In Arabia, seasonal migration between highlands and lowlands was common. Taif (1,879 meters above sea level) as an example could act as a summertime refuge for the residents of Mecca (277 m). Nomads and semi-nomads could of course make this journey as they pleased, depending on the relative status and privileges of their tribe to do so. Access to fresh water, whether from temporary rivers in the mountainous wadis during the monsoon rains, from dams like Marib, cisterns or oases was vital to that movement and the survival of people in the desert.

At the more technological end of the spectrum, the region also had knowledge and understanding of various architectural and other means to improve ventilation and lower temperatures. "Wind catchers" were one example. These could also be use in conjunction with Qanat systems to create a refrigeration effect robust enough to preserve ice in the the middle of Persian summers.

It also almost certainly helped that in some of the hottest parts of eastern Arabia that one of the primary economic activities was pearl diving.

That being said, there's no getting around the fact that it was intensely hot. Pre-modern population figures were low and in certain senses they remain quite low. The population figures for Qatari nationals, for instance, remain a state secret, most likely because it would be embarrassingly small. In 1904 the population was estimated at 27,000 and had declined to 16,000 by the 1940s and is today plausibly somewhere around a quarter million. As a brief contemporary aside, among other things this makes Qatar's claims of troop deployments in Yemen seem improbable.

Returning to the subject at hand, clothing was also suited (pun intended) for purpose. Long bedouin robes and headdresses are much more appropriate for such a hot climate than western clothing. The man in the middle of that photo is the photographer, Wilfred Thesiger.

Churchill, in history of the Second World War noted his surprise at the intense tan lines of British soldiers wearing shorts, commenting wryly that during his participation in the march to Khartoum that the he was covered from head to toe, I believe as can be seen in this photo: https://undereveryleaf.files.wordpress.com/2013/05/churchill-sudan.jpg

I'm inclined to agree with that, personally, that long clothing is better suited to intense heat than shorts and t-shirts, but that's my opinion. The other piece of gear that he would have had though is the stereotypically imperialist pith helmet, which was so widely used because it can be soaked in water, again providing an evaporative cooling effect.

Somali clans have throughout the ages inhabited one of the hottest regions on earth centered right in the semi-desert and desert expanse of Greater Somalia in the Horn of Africa, a mostly desert climate were tempratures frequently hit 120 degrees. Of note, prior to the 50's - 80's, Somalis were also a vastly (+90%) nomadic population who ranged the scorching Somali deserts and plains of the Greater Somalia region, practicing the same prehistoric nomadic lifestyle and transhumance in the same geographic area for several millenia.

Aside from clear physical adaption to such enviroments with leaner, dark, and taller physiques, material adaption was manifested in clothing styles. Somali men wore white skirts that went right above the knee with a camel-skin belt (kiishaad) to sometimes hold the knee-length white wrap in place. In more formal occaisions, a white full-length robe was worn by being thrown across one or two shoulders and reaching the ground. Somali young women wore short white knee-length dress almost exactly like the Greek chiton but consisting of white robes being thrown the over both shoulders. The aesthetics of Somali dresss emphasized a clear adaption to and were well-suited to desert climate and the physical flexibility and atheletcism demanded by a nomadic lifestyle in such climate. The clothing material was made from a very light white cloth produced by Somali weavers in Mogadishu who produced the famed 'toob Benadir' that garned popularity and high demand in Levantine and Egyptian markets and served as one of the most important Somali exports in the Middle Ages.

Nomadic homes were portable tent-like 'Aqals' made up of branches and covered with mats that would always be transported by camels. Inside an Aqal, air circulation was pleasant enough to cool off the nomads from the heat of the deserts climate. Aside from the Aqal, the scarcity of trees and the refuge offered by their shade were so important to nomads that individual trees were given names and served as important geographic primers and landmarks.

An abudance of natural limestone pools and other geologic formations in Greater Somalia - especially the north - were heavily utilized by nomads for showers and bathing, and showering and grooming were very common throughout the day so people would could cool off whislt also adhering to traditional cleaniness standards.

The hot inland climate coupled with a very long coastline also encouraged a very extensive maritime tradition of seafaring and seaborne travel partially as a way to avoid unbearable inland routes. As such, Somali clans would resort to seaborne raids for both long distance and shorter distance raids and migrations, as exemplified by the Darod Warsengeli and their long history of seaborne raids and invasions of Isaaq coastal settlements and inland territory, or the Darod Majeerteen/Harti clans maritime seaborne invasions and subsequent seaborne migration waves into southern Somalia from modern-day Puntland and Somaliland.

Also, permanent structures and ancient Somali city-states and inland cities were constructed of limestone and coral, both of which were ideal for cool air circulation and ventilation indoors. Coastal sites for cities and permanent settlement was also preferred as monsoon coastal winds made the Somali coast significantly cooler than the more humid and desert-climate inland regions.

Also as a response to heat waves, Somali clans practiced seasonal migration patterns which were determined by using the traditional Somali calandar. Somali calandrical terminology reflects a cold-to-hot and rainy-to-dry diachtomy and a mastery of inland winds and climate patterns. As such, Somali clans migrated with their camels and horses into cool mountanious areas and more upland or even cooler coastal regions before moving back into the burning humid inland lowland plateaus and plains.

The location of wells and important waterworks were also extremely vital to the nomadic seasonal migrations, and Somali states and empires like the early-to-late medieval Ajuran are famous for their hydraulic engineering and legacy with the extensive waterworks they built that are still used today.

It is important to note that the Somali population was very small prior to the 70's, with the early 1900's British Somaliland population being the same as Bhutan, and French Somaliland having a mere 50,000 people. Italian Somaliland, being the most urban with the major Benadir urban areas of Mogadishu and Brava, and other cities like the more northerly Merca, in addition to having a large non-Somali argicultural population of free and enslaved Bantu, Benadiri, and Italians, with ethnic Somalis likely forming less than 50% of the population had a population of around 400,000. Massive demographic growth of the Somali population right around the period of independence during the late 50's and early 60's and a stable preceding population coincided with an influx of technologies and funding for a larger healthcare sector and the introduction of urban and residential infrastructure more sheltering in a scorching climate. This lead to a heavily reduced % of childhood death and all climate-related fatalities.

As such, Somali clans that numbered in the 10's of thousands (20,000 - 40,000) in early British census now number in contemporary estimates upwards 1,000,000. Similarly, Somali population growth in Somalia skyrocketed, with the greater Mogadishu metropolitan area having a population of at 500,000 by 1980 increasing to almost 2 million in 2016. The 1st Somali Republic had a population that initially stood at a 2.7 million increasing 4x over a span of 5 decades until 2016 were it now stands at around 11.1 million. The 1931 Italian census of Miguirtunia (most of modern day Puntland) had a population under 48,863, with Puntland's current population estimate standing at more than 3 million.