The secret of the temperature of “it’s cold at high altitudes”

Produced by: Science Popularization China

Author: Meteorological Science Team

Producer: China Science Expo

The main heat source of the atmosphere is on the surface of the earth. The farther from the ground, the lower the temperature. The temperature decreases with increasing altitude. No wonder Su Dongpo of the Song Dynasty sighed: "It is too cold at high altitudes." In mountainous areas, the temperature at different altitudes also decreases with altitude. However, the measuring points in the mountains and the measuring points in the low plains are close to the ground heat source.

1. The secret of the temperature that “it’s cold at high altitudes”

Why is there such a temperature difference? The reason is that the mountains protrude from the free atmosphere. In addition to being affected by the ground heat source itself, the temperature on the mountain is also regulated by the temperature of the free atmosphere. The higher the mountain, the greater the difference between the mountain ground temperature and the free atmosphere temperature, and the more obvious the regulation of the free atmosphere on the mountain temperature.

Mount Lushan is shrouded in clouds and mist (Photo source: veer photo library)

For example, Mount Lushan is 1,132 meters higher than Jiujiang, but the average temperature in January in winter dropped from 4.2℃ to -0.2℃, and in July in summer it dropped from 29.4℃ to 22.5℃, a decrease of 4.4℃ in winter and 6.9℃ in summer.

We divide the temperature difference between the two locations by their height difference (in units of 100 meters) to get the temperature gradient between them. The temperature gradient between Jiujiang and Lushan is -0.39℃/100m in January and -0.61℃/100m in July.

The temperature gradient varies not only with the seasons, but also with the specific terrain conditions. For example, the temperature gradient on the northern slope of the Qinling Mountains is lower than that on the southern slope. The annual average temperature gradient on the northern slope is -0.45℃/100m, while on the southern slope it is -0.54℃/100m. The main reason is that in winter, cold air often gathers on the northern slope, reducing the temperature difference between the basin and the mountains.

The monthly temperature gradient on the north slope in winter is only -0.34℃/100m, while the south slope is located in the northerly direction of the cold air flow, and it still has a temperature of -0.54℃/100m in January. However, this situation does not exist in summer, and the temperature gradient on the north and south slopes is both -0.55℃/100m.

In addition, due to the regulation of the free atmosphere, the annual and daily temperature changes in the mountains also decrease with increasing altitude. The difference between the hottest month's temperature and the coldest month's temperature is called the annual temperature difference. The annual temperature difference in Jiujiang is 25.2℃, and it drops to 22.7℃ in Lushan. The annual temperature difference not only decreases with altitude, but also varies with different slopes.

The annual temperature difference in Xi'an, north of the Qinling Mountains, is 27.6℃, which drops to 24.2℃ in Huashan. However, the annual temperature difference in Ankang, south of the Qinling Mountains, is only 24.2℃, which is almost the same as that in Huashan. Of course, this is also related to the fact that Ankang is located in the south, with more clouds, fog and precipitation.

2. Temperature changes from the equator to the poles

Since solar radiation decreases with increasing latitude, the temperature of the entire earth decreases from the equator to the poles. However, this rule is often disturbed by other factors, and the temperature is not necessarily the same at the same latitude. Especially in high latitudes, the temperature difference between land and sea is very large.

The equator (left) and the poles (right) (Image source: veer Gallery)

In order to illustrate the effect of latitude on temperature, people compare the average temperature of latitude. The method is to obtain the temperature of equidistant points on the latitude circle every 10° from the monthly and annual isotherm chart, and then average them to get the average temperature of each latitude. Usually, the average temperature of latitudes of 0°, 10°, 20°, 30° and up to 80° is calculated.

The advantage of this is that the effects other than latitude cancel each other out, leaving only the latitude effect. From the average temperature of latitude, it is very obvious that the temperature decreases with increasing latitude.

For example, the average temperature of the year, whether in the northern or southern hemisphere, gradually decreases from the equator to the poles. The equator is 26.2°C, and it becomes negative near 58° latitude, and it is below -20°C at the poles. However, it is interesting that the hottest latitude on Earth is not the equator, but at 10° latitude in the northern hemisphere, which is called the "hot equator".

The equator is the hottest latitude only in the northern hemisphere winter. By July, the hottest average temperature has moved to 20° north latitude. In the southern hemisphere, due to the large ocean area, the law that the average temperature decreases with increasing latitude is more obvious.

The annual temperature range is the difference between the average temperature of the hottest month and the average temperature of the coldest month in a year. The annual temperature range increases from the thermal equator to the poles. The annual temperature range in Xisha (16°50′N) is only 6.0℃, while that in Mohe (53°28′N) is as high as 50.0℃.

This characteristic is directly related to the fact that the difference in solar radiation between winter and summer increases towards the poles. However, the annual temperature difference at all latitudes in the Southern Hemisphere is smaller than that in the Northern Hemisphere, which is largely related to the fact that the ocean area in the Southern Hemisphere is much larger than the land area.