Changes in the thickness temperature (TT) of the lower atmosphere from 1961 to 2025 were investigated using air pressure data from eight stations in the Central Mountain region of Japan. The stations were located at elevations ranging from 400 m to 1300 m above sea level. The average linear trend in TT at these stations was 0.25 °C per decade, which was close to the air temperature trend at non-urban stations in the surrounding coastal areas. Despite the presence of some divergence in TT trends across the stations, TT is expected to serve as a complementary index for climate change research, since concerns about spatial representativeness and temporal homogeneity exist in air temperature data.

Using surface observation data for the past hundred years, the contributions of year-specific climate anomaly, nationwide warming, and urban warming to hot summers in Japan were evaluated. A number of indices in temperature were defined to indicate the severity of summer heat in each year. Then, the year-to-year time series of each index was divided into a year-specific component and a temporally smoothed component, and the latter was divided into a nationwide non-urban component and an urban component. The results show that the non-urban component began to increase after the 1990s, which is approximately attributable to global warming, although there are some temperature variations on the yearly to multidecadal scales related to the Pacific Decadal Oscillation (PDO) and the Southern Oscillation (SO), whereas urban warming became apparent since the 1960s at stations in highly urbanized areas. For the recent record-breaking summer heat, the contributions of the year-specific temperature anomaly, nationwide warming, and urban warming are all evaluated to be of the order of 1 °C.

Long-term and contiguous records from the rural weather stations are necessary to estimate the rate of global warming without urbanisation bias. This study reconstructed the trends of the annual mean temperatures in rural Japan using digitised monthly maximum and minimum temperature data recorded from 1916 to 2023. Monthly temperature data from 1926 to 1940 were recently digitised from the printed materials of the Japan Meteorological Agency (JMA). While the reconstructed annual mean temperatures over Japan are in good agreement with those from previous studies for rural stations in Japan (population density < 100 people km(-2)), the linear rate of increase in the annual temperature averaged for 15 monitoring stations of the JMA showed a linear trend of + 0.15 degrees C decade(-1) for 1916-2023 higher than that averaged for our rural stations (+ 0.11 degrees C decade(-1)) due to the urbanisation bias. Interestingly, when the urbanisation biases were subtracted, a significant decreasing trend (i.e., a climatic jump) was detected in the rural temperature record during the 1960s. Comparing our rural temperature trend with those in other regions of the Northern Hemisphere and sea surface temperatures around the Japan region, the urbanisation bias likely partially masked the importance of natural variations such as PDO and solar variability.

A statistical study was conducted on weekday-holiday differences in atmospheric pressure in the central part of Tokyo and Osaka, and on surface wind fields in surrounding areas. The analysis for pressure was based on 33-year data from stations of the Japan Meteorological Agency (JMA). The analysis of winds was based on 44-year data from the Automated Meteorological Data Acquisition System (AMeDAS) of the JMA and 28-year data from the Air Pollution Monitoring System (APMS) of the Tokyo Metropolis. It was found that the pressure in central Tokyo was higher during the daytime on holidays than on weekdays, by approximately 0.045 hPa at 1500 JST. The daytime surface winds had a divergent anomaly within several tens of kilometers of the city center, corresponding to a wind speed anomaly of the order of 0.1 m s-1. Similarly, a pressure anomaly of approximately 0.015 hPa was found in the afternoon on holidays in central Osaka, as well as a divergent anomaly in surface winds in the surrounding area.

The climatological characteristics of cold-air damming (CAD) were statistically investigated on the Kanto Plain throughout the year. We detected 397 CAD events from 1991 to 2020 using hourly surface observation data from the Japan Meteorological Agency. The statistical analyses revealed the following characteristics: (1) CAD is most frequent in autumn followed by spring; (2) strong CAD is more frequent in the cold season than in the warm season; (3) The long-term trend in CAD frequency is not obvious; (4) 53% of the events finish in 12 h or less, and 17% of them continue for 24 h or more. CAD with precipitation tended to be stronger than that without precipitation, and the latter was more frequent in the nighttime than in the daytime, indicating the contribution of diabatic processes to the development of CAD. Analysis of synoptic fields in CAD cases using long-term reanalysis data revealed six patterns that differed in the intensity and position of surface highs and lows, corresponding to the typical seasonal sea level pressure fields.

The diurnal variation of boundary layer wind speed in Japan was statistically investigated using wind profiler data for 2002-2013. The analysis was made for 17 stations where the diurnal variation in surface air temperature had an amplitude of 2°C or more. It was found that the wind speed in the upper boundary layer (985 m above the surface) had a minimum in early afternoon in all seasons, in agreement with the general understanding that winds in the daytime boundary layer are reduced due to enhanced frictional force. However, the wind speed in the lower boundary layer (394 m) had a maximum in the afternoon at some stations in contrast to the general feature mentioned above. This afternoon maximum is more conspicuous in summer than in winter and in southern Japan than in northern Japan, and appears to be due to thermally driven local circulations. Nevertheless, the ratio of wind speeds at 394 m with respect to surface had a minimum during the daytime in all stations and seasons, indicating the daytime reduction of surface-relative boundary layer wind speed as a common feature over land.

Temperature trends in Japanese cities were analyzed using data at 433 stations on the AMeDAS network from April 1979 to March 2023. It was found that urban warming, defined by a temperature increase at an urban station relative to the surrounding non-urban stations, had slowed down in the latter part of the analysis period. The deceleration of urban warming was commonly found for northern, eastern, and western Japan, and not only for stations in densely inhabited areas but also those at weakly urbanized sites where the surrounding population density was 100-300 km−2. The deceleration was observed in all seasons and time of the day, although it tended to be more conspicuous in winter than in other seasons, and in the nighttime than in the daytime.

Wind speed differences between weekdays and holidays at urban sites in Japan were investigated in search of the influence of urban anthropogenic heat on surface wind speed using data from the Automated Meteorological Data Acquisition System (AMeDAS) of the Japan Meteorological Agency (JMA) for 44 years. The wind speed was found to be lower on holidays than on weekdays, not only in large cities but also in areas with medium degrees of urbanization, which is interpreted to be due to the stronger stability of the surface boundary layer under lower temperatures with smaller amounts of anthropogenic heat. The rate of decrease is about −3% in central Tokyo, and about −0.5% for the average over stations with population densities between 1000 and 3000 km−2. Additionally, an analysis using the spatially dense data on the Air Pollution Monitoring System of Tokyo Metropolis for 28 years showed that negative anomalies in wind speed on holidays were detected at many stations in the Tokyo Wards Area, although negative temperature anomalies were limited to a few stations in the central area or near big roads, suggesting different spatial scales in the response of temperature and wind speed to anthropogenic heat.

Using data at 724 stations for 12 years (2010-2021) in Japan, the climatological characteristics of diurnal variation in surface wind speed were examined. It was found that wind speed reached a maximum in early afternoon at most sites in all seasons. Even the 99.9th percentile winds have a weak peak in early afternoon as an average over all stations. In the spatial distribution, the amplitude and phase of the first harmonics of wind speed are positively correlated with those of temperature, but have larger variabilities among stations than those of temperature. The features of the diurnal variation are dependent on geographical factors such as the percentage of water surface, topographic convexity, and population density, with smaller amplitudes and earlier phases at coastal sites and later phases at urban sites. Some inland stations have an exceptionally high nighttime wind speed that is comparable to the daytime wind speed, apparently because of dominant mountain breezes.

Surface air temperature can show small-scale variations corresponding to uneven land use. Urban green spaces tend to form cool islands that mitigate the summer heat load, although they are sometimes warmer than the surrounding built-up area in the daytime of winter. Because of inhomogeneous surface heat budget, radiation, and ventilation, temperature can also be affected by microscale land properties and obstacles. This chapter presents the results of several observational studies on small-scale distribution of long-term mean temperature.

In Japan, approximately one thousand people die from heat stroke in a year. The majority of these deaths are among the elderly. This chapter presents the results of analyses on the relationship between heat stroke mortality and temperature based on data from the Vital Statistics of Japan, focusing on regional patterns and temporal variations. The features of the spatial distribution of heat stroke mortality rate differ with age groups, suggesting that mechanisms of action of heat stroke vary with age. For a fixed value of daily maximum temperature, daily heat stroke mortality rates are higher in prefectures with lower average summer temperatures, and are higher in the first half of summer than in the second half, implying acclimatization to high temperatures. Analysis of municipality-wise variation has revealed a negative correlation between heat stroke mortality and average income, suggesting the contribution of social factors for heat stroke casualty.

Along with the mortality, the number of ambulance transports is another measure indicating the number of heat stroke victims. This chapter presents the relationship between the number of heat stroke ambulance transports and temperature in Japan. The results are similar to those obtained for mortality (Chap. 5 ) in many respects. For example, the transport rate is higher for prefectures in cooler climates, and is higher in early summer than in late summer by a factor of two or more, for a fixed value of daily temperature. However, there are many transport cases of young people, with some features different from those of the elderly. The transport rate of young people is less affected by high temperature on the previous few days, suggesting that they are primarily influenced by on-site hotness during outdoor activities rather than accumulated heat load.

An overview of the geography and climate of Japan is provided. Japan consists of four main islands ranging from 30°N to 46°N, and many smaller islands. It is administratively divided into 47 prefectures. The population of Japan is 126 million, half of which are in the three metropolitan areas centered on Tokyo, Osaka, and Nagoya. Temperatures are cooler in the northern part of the country with a larger annual range. The midsummer season from late July to August is characterized by sunny and hot weather, during which the adverse effects of the UHI are experienced, with many heat stroke casualties.

Temperatures in cities have been increasing over time with the development of the UHI. This chapter presents observational facts regarding urban warming in Japan. Anomalous warming trends are found not only in large cities but also at weakly urbanized sites with population density of less than 300 km–2. The urban warming tends to be larger in the nighttime than in the daytime, in no precipitation cases than in precipitation cases, and under weak wind than under strong wind conditions. It is possible that the monitoring of the long-term temperature change in Japan is affected by urban warming, although stations in relatively small cities were used.

The observational features of the UHI are outlined. The urban–rural difference in surface air temperature (ΔTu–r) is largest in calm cloudless nights under strong surface inversion in the rural area, whereas the height of the nighttime UHI is a few hundred meters or less. The variability of ΔTu–r according to city size and weather conditions is described, including some of the problems in defining ΔTu–r. Several observational studies on UHI in Japanese cities are presented, and the mechanism underlying the formation of UHI is discussed with respect to anthropogenic heat release, low evapotranspiration from the surface, and thermal processes around buildings.

Urbanization affects not only temperature but also wind and precipitation. Urban winds are influenced by high surface friction, weak stability of the surface layer, and formation of local circulation because of high temperature. Cities tend to have low relative humidity, and Japanese cities tend to have low water vapor amount as well. Urban influence on clouds and precipitation has long been debated; however, a statistical study using satellite data has confirmed higher cloud amount over urban areas around Tokyo than in the surrounding rural area in the daytime of summer. Additionally, some observational studies and numerical simulations support the possibility of increase in convective precipitation in Tokyo in the warm season.

In Japan, the number of deaths from “exposure to excessive natural cold” reaches around 1,000 per year. Most of them are elderly people. In this chapter, the analysis used in Chap. 5 is applied to cold mortality based on data from the Vital Statistics. The cold mortality rate tends to be higher in prefectures with lower average winter temperatures, although the temperature dependence of cold mortality is weaker than that of heat stroke deaths. In terms of day-to-day variation, the cold mortality rate changes by 13–15% with the daily mean temperature, whereas no regional differences in mortality are found for a fixed value of daily temperature. The municipality-wise cold mortality rate correlates negatively with average income, and is particularly high in some wards of large cities with areas of poor living conditions.

Meteorological observations in Japan are primarily conducted by the Japan Meteorological Agency (JMA). In recent years, observations have been mostly automated, and the data are provided in digital formats. In addition, the data before the mid-twentieth century have been partially digitized. However, the data may not be perfectly homogeneous owing to changes in observation sites, instruments, and methods; hence, they should be used with care.

The relationship between summer mortality and mean temperature in Japan was analyzed using monthly data from 1951 to 2020. During this period, the mortality rate decreased greatly and the elderly population has largely increased in Japan. A positive correlation between mortality rate and temperature was detected for all the analysis periods except for a highly disturbed feature before 1970, whereas the range of age-adjusted mortality variation per 1 degrees C temperature anomaly has decreased by a factor of 5-10, indicating that the sensitivity of mortality to temperature has weakened over time. However, the mortality rate for deaths directly caused by heat decreased from the 1950s to the 1980s and then increased, showing a V-shaped change over the entire analysis period.

Warm season precipitation in most parts of Japan comprises early summer (Baiu) and autumn (Shurin) rainy periods with a relatively dry mid-summer. We aimed to determine details on the features of the seasonal progress of precipitation during the warm season in Japan. We assessed the timing of maximum and minimum precipitation based on daily records of Japanese dense local Kunai observations and AMeDAS networks collected at 522 stations for 95 years from 1926 to 2020. The maximum precipitation during Baiu has a northward delay, with a transition zone from the end of June to around July 10 at about 37 degrees N, whereas Shurin has three precipitation peaks corresponding to late August, mid-September, and early October over a wide area of Japan. The timing and intensity of the precipitation maximum varies according to the El Nino-Southern Oscillation (ENSO) phase, but the northward delay of the Baiu peak and multiple Shurin peaks are found both in El Nino and La Nina years. Toward the past 30 years, the Baiu has lengthened, the mid-summer minimum is earlier, and the main peak of Shurin precipitation has become less distinct.