What are Heating Degree Days?

The highest temperature in meteorology generally refers to the highest value of temperature in a certain period of time. The daily maximum temperature refers to the highest temperature reached on the surface of the measurement site that day. The highest temperature in a day generally appears at 14-15 ^[1] . This time is the hottest period of the day.

The main factor affecting air temperature is the intensity of solar radiation, and solar heat cannot directly increase air temperature. Various gases in the air directly absorb about 14% of the thermal energy of the sun, and about 43% are absorbed by the ground. The ground absorbs solar radiation heat and then conducts it to the air through radiation, convection, turbulence, etc. The result of this upward transfer of heat is the main reason for increasing the temperature. Therefore, the increase in ground temperature is mainly the result of absorbing solar radiation energy.

The sun shines on the ground and heats the ground. The heat absorbed by the ground is released into the hot air. The temperature of the air is mainly obtained indirectly from the ground. Although at noon in summer, the sun's rays are closest to the ground, and the ground and air receive strong heat, but the heat emitted from the ground is less than the heat supplied by the sun, so it is not hot at this time. After noon, the ground temperature can continue to rise, and the ground temperature can reach its maximum when the heat released from the ground is equal to the heat supplied by the sun. The increase in the temperature near the ground must be obtained from the heat emitted from the ground. It takes time, so the hottest time in summer is not noon, but at 2-3 o'clock in the afternoon.

In the same way, after the sun goes down, both the air and the ground lose the supply of solar heat, so they start to lose heat continuously, and the temperature continues to decrease. By the early morning of the next day, the ground temperature drops to the lowest value. So generally the air temperature is the lowest before sunrise. ^[1]

In order to improve the accuracy of regional climate simulation, it is necessary to convert GCM information into regional-scale surface information through downscaling methods. There are two main downscaling methods commonly used at present: one is a dynamic downscaling method, and the other is a statistical downscaling method. Dynamic downscaling adopts regional models or finite fields to simulate nested forecasts. This method requires better resources and computer conditions. Through nested regional simulations, higher-resolution local climate variables can be obtained to achieve time and space. Downscaling. The statistical downscaling method is relatively simple. It uses years of observation data to establish a statistical relationship between large-scale climate conditions (such as atmospheric circulation) and regional climatic elements, and uses independent observations to test this relationship. It is applied to large-scale climate information output by GCM to obtain regional climate information.

For the statistical downscaling of temperature, many studies have been done at home and abroad over the years. Researchers use ANN, MLR, CCA, SDSM (Statistical Downscaling Model), SDSM and other statistical downscaling methods to simulate the maximum and minimum temperatures.

In terms of daily temperature downscaling, some researchers use typical correlation analysis, principal component analysis, singular value decomposition, and multiple linear regression methods to simulate the daily average temperature in Central Europe. Paulin Couli compared the Genetic Programming (SDS) and SDSM methods for studying daily extreme temperatures in southeastern Canada. The GP method is based on the idea of establishing the relationship between large-scale factors and simulated values. However, unlike the traditional method, the GP method does not determine the form of the model when it builds the model, but iterative calculations are repeated to obtain the final model. The advantage of this method is that the model is more accurate, but correspondingly, repeated experiments lead to a large amount of calculation.

It can be seen that, for the statistical downscaling of daily temperature, previous researches mainly used some kind of statistical method to target smaller areas, which is more tedious to calculate for a wide range of downscaling problems; SDSM method is mainly for a period of time. The random reproduction of elements subject to a certain distribution characteristic is not a suitable method for the specific downscaling of a certain day's temperature. Therefore, it is very important to develop a simple and effective downscaling method. The interpolation method is undoubtedly the simplest downscaling method. Foreign scholars have also applied different interpolation methods for some downscaling studies, but because it treats all adjacent information completely and equally, it is difficult to become a very effective one. Downscaling method. ^[3]

What we generally mean by the maximum / minimum temperature refers to the maximum and minimum temperature meters used by meteorological stations, and the instantaneous maximum and minimum values of air temperature in a day at a height of 1.5m from the ground observed in a louver. The maximum and minimum temperature values, and the appearance time cannot be accurately observed due to the limitation of the observation instrument. The daily maximum and minimum temperature and the occurrence time are closely related to atmospheric physical processes such as water vapor evaporation and condensation. It is a judgment of the cold wave. , Frost, high temperature, cold damage and other important indicators of the intensity of extreme weather events. Studying the characteristics of the changes in the daily maximum and minimum temperatures at different heights near the ground and the rules of their occurrence time, we can accurately understand when the extreme weather events occur, the greatest harm, and when the chemical reactions in the atmosphere are most intense Has a very important role. In addition, it is very important to study the changes in the daily maximum and minimum temperature and the occurrence time of global and regional climate change, the conversion of energy between ground and atmosphere, changes in atmospheric chemical composition, and concentration. In recent years, the research on global climate change has become a hotspot for scientists. There are people focusing on the interannual changes in the maximum and minimum temperatures, regional distribution characteristics, asymmetric changes, and the relationship between the changes in maximum and minimum temperatures and other meteorological elements Related research has been carried out. In general, the daily maximum and minimum temperature levels and their occurrence times are mainly affected by the diurnal variation of the sun. However, due to the difference in surface conditions at different heights near the ground, the weather conditions are different. ^[2]

As global warming intensifies, people are increasingly aware that meteorological factors such as temperature play a significant role in human health. Cardiovascular and cerebrovascular disease is a common disease that endangers human life and health, and its relationship with temperature has become the focus of research. Relevant research points out that the effects of extreme temperature on myocardial infarction in May-September are as follows: the extreme temperature makes the cumulative mortality rate increase by 1.24% (95% CI: 1.57) % ~ 4.73%), 4.68% (95% CI: 1.56% to 7.89%), 4.37% (95% CI: 0.15% to 8.78%), this The results suggest that the summer maximum temperature is one of the indicators that can more sensitively reflect the effect of temperature on cardiovascular and cerebrovascular diseases.

With the simultaneous control of relative humidity, average daily air pressure, and average wind speed, deaths of cardiovascular and cerebrovascular diseases increased by 17.3% (OR = 1.173, 95% CI: 1.149 ~ 1.197) and 4.2% (OR = 1.042, 95%, CI: 1.011 ~ 1.074); and every 1 increase in the maximum daily temperature in autumn, cardiovascular and cerebrovascular diseases die. The cases decreased by 25% (OR = 0.750, 95% CI: 0.727 ~ 0.73).

At present, the results of studies on the relationship between air temperature and cardiovascular and cerebrovascular disease at home and abroad can be roughly summarized as the "u", "V" or "J" relationship between temperature and the incidence or mortality of cardiovascular and cerebrovascular disease, ie When the air temperature is lower than or higher than a certain critical temperature, the incidence and mortality of cardiovascular and cerebrovascular diseases gradually increase as the temperature decreases or increases. In terms of the mechanism of the effects of air temperature on cardiovascular and cerebrovascular diseases, human body temperature is achieved by the body adjusting the dynamic balance of heat production and heat dissipation. The temperature adjustment ability and temperature sensitivity of patients and weak people are poor, which reduces the ability to maintain normal body temperature, increases the risk of heterogeneous exposure to high or low temperature, and suffers from cardiovascular and cerebrovascular disease or People with obstacles to the circulatory system. In summer, the high temperature puts their body temperature regulation system in an "overload" state, feeling "heat stress" and increasing the risk of death. Therefore, hot weather often leads to excess death, indicating that the heat increases the load on the originally damaged system and is an external cause of death. ^[4]