Excuse me, what are the highest and lowest temperatures reported in the weather forecast?

The above picture shows the weather forecast data of Tianjin for one day. The highest temperature in a day is 7℃; The lowest temperature is MINUS 2 degrees Celsius.

2. Principle: Before explaining the maximum and minimum temperature of a day, let's talk about the warming process of the atmosphere. The temperature of the near-surface atmosphere (air temperature for short) does not come directly from solar radiation.

(1) During the day (as shown below), solar radiation passes through the thick atmosphere of the earth. Although the atmosphere can weaken the solar radiation ("reflection", "absorption" and "scattering" in the figure), most of the solar radiation can still reach the ground (solar radiation belongs to short-wave radiation, and the absorption capacity of the atmosphere for short-wave solar radiation is limited, so the warming effect is small). At this time, the atmosphere near the ground has not been effectively warmed up. When the ground absorbs solar radiation, it is released to the outside world in the form of ground (long wave) radiation. The ability of the atmosphere to absorb ground (long wave) radiation is much stronger than that of the sun (short wave) radiation. The near-surface atmosphere began to absorb heat and the temperature rose slowly. As heat transfer takes time, the following conclusions can be drawn:

① Solar (short wave) radiation plays the role of "the sun warms the earth"; Ground (long wave) radiation plays the role of "warming the atmosphere". The direct heat source near the ground should be ground (long wave) radiation;

② The solar (short wave) radiation is the strongest at 12, the ground (long wave) radiation is the strongest at 13, and the temperature near the ground is the highest at 14.

(2) At night (as shown below), the solar (short wave) radiation disappears, and the ground still provides energy to the near-surface atmosphere through the ground (long wave) radiation accumulated during the day. But with the deepening of the night, the intensity of ground radiation will become weaker and weaker, and the temperature will become lower and lower. Until sunrise the next day, there was still no solar radiation supply on the ground. It can be concluded that the lowest temperature is often before sunrise.

3. Summary: Based on the above, it can be concluded that the highest temperature in a day appears around 14, and the lowest temperature appears before sunrise. (The picture below shows the temperature statistics of some time periods in Tianjin on a certain day for reference. )

(Note: The wavelength of radiation depends on the temperature of the radiation object. The higher the temperature, the shorter the wavelength of radiation. If the surface temperature of the sun is above 5000, solar radiation is also called solar short-wave radiation. In contrast, the ground temperature is much lower, so ground radiation is also called ground long wave radiation)