@article{22537,
  abstract     = {Urban heat islands (UHIs) are a widely studied phenomenon, while research on urban-rural differences in humidity, the so called urban dry or moisture islands (UDIs, UMIs), is less common and a large-scale quantification of the seasonal and diurnal patterns of the UDI is still lacking. However, quantification of the UDI/UMI effect is essential to understand the impacts of humidity on outdoor thermal comfort, building energy consumption, and urban ecology in cities worldwide. Here, we use a set of globally distributed air temperature and humidity measurements (1089 stations) to quantify diurnal and seasonal patterns of UHI and UDI resulting from rapid urbanization over many regions of the world. The terms ‘absolute UDI’ and ‘relative UDI’ are defined, which quantify urban–rural differences in actual and relative humidity metrics, respectively.

Results show that absolute UDI is largest during daytime with the peak humidity decrease in urban areas occurring during late afternoon hours. In contrast, relative UDI is largest during night and the peak urban relative humidity (RH) decrease and vapor pressure deficit (VPD) increase occurs in the late evening hours with values of around −10% to −11% for RH and 2.9–3.6 hPa for VPD between 20–00 local time during summer. Relative and absolute UDIs are largest during the warm season, except for daytime RH UDI, which does not show any seasonal pattern. In agreement with literature, canopy air UHI is shown to be a nighttime phenomenon, which is larger during summer than winter. Relative UDI is predominantly caused by changes in actual humidity during day and UHI during nighttime.},
  author       = {Meili, Naika and Paschalis, Athanasios and Manoli, Gabriele and Fatichi, Simone},
  issn         = {1748-9326},
  journal      = {Environmental Research Letters},
  keywords     = {Urban dry island, Urban moisture island, Urban heat island, Urban climate, Urbanization effects, Humidity},
  number       = {5},
  publisher    = {IOP Publishing},
  title        = {{Diurnal and seasonal patterns of global urban dry islands}},
  doi          = {10.1088/1748-9326/ac68f8},
  volume       = {17},
  year         = {2022},
}

@article{22564,
  abstract     = {It is well known that cities increase air and surface temperatures compared to their rural surroundings, the so-called urban heat island (UHI) effect. However, the associated changes in atmospheric humidity (also known as urban dry island, UDI) and convection triggering remain largely unexplored and it is still unclear how urban modifications of the surface energy budget Influence the diurnal evolution of temperature and humidity in the Atmospheric Boundary Layer
(ABL) and ultimately control the initiation of convective clouds.
Here we quantify the impact of different urban settings and free atmospheric conditions on
UHI, UDI, and convection triggers by means of a zero-order model of the ABL. Specifically, we
derive an approximate solution for urban-rural changes in surface energy fluxes and ABL potential
temperature and humidity and we investigate the crossing between the ABL height and the lifting
condensation level (LCL) which is a proxy for the triggering of convective clouds. We show that
urban areas are generally warmer and drier, thus causing an increase in both ABL and LCL
heights. However, the response of the ABL-LCL crossing to surface conditions is non-linear and
there exists a range of free atmosphere conditions for which changes in imperviousness can
impact convective clouds.},
  author       = {Chiu, Clinton T.F. and Wang, Kai and Paschalis, Athanasios and Erfani, Tohid and Peleg, Nadav and Fatichi, Simone and Theeuwes, Natalie and Manoli, Gabriele},
  issn         = {2212-0955},
  journal      = {Urban Climate},
  keywords     = {Urban heat island, Urban dry island, Boundary layer, Convective clouds, Analytical model},
  publisher    = {Elsevier},
  title        = {{An analytical approximation of urban heat and dry islands and their impact on convection triggering}},
  doi          = {10.1016/j.uclim.2022.101346},
  volume       = {46},
  year         = {2022},
}

@article{22542,
  abstract     = {Temporal dynamics of urban warming have been extensivelystudied at the diurnal scale, but the impact of background cli-mate on the observed seasonality of surface urban heat islands(SUHIs) remains largely unexplored. On seasonal time scales, theintensity of urban–rural surface temperature differences (∆Ts)exhibits distinctive hysteretic cycles whose shape and loopingdirection vary across climatic zones. These observations high-light possible delays underlying the dynamics of the coupledurban–biosphere system. However, a general argument explain-ing the observed hysteretic patterns remains elusive. A coarse-grained model of SUHI coupled with a stochastic soil waterbalance is developed to demonstrate that the time lags betweenradiation forcing, air temperature, and rainfall generate a rate-dependent hysteresis, explaining the observed seasonal varia-tions of ∆Ts. If solar radiation is in phase with water availability,summer conditions cause strong SUHI intensities due to highrural evaporative cooling. Conversely, cities in seasonally dryregions where evapotranspiration is out of phase with radia-tion show a summertime oasis effect controlled by backgroundclimate and vegetation properties. These seasonal patterns ofwarming and cooling have significant implications for heat mit-igation strategies as urban green spaces can reduce ∆Ts duringsummertime, while potentially negative effects of albedo man-agement during winter are mitigated by the seasonality of solarradiation.},
  author       = {Manoli, Gabriele and Fatichi, Simone and Bou-Zeid, Elie and Katul, Gabriel G.},
  issn         = {1091-6490},
  journal      = {Proceedings of the National Academy of Sciences},
  keywords     = {Cities, Hysteresis, Seasonality, Surface temperature, Urban heat island},
  number       = {13},
  pages        = {7082--7089},
  publisher    = {National Academy of Sciences},
  title        = {{Seasonal hysteresis of surface urban heat islands}},
  doi          = {10.1073/pnas.1917554117},
  volume       = {117},
  year         = {2020},
}