Humidity regime in the world's mega cities

As urbanization continues to grow, it is expected that by 2050, the currently 55% of the world population living in urban areas will reach 68% (UN, 2016). The 'Urban Heat Island' is the most studied phenomenon in urban areas, but changes are expected also in the atmospheric humidity regime (Ziv & Saaroni, 2011). Generally, an urban dry island has been noted in previous studies (Luo & Lau, 2019).

Following the authors' findings in the field of hygroelectricity (Lax, Price & Saaroni, 2020)– energy extracted from isolated metals exposed to high relative humidity (RH) conditions – as spontaneous voltage accumulated on isolated metals starting from RH > 60%, investigation of suitable regions with high RH and its durations is needed. Such analysis is also applicative for numerous aspects, related to negative impacts caused by high RH, i.e., thermal comfort & health aspects, when associated with high temperatures, allergies related to dust mites & mold, fungi & bacteria survival, corrosion development, etc. However, high RH has advantages as well, moist-reliant renewable energies and moist harvesting for drinking water (Shen et. al., 2020). This is especially relevant in a warming world due to climate change.

Our study explored the climatology of the world's largest 33 mega-cities; High RH distribution & duration spells are analyzed on a seasonal & annual scales, based on a minimum of 10-years hourly data. Moreover, for cities with several stations, a spatial comparison is performed. The atmospheric variables included are not only the cities' RH, but also the specific humidity, dry and wet bulb temperatures and heat load. Cities are ranked in terms of their potential contribution to the above-mentioned technologies on the one hand and to their disadvantages in terms of human comfort on the other. Finally, we propose a tool to determine the potential of these cities for moist-reliant technologies.