The sun is hot in summer, unbearable, but the temperature below ground is very cool and comfortable. Take, for example, the average July temperature in Shanghai is 27.8 ° C, 0.8 m from ground level is 24 ° C, 1.6 m from only 20.6 ° C, 3.2 m lower down to 16.9 ° C. So when the ground is hottest, the deep ground is cool. In contrast, the average January temperature in Shanghai is 3.4 ° C, 0.8 m from the ground is 9.8 ° C. 13.1 ° C away 1.6 m, 3.2 m high to 17.3 ° C.
Ground and soil temperatures vary, which is related to the rock’s ability to transfer heat. Summer is at its hottest; the heatwave of the ground travels down very slowly. The hottest period near the ground occurs a little bit later than the hottest period, while the area is far from the ground. For example, the hottest period in Shanghai appeared in July, 0.8 m from the ground appeared in August, 1.6 meters from the ground appeared in September, 3.2 meters from the ground appeared in November. Similarly, when the coldest weather is on the ground, the cold waves cannot quickly affect the soil’s deep layer, so the coldest period occurs much slower underground than on the ground. In Shanghai, the coldest ground period appeared in January, 0.8 m from the ground appeared in February, 1.6 m away in March, 3.2 m away in April.
The above situation makes the cities in the high latitude region very similar to Shanghai. As the Russian city of Petecbua, 3 m from the ground, the coldest period in a year to 76 days behind the ground, when the hottest ground is in July, it is 3 m from the ground, the hottest in October. The coldest period comes 108 days slower than the ground. The coldest ground is in November; the coldest ground is 3 m from the ground in May.
In the heat transfer process deep into the soil, the soil layers must absorb some heat, so the deeper it goes, the less obvious the annual temperature change. At a certain depth, the temperature will remain constant. For example, the Paris Observatory, France, located 28 meters deep from the ground, has a thermometer that has remained only 11.7 ° C for over 200 years.
Because the hottest ground period, the deep ground is still cool, the coldest ground and the deep ground are still warm, so the water of some deep wells is warm in winter and cool in summer. Winter because underground is much warmer than the ground, so people in the North often dig deep tunnels to store vegetables very well.