Source丨Science Compound(ID:kexuedayuan)
Author 丨 Chen Kexin
On December 26, the Central Meteorological Observatory issued its first low temperature warning since the beginning of winter this year. It's been several days since the winter solstice, and the sunshine hours are gradually increasing, why is it still so cold?
(Source: China Meteorological Observatory)
Indeed, the winter solstice is the day of the year when the direct sun reaches the southernmost position of the Earth and the northern hemisphere has the shortest insolation hours. After the winter solstice, the direct solar point gradually moves north from the Tropic of Cancer, the length of the day in the northern hemisphere gradually increases, and the solar altitude angle (that is, the angle between the direction of the incidence of sunlight and the ground level) in most areas also gradually increases, and the northern hemisphere gradually returns to the "embrace" of the sun's warmth. In this way, the temperature should rise after the winter solstice, but it is not?
(Image source: veer)
Where does the Earth's temperature come from?
Before answering the above questions, we need to understand the energy sources behind the Earth's temperature.
The sun is the source of the earth's energy, it continuously transmits energy to the earth in the form of short-wave radiation, and is the fundamental driving force that drives the formation of the earth's weather and climate and its changes. But the Earth's temperature does not rise indefinitely because it absorbs solar energy. Because while the earth absorbs the amount of solar energy, it also emits energy into the space of the universe in the form of long-wave radiation.
While the Earth absorbs energy, it also releases energy
(Image source: NASA Robert Simmon)
According to the Stefan-Boltzmann law, the heat radiated by the surface of an object is proportional to the quaternary of its temperature, i.e. if the temperature of an object doubles (Kelvin temperature), the energy it radiates increases by a factor of 16 (quaternary of 2).
So if the Earth's temperature rises, the Earth will quickly radiate more heat into space. That is, the increase in temperature due to the absorption of solar energy will cause the Earth to produce greater heat loss, and this radiant cooling method is the main mechanism for preventing the Earth from heating out of control.
By calculating the input and output of energy, we can calculate the energy balance of the Earth system. The complex Earth system can be simply divided into three structures, from top to bottom: the top of the atmosphere, the atmosphere, and the earth's surface.
When the Sun reaches the top of the atmosphere, about 29 percent of the energy is reflected back into space by clouds, atmospheric particles, and bright ground such as sea ice and snow. The atmosphere and the Earth's surface absorb about 71% of the total incident solar energy (of which the atmosphere absorbs 23% and the surface absorbs 48%), and the Earth system absorbs an average of about 240 watts of solar energy per square meter in a year, so it must radiate just as much energy into space to keep the Earth's average temperature stable.
To maintain the balance of surface radiation, the surface radiates energy outward through evaporation (25%), convection (5%), and thermal radiation (17%). In addition to the 23% of the solar energy directly absorbed by the atmosphere, it also has the energy (25% + 5% + 17%) transmitted by the ground, so it must radiate outward at the top of the atmosphere to accumulate about 71% of the energy. On a long-term average, the amount of solar energy absorbed by the Earth is in equilibrium with the energy output to space, that is, the Earth is in radiation balance and the global temperature is relatively stable.
Map of Earth's energy budget (Image source: NASA)
Earth's radiation balance
So why isn't the world as hot?
Since the Earth is a spherical body, the Sun does not heat every place on Earth evenly. The length of the sun and the magnitude of the sun's altitude angle will affect how much solar energy is obtained somewhere on the earth, so the sun heats up the lower latitudes much more than the high latitudes. The long-wave radiation emitted by the earth is similar at all latitudes, which leads to energy surpluses in low latitudes and energy losses in high latitudes. This energy imbalance that occurs within the earth's gas system drives global atmospheric and oceanic circulation, transporting energy surpluses at low latitudes to higher latitudes to maintain the global energy balance.
Image credit: NASA Robert Simmon
So, when we discuss the temperature of a place, in addition to considering the solar energy absorbed by the internal gas system and the energy radiating outward during this time, we also need to consider the global transmission of energy. If during this period, the energy surplus of the earth gas system, the surplus energy can heat the surface and the atmosphere, so that the temperature rises; conversely, the energy of the earth gas system is lost, and the temperature is naturally reduced.
What makes the sun after the winter solstice
Become a "light bulb in the refrigerator"?
With all this knowledge, we can answer the question of why the weather is getting colder after the winter solstice.
From the winter solstice to the summer solstice of the following year, although the direct point of the sun gradually moves towards the northern hemisphere, the recovery of energy is a slow process. For some time after the winter solstice, although the amount of solar energy absorbed by the northern hemisphere increased, this energy could only reduce the "deficit" of lost energy.
Changes in the location of the sun's direct point over time (Image: Network)
Overall, the energy of every day in the northern hemisphere during this period is in a state of "not being able to make ends meet", so the temperature drops a little bit every day, so we will feel colder and colder. If the cold air from the north moves south, the temperature will be further reduced during this period.
Just as the so-called "heat is in three volts, cold is in three nines", the summer solstice and winter solstice are not the hottest or coldest times of the year. Due to the slow loss of heat, the third nine days after the winter solstice are the coldest period of the number nine mid-winter, that is, the "three nine four nine ice walk" chanted by the elders. If there is no snow and ice in the "Three Nines", and people do not feel particularly cold, then it is likely to usher in a warm winter.
(Image: NASA University of Maryland Steven Fons)
Finally, I have to sigh the depth of the ancients' perception of natural changes, although the sky can know the increase of sunlight after the winter solstice, but combined with the natural rhythm of continuous temperature decline, so the name "winter solstice" has the meaning of "winter is coming". Therefore, although the temperature in winter occasionally rises, friends do not easily change out of cotton pants, and still need to guard against cold waves cooling after "three nine four nine". After 9981 days (the ninth "nine days after the winter solstice"), you can dress lightly and embrace the spring.
bibliography
[1] Stephens,G.,Li,J.,Wild,M. etal. AnupdateonEarth'senergybalanceinlightofthelatestglobalobservations. NatureGeosci 5, 691696(2012)https://doi.org/10.1038/ngeo1580
[2] https://earthobservatory.nasa.gov/features/EnergyBalance
[3] https://baike.baidu.com/item/%E5%86%AC%E8%87%B3/9381?fr=aladdin
[4] https://zh.wikipedia.org/wiki/ earth's energy budget
Author Affilications:Institute of Atmospheric Physics, Chinese Academy of Sciences