Geodiode

A nation’s capital, a sprawling city of 10 million people. This city lies between some of the highest peaks in the world, and the planet’s largest ocean. The world’s only major city to exist in a desert within the tropics, it is host to one of the world’s most peculiar climates. A city of mild temperatures, rolling fogs and almost no rain, this is the tropical desert metropolis. This is Lima.

Lima is the capital of Peru, founded in 1535 during the Spanish conquest, and has a population in its metropolitan area of just over 10 million. It is located midway along that country’s coast with the Pacific Ocean and being at a latitude of only 12 degrees south, is firmly within the tropics. However, due to a number of causes as we shall explore, the city experiences virtually no rain, and so is unlike any other major city within the tropical band that normally experiences heavy rain, either year-round, or interspersed with a dry season. While the city is home to typically tropical high humidity, it only experiences frequent low cloud and fog. And tropical deserts are rare. Hot deserts mostly occur in subtropical regions just beyond the tropics of Capricorn and Cancer, while cool deserts are present in the middle of continental land masses or in the rain shadow of mountains.

So what is going on here? Well, in essence, it is down to two main factors. The first of these is that the Andes mountains, rising above 6,300m (20,000 feet) to the east of the city, block almost all of the moist trade winds that blow down in a south-westerly direction from the equator during summer. In the absence of these mountains, Lima would have experienced a rainy season during the summer, and a Tropical Savannah climate typical of such a latitude. But the Andes prevent this wet season.

The second factor is the presence of the Humboldt ocean current, also known as the Peru current, which brings cold water up from the mid-latitudes of the Pacific all the way to the equator via the coast of Peru. Such is the strength and volume of this current, that water temperatures near the equator at the border with Ecuador are around 16°C, ten degrees lower than waters typically found at this central latitude.

The consequences of unusually cool waters just off the Peruvian coast lead to a number of phenomena.

The first of these is simple, in that the coast is much cooler than would be expected at such a tropical latitude, with winter temperatures falling to 14°C instead of the low twenties and summer highs averaging only 26°C instead of the low thirties. The second factor is further denial of potential rain for the city coming off the Pacific. This is despite high relative humidities, of 80 percent or more, found year round. Instead, frequent low cloud and fog is the norm, especially during winter. In fact Lima is the one of the cloudiest cities to be found outside of mid-latitude Oceanic or Polar climates, with average sunshine hours being only 1,230 per year. By contrast, Bangkok, at a similar latitude north of the equator, has double that number.

In order to understand why this humidity does not precipitate into rain, we need to understand a bit more about the formation of rain and unstable versus stable air. Air at higher temperatures can hold more water vapour than colder air. So a cubic metre of air at 30 degrees with a relative humidity of 80%, for instance, will contain more water vapour than the same block of air at 15 degrees. Typically, the means by which air can be cooled in weather is by lifting the air higher into the atmosphere, since higher altitudes are cooler. This is done by either unstable air circulation, in the form of storm systems or thunderstorms, or through the air being pushed up over a mountain range. As the air is cooled, the relative humidity increases as the capacity of the air to hold the same amount of water vapour is reduced. When the relative humidity approaches 100%, the water will precipitate out of the air to form tiny water droplets. This is what a cloud is, and a cloud that forms near the ground is simply referred to as fog. In turbulent, moving air, these water droplets collide and coalesce, and eventually form much bigger droplets, which then fall to earth as rain.

In the case of Lima, the land, heated by the high tropical sun, is almost always warmer than the nearby cold ocean during the daytime. This can lead to sea breezes that pull moist air from the ocean onto the warmer land. But this is the exact opposite of what we need in order to form rain, since any moisture in such air can comfortably remain as water vapour as the air warms up. If this air is saturated, with a relative humidity of 100%, then it will blow onto the land in the form of fog, and sometimes a light drizzle, before its heated enough several miles inland to dissolve back into the air as water vapour. So such fogs, which occur all along the Peruvian coast, are localised to just a few miles from the ocean.

Such conditions – the presence of the Andes blocking trade winds, and the cold ocean along the coast, lead to very stable atmospheric conditions, and the inability of the tiny water droplets to coalesce into rain. And so we have a desert.

In terms of Koppen Climate Classification, Lima has a “Hot Desert Climate”, with the code “BWh”, B for “Dry Climate Types”, “W” for true desert and not semi-arid, and “h” to distinguish it from cool deserts, having a mean annual temperature over 18°C. However, as just described, summer temperatures are lower than a typical hot desert climate, as can be seen in the comparison with this other well-known desert city. In my Hot Deserts article, I alluded to the Koppen code “BWn” for Lima, describing it as a “mild desert”, but this code is unofficial and is not found in peer-reviewed academic papers such as those of Beck et al of 2018.

Lima is not alone in being a large city within a desert, as it’s clear from the list of cities revealed in my Hot Deserts article. The civic authorities have been able to source sufficient water to provide for the city’s needs, in this case through underground springs in the Andean foothills and from rivers further into the Andes, and pumped down to the coast. This has not acted as a brake to the city’s continued growth – Lima is now the third or fourth largest city on the continent, depending upon how city boundaries are decided.

Unable to rely upon piped water systems, more remote communities along the desert coast of Peru have resorted to more ingenious means of obtaining clean water. Nets are used to collect the tiny droplets of fog. When trapped, these droplets build up and eventually coalesce to form drops of water which then fall, like man-made rain, into gutters below. So through the ingenuity of humans, the fogs of coastal Peru, at last, bring rain.