Extreme Hot Days Research Articles

Decoupled heatwave-tree growth in large forest patches of Larix sibirica in northern Mongolian Plateau

Semiarid forests are experiencing unprecedented hotter drought, which could have a profound impact on tree growth. In addition to increasing temperature, the warming climate may cause more frequent and intense heatwaves further increasing the stress on tree growth. In this study, we measured tree ring width of 1139 cores collected from 12 Siberian larch (Larix sibirica) forest patches in the northern Mongolian Plateau to investigate how heatwaves affect tree growth. Our results showed a coupled relationship between tree growth and heatwaves that indicated by the extreme-hot days before 1996. After 1996, there was an increased frequency of heatwaves, which has strongly reduced tree growth, with an 8.4% growth decline in large forest patches (> 10 ha), 48.1% decline in medium patches (3–10 ha), and 39.2% decline in small patches (< 3 ha), respectively. The tree growth-heatwave relationship remained in medium and small forest patches after 1996, however, it became decoupled in large forest patches (p > 0.05), potentially through the effects of decoupled soil water-heatwave relationship in large forest patches that mostly located on thick soils. Our observations indicated that current climate warming and frequent heatwaves may have exceeded the favorable growth conditions of Siberian larch, but landscape heterogeneity (patch size and slope position) can alleviate the pressure of temperature warming and reduce tree growth rapid decline under high-frequency heatwaves climate in Mongolia Plateau.

A Retrospective and Prospective Examination of the 1960s U.S. Northeast Drought

AbstractAs the most severe drought over the Northeastern United States (NEUS) in the past century, the 1960s drought had pronounced socioeconomic impacts. Although it was followed by a persisting wet period, the conditions leading to the 1960s extreme drought could return in the future, along with its challenges to water management. To project the potential consequences of such a future drought, pseudo‐global warming simulations using the Weather Research and Forecasting Model are performed to simulate the dynamical conditions of the historical 1960s drought, but with modified thermodynamic conditions under the shared socioeconomic pathway SSP585 scenario in the early (2021–2027), middle (2041–2047), and late (2091–2097) 21st century. Our analysis focuses on essential hydroclimatic variables including temperature, precipitation, evapotranspiration, soil moisture, snowpack, and surface runoff. In contrast to the historical 1960s drought, similar dynamical conditions will generally produce more precipitation, increased soil moisture and evapotranspiration, and reduced snowpack. However, we also find that although wet months get much wetter, dry months may become drier, meaning that wetting trends are most significant in wet months but are essentially negligible for extremely dry months with negative monthly mean net precipitation. For these months, the trend toward wetting conditions provides little relief from the effects of extreme dry months. These conditions may even aggravate water shortages due to an increasingly rapid transition from wet to dry conditions. Other challenges emerge for residents and stakeholders in this region, including more extreme hot days, record‐low snow pack, frozen ground degradation and subsequent decreases in surface runoff.

Inhomogeneous trends in the onset date of extreme hot days in China over the last five decades

Using a homogenized daily maximum temperature (Tmax) dataset across China, this study characterized the spatiotemporal variation of the onset date of extreme hot days in a year (i.e., FirstEHD) during 1960–2018. Inhomogeneous trends of FirstEHD over China during 1960–2018 can be found, with the advanced trend of FirstEHD over most parts in China, while a number of stations in North-Central China (NC) show the delayed trend of FirstEHD. Moreover, there exist interdecadal changes of FirstEHD trend, with a remarkable difference in the trend magnitude before and after the 1990s over South China (SC), and the sign of trend can even reverse from negative to positive after the 1990s in Xinjiang (XJ) and Yangtze River Basin (YR), and from positive to negative in NC. The overall trends of FirstEHD over NC, YR, and XJ during 1960–2018 are dominated by the trends before the 1990s, while they are dominated by the sharp advance after the 1990s over SC. It is further found that the trend of FirstEHD can generally be explained by the long-term trend in Tmax over most parts of China, but the contribution from Tmax variabilities is also non-negligible and can even account for more than 75% of the overall trend over NC. The possible factors responsible for the decadal changes in FirstEHD trends are also discussed.摘要基于1960–2018年均一化日最高气温 (Tmax) 资料, 发现中国大部分区域高温初始发生日期 (FirstEHD) 呈提前趋势, 但华北地区一些站点呈推迟趋势, 这主要由上述区域 Tmax 的长期变化趋势决定, 但Tmax 变率的影响也不可忽视. FirstEHD的变化趋势在1990s出现年代际转折, 其中新疆, 华北和长江流域在1990s前后FirstEHD发生变化, 华南地区则表现为1990s前后 FirstEHD趋势幅度的不同, 但符号保持不变; 研究表明FirstEHD趋势的年代际转折转折与大气环流年代际趋势的变化紧密相关.

Effect modification of greenness on temperature-mortality relationship among older adults: A case-crossover study in China

BackgroundClimate change exacerbates temperature-related mortality, but effects may vary by geographic characteristics. We hypothesize that higher greenness may mitigate temperature-related mortality, and that the effect may vary in different areas. ObjectiveWe examined how mortality among older adults in China was associated with temperature for 2000–2014, and how geolocation and residential greenness may modulate this association. MethodsWe used health data from the China Longitudinal Healthy Longevity Survey (CLHLS), and meteorological data from the Global Surface Summary of Day (GSOD) product by National Climate Data Center. We used a case-crossover study design with distributed nonlinear modeling to estimate mortality risks in relation to temperature, and stratified analysis by quartile of greenness. Greenness was estimated by Normalized Difference Vegetation Index (NDVI) from remote-sensed imagery. In addition to the national analysis, we also assessed three provinces (Jiangsu, Guangdong, and Liaoning) to examine differences by climatic regions. ResultsExtreme temperatures had a significant association with higher mortality, with regional differences. Findings from the national analysis suggest that individuals in the lowest quartile of greenness exposure had a ratio of relative risks (RRR) of 1.38 (0.79, 2.42) for mortality risk on extreme hot days at the 95th percentile compared to those at the 50th percentile, compared to those in the highest quartile, which means those residing in the lowest quartile of greenness had a 38% higher RR than those residing in the highest quartile of greenness, where RR refers to the risk of mortality on days at the 95th percentile of temperature compared to days at the 50th percentile. The RRR for the highest to lowest quartiles of greenness for mortality risk on extreme cold days at the 5th percentile compared to the 50th percentile was 2.08 (0.12, 36.2). In Jiangsu and Guangdong provinces, both the heat effects and cold effects were the lowest in the highest greenness quartile, and the results in Liaoning province were not statistically significant, indicating different regional effects of greenness on modulating the temperature-mortality relationship. DiscussionWe elucidated one pathway through which greenness benefits health by decreasing impact from extreme high temperatures. The effects of greenness differed by climatic regions. Policymakers should consider vegetation in the context of climate change and health.

Prediction of summer extreme hot days in China using the SINTEX‐F2

AbstractAccurate prediction of extreme weather and climate is crucial for their devastating impacts on our society and environment. Here, predictability of frequency of summer (June–August) extreme hot days (SEHDs) in China has been assessed in the SINTEX‐F2 seasonal forecast system during the period of 1983–2015. The hindcast had 12 ensemble members and was initialized on the first day of March, April and May, respectively. Results show that overall, the SINTEX‐F2 predicts more regions with good prediction skills at shorter lead time due to its better capture of the linear trends. Whereas, in Southwest China and eastern Tibetan Plateau, the prediction skills are consistently increased at shorter lead time even without the impacts of linear trends; the correlation coefficients between the region‐mean anomalies in the observed and predicted SEHD frequencies are 0.74, 0.68 and 0.61 at 1–3 month lead, respectively, and they remain as high as 0.56, 0.49 and 0.40 when the linear trends are removed. This is because the SINTEX‐F2 can reproduce the observed influences of Indian Ocean Basin Mode (IOBM) on the SEHD frequency. The warm IOBM can cause anomalous ascending and strong divergence in the upper troposphere over the tropical Indian Ocean and Indian subcontinent. The strong divergence causes direct convergence and descending over Southwestern China and eastern Tibetan Plateau. It also causes substantial descending over the western North Pacific that can introduce anomalous cyclonic circulation over northeastern Asia via the Pacific‐Japan or East‐Asia pattern. The latter serves as a source of wave energies, excites the Rossby waves meandering westward in the mid‐latitude, and enhances the anticyclonic circulation anomalies over Southwestern China and eastern Tibetan Plateau.

Changes in the extreme air temperature in Czech Republic

This study is focused on air temperature in Czech Republic in relation to selected climatological indices for extreme hot and extreme cold days for the period 1961- 2017. The number of summer days (SU) and number of tropical days (SU30) have been chosen as the indicators of extreme hot days. For the indicators of extreme cold days, the number of frost days (FD) and number of icing days (ID) have been analyzed. The analyses are based on the daily data for air temperature measured at eight meteorological stations situated at the regions with different geographical and climate conditions. The data are freely available from the website of the Czech Hydrometeorological Institute. The results of the study show negative trend of extreme cold events and positive trend of extreme hot temperatures. With a few exceptions, the trend is statistically significant at p=0.05. The trend values for the indicators of cold days are higher than those for the indicators of warm days. The results show a faster change in air temperature for the cold part of the year

Investigating thermal comfort and energy impact through microclimate monitoring- a citizen science approach

The increasing frequency of extreme hot days has a considerable impact on health and energy. Heat stress related to high temperatures cause increased rates of mortality and morbidity, particularly amongst vulnerable populations such as those belonging to lower socio-economic groups. Most studies on heat related mortality and morbidity generally use temperatures from weather stations that do not consider the urban heat island (UHI) effect, leading to inaccurate predictions, particularly during heat waves. This study uses data collected by citizen scientists in predicting outdoor thermal comfort as well as indoor heat stress and peak cooling energy for low income housing in Australia. The results show that the outdoor Universal thermal climate index (UTCI) values estimated during hot afternoon ranged from 26 °C to 46 °C which equate to moderate to extreme heat stress. The indoor Discomfort index (DI) values calculated inside the living and bedrooms were below the heat stress limits (DI < 28) when Bureau of Meteorology (BoM) data was used as the input for calculation. However, indoor DI exceeded the threshold when actual on-ground data was used in the calculations, indicating that people will be at severe risks of heat related illnesses. It was also found that 60% of the time DI exceeded the threshold value as opposed to 2% with the use of BoM data. Furthermore, the peak cooling load was increased by 24% when on-ground data was used. The results show that low income houses and the neighbourhoods where they are located can cause significant heat related health risks which are normally overlooked in typical simulation studies. This study demonstrated a cost-effective way of collecting microclimate data for urban heat island mitigation and adaptation studies.

An integrated approach to project the future urban climate response: Changes to Lisbon's urban heat island and temperature extremes

Most state-of-the-art climate simulation ensembles misrepresent the effects of urbanization on local climate due to computational restrictions, posing a major limitation for urban climate projections. We circumvent this shortcoming by employing the SURFEX land-surface model coupled to TEB (Town Energy Balance) urban canopy model, forced by EURO-CORDEX regional climate simulations. We demonstrate the added-value of CORDEX-SURFEX-TEB compared to EURO-CORDEX in reproducing Lisbon’s temperature observations, halving the bias and improving the PDF matching by ~5%. Furthermore, the proposed framework allows characterizing the urban heat island (UHI) effect. Over Lisbon, the annual-mean UHI was ~+3 ºC (city warmer than rural) during nighttime, and around -0.5 to -1ºC (city cooler than rural) during daytime. The CORDEX-SURFEX-TEB climate projections for Lisbon, under fixed land-use and RCP8.5 scenario, showed a warming of ~+1ºC by 2021-2050 and ~+3.5ºC by 2070-2099. Despite the large multi-model spread, these long-term temperature changes were roughly constant for all hours and for urban and rural environments (corresponding to roughly constant UHI magnitude). Our results also reveal large seasonality of Lisbon’s UHI effect and of long-term warming. Finally, the projections show a striking growth in the number of extreme hot days and nights, respectively nearly tripling and quintupling its current value by 2070-2099.

Extreme hot days over three global mega‐regions: Historical fidelity and future projection

AbstractUsing a downscaled high‐resolution NASA Earth Exchange Global Daily Downscaled Projections (NEX‐GDDP) dataset based on Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations, this study evaluated and compared extreme hot days (EHDs) over the three mega‐regions [the Eastern United States (EUS), Europe (EU) and Eastern Asia (EA)] during the historical period (1981–2005) against observations, resulting in a subset of models with high skill for the past climatology and trend. The observed EHDs over EU exhibit the largest absolute amount and the most significant increases in frequency (4.0 days·decade−1), intensity (0.22°C·decade−1) and extent (8.4°C·days·ecade−1), while no significant trend over EUS is found. Compared with the observation, the largest bias in NEX‐GDDP is the remarkably overestimated increase in the trend over EUS. In the RCP8.5 projection using six models with relatively high fidelity, the increase of EHDs is evidently enhanced during 2030–2054 over the three regions, particularly EUS. The projected trend of EHDs over EUS could be undetermined because of the modelling biases in aerosol effects and internal variation, which is worthy of further investigation in CMIP6.

Doubling of U.S. Population Exposure to Climate Extremes by 2050

AbstractWe quantify historical and projected trends in the population exposure to climate extremes as measured by the United States National Center for Environmental Information Climate Extremes Index (CEI). Based on the analyses of the historical observations, we find that the U.S. has already experienced a rise in the occurrence of aggregated extremes in recent decades, consistent with the climate response to historical increases in radiative forcing. Additionally, we find that exposure can be expected to intensify under the Representative Concentration Pathway 8.5, with all counties permanently exceeding the baseline variability in the occurrence of extreme hot days, warm nights, and drought conditions by 2050. As a result, every county in the U.S. is projected to permanently exceed the historical CEI variability (as measured by one standard deviation during the 1981–2005 period). Based on the current population distribution, this unprecedented change implies a yearly exposure to extreme conditions for one in every three people. We find that the increasing trend in exposure to the aggregated extremes is already detectable over much of the U.S., and particularly in the central and eastern U.S. The high correspondence between the pattern of trends in our simulations and observations increases confidence in the projected amplification of population exposure to unprecedented combinations of extreme climate conditions, should greenhouse gas concentrations continue to escalate along their current trajectory.

Thermal Performance of Vertical Courtyard System in Office Buildings Under Typical Hot Days in Hot-Humid Climate Area: A Case Study

Due to the different types of courtyards in vertical courtyard system (VCS), their impacts on thermal performance in office buildings may vary. To better understand this issue, this paper investigates the thermal performance impact of three typical vertical courtyards. A field case study was conducted in VCSs during two typical extreme hot days under hot-humid climate conditions. The results show that the vertical courtyards have significant cooling effects under hot-humid climatic conditions. Via testing on linear, integrated, and rooftop courtyard with fusion layout, the fusion one has an obviously positive impact on air temperature reduction (4.3 °C). Compared with the linear and integrated courtyards, the maximum air temperature difference of fusion layout is around 1.6 °C. The thermal radiation environment of the fusion layout was better than that of the other two (linear and integrated). Besides, the surface temperature of the pavements (wood panel) in the vertical courtyards can reach 47 °C, while the vegetation can lower it by 8 °C under the same weather conditions. These findings show that the courtyard with fusion layout is more suitable for extreme hot weather conditions.

Performance prediction and cost-effectiveness analysis of a novel natural draft hybrid cooling system for power plants

Natural draft dry cooling system (NDDCs) utilizes ambient air as cooling medium and is susceptible the ambient conditions. Natural draft wet cooling system (NDWCs) can provide adequate cooling capacity by consuming large amount of water but forms visible plumes. A novel natural draft hybrid cooling system (NDHCs) which consists of both dry and wet components is proposed in this study as a means to be in conflict with the protection and conservation of water resources. With full consideration of its control equations of flow, heat and mass transfer, an iterative algorithm is developed to predict the performance of NDHCs. Coupled to a practical coal-fired power unit, the annual performances of the three systems, as well as the trade-off of the water consumption rate and saved coal consumption rate are explored. The results indicate that during extreme hot days, NDHCs shows more priority over NDDCs and NDWCs. The annual performance of NDHCs is better than NDDCs and NDWCs due to its parallel connection of air, which brings more air passing through the tower. Compared with NDDCs, the playback period of the modification is only 1 year though there is a slightly underestimation due to the structure difference of tower. Furthermore, NDHCs can avoid plumes and further pollution caused by the plumes effectively, since the saturated/supersaturated air will be warmed by the air from dry section and within the sub-saturated region.

Climatology and the Interannual Variability of the High‐Temperature Extremes in Taiwan

AbstractThis study analyzed the temporal/spatial distribution of the high‐temperature extremes (HTEs) in Taiwan. Three major weather types for HTEs were identified: the southerly/southwesterly (S/SW) (47.6%), western North Pacific subtropical high (SH) (35.9%), and tropical cyclone (16.5%). The S/SW type features anomalous warm advection in lower atmosphere and anomalous subsidence in all the altitudes around Taiwan. Different from the S/SW type, the SH type warms the surface primarily with increasing downward shortwave radiation. In June, the S/SW type was the most common. In July, both the S/SW and SH types dominated. In August, the tropical cyclone and SH types were more common. The subseasonal variation of extreme hot days (EHDs) for these three weather types is examined. Taipei and Dawu were two HTE hot spots in Taiwan. EHDs in Taipei occurred mostly in July and August, with the SH being the major type. Most EHDs in Dawu occurred in June, with the S/SW being the predominant type. These differences likely resulted from not only the subseasonal monsoon variation but also geographical differences. The interannual variability of the summer EHDs in Taiwan was found to be related to the phase and intensity of the El Niño‐southern oscillation events. The number of EHDs increased in the El Niño decaying summers and decreased in the La Niña decaying summers. The correlation between the number of summer EHDs and the Niño 3.4 SST anomaly reaches its peak in the previous winter (November/December). The associated mechanisms were discussed in the article.

Limiting global warming to 1.5 °C will lower increases in inequalities of four hazard indicators of climate change

Clarifying characteristics of hazards and risks of climate change at 2 °C and 1.5 °C global warming is important for understanding the implications of the Paris Agreement. We perform and analyze large ensembles of 2 °C and 1.5 °C warming simulations. In the 2 °C runs, we find substantial increases in extreme hot days, heavy rainfalls, high streamflow and labor capacity reduction related to heat stress. For example, about half of the world’s population is projected to experience a present day 1-in-10 year hot day event every other year at 2 °C warming. The regions with relatively large increases of these four hazard indicators coincide with countries characterized by small CO2 emissions, low-income and high vulnerability. Limiting global warming to 1.5 °C, compared to 2 °C, is projected to lower increases in the four hazard indicators especially in those regions.

A surface modelling approach for attribution and disentanglement of the effects of global warming from urbanization in temperature extremes: application to Lisbon

Attribution and disentanglement of the effects of global greenhouse gas and land-use changes on temperature extremes in urban areas is a complex and critical issue in the context of regional-to-local climate change mitigation and adaptation. Here, an innovative modelling framework based on a large ensemble of urban climate simulations, using SURFEX (a land-surface model) coupled to TEB (an urban canopy model), forced by E20C (a GCM-based reanalysis), is proposed, and applied to the capital of Portugal—Lisbon. This approach allowed to disentangle the main drivers of change of extreme temperatures in Lisbon, while also improving the simulated summer temperature variability compared to E20C, using station observations as reference. The improvements were physically linked to the strong sensitivity of summer mean and extreme temperatures to local land-use properties. The sensitivity was systematically investigated and robustly demonstrated here, with built-fraction (buildings + roads), albedo and emissivity emerging as key surface parameters. The results revealed a very strong summer temperature increase between 1951–1980 and 1981–2010 periods: 0.90 °C for daily maximum temperature (Tmax), and 0.76 °C for daily minimum temperature (Tmax). These changes were sensitive to considering different (but constant throughout the simulation) land-uses, varying by about 10% for Tmax, and around 17% for Tmin. Regarding the temperature extremes (quantified by extreme hot days, EHD, and extreme hot nights, EHN, respectively defined as exceeding the 95th-percentile of Tmax and Tmin) the changes and their dependencies with the land-use are much more drastic. The isolated effect of changing land-use (keeping the climate forcing unchanged) from rural/natural (low built-fraction) towards dense urbanization (high built-fraction) caused a significant increase in EHN (up to ∼+130 d per 30 years, larger than the effect due to climate forcing alone), and in EHD (∼+60 d per 30 years, which is similar to the effect due to climate forcing alone).

Skillful Seasonal Prediction of Eurasian Winter Blocking and Extreme Temperature Frequency

AbstractAtmospheric blocking is a major producer of extreme weather events in midlatitudes that have profound socioeconomic impacts. However, few strides toward seasonal prediction of atmospheric blocking have been made. Here, we developed a new statistical model for prediction of the winter seasonal blocking frequency over Eurasia 1 month in advance using sea surface temperature, geopotential height at 70‐hPa, and sea ice concentration as predictors, and the model captures more than 65% of the interannual variance. Furthermore, we applied the same predictors used for blocking prediction to predict the seasonal occurrence of winter extreme hot and cold days, and skillful prediction was achieved over Greenland and large portions of Eurasia. The predictive models provide insight into the seasonal predictability of atmospheric blocking and extreme temperature and also aide in valuable decisions across a variety of sectors.

Optimization and performance of a small scale adsorption cooling system fully powered by solar energy for hot arid areas

The aim of this study is to identify the optimal design parameters of a fully solar-powered adsorption cooling system for residential buildings located in hot arid areas. The proposed system consists of solar collectors, hot water tank, water–silica gel adsorption chiller of 8 kW cooling capacity, cold water tank, fan coil unit, and cooling tower. An optimization based on computer simulation has been conducted for achieving the aim of this study. The proposed system was applied to Assiut, Egypt as a case study. The optimization shows that the optimal design parameters of the proposed system are as follows: 58 m2 solar collecting area, 5.5 m3 hot water tank and 1 m3 cold water tank. The simulation of the proposed system with the optimal design parameters during the design day shows that the chilled water temperature leaving the chiller is between 17 and 18 °C with high cooling water temperature of 34.5 °C. This makes the system unable to achieve the indoor thermal design conditions. However, the system is able to meet the cooling demand in most days of the cooling season except for about 9.15% representing extreme hot days. The simulation also shows that during the design day, the proposed system achieves daily solar collecting efficiency of 56% and can produce chilling energy of 113.3 kWh with 7.35 kW energy rate and daily COP of 0.41. In addition, the cold water tank enables the system to extend the operation period for 110 min after switching off the chiller.

Impact of Extremely Hot Days on Emergency Department Visits for Cardiovascular Disease among Older Adults in New York State.

Prior studies have reported the impact of ambient heat exposure on heat-related illnesses and mortality in summer, but few have assessed its effect on cardiovascular diseases (CVD) morbidity, and the association difference by demographics and season. This study examined how extremely hot days affected CVD-related emergency department (ED) visits among older adults from 2005–2013 in New York State. A time-stratified case-crossover design was used to assess the heat–CVD association in summer and transitional months (April–May and September–October). Daily mean temperature >95th percentile of regional monthly mean temperature was defined as an extremely hot day. Extremely hot days were found to be significantly associated with increased risk of CVD-related ED visits at lag day 5 (OR: 1.02, 95% CI: 1.01–1.04) and lag day 6 (OR: 1.01, 95% CI: 1.00–1.03) among older adults in summer after controlling for PM2.5 concentration, relative humidity, and barometric pressure. Specifically, there was a 7% increased risk of ischemic heart disease on the day of extreme heat, and increased risks of hypertension (4%) and cardiac dysrhythmias (6%) occurred on lag days 5 and 6, respectively. We also observed large geographic variations in the heat–CVD associations.

Climate Change Projections of Extreme Temperatures for the Iberian Peninsula

The comprehensive characterization of heat waves and extreme hot days is fundamental for policymakers due to its vast implications for human health. This study evaluates extreme temperature changes over the Iberian Peninsula for the present climate and future projections, considering extreme temperature indices, cold/heat waves, and a recovery factor, using the Weather Research and Forecasting model. The projected temperatures show an increase of over 6 °C. An increase in the number of summer days and tropical nights and a decrease in frost days is expected. The number of heat waves and their duration and intensity are expected to increase. The number of heat wave days are expected to increase, with much of the average summer season being under heat wave conditions. The recovery factor is expected to decrease. Cold spells are projected to decrease in terms of number, intensity, duration, and number of spell days, whereas the recovery factor is expected to increase. Heat wave analysis was combined with maximum temperature thresholds to isolate extreme heat waves. The results show an increase in extreme heat wave days, with regions experiencing over 10 heat wave days with maximum temperature surpassing 45 °C for the long-term future.

Detecting spatial differences in thermal stress across China

In this study, we investigate spatial differences in thermal comfort conditions using the net effective temperature (NET) considering the synthetic effects of air temperature, relative humidity, and wind speed. Using a daily-scale dataset of maximum air temperature (Tmax), relative humidity, and wind speed from 518 stations during 1960–2016 across China, we analyze the influence of different climate conditions on NET or Tmax at three different levels of hot conditions (35 °C < Tmax < 37 °C, NETTmax35 for HT35 cases; 37 °C < Tmax < 40 °C, NETTmax37 for HT37 cases; NET = 27 °C, NET27 cases). In HT35 (HT37) cases, NETTmax35 (NETTmax37) can reach up to 32 °C (34 °C) in southern China and also can be less than 29 °C (31 °C) in western Northwest China. In NET27 cases as the threshold for the thermal sensation of very hot, Tmax should be over 33 °C in western Northwest China and was less than 30.5 °C in southern China, by contrast. With global warming, there is an increasing trend in the number of extreme hot days in most part of China, but a decreasing trend is detected in the part of Jianghuai region, partly due to the decreasing trend in Tmax.