Why are the hottest days during the "dog days" instead of the summer solstice?

We are lucky to live on the paradise of life - the earth. Here, there are not only conditions suitable for the survival and reproduction of organisms, but also the changes of the four seasons. There are colorful flowers in spring, and clusters of red leaves and abundant fruits in autumn. In summer, you can swim and fight the waves; in winter, you can challenge the severe cold and skate and ski. Why can we experience periodic changes in temperature throughout the year in the same place on the earth?

The orbit of the earth around the sun is not a perfect circle, but an ellipse. So sometimes the earth is closer to the sun, and sometimes farther away. Could it be that the change in the distance between the earth and the sun causes the change of seasons? The earth is closest to the sun in early January, at a distance of 147 million kilometers. That is when the northern hemisphere is coldest. The earth is farthest from the sun in early July, at a distance of 152 million kilometers, which is when the northern hemisphere is very hot. So the change in the distance between the earth and the sun is not the reason for the formation of the four seasons.

The colorful changes of the four seasons on Earth are caused by the Earth's rotation axis being tilted 23.5 degrees relative to the plane of its revolution around the sun. In summer, the Northern Hemisphere is inclined toward the sun, and the direction of the sunlight is nearly perpendicular to the ground, so we see the sun is very "high" in the sky. In winter, the Southern Hemisphere is inclined toward the sun, so people in the Northern Hemisphere see the sun very "low" in the sky. The sunlight is very "slanted".

The difference between the high and low points is obvious. If you shine a flashlight on the ground at night, you will find that when the flashlight is shone vertically downward, the illuminated area is the smallest. The more obliquely it is shone, the larger the illuminated area is. When the sun is "high", the heat in the sunlight is concentrated in a small area, so naturally the ground in this area receives more heat. When the sunlight is "oblique", the same amount of heat has to be dispersed over a larger area, so the amount of heat received per unit area is naturally less. Moreover, when the sun is "high", the sunshine time is also longer. The more "oblique" the sun is, the shorter the sunshine time is. These two factors together are the cause of the four seasons on Earth. Not only the Earth, but other planets in the solar system also have tilted axes of rotation, such as Mars (25.2 degrees) and Saturn (26.7 degrees). They also have four seasons.

At this point, everything seems clear. But think about it carefully, and the question arises again. On the summer solstice, the sun is at its "highest" in the sky, so it should be the hottest day. But in fact, the hottest day is not the summer solstice (around June 22), but the "dog days" more than a month later, which is about July 12 to August 27. On the winter solstice, the sun is at its "lowest" in the northern hemisphere, so it should be the coldest day. But the coldest day here is not the winter solstice (around December 22), but the "three-nine days" a month later, which starts in late January of the following year. What's going on?

The main reason is that the earth is wrapped in a dense atmosphere. Sunlight does not directly heat the air near the ground, but heats the ground first. The ground then transfers heat to the air through infrared radiation, air convection and water evaporation. There are about 10 tons of air above each square meter of the ground surface, which can store a lot of heat. This keeps the surface temperature from being too high. At night, this heat keeps the surface temperature from being too low. In this way, the temperature difference between day and night on the earth is only a few degrees to more than ten degrees. The air on Mars is thin, and the temperature difference between day and night can exceed 100 degrees. For artificial satellites operating in space, the temperature in the places where they are exposed to the sun can reach 100 to 200 degrees, while the temperature in the parts that are not exposed to the sun can drop to minus 100 to 200 degrees, because there is no air around to buffer it.

However, in this case, the average daily temperature does not depend entirely on how much heat is received from the sun, but on the difference between the heat received and the heat lost each day, which is the net change in accumulated heat. On the summer solstice, the northern hemisphere does receive the most heat from the sun, but in the dozens of days after the summer solstice, although the sun is not at its highest position, it is still quite high. The sunshine is not the longest, but it is still quite long. The heat received every day still exceeds the heat lost, so the daily average temperature continues to rise and reaches the highest in the "dog days". Only when the sun is "low" enough and the heat received every day is less than the heat lost, the temperature begins to drop. Similarly, after the winter solstice, although the sunshine gradually increases, the heat received every day is not enough to make up for the heat lost, so the "three nines" are the coldest. Only when the sunshine is strong enough to exceed the daily heat loss, it is spring when "the five nines and six nines, watching the willows along the river" arrives.

It's like boiling a pot of water. Using the biggest fire will naturally heat the water quickly, but using a slightly smaller fire will still increase the water temperature because the heat lost through the outer wall of the pot and the surface of the water is still less than the heat brought in by the fire. Only when the fire is small enough (if the stove can do that) will the water in the pot gradually cool down. When the fire is the smallest, the water temperature will naturally drop. But a slightly larger fire than the smallest fire will not stop the water temperature from continuing to drop.

By the same token, the sun is at its "highest" at 12 noon every day, but the highest temperature of the day is not at noon, but around 2 p.m.

If you are indoors and don't have air conditioning, the temperature changes are even more out of sync with the height of the sun because the house (walls plus furniture) can store a lot of heat. In summer, the hottest time indoors is not at noon, but in the evening. At that time, even though it is cooler outside, the walls and furniture are still constantly releasing the heat stored during the day, "roasting" us in the form of infrared radiation, so even if there is a cross-wind, it doesn't feel cool. Places near large rivers, such as Chongqing, Wuhan, and Nanjing in China, are particularly hot in the evening because the river water is heated by the sun during the day and the heat is released again after the sun "sets", forming famous "furnace" cities.

Interestingly, the temperature underground is not synchronized with the temperature on the surface. For example, August 6 is usually the highest average daily temperature on the surface. However, at 60 centimeters underground, the temperature is highest on August 20. At 1.5 meters underground, it will be September 10. At 3.5 meters underground, it will be October 29. At 9.4 meters underground, summer and winter are reversed from the surface. This is why cave dwellings are warm in winter and cool in summer. Moreover, because the time difference between winter and summer underground and the surface can be very large, the reproduction of some plant root cells actually takes place in winter and stops in summer.

The air on Mars is thin, and not only is the temperature difference between day and night large, but there is no obvious lag between the daily average temperature and the sunshine. Venus is a different story. Because the atmosphere is too dense (90 times the atmospheric pressure on Earth) and is mainly composed of greenhouse gas carbon dioxide, it is difficult for the heat from the sun to "escape", making the surface of Venus as hot as purgatory, and even lead will melt on it. In comparison, we are really lucky to live in the Earth's atmosphere.