can collect some moisture when passing over the sea.
Here is the average precipitation.
Average precipitation - Socotra
SocotraJanFebMarAprMayJunJulAugSepOctNovDecYear
SocotraJFMAMJJASONDY
Prec. (mm)252015202010202510202020235
Days222346109533352

Even the Arabian Sea is warm enough for swimming all year round, but in summer it's warm as in winter, because of the wind related to the monsoon, which lowers a bit the temperature; this also applies to the eastern coast of the mainland (see Al Ghaydan).
Sea temperature - Socotra
SocotraJanFebMarAprMayJunJulAugSepOctNovDec
SocotraJFMAMJJASOND
Sea (°C)262627293028262526272726

Socotra Island

When to go

The best time to visit Yemen as a whole, and also the island of Socotra, runs from December to February: along the coast and at lower altitudes it's a warm and sunny period; in the desert it's pleasant during the day, while it may get a little cold at night, but at least you avoid the blistering heat of the long summer months. In Sana'a and at high altitudes, winter is equally sunny, but it's mild during the day and a bit cold at night.
Due to the mild temperatures, Sana'a can be visited all year round, but in summer, especially in July and August, it is often cloudy and may experience some showers, at times heavy. Certain areas of the plateau are even more rainy and may experience flooding and landslides. Even between March and May, the plateau can see some showers, however, wanting to visit only the plateau, in this area the best months are March-April and October-November, to avoid both the cold of winter nights and the summer rains. October is probably the best month.
As we have seen, the sea in Yemen is always warm enough for swimming, both in Socotra and along the coasts of the mainland.

What to pack

In winter: for the plateau and Sana'a, spring/autumn clothes, light for the day, sun hat, sweater and jacket for the evening; for the mountain peaks, warm clothes, down jacket, sweater, gloves, hat; for the coast, summer clothes, a sweatshirt for the evening. Snorkeling gear, water shoes or rubber soled shoes for the reef. In the desert, light clothes for the day, scarf or turban, desert boots or sandals, sweater or jacket for the evening and early morning.

In summer: for the plateau and Sana'a, light clothing, a sweatshirt for the evening, raincoat or umbrella; for the mountain tops, spring/autumn clothes for the day, sweatshirt and jacket, raincoat. For the desert and the Red Sea coast, lightweight clothes, made of natural fabric (cotton or linen), desert turban; for the coast of the Arabian Sea and Socotra, light clothes, light sweatshirt and scarf for the breeze, light raincoat.
For women, it is best to avoid shorts and miniskirts outside tourist clubs.