How do fronts and air masses affect the weather?

1. The concept of air mass

Generally speaking, due to the differences of latitude, underlying surface, topography and vegetation, soil water content and other factors, the temperature, humidity and stability on the ground are different and uneven in horizontal and vertical directions, which is also an important feature of the troposphere. But as far as the vast area is concerned, there is still a large area of air with uniform physical properties (temperature, humidity, stability, etc.). ) the consistent change in horizontal and vertical directions is called air mass. Generally, the horizontal range of air mass can reach several thousand kilometers, and the vertical range is several kilometers to dozens of kilometers, and it can often develop to the tropopause.

The air mass is formed under a wide range of uniform underlying surface and suitable circulation conditions. Therefore, to form an air mass, we must first have a large-scale and relatively unified underlying surface, that is, the source of the air mass, such as a vast ocean, a huge desert or frozen land. Different sources form air masses with different physical properties; Besides the source, the formation of air mass also needs suitable circulation conditions, that is, relatively stable circulation, so that a large range of air can stay on this underlying surface for a long time, and gradually obtain relatively uniform physical properties suitable for the underlying surface through physical processes such as radiation, convection, evaporation and condensation.

After the formation of the air mass, when the circulation situation changes and it is far from the origin, the physical properties of the air mass also change due to the change of the underlying surface. This is called air-mass property, also known as denatured air mass.

2. Air mass classification

According to the different physical properties of air masses or the geographical differences of air masses in the source area, there are two kinds of thermal classification and geographical classification.

Thermal classification is to divide air masses into cold air masses and warm air masses according to the temperature contrast between air masses and the underlying surface. If the temperature of an air mass is higher than that of an adjacent air mass, or moves to the underlying surface colder than it, so that the ground it passes through warms up and it gradually cools down, this air mass is called a heating air mass; If the temperature of an air mass is lower than that of an adjacent air mass, or it moves to a warmer underlying surface, so that the ground it passes through becomes cold and gradually warms up, this air mass is called a cold air mass. When the warm air mass contacts with the cold underlying surface, it transfers heat to the underlying surface, and the lower air of the air mass cools rapidly due to the heat transfer, forming an inversion temperature and increasing the stability, so the warm air mass belongs to a stable air mass. Under the control of the warm air mass, water vapor condenses due to the cooling of the lower air, resulting in weather phenomena such as advection fog, low clouds or Mao Mao rain. The cold air mass is in contact with the hot underlying surface, which absorbs heat from the underlying surface, and the air in the lower layer becomes hot, forming an unstable state, so the cold air mass belongs to an unstable air mass. Under the control of unstable air mass, convective movement is easy to occur, resulting in convective clouds and precipitation or thunderstorm weather.

According to geographical classification, air masses can be divided into four categories: Arctic (Antarctic) air masses, polar air masses, tropical air masses and equatorial air masses. Among them, the first three types of air masses can be divided into continental air masses and marine air masses. The equatorial region only has the conditions to form air masses in the ocean, so there are only equatorial ocean air masses, as shown in the following table.

3. Main air masses affecting China.

(1) Denatured polar continental air mass from Siberia and Mongolia, or Denatured Siberian air mass, can affect all parts of China in winter, and can only reach North China and Northwest China in summer, and occasionally reach Huanghuai Valley. Under the control of this kind of air mass, in winter, the continental radiation cools strongly, the temperature is low, the water vapor content is low, the inversion layer often appears, and the weather is fine, especially in the area near the center of the metamorphic polar continental air mass.

(2) Tropical marine air mass from tropical Pacific Ocean and South China Sea, also known as tropical Pacific air mass. In summer, all parts of the country can be affected by it except some parts of the northwest; Winter only affects South China and Southwest China, and sometimes it can also affect East China and South China. Under the control of this air mass, the lower layer is generally unstable, the middle layer often has a sinking inversion layer, and the atmosphere is generally good. Cumulus clouds often appear on land in the afternoon. When the vertical movement develops in summer, the inversion layer above it is often destroyed, and convective weather may occur. (3) In summer, the tropical continental air mass from the subtropical region of the European continent affects the western part of China and sometimes North China. Under the control of this air mass, the weather is generally hot and dry, the ground temperature is high, reaching above 35 ~ 40℃, and the air layer is unstable. Because of the low water vapor content in the air, it is sunny and cloudy.

(4) The equatorial air mass from the Indian Ocean, also known as monsoon air mass, can affect South China in summer, causing a lot of precipitation in the south of the Yangtze River basin.

Second, the front.

1. The previous concept

When the cold air mass and the warm air mass meet, a narrow and inclined transition zone is formed between them. Its width is about tens of kilometers near the ground air layer, and it can reach 200~400 kilometers at high altitude. Compared with large-scale air mass, the width of transition zone is very narrow, which can be regarded as a geometric surface, called front. The air mass properties on both sides of the front are very different, and the meteorological elements and weather phenomena change greatly. The intersection of the front and the ground is called the front. It is customary to call the front and the front the front.

The front line is thousands of kilometers long and hundreds of kilometers short. The front is a weather system with a three-dimensional spatial structure. Because of the high density of cold air, the front is inclined to the side of the cold air mass with height in space, so the cold air mass is below the front and the warm air mass is above the front. Usually warm air will climb up along the front, adiabatic cooling, easy to condense water vapor. Therefore, the front indicates rainy weather.

2. Frontal and weather classification

According to the movement of fronts, fronts can be divided into four types: warm fronts, cold fronts, quasi-static fronts and occluded fronts. (1) During the movement of the warm front, the warm air mass plays a leading role, pushing the front to move to the side of the cold air mass, which is called the warm front. On the warm front, warm air climbs slowly along the front, generally rising to 5 ~ 6 kilometers or even reaching the tropopause. The gradient of a warm front (the inclination of the front) is generally around1150, so the front covers a wide area on the ground. Warm air is adiabatically cooled in the rising process. If the air humidity is high at this time, a series of clouds will form on the front, which is called frontal cloud system. The warm front cloud systems are cirrus (Ci), cirrostratus (Cs), stratosphere (As) and nimbostratus (Ns) in turn. As the front moves, these changing clouds will be observed at a certain point on the ground, and the weather will change from sunny to cloudy to rainy. Because the gradient of warm front is small and the warm air convection is weak, the precipitation range is wide, with an average width of about 300 ~ 400 km, while the precipitation intensity is small and the precipitation duration is long. In the precipitation area, when the water vapor content in the cold air mass is saturated by the evaporation of rain in the lower air mass, some broken rain clouds (Fn) may appear in the low altitude, and frontal fog may be formed. After the ground front moved over some areas, the weather gradually cleared up, the temperature rose and the air pressure dropped.

When the warm air itself is dry or affected by the underlying surface such as topography, there may be no clouds on the front, or only some high clouds and no precipitation. However, in summer, if the warm air itself is humid and unstable, and the internal disturbance is strong, there may be cumulonimbus clouds (Cb) and thunderstorms on the front.

Warm fronts are relatively rare in China, generally appearing in the Jianghuai Basin and Northeast China in spring and autumn, and mostly appearing in the Yellow River Basin in summer. (2) During the cold front movement, the cold air mass plays a leading role, pushing the front to move to the side of the warm air mass, which is called the cold front. According to the moving speed of cold front, it can be divided into type I cold front (slow moving cold front) and type II cold front (fast moving cold front).

A.I Cold Front This type of cold front moves slowly, and warm air rises slowly along the front, and the slope of the front is about1100. Frontal cloud system is similar to warm front, but its arrangement order is just the opposite. Rain clouds appeared in the sky and began to rain. As the front moves, the clouds gradually rise, the precipitation stops, the air pressure rises, the temperature drops and the wind decreases.

The precipitation formed by the first cold front is similar to that of the warm front, and its precipitation (cloud shape and cloud amount) is related to the warm air itself. The difference is that the precipitation area of the warm front is located in front of the front, and the cold front is located behind the front. Moreover, because the gradient of the cold front is larger than that of the warm front, its precipitation range is narrower than that of the former, with an average width of about 150 ~ 200 km.

B.type Ⅱ cold front This type of cold front moves faster and has a large front slope, which is generally 1/40 ~ 1/80. The warm air in front of the front was forced to rise sharply, which led to severe weather changes, but the range was narrow.

In summer, due to the moist and unstable warm air mass, strong cumulonimbus clouds and thunderstorms often appear before the ground front. When the cold front crosses the border, there are often dark clouds, storms and thunder, but the time is short, and the sky is clear after the cold front crosses the border; In winter, especially in the north, because the warm air is relatively dry and stable, precipitation clouds only appear near the front line of the ground, and there is continuous precipitation. After the front, the clouds disappear quickly, but the wind speed increases rapidly, and windy weather often occurs; For particularly dry areas or seasons, when the second cold front comes, sandstorms often occur, with no or little precipitation.

Cold front is one of the most important and common weather systems that affect the weather in China. Most of its cold air comes from Siberia or the Mongolian Plateau. The cold air activity is weak in summer, and when it moves southward, it often forms a slow-moving cold front in North China, and the affected area is mainly North China. In winter, cold air frequently goes south in half a year, and its power is stronger in the north, which often forms a second type of cold front, especially when there is continuous drought in winter and spring in the north, which often forms sandstorms, but the cold air gradually degrades and its power weakens in the process of going south. After moving to the Yangtze River basin or South China, it often turns into a slow-moving cold front or quasi-static front.

(3) When the cold and warm air masses meet, the quasi-static front is evenly matched, or the front rarely moves or swings back and forth in the same place due to terrain obstruction. This kind of front is called quasi-static front (static front). Generally speaking, a front whose position changes less than one latitude within six hours (two consecutive weather charts) is defined as a quasi-static front. The quasi-static front is mostly formed by the cold front heading south, the cold air gradually degenerating and the strength weakening. The quasi-static front weather is similar to the slow-moving cold front weather, except that the front slope is small (1150 ~1/200), so the surface rain area is wide, with an average width of 400 ~ 600 km. Generally, the rainfall intensity is small, and it is mostly continuous rain in Mao Mao for a long time. When the cold air or warm air is strengthened, the quasi-stationary front turns into a cold front or a warm front, which can produce short-term heavy rainfall.

The quasi-static front active in China mainly includes South China quasi-static front, Kunming quasi-static front, Tianshan quasi-static front and Qinling quasi-static front.

The quasi-static front in South China is an important weather system affecting the south of the Yangtze River in China: the cold air force is strong in winter and spring, and the ground front can extend southward to the sea surface in the north-central part of the South China Sea, which is called quasi-static front in the South China Sea; In summer, the frequency of activity is relatively less, and the location is relatively northerly, generally around 30 N, which is called Jiangnan quasi-static front.

After the formation of quasi-static front in South China, it generally lasts for a long time, with an average of about 10 day. Especially when cold air frequently supplements southward, the quasi-static front stays in a certain area for a long time, resulting in continuous rainy weather, such as low temperature and less sunshine in winter in South China. The sunshine percentage in Nanling mountain area, 1 and February is less than 20% in some places.

The quasi-static front in Kunming is mainly formed by the cold air from the south being blocked by the Yunnan-Guizhou Plateau, and it is often connected with the quasi-static front in Nanling, with similar weather characteristics.

(4) Blocking front Because the cold front moves faster than the warm front, the cold front catches up with the warm front or two cold fronts meet, and the warm air is lifted to the sky, forming a new front under the original front, which is called blocking front. If the cold air mass before the front is colder than the cold air mass behind the front, it is called a warm blocking front. If the cold air mass behind the front is colder than that before the front, it is called a cold blocking front. Because the blocked front is formed by merging two fronts with different moving speeds, it retains some weather characteristics of the original two fronts. However, after confinement, the warm air gradually rises and spreads along the front, so in the early stage of confinement, the clouds gradually thicken, the cloud area expands, the precipitation area also expands, and the precipitation increases. With the development of confinement, the warm air is lifted high, the water vapor content is reduced, the clouds are thinned and dissipated, the precipitation gradually weakens and stops, and the confinement front becomes a single warm front or cold front, and the confinement disappears.

Occlusion fronts mainly appear in Northeast China and North China, and are most common in spring.

These fronts are the main weather systems that affect the weather in China. Due to our special geographical location, cold and warm air masses are active and frontogenesis is very active. The formation and movement of fronts bring abundant precipitation to many areas, which is one of the main reasons why China is warm and humid in summer and cold and dry in winter.